Abstract:
A feasible method of overcoming the stringent certification and ecological
sustainability objectives within the aerospace sector is to substitute conventional
engineering alloys with high-performance lightweight composite materials for primary
structures. Although, widespread utilisation of laminated composites are constrained
by the significant reduction of the residual compressive strength stemming from a
lack of through-thickness reinforcement leading to interface delamination. This work
presents the detailed investigations on the characteristics of the fibre-matrix
interactions in conjunction with ply interface enhancements; to develop new
knowledge for damage tolerance enhancement of thermoset composite laminates
modified with thermoplastic veil interleaves.
The influence of fracture initiation and propagation is the focus of this work, through
the experimental exploration of various failure mechanisms. The failures of
toughened carbon-fibre/epoxy laminates with poly-phenylene sulfide (PPS) veil
interlayers are considered under; inter-laminar, intra-laminar, fracture migration and
low-velocity impact damage.
Conservative test standards for the interlaminar fracture resistance present veil
interleaves as having a 48% enhancement on the interlaminar fracture parameters
for longitudinal propagation of unidirectional (UD) fibre reinforced polymer (FRP)
laminates. However, interleaving enhancements are revealed to be over-projected
when fracture propagation is transverse to the interfacial fibre orientation with
enhancements being <2%. Under mode I loading, intra-laminar fracture plane
migration is the dominant damage mechanism for transverse yarn interfaces, which
trigger migration away from the toughened interface.
In contrast, woven lamina cannot permit through-thickness migration due to yarn
interlacing, where 136% for longitudinal bias and 20% for transverse biased
enhancements are apparent using 20 gm-2 PPS. Furthermore, reductions of
interfacial fibre orientation bias sensitivity were demonstrated for veil interleaved
interfaces. Similarly, under mode II loading, transverse biased interfaces exhibited
up to 75% improvement in delamination resistance with gradual growth
characteristics resulting from the veil and transverse fibre interactions.
Interleaved laminates were subsequently explored for low-velocity impact damage,
emphasising on veil distributions within the laminate. Interleaving the outside
interfaces transformed the damage from inter-laminar to intra-laminar and increased
the delamination threshold load (DTL) by 9% or entirely removed DTL occurrences
for heavily interleaved layups. The veil damage mechanisms resulting in improved
fracture resistance and enhanced damage tolerance are due to the inclusion of lower
modulus PPS fibres. These discrete thermoplastic regions within the laminate provide
a cushioning effect while increasing the localised ductility resulting in increased fibrematrix interactions at the fracture process zone.