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Modern aircraft such as the Boeing 787 Dreamliner are comprised of more than fifty percent carbon fiber composite, requiring the addition of expanded metal foil for lightning strike protection.

The Boeing 787 Dreamliner is comprised of more than fifty percent carbon fiber reinforced plastic (CFRP) due to the material’s light weight and exceptional strength. Figure 1 shows the extensive use of composite materials throughout the aircraft. Although CFRP composites inherently have many 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 simulation and physical measurements to investigate the effect of the EMF design

Figure 1. Advanced composites used throughout the Boeing 787 account for more than fifty percent of the aircraft body1. (Copyright © Boeing)

parameters on thermal stress and displacement in each layer of the composite structure layup shown at left in 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 cause corrosion of the EMF, thereby reducing its electrical conductivity and ability to perform its protective function. 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 Aircraft Composites

Figure 2. At left is the composite structure layup from the COMSOL model and, at right, the geometry of the expanded metal foil. SWD and LWD correspond to short way of the diamond and long way of the diamond. The mesh aspect ratio: SWD/LWD is one of the parameters varied in the simulations. (Copyright © Boeing)

In the surface protection scheme shown at left in Figure 2, 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 over time as they are subject to thermal cycling. The geometry in the figure is from the coefficient of thermal expansion (CTE) model developed by Robert Greegor2,3 and his colleagues using COMSOL Multiphysics® in order to evaluate the thermal stress and displacement in each layer of a one-inch square sample of the composite structure layup.

The structure of the EMF layer is shown at right in Figure 2. 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 model as custom-defined values and are summarized in 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.

Figure 3. Ratio of each material parameter relative to the paint layer. The paint layer shows higher values of CTE, heat capacity, and Poisson’s ratio indicating that it will undergo compressive stress and tensile strain upon heating and cooling. (Copyright © Boeing)

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 heating of the composite structure layup as experienced upon descent in an aircraft where final and initial temperatures were defined in the model to represent the ground and altitude temperatures, respectively.

Impact of EMF on Stress and Displacement

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. An example of the simulation results is shown in Figure 4.

Figure 4. Top & middle: top-down and cross-sectional views of the von Mises stress and displacement in a one-inch square sample of a composite structure layup. At bottom, transparency was used to show the high stress in the composite structure and EMF. Stress was evaluated along the vertical line extending through the depth of the sample. (Copyright © Boeing)

Through the paint layer at the top of Figure 4, it is possible to observe the displacement pattern of the underlying EMF. The magnified cross-sectional view clearly shows the variations in displacement above the mesh and voids in addition to the trend in stress reduction in the uppermost protective layers. Figure 5 shows the relative stress for each layer in surface protective schemes that incorporate either copper or aluminum EMF. 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 percent 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 tradeoff 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.