The Boeing 787 Dreamliner is comprised of more than 50% carbon fiber reinforced plastic (CFRP) (Figure 1). Although CFRP composites have many inherent advantages, they cannot mitigate the potentially damaging electromagnetic effects from a lightning strike. To solve this problem, electrically conductive expanded metal foil (EMF) can be added to the composite structure layup to rapidly dissipate excessive current and heat for lightning protection of CFRP in aircraft.
Engineers at Boeing Research and Technology (BR&T) are using multiphysics simulations and physical measurements to investigate the effect of the EMF design parameters on thermal stress and displacement in each layer of the composite structure layup shown (Figure 2). Stress accumulates in the protective coating of the composite structure as a result of thermal cycling due to the typical ground-to-air flight cycle. Over time, the protective coating may crack, providing an entrance for moisture and environmental species that can corrode the EMF, reducing its electrical conductivity and ability to perform.
The research effort at BR&T is led by Jeffrey Morgan from Sealants and Electromagnetic Materials, associate technical fellow Robert Greegor from Applied Physics leading the simulation, Dr. Patrice Ackerman from Sealants and Electromagnetic Materials leading the testing, and technical fellow Quynhgiao Le. Through their research, they aim to improve overall thermal stability in the composite structure and therefore reduce the risks and maintenance costs associated with damage to the protective coating.
Simulating thermal expansion
In the surface protection scheme, each layer including the paint, primer, corrosion isolation layer, surfacer, EMF, and the underlying composite structure contribute to the buildup of mechanical stress in the protective coatings as they are subject to thermal cycling over time. The geometry in the figure is from the coefficient of thermal expansion (CTE) model developed by Greegor1,2 and his colleagues using COMSOL Multiphysics in order to evaluate the thermal stress and displacement in each layer of a 1" square sample of the composite structure layup.
In this study, the EMF height, width of the mesh wire, aspect ratio, metallic composition, and surface layup structure were varied to evaluate their impact on thermal performance throughout the entire structure. The metallic composition of the EMF was either aluminum or copper where an aluminum EMF requires additional fiberglass between the EMF and the composite to prevent galvanic corrosion.
The material properties for each layer – including the coefficient of thermal expansion, heat capacity, density, thermal conductivity, Young’s modulus, and Poisson’s ratio – were added to the COMSOL multiphysics model as custom-defined values (Figure 3). The coefficient of thermal expansion of the paint layer is defined by a step function that represents the abrupt change in thermal expansion at the glass-transition temperature of the material.
In the CTE model, the thermal stress multiphysics interface couples solid mechanics with heat transfer to simulate thermal expansion and solve for the displacement throughout the structure. The simulations were confined to the heating of the composite structure layup that an aircraft would experience upon descent, where final and initial temperatures were defined to represent the ground and altitude temperatures, respectively.
Impact of EMF
The results of the COMSOL simulations were analyzed to quantitatively determine the stress and displacement in each layer upon heating and for varied properties of the expanded metal foil (Figure 4).
The magnified cross-sectional view from the simulating shows variations in displacement above the mesh and voids in addition to the trend in stress reduction in the uppermost protective layers. Researchers also simulated the relative stress for each layer in surface protective schemes that incorporate either copper or aluminum EMF (Figure 5). The fiberglass corrosion isolation layer required by the aluminum EMF acts as a buffer, causing the stress to be lower in the aluminum than it is in the copper EMF.
Despite the lower stress in the aluminum EMF, simulation results from the variation of the EMF design parameters reveal a consistent trend toward higher displacements in the surface protective scheme with the aluminum EMF when compared to copper. The larger displacements generally caused by the aluminum EMF can be attributed, in part, to the relatively higher CTE of aluminum.
Further analysis of the impact of the EMF design parameters was performed to confirm the effect of varying the height, width, and mesh aspect ratio on displacement in the protective layers. When varying the mesh aspect ratio, it was found that an increased ratio led to a modest decrease in displacement of about 2% for both copper and aluminum EMF, where higher ratio values correspond to a more open mesh structure. For any EMF design parameter, there is a trade-off between current carrying capacity, displacement, and weight. In the case of mesh aspect ratio, while choosing an open mesh structure can reduce displacement and weight, the current carrying capacity that is critical to the protective function of the EMF is reduced as well and needs to be taken into account.
Similarly with regard to the mesh width, varying the width by a factor of three led to a relatively minor increase in displacement of about 3% for both copper and aluminum EMF. However, varying the height of the EMF by a factor of four led to an increase in displacement of approximately 60% for both aluminum and copper (Figure 6). Due to the lower impact on displacement, increasing the mesh width or decreasing the aspect ratio are better strategies for increasing the current carrying capacity of the EMF for lightning strike protection.
Relating displacement with crack formation
Greegor and his colleagues at BR&T qualitatively regard any projected increase in displacement as an increased risk for developing cracks in the protective layers since mechanical stress due to thermal cycling accumulates over time.
Experimental evidence supports this logic (Figure 7) in photo micrograph cross-sections of surface protection schemes with aluminum and copper EMF after prolonged exposure to moisture and thermal cycling in an environmental test chamber. The layup with the copper EMF showed no cracks, whereas the aluminum EMF led to cracking in the primer, visible edge and surface cracks, and substantial cracking in mesh overlap regions.
Throughout the same temperature range, the experimental results correlate well with the results from the simulations that consistently show higher displacements in the protective layers for the aluminum EMF. Both simulation and experiment indicate that the copper EMF is a better choice for lightning strike protection of aircraft composite structures. Multiphysics simulation is therefore a reliable means to evaluate the relative impact of EMF design parameters on stress and displacement to better understand and reduce the likelihood of crack formation.
About the author: Jennifer A. Segui is the senior technical marketing engineer at COMSOL Inc. She can be reached at 781.273.3322 or by email at firstname.lastname@example.org.
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