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LIGHTWEIGHT COMPOSITE BUS SYSTEM HOUSING FOR EXTREME ENVIRONMENTS

Final Report Summary - LIGHTBOX (LIGHTWEIGHT COMPOSITE BUS SYSTEM HOUSING FOR EXTREME ENVIRONMENTS)

Executive Summary:
Conventional racks for avionics are made from metal. Most of the avionics fielded today offer a monolithic architecture in form of a closed box packed with electronics and lots of connectors at the front or the back with corresponding heavy cable harness. More recent developments go for modular avionics, packing electronics with standard dimensions and connectors that assure physical interchangeability.
The amount of electronically controlled tasks in modern aircraft is increasing steadily and also the contribution of racks for avionics to the overall weight of an aircraft has reached a magnitude that requires an analysis to obtain mass reduction.
The proper housing protects the electronics against the environment, ensures EMC and supports the thermal management. In order to reach higher power density and lower costs, the optimisation of the housing is a must.
Modern structures built in composite technology are able to provide important mass savings with respect to conventional designs. The advantages of high performance composites are many, including lighter weight, the ability to tailor lay-ups for optimum strength and stiffness, improved fatigue life, corrosion resistance and, with good design practice, reduced assembly costs due to fewer detail parts and fasteners.
Composite enclosures can be made significantly lighter than machined aluminium enclosures and may be produced at an affordable cost provided a modular approach is followed while possessing equal or better mechanical and thermal performance.
The objective of the present work is the development of a composite “open box” ARINC housing (ARINC standards) which withstands vibration levels C/C1 according to RTCO-DO160.

ARINC 600 housing has been re-engineered applying the composite approach. Specifications have been drawn and architectures, manufacturing processes and materials have been envisaged, evaluated and a traded-off.
The potential problems in the composite design have been anticipated and alternative countermeasures have been evaluated. A design concept integrating as much as possible of the housing parts has been finally selected, having infusion as manufacturing process in order to keep manufacturing costs as low as possible. Regarding the materials, standard high strength carbon fibres together with aero grade infusion epoxy resin have been selected as main materials for the production of the enclosure parts.
Trials at sample level have been performed in order to explore the different material solutions identified. A mechanical characterisation supporting the FEA of the prototype has been also performed.
Detailed analyses indicate the composite housing is able to withstand the dynamic environment. The selected laminate fulfils both lightness and stiffness requirements. Thermal results obtained are in line with the aluminium counterpart. No problems due to CTE mismatch are foreseen.
The manufactured composite structure successfully passed the vibration testing required by ARINC standards for the present application. Thermal and EMI testing was successfully passed, while some improvements would be necessary for the electrical bonding approach (some of the measurements were slightly over the required threshold).

Compared to the current aluminium approach, significant weight reductions have been obtained: 34 % reduction with respect to the aluminium enclosure; 44 % of reduction if only the modified components are considered – not the commercial parts as hook, handle, etc.

Project Context and Objectives:
The amount of electronically controlled tasks in modern aircraft is increasing steadily and also the contribution of racks for avionics to the overall weight of an aircraft has reached a magnitude that requires an analysis to obtain mass reduction.
Conventional racks for avionics are made from metal. Most of the avionics fielded today offer a monolithic architecture in form of Line Replaceable Units (LRU), i.e. a closed box packed with electronics and lots of connectors at the front or the back with corresponding heavy cable harness. More recent developments go for modular avionics, packing electronics into functional Line Replaceable Modules (LRM) with standard dimensions and connectors that assure physical interchangeability. A rack offers a standard backplane for several LRMs and connectors to the aircraft.
The integrated concept of modules and backplane with power and signal bus reduces redundancy in housing and wiring and thus saves weight. For maintenance there is no need to remove aircraft connections, but only modules have to be swapped. But even currently available racks for modular avionics that comply with requirements of flight operation offer potential for weight reduction.
Electronics enclosures are discrete structures designed for the sole purpose of holding printed circuit boards and electrical components in place. The proper housing protects the electronics against the environment, ensures EMC and supports the thermal management. In order to reach higher power density and lower costs the optimisation of the housing is a must.
Modern structures built in composite technology are able to provide important mass savings with respect to conventional designs. The first large scale usage of composites in commercial aircraft occurred in 1985, when the Airbus A320 first flew with composite horizontal and vertical stabilizers. The cost of composites is the number one deterrent to their broader application. However, the modularization and the manufacturing of standard components can drastically decrease the costs reducing recurring machining cost and material waste because a single reusable mould can be used to produce many near-net shape parts.
The advantages of high performance composites are many, including lighter weight, the ability to tailor lay-ups for optimum strength and stiffness, improved fatigue life, corrosion resistance and, with good design practice, reduced assembly costs due to fewer detail parts and fasteners. The specific strength and specific modulus of high strength fibre composites are higher than other comparable aerospace metallic alloys.
Composite enclosures can be made significantly lighter than machined aluminium enclosures while possessing equal or better mechanical and thermal performance. Three groups of functional requirements—mechanical loading and physical attachment specifications, thermal control for installed electronic components and environmental shielding—must be satisfied in the design process for electronics enclosures. Design considerations must include the accurate alignment of printed circuit boards (PCBs) to a motherboard or input/output connection area, and provisions for mechanical attachment to the supporting structure.

There is a tendency for the near future to replace the current aluminium electronics housings by lightweight and standardized composite housings.
The objective of LIGHTBOX project has been the development of a composite “open box” ARINC housing (ARINC standards) which withstands vibration levels C/C1 according to RTCO-DO160.
A lightweight and modular composite solution has been developed. Advanced fibre-reinforced composite materials have several key properties that make them especially useful in aerospace applications. Their high specific strength and stiffness (strength/density, stiffness/density), allow obtaining structure designs that are lighter than traditional metal structures. Using composite materials, an initial 40 % of weight reduction in the housing was estimated.
LIGHTBOX project has developed an innovative concept of composite electronics housing. A multidisciplinary knowledge and experience had to be combined in a single structure; the main breakthrough has resided in the use of advance composite materials and in the combination of different technologies to give answer to the specifications.

The design has been based in the use of carbon fibre composite materials. As aforementioned, it is possible to achieve significant weight reductions face to the current aluminium approach. The main advantages of the proposed lightweight design compared to the current design are summarised below:
• Lower weight, up to a 40% reduction in the housing mass was foreseen.
• Parts integration, meaning less assembly time and fewer fastening elements (additional mass reduction and improved housing robustness)
• Improved fatigue behaviour due to the better fatigue performance of the carbon composite materials. This is particularly useful in a high vibration environment such as the found in an airplane
On the other hand, the composite materials present some weak points for the intended application. One of the main disadvantages of the use of composite materials in structures is their poor EMI-EMC shielding behaviour. EMI shielding requirements are based on the level and quality of continuous electrical conductivity across all housing walls, fixed joints and seams. The main objective of any type of shielding is to prevent the passage of electromagnetic waves into or out of the device. EMI shielding can work by reflection, absorption, or by carrying the electromagnetic radiation to ground. The fundamental aim is to establish a Faraday cage to provide an EMI shield. Composite materials are not as electrically conductive as traditional metal structures. Therefore, extra steps must be taken to mitigate this deficiency.

Vibrations
Related to the mechanical behaviour, the box shall withstand C / C1 levels according to RTCA DO 160. Composite materials are lightweight and stiff. In comparison with metals, advanced composites exhibit superior fatigue performance due to their high fatigue limit and resistance to corrosion. A proper material selection, mechanical design and interface definition has helped reaching/sustaining the specified levels.

Cost
Composite components tend to be higher in cost than their metallic counterparts. Material selection has been carried out having into account the requirements to be fulfilled versus cost. The most appropriate material has been selected.
In many applications composite materials are used for the additional benefits they provide in terms of improved performance (lightweight, specific strength or stiffness, less maintenance needs) that means that considering a life cycle analysis, despite a higher manufacturing cost, composite components are superior in some applications.
In this specific case, materials consumption is very low, thus the raw materials cost is not driving the cost of the final component, which is much more affected by the manufacturing process and the required secondary operations. Thus, an application where a modular/standardised approach can be followed can benefit from reduced manufacturing costs. Therefore, following ARINC standards, the costs associated to the required manufacturing tooling and the more complex design and manufacturing set up of composite materials has had a high reduction potential.
In addition, if manufacturing of large series is required, alternative manufacturing processes as RTM could be used. The tooling costs for RTM process are much higher than the required for an infusion process because a closed two parts mould that has to withstand the high injection pressures is required. However, RTM has a clear automation potential. A specific press could be used for holding the two mould halves in order to provide the required closing force and the mould movements. Therefore, for large series, RTM process could be a more suitable process.

The innovation of the project lies on taking advantage of commercially available materials and technologies (at an affordable cost) and combining them within composite processing techniques in order to yield a multifunctional design based on CFRP laminates, with same or improved functionality (mechanical, thermal and electrical), lighter weight and reasonable initial manufacturing cost.
The project has been divided in work packages were the following activities have been carried out:
• Definition of specifications and test matrix.
• Conceptual design of the bus system housing for extreme environment.
• Identification of the critical aspects that need further study / development.
• Identification of the contingency techniques / strategies
• Development at sample level.
• Detail design of the housing
• Manufacturing of the housing
• Validation of the housing

The work has been carried out by TECNALIA and TECNOBIT, having Liebherr-Elektronik GmbH (LEG) as Topic Manager.

Project Results:
This chapter summarizes the activities carried out and results achieved under LIGHTBOX Project.

The housing to be redesigned corresponds to an ARINC housing housing which is used for a 4 MCU LRU in an existing application.
As a first step and based on the specifications of the current ARINC 600 housing and the ones provide by the Topic Manager, the specifications for LIGHTBOX housing have been drawn. The tests to be performed to validate the housing have been identified and specified. The test matrix includes vibration, thermal, EMI-EMC and electrical bonding tests.

A conceptual design based on composite material was identified. Different architectures, manufacturing processes and materials were envisaged, evaluated and traded-off. To perform the comparison, criteria as cost, simplicity in manufacturing, simplicity in assembly and weight, among others, have been considered and weighted.

The conceptual design of the electronic housing has been defined. As a conclusion of the trade-offs performed, the following features were selected:
• Architecture: central single part box and two covers. It offers a good compromise between mechanical behaviour, light weighting and easy of manufacturing.
• Infusion manufacturing process, either liquid or film infusion
• Carbon fibre: a preselection of fibres has been performed as a first step. The final selection was based on the results obtained in the simplified thermal and mechanical model.
• Resin: Aero grade epoxy resin specific for liquid impregnation processes.

On the other hand, aspects / issues (risks) that should be further studied and analysed has been identified (EMI/EMC behaviour, electrical bonding, galvanic corrosion, assembly and integration) and solutions proposed.
In summary, the following solutions for the points of concern have been studied:
• Material basic characterization of the laminates in order to feed the FEA model and to demonstrate that the thermal environment will be supported by the selected resin:
• Physical characterisation: Density, fibre weight and volume content
• Thermal characterisation: Glass transition temperature (dry conditions)
• Basic mechanical characterisation: tensile, compressive and shear properties in order to obtain the properties of the elementary lamina required for analysis software

Overall, the obtained results are in the expected range.
• EMI/EMC aspects: several approaches as embedment of conductive fabrics and coatings with both paints and metallic layers have been studied.
The highest shielding effectiveness is achieved using an electrically conductive paint that can be applied using standard painting techniques.
• Electrical bonding issues: The same solutions used for EMI-EMC shielding have been tested for electrical bonding. As a first step, electrical volume conductivity and contact resistance measurements for different material options and processes parameters have been carried out. In a second step, electrical bonding measurements for assemblies representing real mating parts have also been conducted.
• Assembly: two different alternatives for the assembly of the different parts of the housing have been compared. The behaviour of the riveted anchor nuts used in the current aluminium design and the proposed adhesive bonded rivetless floating nutplates have been compared.
Two different types of tests have been carried out, pull-out test and torque test.The results indicated that pull out strength for the rivetless nutplates is lower than the measured for the standard anchor nuts. However, these inserts are mechanically loaded just during the screw assembly operation. Once the parts have been assembled, the bolt is applying a compression force that retains the nutplate into position. Therefore, it is considered that this reduction does not significantly affect the resistance of the joint. Moreover, an additional benefit for the rivetless nutplates is that they are assembled by means of adhesive and therefore the two holes for riveting each of the inserts are not needed. This means lower stress concentrations on the laminate and better galvanic corrosion properties (adhesive act as an insulation layer between carbon laminate and insert).
Regarding the torque testing of inserts, it was found that the minimum failure torque values measured were much higher than the recommended tightening torques for M3 stainless steel bolts and so there is no concern for using the rivetless nutplates for assembly purposes.

Based on the results obtained in the characterization campaign and on the requirements to be fulfilled detailed thermal, mechanical and thermo-mechanical analysis of the housing have been carried out.
Through preliminary analysis it was concluded that the thermal conductivity of the carbon fibres was not affecting in a significant fashion the thermal behaviour of the housing. It was also demonstrated that standard non expensive high strength carbon fibres are able to fulfil the mechanical requirements. Therefore, standard high strength carbon fibres in a Non Crimp Fabric (NCF) format were selected for the prototype.
Regarding the proposed laminate, an optimisation was required in order to find a suitable stacking sequence. Two material options have been explored for an easy manufacturing, a triaxial NCF [45/0/-45] (460 gsm) and an UD NCF (300 gsm). The combination of these two fabrics has allowed obtaining the required laminate using a small amount of layers (easy of manufacturing is also required).
The housing is considered to be clamped in the rear connector and the hooks located in the front cover. Up to five different laminate configurations have been analysed in order to obtain the optimum one for each component.
From the thermal point of view, natural convection (5W/m2K) and simple radiation conditions have been established in the outer surfaces of the equipment. For the simulation of the internal heat exchange by convection between the PCBs and the air, an air flow has been modelled. Openings have been defined at the top and bottom of the housing. The exact geometry and the configuration of the holes have been considered for the definition of the cross-sectional area of the openings (in line with ARIC 600 standard).

Analysis results obtained indicate:
• Minor differences are observed if thermal results obtained for carbon fibres with different thermal conductivity are compared. A high strength carbon fibre in a Non Crimp Fabric (NCF) format was finally selected.
• Results from detailed dynamic analyses show that strength ratios are high enough for all the proposed laminates. In order to obtain a laminate with the desired stiffness as well as weight reduction properties, several stacking sequences have been analysed and the one included above has been selected.
• The thermal analysis conducted shows that the maximum thermal temperatures at the PCB material are quite high and might compromise the correct operation of the equipment. However, this result is in line with the one obtained for the reference aluminium housing and should be addressed as a separate study.
• In the housing, the maximum temperature is below the continuous service temperature of the selected resin. Temperatures obtained are very similar to the ones observed with the aluminium housing.

A detailed manufacturing plan, procedures and drawings for the development of the CFRP housing prototype have also been prepared.
The selected manufacturing process is the infusion of liquid resin, providing high quality components at an affordable cost. According to the design, three composite parts have to be produced:
o Box Main Body.
o Front Cover
o Rear cover
In the case of the Main Body, a male tooling is needed, providing accurately the inner dimensions required by the electronic components to be mounted. In the case of the two covers, the same tool has been used for the lamination, female in this case. In this way, the accurate dimensions will be the ones fitting the inner dimensions of the Main body.
Once the hand lamination of the required dry fabrics is completed, the infusion and vacuum consumables have to be installed. A semipermeable membrane has to be used under the vacuum valve in order to keep the vacuum compaction along the resin curing process
Once released, the parts have been trimmed for removing the extra material and drilled in order to machine the holes required for the assembly. In order to improve the EMI behaviour and the electrical bonding, the parts have been coated with a metallic paint. In addition, the painting also improves the outer aesthetics of the part.
Once the parts were prepared for the assembly, the adhesion of the floating nuts followed by the adhesion of the metallic card retainers took place.

The following tests have been carried out to validate the housing:
- Vibration: Vibration tests for levels C/C1: The equipment has successfully passed the test.
- Mechanical: Mass and geometry: In order to assure a proper assembly of the housing, the positioning of the PCB boards has been based on the one corresponding to the aluminium counterpart. The electronics are correctly assembled in the housing and the drawings are correct for the equipment developed by LEG. However, there are some measurements that do not fulfil with the ARINC 600 standard. This is a minor defect as it relates to the final machining of the housing.
Mass savings of 34 % have been achieved.
- Thermal: High temperature thermal test has been passed successfully. The results obtained in the composite housing are comparable with its aluminium counterpart.
- EMI-EMC: tests have been carried out based on RTCA-DO-160 standard. Tests corresponding to radiated emissions and radiated susceptibility have been carried out. The composite housing has successfully passed the test. Results are comparable with its aluminium counterpart.
- Electrical bonding: Some measurements carried out between the bonding stud and the composite parts are slightly above the specified value (20 mΩ).

Hence, it can be concluded that a significant weight reduction has been achieved by LIGHTBOX CFRP prototype. Considering the enclosure weight, a 34 % weight reduction with respect to the aluminium enclosure (44 % of reduction if only the modified components are considered – not the commercial parts as hook, handle, etc) has been obtained. This can bring about a fuel saving of 1320 litres of fuel per year for single equipment. Considering a fuel cost of 0.5 €/l, this means a cost saving on operation costs of about 660 € per year (for one redesigned equipment).
The composite structure successfully passed the vibration testing required by ARINC standards for the present application.
The weak point for the CFRP design is the electrical bonding behaviour. The obtained results are close to the acceptance levels. That means that a robust bonding strategy should be implemented. However, this point is not considered to be critical. It is thought that a better bonding strategy could be easily adopted using the same overall design concept.
From the manufacturing point of view, cost estimation has been conducted considering an industrial process based on an annual run of 1000 enclosures and a total production of 10.000 units. The production cost obtained is 325 € per enclosure including manufacturing, finishing operations and assembly of the enclosure (mechanical parts).

Potential Impact:
In 2012 Europe’s aeronautical sector (civil and military activities) recorded a turnover of €127,5 bn. Civil aeronautics represents nearly 64% of the European aeronautics industry in terms of turnover. It is by far the most important sector of ASD industries.
One of the impacts requested by the European Commission, is to reduce fuel consumption and hence CO2 emissions by 50% per passenger km as CO2 is a significant contributor to global warming and is directly related to fuel burn (considered in the Strategic Research Agenda). Eco-efficient aircrafts, which involve producing highly efficient aircrafts, are needed. Therefore, it is crucial to develop lightweight technologies that will imply important mass savings and, as a consequence, a reduction of emissions.
Materials and systems need to be smaller, lighter, stronger, more resistant to the environment and longer lasting. As an indication of the benefit of such weight saving it has been estimated that 1 kg weight reduction saves over 2900 l of fuel per year.
The current trend is to reduce weight, so lightweight composite materials have been increasingly used. Successful composite designs can provide design flexibility, lightweight parts, ease of fabrication and installation (generally fewer parts), corrosion resistance, impact resistance, high fatigue strength (compared to metal structures with the same dimensions) and product simplicity when compared to conventional fabricated metal structure. Composite materials and their manufacturing processes can be tailored specifically to given design constraints.
The main challenges restricting their use are material and processing costs, impact damage and damage tolerance, electrical properties, repair and inspection, dimensional tolerance, size effects on strength and conservatism associated with uncertainties about relatively new and sometimes variable materials. Significant weight savings can be achieved.
With the lightweight and modular composite solution developed, LIGHTBOX project has achieved a mass reduction of around 34 % in the electronics housing (455 grams saved in a single housing), promoting actively the decrease of fuel consumption and, as a consequence, of detrimental gas emissions. According to the estimations obtained in literature, this can bring about a fuel saving of 1.320 litres per year and considering a fuel cost of 0.5 €/l, this means a cost saving of about 660 € per year. According to literature, this would mean 3.381 kg/year CO2, 22 kg/year NOx, 1,04 kg/year SO2 and 0.59 kg CO
New technologies may open the door to high performance, environment friendly and economic aircraft operation by better exploiting available weight reduction potentials of new design philosophies without compromising the existing, high aerospace safety requirements.

Exploitation
TECNALIA
Knowledge gained in the project has strengthened TECNALIA’s position as a niche technology provider of advanced composite components. This project has enabled to extend the validation of the boxes under aeronautic standards
Developments carried out in the area of lightweight electronics housing has allowed proving the feasibility of profitably producing an avionics box that meets the requirements in terms of functionality and at relatively low cost.
TECNALIA has reached a TRL5 (Component and/or breadboard validation in relevant environment, ESA definition) in the development of lightweight composite boxes. Breadboards have been qualified in the representative environment on ground. The current project has allowed increasing the applications for the lightweight housings in avionics, with specific qualification under ARINC-standards.
The project has also strengthen the collaboration between TECNALIA and TECNOBIT and LEG in the re-engineering of avionics systems by using lightweight boxes complementing the supply chain between technology provider and equipment manufacturer.