Final Report Summary - SAFEJOINT (Enhancing structural efficiency through novel dissimilar material joining techniques)
Executive Summary:
Structural engineers are constantly pushing the boundaries, aiming to design increasingly energy efficient and safe structures. This requires a multi-material structural design approach that introduces the significant challenge of joining materials, which often possess fundamentally different physical properties, so that the operational loads are transferred efficiently and safely through the structure with minimum presence of hot spot stresses. Bolting, riveting and adhesive bonding have been the most widely used techniques to perform such joints with the latter considered the most efficient both in terms of light weight design and efficient stress transfer. However, adhesives technologies have now reached a limit in terms of bond strength and new approaches are necessary to enhance adhesive strength, raise safety factors and ensure the long term integrity of adhesively bonded joints. SAFEJOINT has undertaken R&D work to develop novel processes in adhesive bonding of dissimilar materials through the development of techniques such as the incorporation of carbon nanotubes at the joint interface in an attempt to enhance bonding strength of joints, the use of functionally graded adhesives to tailor the joint strength based on the distribution of peel stresses along the joint and to optimise performance though mechanically interlocking elements introduced at the interface of mainly metal to composite joints. SAFEJOINT also investigated the possibility of joining of dissimilar metals by exploiting the strengthening effects of carbon nanotubes at the weld pool of friction stir welds (FSW). The work also developed nano-containers to provide corrosion inhibitors at the weld pool thus eliminating any preferential corrosion effects at the weld pool.
SAFEJOINT also addresses the issue of non-destructive inspection (NDE) of bonded joints, an issue of concern for all bonded joints as it has always been a challenge to assess the integrity of the joint interface non-destructively. A novel approach of signal processing from ultrasonic inspection has been developed to reliably inspect the condition of the joints. The major impact of the approach is that ultrasonics is a widely used technique in industry and the uptake of the developed technologies will be beneficial to industry in the short term.
All the technologies developed within SAFEJOINT have addressed joining practices of interest to a wide range of applications, namely; rail car-body construction (e.g. lightweight composite roofs on aluminium body frames), marine applications (composite superstructures on steel hulls), cold repair technologies (composite repairs on steel pipelines) and aeronautics applications (bonding of wing skins and semi-structural fuselage parts).
Project Context and Objectives:
Lightweighting in structural design has in recent years seen a significant expansion with the use of lightweight metallic alloys and polymer composite materials becoming ever more prevalent in engineering construction. However, there are limits to what can be achieved by the use of a single lightweight material and designers have introduced the concept of “hybrid” structures, involving the combination of two or more lightweight materials, each possessing unique mechanical and physical properties, which can result in high performance lightweight structures not achievable by the use of a single material alone. This design approach however, introduces the problem of “joining” of materials with fundamentally different physical properties, which requires the introduction of innovative techniques to establish strong and durable bonds able to withstand service loads throughout the design life of a structure.
The SAFEJOINT project addressed this problem and its main aim is to develop novel joining techniques for metals to polymer composites as well as dissimilar metal alloys, exploiting the unique characteristics of carbon nanotubes (CNT) introduced at the joint interface (bond line), which is expected to enhance the mechanical properties of such joints.
Although the main idea of SAFEJOINT relies in the introduction of carbon nanotubes (CNT) at the joint interface, the method of introduction of the CNTs uses a number of innovative techniques which include; a) Electrophoretic Deposition (EPD) to attach CNTs on the fibre reinforcement network or the metal substrate to manufacture prefabricated joints, b) dispersion of CNTs in epoxy adhesives to manufacture metal composite joints, c) manufacture of CNT thin veils introduced at the interface between the dissimilar bonded joints. In addition SAFEJOINT explores the introduction of functionally graded adhesives for dissimilar material joints through the development of suitable dispensers. Mechanically interlocking metal to composite joints in addition to CNT addition is also explored. Finally the project addresses dissimilar metal joints by adopting the chemical vapour deposition (CVD) method to create in-situ CNTs at the interface of friction stir welded dissimilar metal alloys.
The development of dissimilar joints is also complemented in SAFEJOINT by the development of non-destructive evaluation (NDE) and inspection techniques to facilitate the implementation of the developed joining technologies by industry, by ensuring that quality control and through life inspection are feasible and reliable. The overall objective of the project is to provide designers with; a reliable method of joining of dissimilar materials by providing the information for a range of solutions applicable to a variety of applications in transport and renewable energy sectors; details of the manufacturing procedures for such joints; as well as the methodology to inspect them.
By enhancing the properties of dissimilar material joints, the project makes a step change over the current adhesive bonding technologies for metal to composite joints. In addition by developing techniques for the introduction of CNTs at the interface of friction stir welded dissimilar metal joints enables such joints of good strength to be realised.
Project Results:
SAFEJOINT has addressed the issue of dissimilar material joining by considering metal to polymer composite joints as well as metal-to-metal joints. The main results from the research and the techniques adopted are as follows:
A. Use of carbon nanotubes at the interfacial bond region between the metal substrate and the composite.
This was achieved through mixing and dispersion of CNTs in epoxy resin at the interface. Formulations with different weight contents of CNTs ranging from 0.05% to 0.5%. Ultrasonication and shear mixing have been used for the dispersion of the nanotubes in the resin and tests on resin (with and without CNTs) were carried out showing that the major benefit of the introduction of CNTs has been to enhance the fracture toughness of the resin.
Once the dispersion techniques had been optimized the work then proceeded in the manufacture of Steel to composite (both glass fibre and carbon fibre reinforced) single lap joints has been manufactured in which pure epoxy resin is used as the adhesive. The metal substrates were shot blasted with aluminium oxide grit prior to bonding. The fabric reinforcement used to manufacture the composite laminates was: Woven Glass Cloth [0o/90o] and Woven Carbon Cloth [0o/90o]. The manufacturing process was hand lay-up followed by vacuum bagging. Co-cured single lap joints, i.e. CFRP/Steel and GFRP/Steel were manufactured. Lap shear tests have been conducted with constant crosshead speed equal to 1mm/min and the overlap length is 25mm in all lap joints.
The results have shown that there is a clear benefit to the strength of a bonded joint when the CNTs are introduced at the interface and since the failure mode changes form cohesive to adhesive as the content of the CNTs increases, it is clear that this is due to the improvement of the fracture properties of the adhesive layer through the introduction of the CNTs. What has also been observed that the beneficial effect of the introduction of CNTs at the interface drops as the weight content increases beyond 0.3%. This is mainly due to the increased agglomeration of CNTs in the resin for higher percentage content. It is expected that this may improve if different regimes for more efficient dispersion of CNTs are used.
B. Functionally graded adhesives
An alternative approach investigated in SAFEJOINT has been the implementation of joints using adhesives of different properties using two (or more) compatible adhesives (base adhesives) with dissimilar mechanical behaviours that can be mixed to generate new phases with intermediate/tailored properties. For that, two epoxy adhesives have been selected and characterized (in-bulk tensile tests). Moreover, different mixtures of them have been also tested. In this sense, these experimental activities have required:
• Determination of a common curing cycle for the mixtures.
• Fabrication of samples.
• Execution of tensile tests to evaluate the resultant stiffness and the toughness.
Regarding the calculations for the design and optimization of the ‘functionally graded adhesive joints”, the activities performed by ITA in this first period of the project comprise:
• Analysis of the influence of different geometric parameters (fillet radius, adhesive thickness, overlap length, etc) for mono-adhesive solutions considering the two base adhesives selected (rigid and flexible) and various mixtures of them.
• Study of the influence of different geometric parameters for the functionally graded solutions.
• Optimizations for different gradings approaches (continuum gradings and “banded” or discrete variations) comparing with mono-adhesive solutions, (with aluminium substrates).
• Analysis of the optimum gradings for dissimilar substrates. This is an on-going activity performed for a general metal/composite joint. The CAF substrate was characterized in July 2014 and IFAM sent ITA the properties of their substrates just prior to the Athens meeting. Some of the results were presented at the SAFEJOINT workshop in Athens. This optimization analysis will be repeated considering the properties of the composites substrates of CAF and IFAM.
• Study of the effect of the squeeze flow during the manufacture of the joints (integrations of CFD and FEM simulations).
All these tasks have been performed through detailed 2D FE models.
The FEM analyses performed show how the overlap length, the fillet radius, as well as overlap length and adhesive thickness strongly affect the joints maximum load capacity. In this sense, the curves of dependencies obtained can be used as criterion for the joints definition/design. Moreover, the influence of the substrates materials models has been also studied.
The results of the FE analyses performed for the optimization of adhesive joints considering different grading strategies have been presented in the Industrial Workshop being held in Athens in July 2014.
Finally, regarding the study of the effect of the squeeze flow during the manufacture of the joints, the activity is still on-going. The methodology to integrate the fluid-dynamic simulations (CFD) with the 2D FE models has been established and some preliminary analyses have been performed for different adhesive thicknesses for both the continuum and the “banded” approaches. These are only preliminary simulations and future optimisations will consider both gradings and dissimilar substrates to obtain definitive conclusions. In both cases the squeeze from an initial “banded” adhesive configuration generates a shape of the grading that reduces the joints strength comparing with the non-squeezed joints. The failure criteria being used was that the simulations were stopped at maximum strain, prior to any cracking occurring. The execution of the optimizations considering this phenomenon will be carried out in the next months.
To be able to manufacture this type of joints, a dispenser + mixing/deposition system has been developed. The system is shown in the following pictures and consists of a series of actuators and mixers that are digitally controlled to dispense the exact amount of adhesive and at the required flow velocity.
C. Prefabricated titanium/ composite joints
This approach adopted in SAFEJOINT investigated the manufacture of dissimilar material joints where the metal substrate features pins where mechanical interlocking with the fibre reinforcement of the composite is the mean to enhance strength. The innovative aspect in this part of the work is that the interlocking pins are laid on the substrate surface through additive manufacturing technologies and can therefore be of any design and shape required for optimum performance at a faction of the cost of any machining technique.
The results show that the incorporation of the pins in the design gave a significant improvement to failure strength of a joint. What is interesting is that the initiation of a delamination crack between the substrate and the composite does not affect the stiffness of the joint (i.e. the slope of the load displacement curve remains the same) and failure is not sudden but progressive.
D. Dissimilar metal Joints.
The feasibility of joining dissimilar alloys by exploiting the strengthening effects of CNTs at the weld pool of a friction stir weld (FSW) was investigated within SAFEJOINT. Dissimilar friction stir welding was conducted using base metals aluminium alloys AA 6082 and AA5083. Carbon nanotubes (CNTs) and functionalized CNTs were used as reinforcing fillers in the stir zone to produce joints with enhanced mechanical properties, improved tribological characteristics and increased creep and fatigue strength. Also, hybrid organic-inorganic layers were used in the stir zone to produce dissimilar joints with improved corrosion protection properties. The hybrid organic-inorganic coatings including loaded containers for corrosion protection were applied to aluminium alloys AA6082 and AA5083 panels via roller application prior to friction stir welding.
The welding direction was parallel to the rolling direction of the aluminium alloys plates. In order to incorporate the fillers, half grooves were machined at the joining faces of the plates. When the plates were located on the back plate and were rigidly connected on the clamping arrangement, a groove with dimensions of 180 mm length, 1 mm width and 2.5 mm depth was created.
In addition to optimisation of the FSW techniques incorporating CNTs, the manufacture of the metal/metal joints using Hybrid Organic-Inorganic Layers for corrosion protection was also carried out. The aim of this work was to improve corrosion protection properties of the joined part between the metal alloys. Amounts of CeMo containers loaded them with corrosion inhibitor 2-mercaptobenzothiazole and incorporated them into hybrid organic-inorganic coatings. The containers were produced and loaded according to the following process. At first, anionic charged organic nanospheres were synthesized via polymerization in suspension using acetonitrile as a solvent, methacrylic acid as monomer and potassium persulphate as an initiator. Then, the fabricated organic nanospheres were used as templates and coated with cerium (III) acetylacetonate and via the sol gel process. After that, the produced CeMo composites were centrifuged and washed with acetonitrile. Finally, the CeMo containers were synthesised after removal of the templates via dispersion of the CeMo composites into ethanol 96%. The produced containers are hollow spheres and their diameter ranges from 370 nm to 600 nm.
The energy dispersive X ray spectroscopy (EDS) confirms that the produced containers consist of cerium, molybdenum and oxygen. The presence of carbon is attributed to template residues that have not been completely removed.
The loading of the containers with 2-mercaptobenzothiazole was accomplished according to the loading process described in the previous report. Briefly, the CeMo containers were placed in a sealed flask under vacuum conditions. A saturated solution of MBT in acetone was added into the sealed flask under vigorous stirring. Finally, the containers were centrifuged and washed with ethanol. The loading of the containers was 60%w/w with 2-mercaptobenzothiazole.
The hybrid organic-inorganic coatings including loaded containers were applied to aluminium alloys AA6082 and AA5083 panels via roller application.
E. NDE of dissimilar material joints
Metal/composite joints produced from different metals to fibre reinforced composites with different acoustic properties (metal – composite), complicated geometry (pins for better grip), rough inhomogeneous surface: porous titanium part; woven CFRP, multi-adhesive joints presented a challenge to the NDE technology to be implemented. The conventional NDE techniques used for inspection of joints were not suitable. Novel signal processing techniques had to be developed because new type of joints having complex geometry required novel signal processing techniques to be implemented.
It was determined, that for inspection of metal-composite joints pulse-echo inspections using focused transducers can be used, but signals have to be processed additionally in order to eliminate the influence of the complicated geometry. This identified defects in joints and even between layers of composite.
In most cases if the pulse echo inspection is preferred the inspection from the composite side is advisable, but there are exceptions – in case of some types of GFRP used the reflections from the fibres were so strong, that it is not possible to reconstruct the image of the adhesion zone. For some type of defects it has to be taken into account that amplitude differences at the defected and defect free areas are very minor and accurate signal analysis has to be performed. In case of samples with complicated geometry, delaminations can be detected in different layers (between metal and composite and between composite layers) and in different positions - even between the pins. The through transmission and pulse echo results are influenced by the flatness of the metal substrate as well as the rough surface of the composite. To compensate the plate misalignment and surface roughness the correction of signal arrival time has to be performed. Arrival time correction allows picking up the ultrasonic signals in the zone of the interest (i.e. joint zone) using time window. It can be noticed, that differently prepared aluminium and steel (polished, ground, metal blasted) and different composite materials (CFRP and GFRP) used don’t influence the quality of the joints and the inspection procedures. However, in all cases the post processing of the ultrasonic data has to be performed.
Also, experiments on the titanium-CFRP samples to investigate the influence of the carbon nanotubes to the attenuation of the ultrasonic signals were carried out, using the pulse echo and through-transmission techniques, with 15MHz focused transducers.
C-scan images were obtained using through-transmission measurements. From the results the follow, the CNT introduces some amplitude damping since the thicknesses and materials were the same for both samples. The comparison of the waveforms at the position (x=21.3mm; y=18.5mm and x=105.1m; y=18.5mm) was seen.
On the CNT sample a small region can be seen where the amplitude of the transmitted signal is lower compared to neighbouring positions. This might be due to some air-bubbles or some other structural features.
C-scan images were obtained using pulse-echo measurements. The results showed some amplitude variations, which are possibly caused by inhomogeneous structure of the composite. A possible air-bubble can also be detected on the CNT sample
Potential Impact:
Joining of dissimilar materials has always been a challenge for several industrial applications and over the years methods and technologies to improve joining as well as methods to reliably inspect those joints have constantly been sought by industry. SAFEJOINT has therefore provided a major step forward in this respect by developing novel technologies to improve the joining strength of dissimilar material joints (both metal to metal and metal to composite) as well as the techniques to reliably inspect those.
The project has generated a lot of interest in both the research community and industry and since the first SAFEJOINT Workshop there have been several enquiries about the technologies involved and several research establishments in Europe and elsewhere are experimenting with the approaches developed in SAFEJOINT and undertaking R&D work to extend the techniques further. The project has therefore made a significant impact in terms of generating new ideas and approaches on the use of carbon nanotubes (CNTs) to enhance bonded joint strength. Several laboratories are now investigating other nanoparticles, such as single wall carbon nanotubes and graphene as the strengthening medium.
In terms of industrial uptake already one of the Industry Advisory Group members has been conducting experiments for implementation of the use of CNTs in its pipeline repair process and partner CAF is planning to implement the use of functionally graded adhesives for rail vehicle manufacture. For the transport applications the implementation is expected to be in the medium to long term as the process of adoption of new technologies requires the undertaking of lengthy qualifications tests, since it is related to structural designs involving public safety.
The output from SAFJOINT has however generated interest to further explore the techniques involved which resulted in the safeguard of existing jobs of the industrial partners involved as the benefits of implementing the developed technologies constitute a clear product advantage over the competition without any significant financial penalty to them over implementation.
The project as mentioned earlier has also generated significant interest in the scientific community and has therefore generated research opportunities for young graduates and a renewed interest in this field of research. Also through a range of publications in scientific journals and conferences the research output from SAFEJOINT has enhanced the position of the EU at the leading edge of scientific excellence.
General Dissemination
The SAFEJOINT had a clear outline of dissemination activities, which were outlined in a Dissemination Strategy. This was updated on an annual basis to take into account activities that had taken pace during the previous year and the impact the success of these had on the coming years dissemination activities.
The strategy focused on a series of key objectives in both internal and external dissemination emphasising the strategic and operational direction of the strategy to fulfil the aim of raising awareness of the project and entice industry to participate in the practical aspects of the project. To facilitate this, there was an Industry Advisory Group (IAG) to monitor the progress of the project and industry participation and advice. Other internal Groups also had an input into the dissemination activities to ensure the maximum high quality exposure to the project is achieved.
Internal dissemination activities were the responsibility of the coordinator who then filtered down individual responsibility to partners. These activities included:
• On-line media
o Website
o Newsletters
o LinkedIn forum
o CORDISWIRE
o Industry targeted
• Project brochures
• Press releases
• Academia targeted
o Journal publications
o Presentations at conferences
• Dedicated Workshop/Conference
o Workshop to be held in M18
o Conference to be held in M34 – “Joining of Dissimilar Materials”
Workshop
An Industrial Workshop was organised in Athens on 3-4 July 2014, which was halfway through the project (Month 18) and was a forum for presenting the results from the SAFEJOINT research so far and how these could be used in an industrial setting, thereby leading to closer collaboration between the project partners and industry.
The Workshop included presentations by the SAFEJOINT partners, as well as from projects Ybridio and PMJoin which form a cluster of projects on the topic of dissimilar projects supported by the EC and other invited experts from industry. The coordinators UNEW have strong associations and links to industry associations such as UNIFE (Rail manufacturers association), EASN (European Aeronautics Science Network), ACARE (Aircraft manufacturer’s association and WEGEMT (Marine Technologies Association) and other associations who were all contacted and invited to make a presentation as well as attend as a delegate.
During the workshop open discussions were encouraged between industry and the research community in dedicated sessions to explore the future needs of industry in lightweight design and the future direction of research in the area of dissimilar materials joining. The workshop was free to participants and open to all to attend.
Since the input of industry was a key component of the workshop outcome it was decided to have facilitated sessions to encourage discussion, feedback and knowledge transfer from the delegates.
The advantages of having a facilitator were seen to be:
• The facilitator helped to make sure that the right people were at the workshop and drew out their expertise to share it with the rest of the participants.
• By having a facilitator lead a section of the workshop, the project coordinator became a participant, rather than the workshop leader. This was an invaluable aid in developing collaboration and allowed him to participate fully, to view the project objectively and listen to the opinions of others in the session in a way that they could not if they were leading.
• There was consistently high output throughout the 2 days giving credible responses.
• A high level of knowledge transfer was achieved because it was specifically built into the workshop sessions via the topics. The delegates left the workshop with a better understanding of the activities and issues within the area of joining technologies and how the project was proposing to overcome these plus a clear vision of how the project was moving forward.
The Workshop sessions over the two days were made up of the following topics:
1. Metal to Composite joining
2. Modelling of dissimilar material joints
3. NDE of dissimilar materials joints
4. Environmental degradation of dissimilar material joints
5. Dissimilar metal joints
6. General Discussion
Day 1
Session 1. Metal to composite joining
The first session was designed to showcase the developments on dissimilar material joining by the three projects, SAFEJOINT, YBRIDIO and PMJoin.
At the end of the session presentations, the following issues were raised:
• The response from the delegates during the discussion session was overall positive on the new technologies been presented and the diverse applications addressed by the three projects. In the words of the Alstom Transportation delegate “There is a lot of information and innovation presented here that is of interest directly to us and I am sure there is the potential for these technologies to will be taken up by industry in the medium term”
• Use of specimen geometry. In the majority of presentations of SAFEJOINT, Ybridio and PMJoin, a lap shear specimen has always been used for the assessment of the strength of the joint. It has been questioned by some industry representatives that this may not be the best approach as different applications require different configurations. A discussion ensued and it became apparent that indeed adhesive bond strength varies with different loading configurations. However, a bonded joint always exhibits best performance
• The rest of the discussions were mainly concerned with explanatory details of aspects of the work presented. One of the questions that arose was that of cost of the new technologies. It was explained that the approaches used do not require any significant capital expenditure and could be easily incorporated into existing operations where adhesive bonding is already in use. The consortia of all projects are well aware of the commercial demands of industry and they aim to achieve a step change in adhesive bonding performance without any significant cost penalties.
Session 2. Modelling of dissimilar material joints
The lunch break provided an area for eating and networking before the afternoon sessions began with Modelling of dissimilar joints again facilitated by George Kotsikos. The topics covered were:
• Failure modes in composite joints: a finite element study.
• “Mixed adhesive joints”
• Integrated modelling of 6xxx series friction stir welds: from thermal cycles to mechanical performances.
• Ultrasonic NDT of dissimilar joints
• Development of Piezoelectric sensors
The open discussion at the end of these topics brought the end of day one of the workshop to a close, covered the following issues and prepared the delegates for the next days activities:
• The presentation of functionally graded adhesives also drew an interest focusing on the effect of (or degree of) mixing of the adhesives once they have been placed at the joint. It was noted that this should not have a significant effect on the performance if it was viewed in terms of the stress distribution along a lap shear joint (i.e. peak load at the edges of the lab and dropping to a small level along the length of the lap. It was suggested that it might be sufficient to have adhesive placed only at the lap edges but it was explained that this would not have worked because of the nature of the stress distribution which will create stress raisers along the lap joint. It was also mentioned that the embedded strain sensor technology developed within SAFEJOINT may provide quantitative information of the stress distribution within a joint and help optimise the placement of graded adhesives.
• The presentation on ultrasonic inspection of dissimilar material joints also drew significant attention as it is already a topic of interest on adhesively bonded joints in industry. The presentation showed that it is possible to detect at least artificial defects along the interface of dissimilar material joints and this led to a discussion of the capability of the technique to give a measure of the residual strength in a joint following the detection of defects. It was explained that at this stage it is a major achievement that defects have been identified as this was not possible by previous methods. Further work is required to develop a reliable residual strength detection tool.
• Another point that was raised was that of “kissing bonds”, namely bonds that appear sound (no void detected) but they are of significantly reduced strength as a result of contamination, environmental degradation or poor surface preparation of the joint. It was explained that at present this is not possible with current techniques. It was mentioned though that the thin film strain sensor technology developed in SAFEJOINT may be able to detect that but the sensors cannot provide information on the “global” condition of the joint.
• A question was raised on the environmental degradation of the joints. All projects are aware of the issue and work is underway addressing this issue. However, it is not expected to encounter any environmental degradation effects dissimilar to those of existing bonded joints as the process is dominated by the adhesive resin properties.
Day 2 of the workshop began with a session facilitated by the SAFEJOINT partner KTU (Lithuania) concerning Environmental degradation of dissimilar material joints. The topics covered in this first session were:
• Environmental impact on elastic adhesive joints in marine applications
• Degradation of adhesive joints
• The effects of pre-bond and after-bond contamination on the fracture toughness of adhesively bonded joints
The outcomes of the Open Discussion were:
• There are tools and procedures for assessing the long-term performance of bonded joints and those are addressed within SAFEJOINT. The discussion was already made on the previous day and it was re-iterated that the performance of bonded joints developed in SAFEJOINT, YBRIDIO and PMJOINT should not display behaviours overly dissimilar to existing bonded joints. The controlling factor is the moisture diffusion in the adhesive resins. In terms of dissimilar metal joints, the effect of environmental degradation (corrosion) is addressed through an innovative idea of incorporating in the weld pool nanocontainers that include corrosion inhibitors. This work is currently underway but preliminary results look promising.
• The issue of contamination of joint surfaces prior to bonding was also discussed as it is in the majority of cases the most important problem affecting the integrity of bonded joints and is related to the manufacturing procedures of the joints. It is a problem that requires extensive training of personel manufacturing those joints who are not always aware of the implications of inadequate surface preparation and cleanliness. This is also the source of “kissing bonds” for which the development of some type of NDE method needs to be developed. This problem is currently addressed in the standards by a combination of bonding and bolting of dissimilar materials for safety critical structures and components in the transportation industry.
After the morning coffee break, the next session titled Dissimilar metal joints was facilitated by Simon Frost of Walker Associates who is a member of the SAFEJOINT IAG. There were 2 topics covered in this time slot:
• Microstructure and mechanical properties of Al-to-steel welds performed by friction melt bonding.
• Micro structural and mechanical study of AA 5083 and AA 6082 friction stir welds reinforced with SiC, TiC and CNTs
Issues raised from the presentations were:
• The main issue affecting this type of work concerned NDE inspection and environmental degradation. KTU presented some images of dissimilar metal joints developed within SAFEJOINT utilising a new technique “ultrasonic spectroscopy” demonstrating that the detection of both defects as well as the distribution of nanoparticles in the weld of dissimilar metals. This technique is an invaluable tool for both inspection and weld quality, the latter allowing for the optimisation of the welding process.
The second day of the workshop was rounded-up with a general discussion forum facilitated by George Kotsikos about the Industry perspective, needs, comment and input on the future direction of research in dissimilar material joining followed by a round-up of discussions and points raised throughout the workshop.
The conclusions of the workshop were:
Industry have reiterated that their main motivation for the dissimilar material joining is lightweighting. The major concern over bonded joints was raised by the aeronautics industry where certification can vastly delay the implementation of any new technology due to the public safety critical application. In particular AIRBUS have expressed the view that riveting (or bolting will always by implemented in the industry due to the design rules being applied in aircraft manufacture. The design rules were then presented to the delegates by Dr Domenico Furfari of AIRBUS to reinforce this point.
The design rules document was looked at in detail by all delegates and a closer examination of the wording in this document showed that bonding alone is not excluded as a means of joining of dissimilar materials.
Any joining technique would be acceptable if it is accompanied by:
• precise manufacturing rules,
• reproducibility of the bond,
• detailed inspection techniques
• reliable through life structural integrity monitoring,
• methodologies to determine residual strength (or remaining life) of a joint following the detection of damage and
• prediction of long term performance of the joints.
What is important for all the above is that the degree of uncertainty must be extremely low and similar to the levels for existing joining techniques such as riveting or bolting.
However, it was also pointed out that the surface transport sector (rail, marine, automotive) is not constrained by such strict design rules as aeronautics. Notably for those industries, the issues of ability to reliably inspect the condition of the joints, long-term structural integrity and environmental degradation are deemed important.
Perhaps the most important issue for land transport is cost. Any developed techniques through SAFEJOINT have to be comparable in cost to existing methodologies for manufacture and inspection.
Another issue mentioned in one of the sessions on the previous day was reiterated at the general discussion session and deals with the ability to detect “weak bonds” (or “kissing bonds” as they are also called) as there is currently no detection techniques able to assess this type of defect in bonded joints and can be the source of premature failures and consequently uncertainty over the adoption of purely bonded joints which are really the goal for lightweighting.
Final Conference
The Conference was due at M34 when the project has reached its final conclusions. Initially, it was thought that the final conference, scheduled for M34 – October 2015, could be held at Newcastle University since there are the required facilities. However, the timing of the Conference conflicted with the start of the Academic term and this made booking a suitable venue difficult. Dr George Kotsikos, the SAFEJOINT project Coordinator was on the Scientific Committee of the forthcoming ICEAF Conference, (International Conference of Engineering Against Failure) being held in Skiathos on 24-26 June 2015 and negotiated a session for dissimilar joining techniques.
Partners were requested to submit abstracts concerning Dissimilar Metal Joints along with the Ybrido and PMJoin projects as part of the project clustering activities. The papers submitted by SAFEJOINT partners were presented on the first day of the Conference, Wednesday 24th June 2015 were:
SAFEJOINT – CONFERENCE PAPERS – SKIATHOS 2015
SESSION 9 - Wednesday 24 June 2015 - 16:00 – 17:40
Chairmen – Dr. George Kotsikos & Professor Geoff Gibson
SAFEJOINT
PARTNER PRESENTER PAPER TITLE
IFAM A. Wulf, M. Brede,
C. Nagel High cycle fatigue behaviour of structural adhesive joints
ITA Chiminelli, R. Breto, E.Duvivier
M.A. Jiminez Functionally graded bond-lines for metal/composite joints
SICOMP V. Churchill SC, E. Julin, D. Mattsson, D. Ramantani,
S. Tampas Mechanical performance of prefabricated hybrid joints
KTU E. Jasiuniene,
L. Mazeika,
E. Zukauskas,
V. Samaitis,
V. Cicenas
NDT of dissimilar joints using ultrasonic and X-ray CT methods
UNEW M. Konstantakopoulou Failure Mode and Strength of FRP/Steel Joints Bonded with MWCNT Epoxy Adhesive
UNEW A. Deligianni, J. Hale,
G. Kotsikos
Development of piezoelectric strain sensors embedded in bonded dissimilar material joints
Conclusions of the conference
All papers presented were well received by the conference audience and several questions were raised during the discussion time after each paper presentation. As this was organised to be a stand-alone session of “Joining of dissimilar materials”, an “open discussion” time slot was allocated so the audience could discuss the issues involved after they have heard all the presentations. The topic was found to be of outmost interest both to industry representatives and academics.
Joining of dissimilar materials is a very challenging topic and there is not one solution alone for this problem. It was therefore welcomed by the audience that SAFEJOINT addressed a number of approaches, each with its own innovations and challenges, which could be implemented in a range of applications.
It was recognised that SAFEJOINT has been a step forward in addressing the topic of dissimilar materials joining, however, some challenges remain in terms of automation of the processes involved and extensive qualification testing before some of the techniques could be widely adopted by industry (in particular the aerospace industry)
The innovations introduced in the NDE approaches adopted in SAFEJOINT, mainly the use of ultrasonics has also been well received with several questions put to the presenters at the open discussion session and afterwards in private discussions. The attractiveness of the ultrasonics NDE is that it is a well-established method, well understood and widely used in industry and will therefore be a very straightforward process to transfer the knowledge generated in SAFEJOINT to inspections of dissimilar material joints in the marine, rail and aeronautics industries.
Website
The SAFEJOINT website was the main on-line tool for disseminating the project and its progress to a wide audience:
• SAFEJOINT progress
• Promoting external activities such as the SAFEJOINT workshop and conference
• Facility to register directly for the above events
• Point of contact for interested readers
Primary Roles
The initial key objective for the website was to raise awareness of the project. To facilitate this, the website aimed to:
• Present a professional design that was relevant and makes visitors comfortable and attract interest in the project and its objectives.
• Design an architecture, which took into account the importance of being search engine friendly, whilst maintaining the management of web site content and images.
• The ability to find the SAFEJOINT information quickly and simply.
• Present up-to-date news items and relevant content by placing strategic calls to action, visitors were encouraged to actively engage with the website rather than just be a passive visitor.
Secondary Roles
After the initial set-up phase the website continued to:
• Convey and maintain the key messages of the project. Including the facilitation of all project progress information and essential activities.
• The content structure took into account visitors who were seeking information concerning the project and information about forthcoming SAFEJOINT events.
Key Outcomes of the website
Through the design and development of the website with functionality such as Newsletters, Visitors Registration, Personalisation and Online Document Management the following Key Outcomes were delivered:
Providing a source of information and awareness by making visitors aware of the SAFEJOINT website by: presenting relevant content, assisting users in having their questions answered, ensuring relevant information
• Supporting the project by reinforcing the objectives and progress and communicating effectively with existing visitors
• Performing its role effectively by: being built using flexible site architecture, having an appropriate mix of internal and external resources, being supported by the various practice areas.
The domain name safejoint.net was secured and a website created at www.safejoint.net.
The pages are:
• A home page
• Objectives of the project
• The work packages as an overall page linking to the individual WP pages
• A contact page
• A SAFEJOINT Events page
• Downloads of papers and presentations
The website was hosted and maintained by the coordinators UNEW with regular updates and maintenance of links. A visitor counter will recorded the overall number of visitors and at the end of the project this was 17000. The website will be kept live for 2 years after the end of the project.
List of Websites:
www.safejoint.net
Dr George Kotsikos
+44 191 2085889
Structural engineers are constantly pushing the boundaries, aiming to design increasingly energy efficient and safe structures. This requires a multi-material structural design approach that introduces the significant challenge of joining materials, which often possess fundamentally different physical properties, so that the operational loads are transferred efficiently and safely through the structure with minimum presence of hot spot stresses. Bolting, riveting and adhesive bonding have been the most widely used techniques to perform such joints with the latter considered the most efficient both in terms of light weight design and efficient stress transfer. However, adhesives technologies have now reached a limit in terms of bond strength and new approaches are necessary to enhance adhesive strength, raise safety factors and ensure the long term integrity of adhesively bonded joints. SAFEJOINT has undertaken R&D work to develop novel processes in adhesive bonding of dissimilar materials through the development of techniques such as the incorporation of carbon nanotubes at the joint interface in an attempt to enhance bonding strength of joints, the use of functionally graded adhesives to tailor the joint strength based on the distribution of peel stresses along the joint and to optimise performance though mechanically interlocking elements introduced at the interface of mainly metal to composite joints. SAFEJOINT also investigated the possibility of joining of dissimilar metals by exploiting the strengthening effects of carbon nanotubes at the weld pool of friction stir welds (FSW). The work also developed nano-containers to provide corrosion inhibitors at the weld pool thus eliminating any preferential corrosion effects at the weld pool.
SAFEJOINT also addresses the issue of non-destructive inspection (NDE) of bonded joints, an issue of concern for all bonded joints as it has always been a challenge to assess the integrity of the joint interface non-destructively. A novel approach of signal processing from ultrasonic inspection has been developed to reliably inspect the condition of the joints. The major impact of the approach is that ultrasonics is a widely used technique in industry and the uptake of the developed technologies will be beneficial to industry in the short term.
All the technologies developed within SAFEJOINT have addressed joining practices of interest to a wide range of applications, namely; rail car-body construction (e.g. lightweight composite roofs on aluminium body frames), marine applications (composite superstructures on steel hulls), cold repair technologies (composite repairs on steel pipelines) and aeronautics applications (bonding of wing skins and semi-structural fuselage parts).
Project Context and Objectives:
Lightweighting in structural design has in recent years seen a significant expansion with the use of lightweight metallic alloys and polymer composite materials becoming ever more prevalent in engineering construction. However, there are limits to what can be achieved by the use of a single lightweight material and designers have introduced the concept of “hybrid” structures, involving the combination of two or more lightweight materials, each possessing unique mechanical and physical properties, which can result in high performance lightweight structures not achievable by the use of a single material alone. This design approach however, introduces the problem of “joining” of materials with fundamentally different physical properties, which requires the introduction of innovative techniques to establish strong and durable bonds able to withstand service loads throughout the design life of a structure.
The SAFEJOINT project addressed this problem and its main aim is to develop novel joining techniques for metals to polymer composites as well as dissimilar metal alloys, exploiting the unique characteristics of carbon nanotubes (CNT) introduced at the joint interface (bond line), which is expected to enhance the mechanical properties of such joints.
Although the main idea of SAFEJOINT relies in the introduction of carbon nanotubes (CNT) at the joint interface, the method of introduction of the CNTs uses a number of innovative techniques which include; a) Electrophoretic Deposition (EPD) to attach CNTs on the fibre reinforcement network or the metal substrate to manufacture prefabricated joints, b) dispersion of CNTs in epoxy adhesives to manufacture metal composite joints, c) manufacture of CNT thin veils introduced at the interface between the dissimilar bonded joints. In addition SAFEJOINT explores the introduction of functionally graded adhesives for dissimilar material joints through the development of suitable dispensers. Mechanically interlocking metal to composite joints in addition to CNT addition is also explored. Finally the project addresses dissimilar metal joints by adopting the chemical vapour deposition (CVD) method to create in-situ CNTs at the interface of friction stir welded dissimilar metal alloys.
The development of dissimilar joints is also complemented in SAFEJOINT by the development of non-destructive evaluation (NDE) and inspection techniques to facilitate the implementation of the developed joining technologies by industry, by ensuring that quality control and through life inspection are feasible and reliable. The overall objective of the project is to provide designers with; a reliable method of joining of dissimilar materials by providing the information for a range of solutions applicable to a variety of applications in transport and renewable energy sectors; details of the manufacturing procedures for such joints; as well as the methodology to inspect them.
By enhancing the properties of dissimilar material joints, the project makes a step change over the current adhesive bonding technologies for metal to composite joints. In addition by developing techniques for the introduction of CNTs at the interface of friction stir welded dissimilar metal joints enables such joints of good strength to be realised.
Project Results:
SAFEJOINT has addressed the issue of dissimilar material joining by considering metal to polymer composite joints as well as metal-to-metal joints. The main results from the research and the techniques adopted are as follows:
A. Use of carbon nanotubes at the interfacial bond region between the metal substrate and the composite.
This was achieved through mixing and dispersion of CNTs in epoxy resin at the interface. Formulations with different weight contents of CNTs ranging from 0.05% to 0.5%. Ultrasonication and shear mixing have been used for the dispersion of the nanotubes in the resin and tests on resin (with and without CNTs) were carried out showing that the major benefit of the introduction of CNTs has been to enhance the fracture toughness of the resin.
Once the dispersion techniques had been optimized the work then proceeded in the manufacture of Steel to composite (both glass fibre and carbon fibre reinforced) single lap joints has been manufactured in which pure epoxy resin is used as the adhesive. The metal substrates were shot blasted with aluminium oxide grit prior to bonding. The fabric reinforcement used to manufacture the composite laminates was: Woven Glass Cloth [0o/90o] and Woven Carbon Cloth [0o/90o]. The manufacturing process was hand lay-up followed by vacuum bagging. Co-cured single lap joints, i.e. CFRP/Steel and GFRP/Steel were manufactured. Lap shear tests have been conducted with constant crosshead speed equal to 1mm/min and the overlap length is 25mm in all lap joints.
The results have shown that there is a clear benefit to the strength of a bonded joint when the CNTs are introduced at the interface and since the failure mode changes form cohesive to adhesive as the content of the CNTs increases, it is clear that this is due to the improvement of the fracture properties of the adhesive layer through the introduction of the CNTs. What has also been observed that the beneficial effect of the introduction of CNTs at the interface drops as the weight content increases beyond 0.3%. This is mainly due to the increased agglomeration of CNTs in the resin for higher percentage content. It is expected that this may improve if different regimes for more efficient dispersion of CNTs are used.
B. Functionally graded adhesives
An alternative approach investigated in SAFEJOINT has been the implementation of joints using adhesives of different properties using two (or more) compatible adhesives (base adhesives) with dissimilar mechanical behaviours that can be mixed to generate new phases with intermediate/tailored properties. For that, two epoxy adhesives have been selected and characterized (in-bulk tensile tests). Moreover, different mixtures of them have been also tested. In this sense, these experimental activities have required:
• Determination of a common curing cycle for the mixtures.
• Fabrication of samples.
• Execution of tensile tests to evaluate the resultant stiffness and the toughness.
Regarding the calculations for the design and optimization of the ‘functionally graded adhesive joints”, the activities performed by ITA in this first period of the project comprise:
• Analysis of the influence of different geometric parameters (fillet radius, adhesive thickness, overlap length, etc) for mono-adhesive solutions considering the two base adhesives selected (rigid and flexible) and various mixtures of them.
• Study of the influence of different geometric parameters for the functionally graded solutions.
• Optimizations for different gradings approaches (continuum gradings and “banded” or discrete variations) comparing with mono-adhesive solutions, (with aluminium substrates).
• Analysis of the optimum gradings for dissimilar substrates. This is an on-going activity performed for a general metal/composite joint. The CAF substrate was characterized in July 2014 and IFAM sent ITA the properties of their substrates just prior to the Athens meeting. Some of the results were presented at the SAFEJOINT workshop in Athens. This optimization analysis will be repeated considering the properties of the composites substrates of CAF and IFAM.
• Study of the effect of the squeeze flow during the manufacture of the joints (integrations of CFD and FEM simulations).
All these tasks have been performed through detailed 2D FE models.
The FEM analyses performed show how the overlap length, the fillet radius, as well as overlap length and adhesive thickness strongly affect the joints maximum load capacity. In this sense, the curves of dependencies obtained can be used as criterion for the joints definition/design. Moreover, the influence of the substrates materials models has been also studied.
The results of the FE analyses performed for the optimization of adhesive joints considering different grading strategies have been presented in the Industrial Workshop being held in Athens in July 2014.
Finally, regarding the study of the effect of the squeeze flow during the manufacture of the joints, the activity is still on-going. The methodology to integrate the fluid-dynamic simulations (CFD) with the 2D FE models has been established and some preliminary analyses have been performed for different adhesive thicknesses for both the continuum and the “banded” approaches. These are only preliminary simulations and future optimisations will consider both gradings and dissimilar substrates to obtain definitive conclusions. In both cases the squeeze from an initial “banded” adhesive configuration generates a shape of the grading that reduces the joints strength comparing with the non-squeezed joints. The failure criteria being used was that the simulations were stopped at maximum strain, prior to any cracking occurring. The execution of the optimizations considering this phenomenon will be carried out in the next months.
To be able to manufacture this type of joints, a dispenser + mixing/deposition system has been developed. The system is shown in the following pictures and consists of a series of actuators and mixers that are digitally controlled to dispense the exact amount of adhesive and at the required flow velocity.
C. Prefabricated titanium/ composite joints
This approach adopted in SAFEJOINT investigated the manufacture of dissimilar material joints where the metal substrate features pins where mechanical interlocking with the fibre reinforcement of the composite is the mean to enhance strength. The innovative aspect in this part of the work is that the interlocking pins are laid on the substrate surface through additive manufacturing technologies and can therefore be of any design and shape required for optimum performance at a faction of the cost of any machining technique.
The results show that the incorporation of the pins in the design gave a significant improvement to failure strength of a joint. What is interesting is that the initiation of a delamination crack between the substrate and the composite does not affect the stiffness of the joint (i.e. the slope of the load displacement curve remains the same) and failure is not sudden but progressive.
D. Dissimilar metal Joints.
The feasibility of joining dissimilar alloys by exploiting the strengthening effects of CNTs at the weld pool of a friction stir weld (FSW) was investigated within SAFEJOINT. Dissimilar friction stir welding was conducted using base metals aluminium alloys AA 6082 and AA5083. Carbon nanotubes (CNTs) and functionalized CNTs were used as reinforcing fillers in the stir zone to produce joints with enhanced mechanical properties, improved tribological characteristics and increased creep and fatigue strength. Also, hybrid organic-inorganic layers were used in the stir zone to produce dissimilar joints with improved corrosion protection properties. The hybrid organic-inorganic coatings including loaded containers for corrosion protection were applied to aluminium alloys AA6082 and AA5083 panels via roller application prior to friction stir welding.
The welding direction was parallel to the rolling direction of the aluminium alloys plates. In order to incorporate the fillers, half grooves were machined at the joining faces of the plates. When the plates were located on the back plate and were rigidly connected on the clamping arrangement, a groove with dimensions of 180 mm length, 1 mm width and 2.5 mm depth was created.
In addition to optimisation of the FSW techniques incorporating CNTs, the manufacture of the metal/metal joints using Hybrid Organic-Inorganic Layers for corrosion protection was also carried out. The aim of this work was to improve corrosion protection properties of the joined part between the metal alloys. Amounts of CeMo containers loaded them with corrosion inhibitor 2-mercaptobenzothiazole and incorporated them into hybrid organic-inorganic coatings. The containers were produced and loaded according to the following process. At first, anionic charged organic nanospheres were synthesized via polymerization in suspension using acetonitrile as a solvent, methacrylic acid as monomer and potassium persulphate as an initiator. Then, the fabricated organic nanospheres were used as templates and coated with cerium (III) acetylacetonate and via the sol gel process. After that, the produced CeMo composites were centrifuged and washed with acetonitrile. Finally, the CeMo containers were synthesised after removal of the templates via dispersion of the CeMo composites into ethanol 96%. The produced containers are hollow spheres and their diameter ranges from 370 nm to 600 nm.
The energy dispersive X ray spectroscopy (EDS) confirms that the produced containers consist of cerium, molybdenum and oxygen. The presence of carbon is attributed to template residues that have not been completely removed.
The loading of the containers with 2-mercaptobenzothiazole was accomplished according to the loading process described in the previous report. Briefly, the CeMo containers were placed in a sealed flask under vacuum conditions. A saturated solution of MBT in acetone was added into the sealed flask under vigorous stirring. Finally, the containers were centrifuged and washed with ethanol. The loading of the containers was 60%w/w with 2-mercaptobenzothiazole.
The hybrid organic-inorganic coatings including loaded containers were applied to aluminium alloys AA6082 and AA5083 panels via roller application.
E. NDE of dissimilar material joints
Metal/composite joints produced from different metals to fibre reinforced composites with different acoustic properties (metal – composite), complicated geometry (pins for better grip), rough inhomogeneous surface: porous titanium part; woven CFRP, multi-adhesive joints presented a challenge to the NDE technology to be implemented. The conventional NDE techniques used for inspection of joints were not suitable. Novel signal processing techniques had to be developed because new type of joints having complex geometry required novel signal processing techniques to be implemented.
It was determined, that for inspection of metal-composite joints pulse-echo inspections using focused transducers can be used, but signals have to be processed additionally in order to eliminate the influence of the complicated geometry. This identified defects in joints and even between layers of composite.
In most cases if the pulse echo inspection is preferred the inspection from the composite side is advisable, but there are exceptions – in case of some types of GFRP used the reflections from the fibres were so strong, that it is not possible to reconstruct the image of the adhesion zone. For some type of defects it has to be taken into account that amplitude differences at the defected and defect free areas are very minor and accurate signal analysis has to be performed. In case of samples with complicated geometry, delaminations can be detected in different layers (between metal and composite and between composite layers) and in different positions - even between the pins. The through transmission and pulse echo results are influenced by the flatness of the metal substrate as well as the rough surface of the composite. To compensate the plate misalignment and surface roughness the correction of signal arrival time has to be performed. Arrival time correction allows picking up the ultrasonic signals in the zone of the interest (i.e. joint zone) using time window. It can be noticed, that differently prepared aluminium and steel (polished, ground, metal blasted) and different composite materials (CFRP and GFRP) used don’t influence the quality of the joints and the inspection procedures. However, in all cases the post processing of the ultrasonic data has to be performed.
Also, experiments on the titanium-CFRP samples to investigate the influence of the carbon nanotubes to the attenuation of the ultrasonic signals were carried out, using the pulse echo and through-transmission techniques, with 15MHz focused transducers.
C-scan images were obtained using through-transmission measurements. From the results the follow, the CNT introduces some amplitude damping since the thicknesses and materials were the same for both samples. The comparison of the waveforms at the position (x=21.3mm; y=18.5mm and x=105.1m; y=18.5mm) was seen.
On the CNT sample a small region can be seen where the amplitude of the transmitted signal is lower compared to neighbouring positions. This might be due to some air-bubbles or some other structural features.
C-scan images were obtained using pulse-echo measurements. The results showed some amplitude variations, which are possibly caused by inhomogeneous structure of the composite. A possible air-bubble can also be detected on the CNT sample
Potential Impact:
Joining of dissimilar materials has always been a challenge for several industrial applications and over the years methods and technologies to improve joining as well as methods to reliably inspect those joints have constantly been sought by industry. SAFEJOINT has therefore provided a major step forward in this respect by developing novel technologies to improve the joining strength of dissimilar material joints (both metal to metal and metal to composite) as well as the techniques to reliably inspect those.
The project has generated a lot of interest in both the research community and industry and since the first SAFEJOINT Workshop there have been several enquiries about the technologies involved and several research establishments in Europe and elsewhere are experimenting with the approaches developed in SAFEJOINT and undertaking R&D work to extend the techniques further. The project has therefore made a significant impact in terms of generating new ideas and approaches on the use of carbon nanotubes (CNTs) to enhance bonded joint strength. Several laboratories are now investigating other nanoparticles, such as single wall carbon nanotubes and graphene as the strengthening medium.
In terms of industrial uptake already one of the Industry Advisory Group members has been conducting experiments for implementation of the use of CNTs in its pipeline repair process and partner CAF is planning to implement the use of functionally graded adhesives for rail vehicle manufacture. For the transport applications the implementation is expected to be in the medium to long term as the process of adoption of new technologies requires the undertaking of lengthy qualifications tests, since it is related to structural designs involving public safety.
The output from SAFJOINT has however generated interest to further explore the techniques involved which resulted in the safeguard of existing jobs of the industrial partners involved as the benefits of implementing the developed technologies constitute a clear product advantage over the competition without any significant financial penalty to them over implementation.
The project as mentioned earlier has also generated significant interest in the scientific community and has therefore generated research opportunities for young graduates and a renewed interest in this field of research. Also through a range of publications in scientific journals and conferences the research output from SAFEJOINT has enhanced the position of the EU at the leading edge of scientific excellence.
General Dissemination
The SAFEJOINT had a clear outline of dissemination activities, which were outlined in a Dissemination Strategy. This was updated on an annual basis to take into account activities that had taken pace during the previous year and the impact the success of these had on the coming years dissemination activities.
The strategy focused on a series of key objectives in both internal and external dissemination emphasising the strategic and operational direction of the strategy to fulfil the aim of raising awareness of the project and entice industry to participate in the practical aspects of the project. To facilitate this, there was an Industry Advisory Group (IAG) to monitor the progress of the project and industry participation and advice. Other internal Groups also had an input into the dissemination activities to ensure the maximum high quality exposure to the project is achieved.
Internal dissemination activities were the responsibility of the coordinator who then filtered down individual responsibility to partners. These activities included:
• On-line media
o Website
o Newsletters
o LinkedIn forum
o CORDISWIRE
o Industry targeted
• Project brochures
• Press releases
• Academia targeted
o Journal publications
o Presentations at conferences
• Dedicated Workshop/Conference
o Workshop to be held in M18
o Conference to be held in M34 – “Joining of Dissimilar Materials”
Workshop
An Industrial Workshop was organised in Athens on 3-4 July 2014, which was halfway through the project (Month 18) and was a forum for presenting the results from the SAFEJOINT research so far and how these could be used in an industrial setting, thereby leading to closer collaboration between the project partners and industry.
The Workshop included presentations by the SAFEJOINT partners, as well as from projects Ybridio and PMJoin which form a cluster of projects on the topic of dissimilar projects supported by the EC and other invited experts from industry. The coordinators UNEW have strong associations and links to industry associations such as UNIFE (Rail manufacturers association), EASN (European Aeronautics Science Network), ACARE (Aircraft manufacturer’s association and WEGEMT (Marine Technologies Association) and other associations who were all contacted and invited to make a presentation as well as attend as a delegate.
During the workshop open discussions were encouraged between industry and the research community in dedicated sessions to explore the future needs of industry in lightweight design and the future direction of research in the area of dissimilar materials joining. The workshop was free to participants and open to all to attend.
Since the input of industry was a key component of the workshop outcome it was decided to have facilitated sessions to encourage discussion, feedback and knowledge transfer from the delegates.
The advantages of having a facilitator were seen to be:
• The facilitator helped to make sure that the right people were at the workshop and drew out their expertise to share it with the rest of the participants.
• By having a facilitator lead a section of the workshop, the project coordinator became a participant, rather than the workshop leader. This was an invaluable aid in developing collaboration and allowed him to participate fully, to view the project objectively and listen to the opinions of others in the session in a way that they could not if they were leading.
• There was consistently high output throughout the 2 days giving credible responses.
• A high level of knowledge transfer was achieved because it was specifically built into the workshop sessions via the topics. The delegates left the workshop with a better understanding of the activities and issues within the area of joining technologies and how the project was proposing to overcome these plus a clear vision of how the project was moving forward.
The Workshop sessions over the two days were made up of the following topics:
1. Metal to Composite joining
2. Modelling of dissimilar material joints
3. NDE of dissimilar materials joints
4. Environmental degradation of dissimilar material joints
5. Dissimilar metal joints
6. General Discussion
Day 1
Session 1. Metal to composite joining
The first session was designed to showcase the developments on dissimilar material joining by the three projects, SAFEJOINT, YBRIDIO and PMJoin.
At the end of the session presentations, the following issues were raised:
• The response from the delegates during the discussion session was overall positive on the new technologies been presented and the diverse applications addressed by the three projects. In the words of the Alstom Transportation delegate “There is a lot of information and innovation presented here that is of interest directly to us and I am sure there is the potential for these technologies to will be taken up by industry in the medium term”
• Use of specimen geometry. In the majority of presentations of SAFEJOINT, Ybridio and PMJoin, a lap shear specimen has always been used for the assessment of the strength of the joint. It has been questioned by some industry representatives that this may not be the best approach as different applications require different configurations. A discussion ensued and it became apparent that indeed adhesive bond strength varies with different loading configurations. However, a bonded joint always exhibits best performance
• The rest of the discussions were mainly concerned with explanatory details of aspects of the work presented. One of the questions that arose was that of cost of the new technologies. It was explained that the approaches used do not require any significant capital expenditure and could be easily incorporated into existing operations where adhesive bonding is already in use. The consortia of all projects are well aware of the commercial demands of industry and they aim to achieve a step change in adhesive bonding performance without any significant cost penalties.
Session 2. Modelling of dissimilar material joints
The lunch break provided an area for eating and networking before the afternoon sessions began with Modelling of dissimilar joints again facilitated by George Kotsikos. The topics covered were:
• Failure modes in composite joints: a finite element study.
• “Mixed adhesive joints”
• Integrated modelling of 6xxx series friction stir welds: from thermal cycles to mechanical performances.
• Ultrasonic NDT of dissimilar joints
• Development of Piezoelectric sensors
The open discussion at the end of these topics brought the end of day one of the workshop to a close, covered the following issues and prepared the delegates for the next days activities:
• The presentation of functionally graded adhesives also drew an interest focusing on the effect of (or degree of) mixing of the adhesives once they have been placed at the joint. It was noted that this should not have a significant effect on the performance if it was viewed in terms of the stress distribution along a lap shear joint (i.e. peak load at the edges of the lab and dropping to a small level along the length of the lap. It was suggested that it might be sufficient to have adhesive placed only at the lap edges but it was explained that this would not have worked because of the nature of the stress distribution which will create stress raisers along the lap joint. It was also mentioned that the embedded strain sensor technology developed within SAFEJOINT may provide quantitative information of the stress distribution within a joint and help optimise the placement of graded adhesives.
• The presentation on ultrasonic inspection of dissimilar material joints also drew significant attention as it is already a topic of interest on adhesively bonded joints in industry. The presentation showed that it is possible to detect at least artificial defects along the interface of dissimilar material joints and this led to a discussion of the capability of the technique to give a measure of the residual strength in a joint following the detection of defects. It was explained that at this stage it is a major achievement that defects have been identified as this was not possible by previous methods. Further work is required to develop a reliable residual strength detection tool.
• Another point that was raised was that of “kissing bonds”, namely bonds that appear sound (no void detected) but they are of significantly reduced strength as a result of contamination, environmental degradation or poor surface preparation of the joint. It was explained that at present this is not possible with current techniques. It was mentioned though that the thin film strain sensor technology developed in SAFEJOINT may be able to detect that but the sensors cannot provide information on the “global” condition of the joint.
• A question was raised on the environmental degradation of the joints. All projects are aware of the issue and work is underway addressing this issue. However, it is not expected to encounter any environmental degradation effects dissimilar to those of existing bonded joints as the process is dominated by the adhesive resin properties.
Day 2 of the workshop began with a session facilitated by the SAFEJOINT partner KTU (Lithuania) concerning Environmental degradation of dissimilar material joints. The topics covered in this first session were:
• Environmental impact on elastic adhesive joints in marine applications
• Degradation of adhesive joints
• The effects of pre-bond and after-bond contamination on the fracture toughness of adhesively bonded joints
The outcomes of the Open Discussion were:
• There are tools and procedures for assessing the long-term performance of bonded joints and those are addressed within SAFEJOINT. The discussion was already made on the previous day and it was re-iterated that the performance of bonded joints developed in SAFEJOINT, YBRIDIO and PMJOINT should not display behaviours overly dissimilar to existing bonded joints. The controlling factor is the moisture diffusion in the adhesive resins. In terms of dissimilar metal joints, the effect of environmental degradation (corrosion) is addressed through an innovative idea of incorporating in the weld pool nanocontainers that include corrosion inhibitors. This work is currently underway but preliminary results look promising.
• The issue of contamination of joint surfaces prior to bonding was also discussed as it is in the majority of cases the most important problem affecting the integrity of bonded joints and is related to the manufacturing procedures of the joints. It is a problem that requires extensive training of personel manufacturing those joints who are not always aware of the implications of inadequate surface preparation and cleanliness. This is also the source of “kissing bonds” for which the development of some type of NDE method needs to be developed. This problem is currently addressed in the standards by a combination of bonding and bolting of dissimilar materials for safety critical structures and components in the transportation industry.
After the morning coffee break, the next session titled Dissimilar metal joints was facilitated by Simon Frost of Walker Associates who is a member of the SAFEJOINT IAG. There were 2 topics covered in this time slot:
• Microstructure and mechanical properties of Al-to-steel welds performed by friction melt bonding.
• Micro structural and mechanical study of AA 5083 and AA 6082 friction stir welds reinforced with SiC, TiC and CNTs
Issues raised from the presentations were:
• The main issue affecting this type of work concerned NDE inspection and environmental degradation. KTU presented some images of dissimilar metal joints developed within SAFEJOINT utilising a new technique “ultrasonic spectroscopy” demonstrating that the detection of both defects as well as the distribution of nanoparticles in the weld of dissimilar metals. This technique is an invaluable tool for both inspection and weld quality, the latter allowing for the optimisation of the welding process.
The second day of the workshop was rounded-up with a general discussion forum facilitated by George Kotsikos about the Industry perspective, needs, comment and input on the future direction of research in dissimilar material joining followed by a round-up of discussions and points raised throughout the workshop.
The conclusions of the workshop were:
Industry have reiterated that their main motivation for the dissimilar material joining is lightweighting. The major concern over bonded joints was raised by the aeronautics industry where certification can vastly delay the implementation of any new technology due to the public safety critical application. In particular AIRBUS have expressed the view that riveting (or bolting will always by implemented in the industry due to the design rules being applied in aircraft manufacture. The design rules were then presented to the delegates by Dr Domenico Furfari of AIRBUS to reinforce this point.
The design rules document was looked at in detail by all delegates and a closer examination of the wording in this document showed that bonding alone is not excluded as a means of joining of dissimilar materials.
Any joining technique would be acceptable if it is accompanied by:
• precise manufacturing rules,
• reproducibility of the bond,
• detailed inspection techniques
• reliable through life structural integrity monitoring,
• methodologies to determine residual strength (or remaining life) of a joint following the detection of damage and
• prediction of long term performance of the joints.
What is important for all the above is that the degree of uncertainty must be extremely low and similar to the levels for existing joining techniques such as riveting or bolting.
However, it was also pointed out that the surface transport sector (rail, marine, automotive) is not constrained by such strict design rules as aeronautics. Notably for those industries, the issues of ability to reliably inspect the condition of the joints, long-term structural integrity and environmental degradation are deemed important.
Perhaps the most important issue for land transport is cost. Any developed techniques through SAFEJOINT have to be comparable in cost to existing methodologies for manufacture and inspection.
Another issue mentioned in one of the sessions on the previous day was reiterated at the general discussion session and deals with the ability to detect “weak bonds” (or “kissing bonds” as they are also called) as there is currently no detection techniques able to assess this type of defect in bonded joints and can be the source of premature failures and consequently uncertainty over the adoption of purely bonded joints which are really the goal for lightweighting.
Final Conference
The Conference was due at M34 when the project has reached its final conclusions. Initially, it was thought that the final conference, scheduled for M34 – October 2015, could be held at Newcastle University since there are the required facilities. However, the timing of the Conference conflicted with the start of the Academic term and this made booking a suitable venue difficult. Dr George Kotsikos, the SAFEJOINT project Coordinator was on the Scientific Committee of the forthcoming ICEAF Conference, (International Conference of Engineering Against Failure) being held in Skiathos on 24-26 June 2015 and negotiated a session for dissimilar joining techniques.
Partners were requested to submit abstracts concerning Dissimilar Metal Joints along with the Ybrido and PMJoin projects as part of the project clustering activities. The papers submitted by SAFEJOINT partners were presented on the first day of the Conference, Wednesday 24th June 2015 were:
SAFEJOINT – CONFERENCE PAPERS – SKIATHOS 2015
SESSION 9 - Wednesday 24 June 2015 - 16:00 – 17:40
Chairmen – Dr. George Kotsikos & Professor Geoff Gibson
SAFEJOINT
PARTNER PRESENTER PAPER TITLE
IFAM A. Wulf, M. Brede,
C. Nagel High cycle fatigue behaviour of structural adhesive joints
ITA Chiminelli, R. Breto, E.Duvivier
M.A. Jiminez Functionally graded bond-lines for metal/composite joints
SICOMP V. Churchill SC, E. Julin, D. Mattsson, D. Ramantani,
S. Tampas Mechanical performance of prefabricated hybrid joints
KTU E. Jasiuniene,
L. Mazeika,
E. Zukauskas,
V. Samaitis,
V. Cicenas
NDT of dissimilar joints using ultrasonic and X-ray CT methods
UNEW M. Konstantakopoulou Failure Mode and Strength of FRP/Steel Joints Bonded with MWCNT Epoxy Adhesive
UNEW A. Deligianni, J. Hale,
G. Kotsikos
Development of piezoelectric strain sensors embedded in bonded dissimilar material joints
Conclusions of the conference
All papers presented were well received by the conference audience and several questions were raised during the discussion time after each paper presentation. As this was organised to be a stand-alone session of “Joining of dissimilar materials”, an “open discussion” time slot was allocated so the audience could discuss the issues involved after they have heard all the presentations. The topic was found to be of outmost interest both to industry representatives and academics.
Joining of dissimilar materials is a very challenging topic and there is not one solution alone for this problem. It was therefore welcomed by the audience that SAFEJOINT addressed a number of approaches, each with its own innovations and challenges, which could be implemented in a range of applications.
It was recognised that SAFEJOINT has been a step forward in addressing the topic of dissimilar materials joining, however, some challenges remain in terms of automation of the processes involved and extensive qualification testing before some of the techniques could be widely adopted by industry (in particular the aerospace industry)
The innovations introduced in the NDE approaches adopted in SAFEJOINT, mainly the use of ultrasonics has also been well received with several questions put to the presenters at the open discussion session and afterwards in private discussions. The attractiveness of the ultrasonics NDE is that it is a well-established method, well understood and widely used in industry and will therefore be a very straightforward process to transfer the knowledge generated in SAFEJOINT to inspections of dissimilar material joints in the marine, rail and aeronautics industries.
Website
The SAFEJOINT website was the main on-line tool for disseminating the project and its progress to a wide audience:
• SAFEJOINT progress
• Promoting external activities such as the SAFEJOINT workshop and conference
• Facility to register directly for the above events
• Point of contact for interested readers
Primary Roles
The initial key objective for the website was to raise awareness of the project. To facilitate this, the website aimed to:
• Present a professional design that was relevant and makes visitors comfortable and attract interest in the project and its objectives.
• Design an architecture, which took into account the importance of being search engine friendly, whilst maintaining the management of web site content and images.
• The ability to find the SAFEJOINT information quickly and simply.
• Present up-to-date news items and relevant content by placing strategic calls to action, visitors were encouraged to actively engage with the website rather than just be a passive visitor.
Secondary Roles
After the initial set-up phase the website continued to:
• Convey and maintain the key messages of the project. Including the facilitation of all project progress information and essential activities.
• The content structure took into account visitors who were seeking information concerning the project and information about forthcoming SAFEJOINT events.
Key Outcomes of the website
Through the design and development of the website with functionality such as Newsletters, Visitors Registration, Personalisation and Online Document Management the following Key Outcomes were delivered:
Providing a source of information and awareness by making visitors aware of the SAFEJOINT website by: presenting relevant content, assisting users in having their questions answered, ensuring relevant information
• Supporting the project by reinforcing the objectives and progress and communicating effectively with existing visitors
• Performing its role effectively by: being built using flexible site architecture, having an appropriate mix of internal and external resources, being supported by the various practice areas.
The domain name safejoint.net was secured and a website created at www.safejoint.net.
The pages are:
• A home page
• Objectives of the project
• The work packages as an overall page linking to the individual WP pages
• A contact page
• A SAFEJOINT Events page
• Downloads of papers and presentations
The website was hosted and maintained by the coordinators UNEW with regular updates and maintenance of links. A visitor counter will recorded the overall number of visitors and at the end of the project this was 17000. The website will be kept live for 2 years after the end of the project.
List of Websites:
www.safejoint.net
Dr George Kotsikos
+44 191 2085889
