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    ABSTRACT: The Hellinger–Reissner mixed variational principle in conjunction with Lagrange multipliers is used to model transverse shear stresses, in bending of variable stiffness laminated plates, using a reduced two-dimensional formulation. The effect of transverse shear stresses on the bending deflection of variable angle tow (VAT) laminates is assessed. The novel formulation features multiple shear correction factors that are functions of the bending rigidity terms Dij, their first and second derivatives and the Timoshenko shear factor χ. The new set of governing equations are solved, in their strong form, using the Differential Quadrature Method (DQM) and the accuracy and robustness of the solution technique verified using a 2D “thick” shell and 3D high-fidelity finite element model. The derived theory is superior to the “thick” 2D shell model in capturing transverse shear effects and shows good accuracy compared to the full 3D solution for thicknesses within the range of practical engineering laminates. The derived equations degenerate to Classical Laminate Analysis for very thin configurations but discrepancies as large as 43% are observed for span-to-thickness ratios of 10:1. Finally, the specific VAT panels under investigation are affected more by transverse shear deformation than a corresponding homogeneous quasi-isotropic laminate.
    Full-text · Article · Jan 2015 · Composite Structures
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    ABSTRACT: A new micro-mechanical model is proposed for describing the bridging actions exerted by through-thickness reinforcement on delaminations in prepreg based composite materials, subjected to a mixed-mode (I-II) loading regime. The model applies to micro-fasteners in the form of brittle fibrous rods (Z-pins) inserted in the through-thickness direction of composite laminates. These are described as Euler-Bernoulli beams inserted in an elastic foundation that represents the embedding composite laminate. Equilibrium equations that relate the delamination opening/sliding displacements to the bridging forces exerted by the Z-pins on the interlaminar crack edges are derived. The Z-pin failure meso-mechanics is explained in terms of the laminate architecture and the delamination mode. The apparent fracture toughness of Z-pinned laminates is obtained from as energy dissipated by the pull out of the through-thickness reinforcement, normalised with respect to a reference area. The model is validated by means of experimental data obtained for single carbon/BMI Z-pins inserted in a quasi-isotropic laminate.
    Full-text · Article · Oct 2014 · International Journal of Solids and Structures
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    ABSTRACT: The High Performance-Discontinuous Fibre (HiPerDiF) method is a new high speed process to produce discontinuous fibre architectures with high volume fraction. It allows the manufacture of tow or tape type prepregs with highly aligned reinforcements directly from short fibres rather than from pre-existing tows. This paper introduces the principle of this unique short fibre alignment method and describes the improved orientation head design for obtaining tape type preforms with high productivity. Using this HiPerDiF method, tensile specimens with 67% of the fibres aligned within the range of ±3° were successfully produced from tape type preforms with 3 mm long carbon fibres. Tensile modulus and strength in the fibre direction of specimens with a fibre volume fraction of 55% were 115 GPa and 1509 MPa, respectively, significantly higher than those of aligned short fibre composites made by conventional methods.
    No preview · Article · Oct 2014 · Composites Part A Applied Science and Manufacturing
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