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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.
    Composite Structures 01/2015; 100:464–478. DOI:10.1016/j.compstruct.2013.01.014
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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.
    International Journal of Solids and Structures 10/2014; 51(19-20). DOI:10.1016/j.ijsolstr.2014.05.017
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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.
    Composites Part A Applied Science and Manufacturing 10/2014; 65:175–185. DOI:10.1016/j.compositesa.2014.06.005
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    ABSTRACT: This work describes the manufacture and characterisation of a PEEK-based zero Poisson’s ratio honeycomb (SILICOMB) produced using Kirigami-inspired cutting and folding techniques. The flatwise compression and transverse shear properties of the structure are determined through ASTM mechanical testing, and the results compared against commercially available honeycombs, including several other zero Poisson’s ratio cellular structures. An analytical model to predict the shear strength is compared to the results. SILICOMB specimens are found to have lower stiffness compared to other honeycomb configurations, but comparable strength. Factors influencing the results and variations to the manufacturing process are discussed.
    Composite Structures 08/2014; 114. DOI:10.1016/j.compstruct.2014.04.001
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    ABSTRACT: An experimental investigation into size effects in notched [45/90/-45/0]4s carbon/epoxy laminates is carried out. The in-plane dimensions of the quasi-isotropic laminates are scaled up by a factor of up to 8. Larger Scale 16 specimens with only their width and notch length being doubled were also tested as a further comparison. Interrupted tests and X-ray Computed Tomography (CT) scanning are carried out to study the damage at the crack tips. Sharp centre-notched tensile tests are compared to open-hole tests of the same notch length (hole diameter), material and stacking sequence. A similar strength reduction scaling trend is found for both configurations at the small sizes, except with higher tensile strength for the centre-notched specimens than the open-hole specimens. However, there is a cross-over point when the sizes increase, with the sharp notched results approaching an asymptote based on Linear Elastic Fracture Mechanics (LEFM), and the open hole results approaching an asymptote based on Weibull theory.
    Composites Science and Technology 08/2014; 100. DOI:10.1016/j.compscitech.2014.06.002
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    ABSTRACT: Automated fibre placement (AFP) is well-known as a cutting-edge technology for manufacturing variable angle tow (VAT) composites with tailored fibre paths. However, its process-induced defects prevent the wide application of VAT composite structures. As an alternative manufacturing method, the continuous tow shearing (CTS) technique, utilising the ability to shear dry tows, has been developed. It was shown that CTS could significantly reduce process-induced defects such as fibre wrinkling, resin rich areas and fibre discontinuities. In this paper, its manufacturing characteristics such as material characteristics, layup accuracy, and thickness variation are investigated experimentally.
    Composites Part A Applied Science and Manufacturing 06/2014; 61. DOI:10.1016/j.compositesa.2014.02.019
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    ABSTRACT: Variable angle tow (VAT) laminates have previously shown enhanced buckling performance compared to conventional straight fibre laminates. In this study, an analytical method is developed for the buckling analysis of a novel blade stiffened VAT panel to allow this potential to be more fully exploited. The prebuckling and buckling analysis, performed on a representative section of a blade stiffened VAT panel, are based on a generalised Rayleigh–Ritz procedure. The buckling analysis includes a first order shear deformation theory by introducing additional shape functions for transverse shear and is therefore applicable to structures with thick skins relative to characteristic length. Modelling of the stiffener is achieved with two approaches; idealisation as a beam attached to the skin’s midplane and as a rigidly attached plate. Comparing results with finite element analysis (Abaqus) for selected case studies, local buckling errors for the beam model and plate model were found to be less than 3% and 2% respectively, whilst the beam model error for global buckling was between 3% and 10%. The analytical model provides an accurate alternative to the computationally expensive finite element analysis and is therefore suitable for future work on the design and optimisation of stiffened VAT panels.
    Composite Structures 05/2014; 111:259–270. DOI:10.1016/j.compstruct.2013.12.029
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    ABSTRACT: This paper presents an experimental characterisation of the mechanical performance and behaviour of through-thickness reinforced composite laminates. To achieve this, composite blocks with individual reinforcing pins were manufactured, quality assessed and tested. Individual specimens were inspected using X-ray Computed Tomography and only the specimens with acceptable quality pin insertions were tested experimentally under a range of mode mixities. Two stacking sequences, uni-directional (UD) and quasi-isotropic (QI) were investigated. It was found that the pins inside the UD samples experienced significantly larger pin/matrix bond strength than those in the QI laminates. The resulting experimental data indicates that a non-UD laminate type may experience pin pull-out and thus increased energy absorption for a wider range of mode mixities than a UD laminate type. Energy plots show a clear transition from a pull-out to a pin rupture region for both laminate types. Specimens that experienced pin rupture during low mode mixity tests exhibited similar failure energies to those loaded in pure mode II.
    Composites Science and Technology 04/2014; 94. DOI:10.1016/j.compscitech.2014.02.001
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    ABSTRACT: This paper proposes a new FE-based approach for modelling all of the possible damage modes in glass/carbon UD hybrid laminates in tensile loading. The damage development is modelled by two sets of cohesive elements, (i) periodically embedded in the carbon layer for modelling carbon fibre failure and (ii) at the glass/carbon interface to capture delamination. The analysis is stopped when the glass layer failure is predicted by integrating the stress distribution over the glass layer to calculate an equivalent stress for unit volume of the glass. The proposed method is validated against the experimental results and then used to simulate the progressive damage process of other hybrid configurations and finally produce a damage-mode map for this material set. The method can easily be applied to other hybrids to assess their performance by producing damage-mode maps.
    Composites Science and Technology 04/2014; 94. DOI:10.1016/j.compscitech.2014.01.013
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    ABSTRACT: Thermally loaded unsymmetric laminates can display separate bending and twisting responses combined with alternating regions of bistability when fabricated with initial curvature in the unstressed state. The twist response is bistable, and represents the minimum potential energy paths that join two orthogonally-oriented bending modes. These two bending modes are each monostable for moderate changes in temperature, but become bistable for large changes. A Rayleigh–Ritz model using von Kármán plate kinematics extended to account for initial curvature is presented. Results are compared against both experimental data and a finite element model. The present model captures the shell-like behavior of initially curved laminates without the need for more complex shell kinematics, and thus represents a useful tool for the initial design of thermally actuated morphing structures.
    Composite Structures 03/2014; 109:240 - 245. DOI:10.1016/j.compstruct.2013.11.005
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