Browsing by Author "Wadley, H. N. G."
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Item Open Access Compressive response of a 3D non-woven carbon- fibre composite(Elsevier, 2017-12-09) Das, S.; Kandan, Karthikeyan; Kazemahvazi, S.; Wadley, H. N. G.; Deshpande, V. S.The compressive response of a three-dimensional (3D) non-interlaced composite comprising three orthogonal sets of carbon fibre tows within an epoxy matrix is analysed. First, the compressive response is measured in three orthogonal directions and the deformation/failure modes analysed by a combination of X-ray tomography and optical microscopy. In contrast to traditional unidirectional and two-dimensional (2D) composites, stable and multiple kinks (some of which zig-zag) form in the tows that are aligned with the compression direction. This results in an overall composite compressive ductility of about 10% for compression in the low fibre volume fraction direction. While the stress for the formation of the first kink is well predicted by a usual micro-buckling analysis, the composite displays a subsequent hardening response associated with formation of multiple kinks. Finite element (FE) calculations are also reported to analyse the compressive response with the individual tows modelled as anisotropic continua via a Hill plasticity model. The FE calculations are in good agreement with the measurements including prediction of multiple kinks that reflect from the surfaces of the tows. The FE calculations demonstrate that the three-dimensionality of the microstructure constrains the kinks and this results in the stable compressive response. In fact, the hardening and peak strength of these composites is not set by the tows in direction of compression, but rather set by the out-of-plane compressive response of the tows perpendicular to the compression direction.Item Open Access Deep penetration of ultra-high molecular weight polyethylene composites by a sharp-tipped punch(Elsevier, 2018-06-05) Liu, B.; Kandan, Karthikeyan; Wadley, H. N. G.; Deshpande, V. S.The penetration of unidirectional (UD) and [0o/90o] cross-ply ultra-high molecular weight polyethylene fibre composites by sharp-tipped cylindrical punches has been investigated. While the measured penetration pressure for both composite types increased with decreasing punch diameter, the pressure was significantly higher for the cross-ply composites and increased with decreasing ply thickness. A combination of optical microscopy and X-ray tomography revealed that in both composites, the sharp-tipped punch penetrated without fibre fracture by the formation of mode-I cracks along the fibre directions, followed by the wedging open of the crack by the advancing punch. In the cross-ply composites, delamination between adjacent 0o and 90o plies also occurred to accommodate the incompatible deformation between plies containing orthogonal mode-I cracks. Micromechanical models for the steady-state penetration pressure were developed for both composites. To account for material anisotropy as well as the large shear strains and fibre rotations, the deformation of the composites was modelled via a pressure-dependent crystal plasticity framework. Intra and inter-ply fracture were accounted for via mode-I and delamination toughnesses respectively. These models account for the competition between deformation and fracture of the plies and accurately predict the measured steady-state penetration pressures over the wide range of punch diameters and ply thicknesses investigated here. Design maps for the penetration resistance of cross-ply composites were constructed using these models and subsequently used to infer composite designs that maximise the penetration resistance for a user prescribed value of fibre strength.Item Open Access High strain rate compressive response of ultra-high molecular weight polyethylene fibre composites(Elsevier, 2019-04-20) Liu, B. G.; Kandan, Karthikeyan; Wadley, H. N. G.; Deshpande, V. S.The mechanisms of deformation during the dynamic in-plane compression of 〖[0^o/〖90〗^o]〗_n (cross-ply) ultra-high molecular weight polyethylene (UHMWPE) fibre composites with polymeric matrices have been investigated for strain rates in the range 0.01 s^(-1) to 4000 s^(-1). The measured strain rate sensitivity was mild for strain rates less than about 100 s^(-1), but increased sharply at higher rates. X-ray computed tomography and optical microscopy revealed that over the range of strain rates investigated here, the deformation mechanism was kinking (micro-buckling) of the plies with a kink band width of about 1 mm. Ply delamination was also observed, but only during softening phase of the response after the peak strength had been attained. To gain a mechanistic understanding of the observed strain rate sensitivity, finite element (FE) simulations were used to model the compression experiments. For these calculations, each specimen ply was explicitly modelled via a pressure-dependent crystal plasticity framework that accounts for the large shear strains and fibre rotations that occur within each ply in the kink band. Calculations were conducted in the limits of perfectly-bonded and completely un-bonded plies. Good agreement between measurements and predictions was obtained when plies were assumed to be perfectly bonded, confirming the hypothesis that ply delamination plays a small role in setting the peak strength as well as the compressive response of the composite at moderate levels of applied strain. The calculations also show that misalignment of the specimen between the compression platens strongly influences the compression response and especially the initial stiffness. Importantly, the FE calculations reveal that over the range of strain rates investigated here, inertial stabilisation has a negligible contribution to the strong rate sensitivity observed for strain rates above 100 s^(-1) and that this sensitivity is primarily associated with the strain rate sensitivity of the polymeric matrix.Item Open Access Indentation response of a 3D non-woven carbon-fibre composite(Cambridge University Press, 2018-01-16) Das, S.; Kandan, Karthikeyan; Kazemahvazi, S.; Wadley, H. N. G.; Deshpande, V. S.The indentation response of a 3D noninterlaced composite comprising three sets of orthogonal carbon-fibre tows in an epoxy matrix is investigated. The 3D composites have a near isotropic and ductile indentation response. The deformation mode includes the formation of multiple kinks in the tows aligned with the indentation direction and shearing of the orthogonally oriented tows. Finite element (FE) calculations are also reported wherein tows in one direction are explicitly modeled with the other two sets of orthogonal tows and the matrix pockets treated as an effective homogenous medium. The calculations capture the indentation response in the direction of the explicitly modeled tows with excellent fidelity but under-predict the indentation strength in the other directions. In contrast to anisotropic and brittle laminated composites, 3D noninterlaced composites have a near isotropic and ductile indentation response making them strong candidates for application as materials to resist impact loading.Item Metadata only The out-of-plane compressive response of Dyneema (R) composites(Elsevier, 2014-06-12) Attwood, J. P.; Khaderi, S. N.; Karthikeyan, K.; Fleck, N. A.; O'Masta, M. R.; Wadley, H. N. G.; Deshpande, V. S.Out-of-plane compression tests were conducted on six grades of ultra high molecular weight polyethylene fibre composites (Dyneema®Dyneema®) with varying grades of fibre and matrix, ply thickness, and ply stacking sequence. The composites with a [0°r/90°] lay-up had an out-of-plane compressive strength that was dictated by in-plane tensile fibre fracture. By contrast, the out-of-plane compressive strength of the uni-directional composites was significantly lower and was not associated with fibre fracture. The peak strength of the [0°/90°] composites increased with increasing in-plane specimen dimensions and was dependent on the matrix and fibre strength as well as on the ply thickness. A combination of micro X-ray tomography and local pressure measurements revealed the existence of a shear-lag zone at the periphery of the specimens. Finite Element (FE) and analytical micromechanical models predict the compressive composite response and reveal that out-of-plane compression generates tensile stresses along the fibres due to shear-lag loading between the alternating 0° and 90° plies. Moreover, the compressive strength data suggests that the shear strength of Dyneema®Dyneema® is pressure sensitive, and this pressure sensitivity is quantified by comparing predictions with experimental measurements of the out-of-plane compressive strength. Both the FE and analytical models accurately predict the sensitivity of the compressive response of Dyneema®Dyneema® to material and geometric parameters: matrix strength, fibre strength and ply thickness.