JEC COMPOSITES MAGAZINE - Issue #99 - September 2015 - 45

thermal expansion

Boeing simulates thermal expansion in composites with expanded metal foil for lightning
protection of aircraft structures
Modern aircraft such as the Boeing 787 Dreamliner
are comprised of more than 50% carbon fibre composite requiring the addition of expanded metal foil
for lightning strike protection. Researchers at Boeing
are using environmental testing and simulation to
verify that protective coatings on the metal foil will
not fail due to thermal stress arising from a typical
flight cycle.

Fig. 1: Advanced composites used throughout the Boeing 787 account for more
than 50% of the aircraft body1

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he Boeing 787 Dreamliner is innovative in that it is comprised
of more than 50% carbon fibre-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 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. Contributing to the research effort
at BR&T are project lead 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 aircraft composites
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 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 aluminium or copper,

Fig. 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 and long way of the diamond, respectively. The mesh aspect ratio: SWD/
LWD is one of the parameters varied in the simulations

No99 September 2015 /

jec composites magazine 45

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #99 - September 2015