With the implementation of major aircraft structures fabricated from carbon-fiber-reinforced plastic (CFRP) materials, lightning protection has become a more complicated issue to solve. One widely used material for lightning strike protection of CFRP structures within the aerospace industry is expanded metal foil (EMF). EMF is currently used in both military and commercial passenger aircraft.

An issue that has historically been an area of concern with EMF is micro cracking of paint on the composite structure, which can result in corrosion of the metal foil and subsequent loss of conductivity. Researchers from Boeing Research and Technology (BR&T) examined the issues of stress and displacement in the composite structure layup, which contribute to paint cracking caused by aircraft thermal cycling (i.e., ground-to-air-to-ground flight cycle).

There are several contributors to the stress buildup, including the paint, primer, corrosion-isolation layer, surfacer, EMF, and the underlying composite substructure. BR&T focused primarily on the EMF contribution to the cracking mechanism, with computer-modeling analysis performed using commercially available COMSOL Multiphysics software that was supported by data from limited experimental testing.

The temperature cycle of the layers was simulated using a coefficient of thermal expansion (CTE) model developed with the software. The simulation allows determination of the thermal stress and displacements that result. Though the full complexity of crack genesis is not included, some insight can be gained regarding what the sensitive parameters of the EMF may be and the variations that can be employed to mitigate the resulting stress and displacements that lead to cracking.

Of particular interest are the EMF width, height, aspect ratio, composition—aluminum (Al) or copper (Cu)—and surface layup structure. In the case of Al used for EMF, fiberglass is needed between the aluminum and the structure to prevent galvanic corrosion.

CTE model and experimental testing

Simulations were conducted over an air-to-ground temperature range, typical of commercial aircraft applications. The model assumed constant material parameters (CTE, heat capacity, density, thermal conductivity, Young’s Modulus, and Poisson’s ratio) over the nominal temperatures with the exception of the paint layer CTE, where a Fermi-Dirac functional form was used.

The objective of the experimental testing was to assess a variety of candidate lightning-strike material systems that have near-term potential of meeting performance requirements for both lightning and long-term durability. The particular tests were limited to the adhesion, salt spray, wedge crack, and thermal cycling environmental durability.

For thermal cycling, test coupons were placed in a test chamber similar to the range used for the simulations. At prescribed cycles, a panel was removed for examination. Surface cracks and finish were evaluated and cross-sections and photo-micrographs were made as necessary.

Three EMF systems were evaluated: heavy expanded copper foil (ECF), light expanded aluminum foil (EAF), and heavy EAF. Overall, the three EMF surface protection systems passed the paint adhesion test. The heavy ECF, light and heavy EAF salt-spray exposure exhibited no paint blistering or corrosion. All three surface protection systems passed the wedge crack evaluation for absence of crack growth over time, demonstrating good adherence of the EMFs to the composite.

The intermediate to longer duration thermal moisture cycling results indicated no surface issues with the expanded copper foil system; however, there was some cracking shown in the micrographs at the primer for the light EAF system, some edge and surface cracks, and extensive cracks in the overlap regions. The heavy EAF system also had some cracking on the surface and significant cracking in the overlap regions.

Evaluating the results

Quantitative determinations of stress and displacement were not conducted in the experimental evaluations; however, qualitative agreement was observed with the simulations since the EAF consistently exhibited greater displacement over the various parameter sets than the ECF displacements. The researchers associate greater thermally induced displacements with increased probability that cracks will eventually become evident. The displacement differences may be small, but over thousands of cycles will eventually generate residual stress, defects, and result in cracking. From this standpoint, both the simulations and testing indicate that Cu would be a better choice for the EMF than Al.

The individual parametric variations also suggest some interesting effects. The parametric variation of SWD/LWD (Short Way of the Diamond/Long Way of the Diamond, as described by Dexmet, a commercial producer of EMF) indicates that larger ratio, more open EMF meshes lead to lower displacements. The dependence is weak, but high thermal cycling has a cumulative effect. From a weight perspective, higher SWD/LWD is also desirable. Provided the EMF function is not seriously degraded, there appears to be benefit with the more open mesh from multiple perspectives.

The effect of the additional layer of fiberglass under the EMF was considered. When the fiberglass was added under ECF, the displacement was significantly increased. The remaining difference between EAF and ECF is most likely due to the larger CTE of aluminum by about 35%.

Examination of the thermally induced displacements suggests that there is little cracking penalty from increasing the width of the EMF. Hence, if greater current-carrying capability is desired from the EMF protection function, increased width appears to be a viable approach. Of course, increased width leads to greater weight penalty, and these conflicting requirements need to be balanced.

Alternatively, the increased current-carrying capacity of the EMF layer may also be realized with increasing height. However, height increase is not as desirable as it leads to greater displacements and hence cracking likelihood.

The researchers also observed that the paint glass transition temperature (t_g) had an influence on the displacement of the surface layer. Moving the t_g above the nominal temperature performance range reduced the displacements by a factor of about 1.6 for the CTE temperature profile used. However, to make any definitive conclusions the temperature dependence of the modulus also needs to be incorporated. These dependencies have not been included here, but are expected to be the subject of future simulations.

This article is based on SAE International technical paper 2013-01-2132 written by Jeffrey Morgan, Robert Greegor, Patrice Ackerman, and Quynhgiao Le of Boeing Research and Technology.