1,720,981 research outputs found
Damage assessment of particle-toughened carbon fibre composites subjected to impact and compression-after-impact using 3D X-ray tomography
No full textIn 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
Quasi-static indentation and compression after impact damage growth monitoring using microfocus X-ray computed tomography
No full textIn 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
The role of particle-toughening in improving post-impact compressive strength
No full textThis 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
Observations of damage development from compression-after-impact experiments using ex situ micro-focus computed tomography
No full textThe development of damage mechanisms leading up to compression-after-impact (CAI) failure is studied in particle-toughened and untoughened systems. Microfocus computed tomography (?CT) enabled non-destructive monitoring of the internal damage development in three-dimensions (3D) by taking scans after impact, after an application of near failure compression loads and after coupon failure. In combination with ?CT work, mechanical CAI testing and ultrasonic C-scans were conducted to determine the effect of the projected damage area on residual CAI strength and to complement the observations made from ?CT scans. The important role of the undamaged “cone” of material immediately under the impact site for out-of-plane sublaminate stability is identified. The implication of delamination growth into this region is discussed. It was found that where particle-toughened systems suppressed delamination growth into this region, greater residual CAI strength was maintained on a like-for-like projected damage are
Investigation of the response to low velocity impact and quasi-static indentation loading of particle-toughened carbon-fibre composite materials
No full textThis work investigates damage caused by low velocity impact and quasi-static indentation loading in four different particle-toughened composite systems, and one untoughened system. For impact tests, a range of energies were used between 25 and 50 J. For QSI, coupons were interrupted at increasing loading point displacement levels from 2 to 5 mm to allow for monitoring of damage initiation and propagation. In both loading cases, non-destructive inspection techniques were used, consisting of ultrasonic C-scan and X-ray micro-focus computed tomography. These techniques are complemented with instrumentation to capture force–displacement data, whereby load-drops are associated with observed damage modes. Key results from this work highlight particular issues regarding strain-rate sensitivity of delamination development and an earlier onset of fibre fracture associated with particle-toughened systems. These issues, in addition to observations on the role of micro-scale events on damage morphology, are discussed with a focus on material development and material testing practices
Composite laminate impact damage assessment by high resolution 3D X-ray tomography and laminography
No full textImprovements to toughening mechanisms in composite materials have hitherto relied on visual inspection techniques that can be rather limited, especially since the inherent damage behaviour is three-dimensional (3D) requiring high resolution to capture micro-cracks and similar damage. To achieve a better understanding of impact damage behaviour, synchrotron radiation computed laminography (SRCL) and computed tomography (SRCT) techniques were used to capture 3D damage mechanisms with voxel sizes of 0.7?m and 1.4?m respectively. Comparisons between impacted toughened and non-toughened carbon fibre reinforced polymer (CFRP) systems were made in which toughening particles were introduced into the matrix of the toughened material. This study has found that at the macro scale, the overall 3D damage pattern in toughened and non-toughened specimens are very similar, however when studied at the micro level, it is clear that significant differences in damage between the two systems exist
Three-dimensional assessment of low velocity impact damage in particle toughened composite laminates using micro-focus X-ray computed tomography and synchrotron radiation laminography
Full text linkResults are presented studying the contribution of particle toughening to impact damage resistance in carbon fibre reinforced polymer materials. Micro-focus X-ray computed tomography and synchrotron radiation computed laminography were used to provide a novel, multiscale approach for assessing impact damage. Thin (1 mm thick) composite plates containing either untoughened or particle-toughened resin systems were subjected to low velocity impact. Damage was assessed three-dimensionally at voxel resolutions of 0.7 ?m and 4.3 ?m using SRCL and ?CT respectively; the former being an innovative approach to the laterally extended geometry of CFRP plates. Observations and measurements taken from ?CT scans captured the full extent of impact damage on both material systems revealing an interconnected network of intra- and inter-laminar cracks. These lower resolution images reveal that the particle-toughened system suppresses delaminations with little effect on intralaminar damage. The higher resolution images reveal that the particles contribute to toughening by crack deflection and bridging
A comparison of multi-scale 3D X-ray tomographic inspection techniques for assessing carbon fibre composite impact damage
Full text linkTomographic imaging using both laboratory sources and synchrotron radiation (SR) was performed to achieve a multi-scale damage assessment of carbon fibre composites subjected to impact damage, allowing various internal damage modes to be studied in three-dimensions. The focus of this study is the comparison of different tomographic methods, identifying their capabilities and limitations, and their use in a complementary manner for creating an overall 3D damage assessment at both macroscopic and microscopic levels. Overall, microfocus laboratory computed tomography (?CT) offers efficient routine assessment of damage at mesoscopic and macroscopic levels in engineering-scale test coupons and relatively high spatial resolutions on trimmed-down samples; whilst synchrotron radiation computed tomography (SRCT) and computed laminography (SRCL) offer scans with the highest image quality, particularly given the short acquisition times, allowing damage micromechanisms to be studied in detail
Micromechanistic analysis of toughened carbon fibre composite laminate failure by high resolution synchrotron computed tomography
No full textSynchrotron 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
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