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Micro mEchanical Characterization of A ThErmoplastic Co-conSolidated / welded joinT for usE in aiRcraft fuSelages

Periodic Reporting for period 2 - MECATESTERS (Micro mEchanical Characterization of A ThErmoplastic Co-conSolidated / welded joinT for usE in aiRcraft fuSelages)

Reporting period: 2020-10-01 to 2022-09-30

High Performance Thermoplastics (HPTP) are an excellent solution to the issues of recyclability, light weight, high performance, and reparability. HPTP composites offer compelling advantages over metal components: improved working life, lower weight, reduced fuel consumption, and longer service intervals. In comparison to thermoset composites, thermoplastic composites also have several advantages: they can be directly joined using welding techniques, and there is hardly any damage or damage growth in the laminate structure.In addition, their thermoplastic nature also opens up numerous possibilities for rework and repair. The use of thermoplastic composites can reduce manufacturing times by as much as 20-30% as compared with metals or thermoset composites.

KVE Composites Group and Rescoll cooperate to create a greater level of understanding of the micromechanical behaviour of the welded interfaces for UD tape based laminates. Issues such as aging, healing, processing parameters, and durability need to be investigated in greater detail in order to develop a coherent approach.

In order to reach this goal, four Key Objectives (KO) have been figured out which will make the overall MECATESTERS project realistic.

- KO1: Evaluate the effect of surface conditions upon interface healing. (WP1)

- KO2: Examine the effects of aging on the durability of carbon UD LM-PAEK and PEEK parts (WP1 – WP3)

- KO3: Perform a process window comparison for methods for welding (WP2)

- KO4: Perform mechanical testing to validate the performance of the bonds
The project was carried out in 3 work packages :

- WP1 : Influence of surface condition of C-PEKK parts on the interface healing characteristics. During this work package we developed a sound understanding of the influencing parameters, on the joint durability for materials that are joined/formed through melting processes.
SLS (single lap shear), pull-off on L-profiles and fissuration test were carried out on induction welded and conduction welded specimen. Both static and dynamic tests were carried out.
The influence of various surface conditions on the weld strength was tested and some different surface treatments has shown some limitations of the welding technologies (for instance with IR degraded). The air plasma treatment has a negative effect on the lap shear strength for either the PEAK and PEKK material. Therefore, this surface treatment is not recommended on the surface that needs to be assembled by a welding. Even if it has been tested for only one material and one weld method, the addition of an extra resin film at the weld interface does not seem to improve the lap shear strength. However, the failure mode is different compared with specimens welded at the same condition without extra film and show an important plies delamination of the laminate at the welded joint. The tested release agent either applied on a PI film or on a caul sheet, do not influence the lap shear strength.

- WP2 : Process window comparison of welded carbon PEKK joints. During this work package we compared the process windows of various welding techniques and evaluated the suitability of the various techniques to a range of design features. The mechanical properties were obtained between different welding methods through static tests (single lap shear tests and pull off tests on L-profile specimens). Two welding methods were used for the material manufacturing, induction welding and conduction welding. Each method was applied with different parameters of temperature, cool down time and pressure. For induction welding, it was found not all materials selected showed adequate heating behavior to result in a robust welding performance. For the single lap shear tests, as regards to the test results the induction welding lap shear test results are not useful due to the huge variation. It seems that the weld width is not constant alongside the weld line due to the poor heating behavior of some materials. More work is needed by the material supplier to improve the heating behavior before having a robust test result. For the conduction welding the results a far more constant (less variation) and can be used. The PEKK over-performs the LM-PAEK in general, both for induction welding as for conduction welding.

- WP3 : Durability aspects of PEKK compound. During this work package we have examined the durability aspects of PEKK compound. On the request of the topic manager, this has been changed to examination of Environmental Stress Cracking, and ratcheting (creep). Creep test on SLS specimen were carried out on inducted and conducted specimens, as well as ratcheting test (ageing with several cycling test phase). To keep at least some research related to PEKK compounds with relevance to the Stunning project, work has been performed to compression mold a window frame from PEKK compound, and investigate weldability of to a UD tape-based laminate. For the environmental stress cracking, the original aim was to test the single lap shear specimens at HTW (80 deg C and saturated at 70 deg C/85%RH) before and after several periods of environmental stress cracking. The residual strength test was not conducted, but all PAEK and most of the PEKK specimens passed the test with the load level and periods of sustained load in the Skydrol/water mixture as indicated. For the Ratcheting, a test program conducted at NIAR in 2020 showed that there was an effect on epoxy specimens. The current measurements of the curvature of the PAEK specimens may indicate a minor effect of ratcheting after 60 hygrothermal cycles similar to those at NIAR. The strength of single lap shear specimens appears to show a significant increase. At NIAR, also a slight increase was found for some of the epoxy single lap shear specimens. No significant effect of ratcheting was found for the PEKK specimens.
The MECATESTERS part is a key part of the overall Multifunctional Fuselage Demonstrator initiative in the Clean Sky Large Passenger Aircraft program. The MFFD, managed under the STUNNING project, focuses on implementing thermoplastic materials in order to obtain lighter, more easy to manufacture, and more ecologically responsible aircraft. As part of this initiative, the MECATESTERS project has evaluated several different types of thermoplastic welding technologies, their performance, and their suitability for eventually scaling to industrial production levels. The MECATESTERS project, led by RESCOLL with the collaboration of KVE and the University of Patras, evaluated several different welding methods, configurations, performed computational modeling, and then evaluated the overall mechanical performance and resistance to aging. Specifically, MECATESTERS demonstrated that of the technologies evaluated, induction welding is one of the most promising, and that conduction welding is also a very promising technology.

The MECATESTERS project is important, as a shift to thermoplastic composites in large passenger aircraft is key to reducing weight. Every kg saved in an aircraft saves 1145 tons of fuel annually, and 73 kg of CO2-equivalent per hour of operational time. In addition, as compared to thermoset composites, thermoplastic composites offer numerous advantages, including recyclability and improved ease for MRO (dewelding, reprocessing, replacement).
Single lapshear specimen fully installed in the creeping cell
Single lap shear test
Plain specimen installed on the support for measuring the stress relaxation (curvature)
Welded test assembly
SLS welded assembly
Autoclave consolidated laminate
L-pull off test