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This work investigates the micromechanisms associated with particle-toughening strategies to improve the damage resistant and damage tolerant performance in carbon fibre reinforced polymer (CFRP) materials. Five material systems were studied; consisting of one untoughened and four particle-toughened systems. Synchrotron radiation computed tomography (SRCT) was used to study the damage micromechanisms in standard 150mm x 100 mm rectangular coupons subjected to 30 J low velocity impact loading. Laboratory based micro-focus computed tomography (?CT) enabled damage assessments and comparison of coupons subjected to low velocity impact, quasi-static indentation and at the onset of compression-after-impact failure. Mechanisms leading to damage resistance and damage tolerance are discussed along with strategies to use these observations to inform finite element models.

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.

In this study interrupted quasi-static indentation and post-impacted compression tests were performed at incremental load steps with X-ray computed tomography performed at each step. This enabled non-destructive, three-dimensional damage assessments to be carried out allowing initiation and propagation of different damage modes to be monitored. Preliminary results from these experiments are reported in this paper.

Synchrotron Radiation Computed Tomography (SRCT) allows for non-destructive identification of fracture mechanisms in materials at very high resolutions. In this work, carbon fibre reinforced plastics (CFRPs) were imaged using SRCT to ascertain fracture micro-mechanisms under both quasi-static Mode I and Mode II dominated loading conditions. This, combined with previous work on impacted coupons, provides mechanistic comparison between the different loading conditions on similar material systems. Initial findings have identified particle/matrix debonding, crack bridging and ligamented behaviour as reported previously, but have emphasized micro-cracks and the extent to which particle/matrix debonding occurs ahead of the crack tip under both Mode I and Mode II loading conditions. Such work is intended to support both material development and more accurate structural performance simulation for the toughened materials that are being increasingly used as primary structures in aerospace applications.

3D X-ray computed tomography (CT) was used to study the effects of particle toughening within unidirectional carbon fibre reinforced polymer (CFRP) materials subjected to impact damage, followed by ex situ CT of compression after impact (CAI) tests at incremental loads. A multi-scale approach utilizing synchrotron radiation CT and laminography was used to study the damage micro-mechanisms of impact-loaded specimens, and micro-focus CT (?CT) assessed damage at meso- and macro-scopic levels. For the ex situ compression after impact testing, plates were CT scanned after progressively larger compressive loads were applied. Particle toughening processes were observed, with damage growth and micro-buckling events being captured close to the final failure in the CAI test.