Browsing by Author "Ramji, Amit"

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    Damage tolerance enhancement of thermoset composites modified with thermoplastic veil interleaves
    (Cranfield University, 2019) Ramji, Amit; Xu, Yigeng
    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.
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    Delamination migration in CFRP laminates under mode I loading
    (Elsevier, 2020-02-10) Ramji, Amit; Xu, Yigeng; Yasaee, Mehdi; Grasso, Marzio; Webb, Philip
    This paper focuses on the effect of interfacial fibre orientation and interleaved veil on the delamination migration of carbon fibre reinforced polymer laminates under Mode I loading. Double cantilever beam specimens with midplane interfacial fibre orientations of 0/0, 90/90, 0/90, 0/45 and 90/45 were tested under two conditions: one with interleaved thermoplastic polyphenylene sulfide veil at the midplane and one without. Results show that, except for the 0/0 configuration, all other orientations exhibit varying levels of migration associated with the interfacial fibre orientation and veil interleaving. The apparent fracture toughness determined with the modified compliance calibration method is closely related to the delamination migration and hence a structural energy dissipation measure dependent on interfacial fibre orientation and the interleaved veil. Distributions of the fibre and matrix materials around the delamination front are found to be closely related to the delamination migration behaviour along its path. The experimental observation and rationalisation presented in this paper provide further knowledge regarding delamination migration and its correlation to the apparent fracture toughness, which is of direct relevance to the damage tolerance design of laminated composite components
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    Effect of interfacial fibre orientation and PPS veil density on delamination resistance of 5HS woven CFRP laminates under mode II loading
    (Elsevier, 2021-02-19) Ramji, Amit; Xu, Yigeng; Grasso, Marzio; Yasaee, Mehdi; Webb, Philip
    This paper presents an experimental study on the effect of interfacial fibre orientation and interleaved thermoplastic veil on Mode II interlaminar fracture toughness of 5-harness satin woven carbon fibre reinforced polymer composite laminates. Three-point End-Notched Flexure tests were carried out to determine delamination resistance, GIIC, of specimens with five fibre orientation biases and two veil densities at the midplane. Results show that delamination resistance of 5-harness satin woven laminates depends on the layup configurations at the midplane with 90/45 fibre orientation bias exhibiting the greatest resistance. The delamination resistance enhancement from polyphenylene sulfide (PPS) veil interleaves is also fibre orientation dependent but a further increase of the veil density from 10 gm−2 to 20 gm−2 offers little extra benefit. Fracture surface morphologies were examined under SEM to understand the failure mechanism and fracture process of the woven laminate under the combined effects of the interfacial fibre orientation and the veil density. Fibre orientation relative to the delamination path, surface texture misfit, and veil density are the three main contributors identified for the variation of delamination resistance of 5HS woven laminates.
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    Implementation of a finite element modelling strategy for the prediction of aircraft tyre response
    (SAGE, 2023-03-11) Ramji, Amit; Grasso, Marzio; Chase, James; Maqbool, Hasher; Krypciak, Michal; Merchante, Veronica; Brighton, James
    This paper presents the finite element modelling (FEM) strategy to identify the structural response of aircraft tyres under quasi-static and taxiing load conditions. The tyre FEM was developed to simulate the aircraft tyre/ground interaction for a range of inflation pressures under vertical, lateral, longitudinal, torsional, yawed and un-yawed rolling. A thorough comparison for validation purposes is made between the test and simulation data extracted from the FEM. The reinforcement plies of the tyres are modelled in a computationally efficient manner whilst also considering the variable fibre volume fractions and ply discontinuities within the tyre. The accurate material characterisation at coupon level combined with the overall modelling approach allowed most simulated responses to match the experimental stiffness within 12% against best fit curves of similar tyre types and within 5% for the majority of test comparisons.
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    Low-velocity impact behaviour of woven laminate plates with fire retardant resin
    (Elsevier, 2019-04-27) Grasso, Marzio; Xu, Yigeng; Ramji, Amit; Zhou, Gang; Chrysanthou, Andreas; Haritos, George; Chen, Yong Kang
    The understanding of the damage mechanisms for woven laminate plates under low-velocity impact is challenging as the damage mechanisms at the interface of adjacent layers are dominated by the fibre architecture. This work presents an experimental investigation of the behaviour of woven glass and carbon fibre composite laminates in a matrix of fire retardant resin under low-velocity impact. The performance is evaluated in terms of damage mechanisms and force time history curves. Six impact energy levels were used to test standard plates to identify the type of damage observed at various energy levels. Scanning electron microscopy (SEM) along with C-scans were used to characterise the damage. It has been observed that in woven composites, the damage occurs mostly between the fibre bundles and matrix. As the impact energy increases, the failure involves extended matrix cracking and fibre fracture. Moreover, due to the fibre architecture, both the contact forces between bundles of fibres and stretching of the bundles are responsible for the dominant matrix cracking damage mode observed at the low-impact energy level. As the impact energy increases, the damage also increases resulting in fibre fracture. The experimental evidence collected during this investigation shows that for both the carbon fibre and the glass fibre woven laminates the low-velocity impact behaviour is characterised by extended fibre fracture without a noticeable sudden load drop.