Investigation Into The Failure Behaviour Of Intraply Woven Hybrid Composites

Hybrid fabrics represent a rapidly emerging branch of reinforcements in composite
materials. A justified use of these textiles requires a good understanding of their role
on the failure behaviour and the mechanical performance of Intraply Woven Hybrid
Composite (IWHC).
This thesis presents a methodology, which helped to describe and explain the failure
behaviour of IWHC using combined experimental and analytical results. In addition it
has been demonstrated that Finite Element (FE) models can be used successfully to
evaluate the extent of damage under impact loading. The efficiency of the models has
been tested against the experimental results and other established micro-mechanical
models. Tensile tests supported by non-destructive techniques such as acoustic
emission, X-ray and strain field measurements have been conducted to characterise
the mechanical behaviour, damage modes and the mechanisms of failure.
The experimental tensile results have identified the non-linear plastic damage
behaviour, while the FE results have indicated the linear elastic damage behaviour.
The overall picture of the failure mechanism has been established by combining the
results gathered from the FE and the experimental analyses.
The results of the longitudinal tensile tests show an increase in stiffness and strength
properties with the increase in the carbon content in the carbon-aramid hybrid
composite. Fibre dominated elastic properties have been predicted with an acceptable
accuracyb ut predictionf or matrix-dominatedb ehaviourh ash ad significante rrors.
This study has identified three major damage events during the tensile loading process
at different strain levels. These include a) onset of damage initiated by debonding of
aramid fibres, b) formation of clusters of damaged carbon fibres c) necking of aramid
fibres at the high stress concentration areas (crimp regions). Eventually, when the
content of unbroken fibres is insufficient to withhold the load, abrupt failure occurs
locally. Furthermore the residual compressive strengths of the hybrids have been assessed
after subjecting them to low velocity impact loading. All the experimental data have
been used to support and test the FE models of impact loading. X-ray and scanning
electron microscopy techniques have been used to identify the extent and mechanism
of damage operating during the impact loading. It has been shown that the lower
stiffness property in the hybrid composite has prevailed over the high toughness
property. Interesting trends have been established for the flexure energy absorption,
impact energy absorption and damage tolerance in compression after impact testing
for hybrid composites with different carbon contents.

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