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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 01/2014;
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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 01/2014;
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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 01/2014;
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    ABSTRACT: A multi-scale approach to modelling is optimal for computationally intensive problems of a hierarchical nature such as 3D woven composites. In this paper an approach capable of modelling feature/component scale fabric deformations and defects is proposed. The proposed technique starts with a meso-scale model for predicting the as-woven geometry of a single unit cell using a high fidelity digital element method. The unit cell geometry is then converted into a macro-scale fabric model by geometric reduction then tessellation. On the macro-scale, two and three dimensional approaches to yarn geometry representation are proposed, with an accompanying yarn mechanical model. Each approach is evaluated based on solution accuracy and computational efficiency. The proposed approach is then verified against experimental results on the meso- and macro-scales. The applicability of this modelling technique to larger scale compaction problems is then investigated. The proposed algorithm was found to be accurate and computationally efficient.
    Composites Part A Applied Science and Manufacturing 01/2014; 57:95–107.
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    ABSTRACT: The fibre-direction tensile strength of carbon/epoxy laminates under the influence of through-thickness compressive stresses has been experimentally investigated. In a unidirectional (UD) laminate, the through-thickness compressive stresses will cause premature longitudinal fibre-splitting, masking the effect of transverse stresses on the fibre-direction strength. Here a cross-ply laminate has been used. With the addition of 90° plies preventing the 0° fibres from splitting, it effectively allows the dependence of fibre-direction tensile strength on high through-thickness stresses to be studied. The severity of the through-thickness loads has been varied using cylindrical indenters of different radii, up to the loads near the through-thickness compressive failure stress of the cross-ply laminate. The results show that there is a linear decrease in fibre-direction strength with the mean through-thickness stress. In all the test cases, the specimens failed in a catastrophic brittle manner, with scanning electron micrography showing primarily a fibre tensile fracture mode. The detailed stress state in the specimens has been calculated via finite element analysis. Two failure criteria are proposed, which can be used as conservative design criteria concerning fibre-dominated failures in multiaxial load scenarios.
    Composites Science and Technology 01/2014; 90:1–8.
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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 01/2014;
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    ABSTRACT: Despite the large number of recent reviews on green composites, limited investigation has taken place into the most appropriate applications for these materials. Green composites are regularly referred to as having potential uses in the automotive and construction sector, yet investigation of these applications reveals that they are often an inappropriate match for the unique material attributes of green composites. This review provides guidelines for engineers and designers on the appropriate application of green composites. A concise summary of the major material attributes of green composites is provided; accompanied by graphical comparisons of their relative properties. From these considerations, a series of complementary application properties are defined: these include applications that have a short life-span and involve limited exposure to moisture. The review concludes that green composites have potential for use in a number of applications, but as with all design, one must carefully match the material to the application.
    Composites Part A Applied Science and Manufacturing 01/2014; 56:280–289.
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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 01/2014;
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    ABSTRACT: The manufacture of advanced composite panels with variable fibre angles can lead to laminates with a flat profile on one side and a smooth, curved profile on the other. When modelling these laminates in two-dimensional form the flat plate assumptions may no longer accurately capture the structural behaviour. In this paper the buckling behaviour of laminates with one-dimensional fibre variations and symmetric stacking sequences is investigated. The assumptions of modelling the three-dimensional profile as a flat plate or a cylindrical panel are assessed, taking into account the effects of transverse shear deformation. The governing differential equations are solved in the strong form using the Differential Quadrature method and validated by 2D finite element models. The validity of the two modelling approaches is assessed by comparing the solutions to a 3D finite element model capturing the actual shape of the laminate. It is suggested that the buckling event of these variable angle tow, variable thickness laminates is characterised more accurately by “shell-like” than by “plate-like” behaviour. The idea of investigating the effects of two-dimensional fibre orientations with their associated doubly curved topologies is proposed.
    Composite Structures. 01/2014; 107:482–493.
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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 01/2014; 111:259–270.
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