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Fernando
L. Bastian Fiber-Metal Laminate Composites: Characteristics and Fracture Toughness Fibre-metal laminates (FMLs)
are a new family of structural composites designed aiming to produce
a damage-tolerant and high strength material. Their main characteristic
is their very low fatigue crack propagation rates when compared to traditional
aeronautical Al alloys. Their application in aeronautical structures
demands a deep knowledge of a wide set of mechanical properties and
technological values, including both fracture toughness and residual
strength. FMLs are made of alternate thin sheets (0.3 to 0.5 in thickness)
of 2024 or 7475 aluminum alloys bonded together by pre-pregs of epoxy
reinforced by aramid or glass fibers. The FMLs reinforced by aramid
fibers are called ARALLÒ whereas the ones reinforced by glass
fibers are called GLAREÒ. In GLARE laminates the fibers can be
applied in unidirectional or bi-directional way depending on the stress
distribution the laminates will support. All FMLs have high specific
mechanical strength and a very high fatigue crack propagation resistance.
Recently, a test methodology based on elastic-plastic fracture mechanics
(J-Integral and CTOD of Schwalbe, d5) which uses small C(T) and SE(B)
test specimens was proposed to evaluate the instability toughness of
FMLs. Residual strengths of middle cracked tension (M(T)) specimens
of unidirectional FMLs were calculated using the fracture toughness
(JC) obtained following the developed methodology. These values were
then compared to experimental ones. The results show that predicted
values are in close agreement with the experimental ones, especially
for W up to 200 mm. There was a tendency to slightly underestimate the
residual strength of wider specimens (up to 400 mm). This behavior was
associated to the higher stable crack growth capacity of wider M(T)
specimens if compared to the small specimens tested.
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