Damage assessment of particle-toughened carbon fibre composites subjected to impact and compression-after-impact using 3D X-ray tomography

Abstract

In this thesis, particle-toughened and untoughened, carbon fibre composite material systems with quasi-isotropic layups were investigated. This was to understand better the toughening behaviour leading to increased impact damage resistance and post-impact compression damage tolerance performance. To achieve this, mechanical testing and conventional ultrasonic C-scan methods were combined with damage assessments using several 3D X-ray computed tomography techniques. These consisted of lab based micro-focus computed tomography (?CT), synchrotron radiation computed tomography (SRCT) and synchrotron radiation computed laminography (SRCL). Mechanical impact and compression-after-impact experiments were undertaken to establish the ranking of damage resistant and damage tolerant properties between material systems. This was followed up by damage assessments from CT scans and laminography to characterise the damage macroscopically and microscopically, linking these observations and quantifications back to the overall damage resistance and damage tolerance of the material systems.

Through qualitative and quantitative assessment of the damage mechanisms it is revealed that particle-toughened systems strongly suppressed the extent of delaminations but had little effect on matrix cracks. The suppression of delaminations was achieved through energy absorption and crack-shielding mechanisms consisting of; particle-matrix debonding, crack deflection and bridging effects, which were observed in the resin-rich regions between the plies. Based on quantification of SRCT data in this study, it is suggested that bridging micromechanisms contributed most significantly to increases in damage resistance over the untoughened material.

Ex situ time-series experiments were also employed in this work. ?CT scans of fully intact test coupons under incremental loads enabled internal damage initiation and propagation to be monitored. This was done for quasi-static indentation (QSI) and compression-after-impact (CAI) experiments.

For QSI work, comparisons between impact and QSI experiments showed both similarities and differences between the two loading conditions. The most significant differences were observed in two material systems which resulted in a lower damage area under QSI loading than low velocity impact at applied energies above 30 J. This behaviour correlated to a larger extent of bridging ligament formation. It is suggested that the extent of bridging micromechanisms are linked to the improved damage resistance under QSI and that this toughening mechanism is potentially sensitive to strain-rate, hence a loss of damage resistance under impact.

For CAI experiments, the sequence of events leading to failure was established. Based on ex situ ?CT scans of material systems subjected to post-impact near-failure compressive loads, it was observed that delaminations propagating into the undamaged cone contributed to failure of the coupon by linking surrounding delaminations. This effect more than doubled the unsupported length of the sublaminates, significantly reducing buckling stability and in-plane load carrying capability. Particle-toughened systems maintained a higher residual compressive strength for a given damage area compared to the untoughened systems. It is suggested that particles suppressed delamination growth into the undamaged cone, increasing stability and enabling more load to be carried prior to failure.

Overall, the experimental findings in this thesis will improve the understanding of the mechanisms contributing to failure and the particle-toughening processes which will support the development of superior carbon fibre-reinforced composite systems. The results also support the development of finite element models to ensure the most important mechanisms are included and captured.

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Identifiers

ORCID for D.J. Bull: orcid.org/0000-0001-6711-6153

ORCID for S.M. Spearing: orcid.org/0000-0002-3059-2014

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