Modelling manufacturing deformations in corner sections made of composite materials

A three-step finite element model has been implemented to predict the spring-in of L-shaped parts. The material property development during the cure has been modelled as step changes during transitions between viscous, rubbery and glassy states of the resin. The tool-part interaction is modelled as a sliding interface with a constant sliding shear stress. The effect of various material and geometric variables on the deformation of L-Section parts are investigated by a parameter sensitivity analysis. The spring-in predictions obtained by the finite element method are compared to experimental measurements for unidirectional and cross-ply parts of various thicknesses and radii. Results indicate that although a 2D plane strain model can predict the spring-in measured at the symmetry plane fairly well, it is not sufficient to capture the complex deformation patterns observed. © The Author(s) 2013 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav

Modelling the effect of gaps and overlaps in automated fibre placement (AFP)-manufactured laminates

AbstractIn automated fibre placement (AFP) process, gaps and overlaps parallel to the fibre direction can be introduced between the adjoining tapes. These gaps and overlaps can cause a reduction in strength compared with pristine conditions. Finite element modelling is an effective way to understand how the size and distribution of such gaps and overlaps influences the strength and failure development. Many modelling work showed that out-of-plane waviness and ply thickness variations caused by gaps and overlaps play an important role in inducing the strength knock-down; however, there has been a lack of effective way to explicitly model the ply waviness, which constrained the relevant research. In this work, 3D meshing tools were developed to automatically generate ply-by-ply models with gaps and overlaps. Intra-ply and inter-ply cohesive elements are also automatically inserted in the model to capture the influence of splitting and delamination. Out-of-plane waviness and ply thickness variations caused by gaps and overlaps are automatically modelled. Models with various sizes and distribution of gaps and overlaps were built to predict the reduction of strength as a function of the magnitude and type of the defects. Results of gap and overlap models will be used to guide future experimental characterization of simulated AFP process defects, manufactured by hand layup from pre-preg tape.</jats:p

Development of pseudo-ductile hybrid composites with discontinuous carbon- and continuous glass prepregs

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Multi-directional hybrid laminates-studying the effect of fragmentation and dispersed delamination on stress-strain curves of unnotched laminates using analytical modelling.

Hybridisation is one of the approaches to introduce pseudo-ductility to brittle composite materials. In this approach, two or more different types of fibre are combined and if the configuration and material constituents are well selected, the tensile response shows a gradual failure and pseudo-ductile strain. Different types of hybrid composites with continuous layers have been studied to produce pseudo-ductile tensile behaviour. However, most hybrid material studies to date have been focused on UniDirectional (UD) laminates which are not usually applied in industry due to poor transverse mechanical properties. In this study, the behaviour of multi-directional hybrid laminates made with UD hybrid sub-laminates is studied. The final goal is to introduce pseudo-ductility to layups with wider industrial applications. The effect of layup as well as the UD building-block stress-strain curve on the final stress-strain curve of the laminate is also studied. A new analytical approach based on Classical Laminate Theory is introduced in which the effect of different damage modes in UD hybrid laminates (fragmentation and dispersed delamination) is taken into account. The output of this method is the non-linear stress-strain curve of a multi-directional laminate with UD hybrid sub-laminates. This method is then used to study the effect of different parameters such as the mechanical properties of the constituents (low and high strain materials) and layup on the pseudo-ductility

Numerical modelling of the damage modes in UD thin carbon/glass hybrid laminates

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. © 2014 Elsevier Ltd

Hybrid specimens eliminating stress concentrations in tensile and compressive testing of unidirectional composites

AbstractTwo novel approaches are proposed for elimination of stress concentrations in tensile and compressive testing of unidirectional carbon/epoxy composites. An interlayer hybrid specimen type is proposed for tensile testing. The presented finite element study indicated that the outer continuous glass/epoxy plies suppress the stress concentrations at the grips and protect the central carbon/epoxy plies from premature failure, eliminating the need for end-tabs. The test results confirmed the benefits of the hybrid specimens by generating consistent gauge-section failures in tension. The developed hybrid four point bending specimen type and strain evaluation method were verified and applied successfully to determine the compressive failure strain of three different grade carbon/epoxy composite prepregs. Stable failure and fragmentation of the high and ultra-high modulus unidirectional carbon/epoxy plies were reported. The high strength carbon/epoxy plies exhibited catastrophic failure at a significantly higher compressive strain than normally observed

Demonstration of pseudo-ductility in unidirectional hybrid composites made of discontinuous carbon/epoxy and continuous glass/epoxy plies

AbstractA new, partially discontinuous architecture is proposed to improve the mechanical performance of pseudo-ductile, unidirectional (UD) interlayer carbon/glass hybrid composites. The concept was successfully demonstrated in different laminates with high strength and high modulus carbon and S-glass epoxy UD prepregs. The novel hybrid architecture provided pseudo-ductile tensile stress–strain responses with a linear initial part followed by a wide plateau and a second linear part, all connected by smooth transitions. The best hybrid configuration showed 60% improvement in modulus compared to pure glass, 860MPa plateau stress and 2% pseudo-ductile strain. The initial modulus, the plateau stress and the overall tensile stress–strain response of each specimen configuration were predicted accurately