: An overview of the virtual crack closure technique is presented. The approach used is discussed, the history summarized, and insight into its applications provided. Equations for two-dimensional quadrilateral elements with linear and quadratic shape functions are given. Formula for applying the technique in conjuction with three-dimensional solid elements as well as plate/shell elements are also provided. Necessary modifications for the use of the method with geometrically nonlinear finite element analysis and corrections required for elements at the crack tip with different lengths and widths are discussed. The problems associated with cracks or delaminations propagating between different materials are mentioned briefly, as well as a strategy to minimize these problems. Due to an increased interest in using a fracture mechanics based approach to assess the damage tolerance of composite structures in the design phase and during certification, the engineering problems selected as examples and given as references focus on the application of the technique to components made of composite materials.

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Virtual crack closure technique: History, approach, and applications

The finite element analysis of delamination in laminated composites is addressed using interface elements and an interface damage law. The principles of linear elastic fracture mechanics are indirectly used by equating, in the case of single-mode delamination, the area underneath the traction/relative displacement curve to the critical energy release rate of the mode under examination. For mixed-mode delamination an interaction model is used which can fulfil various fracture criteria proposed in the literature. It is then shown that the model can be recast in the framework of a more general damage mechanics theory. Numerical results are presented for the analyses of a double cantilever beam specimen and for a problem involving multiple delamination for which comparisons are made with experimental results. Issues related with the numerical solution of the non-linear problem of the delamination are discussed, such as the influence of the interface strength on the convergence properties and the final results, the optimal choice of the iterative matrix in the predictor and the number of integration points in the interface elements. Copyright © 2001 John Wiley & Sons, Ltd.

Finite element interface models for the delamination analysis of laminated composites: mechanical and computational issues

This paper is a review of low-velocity impact responses of composite materials. First the term ‘low-velocity impact’ is defined and major impact-induced damage modes are described from onset of damage through to final failure. Then, the effects of the composite's constituents on impact properties are discussed and post-impact performance is assessed in terms of residual strength.

Review of low-velocity impact properties of composite materials

https://research.birmingham.ac.uk/portal/files/20583243/Oliveux_Dandy_Leeke_Current_status_recycling_fibre_Progress_Materials_Science_2015.pdf

Current status of recycling of fibre reinforced polymers: Review of technologies, reuse and resulting properties

A continuum damage model for composite laminates: Part I - Constitutive model

Unidirectional (UD) carbon fibre reinforced polymers offer high specific strength and stiffness but they fail in a catastrophic manner with little warning. Gas-texturing and non-constrained annealing were used to introduce fibre waviness into UD polyamide 12 composites produced by wet-impregnation hoping to produce composites with a more gradual failure mode and increased failure strain. Both methods increased the variation of fibre alignment angle compared to the control samples. The composites containing wavy fibres exhibited a stepwise, gradual failure mode under strain controlled uniaxial tension rather than a catastrophic failure, observed in control samples. Gas-texturing damaged the fibres resulting in a decrease of the tensile strength and strain to failure, which resulted in composites with lower tensile strength and ultimate failure strain than the control composites. Non-constrained annealing of carbon fibre/PA-12 produced wavy fibre composites with ultimate failure strain of 2%, significantly higher than 1.6% of the control composite.

/pdf/unidirectional-carbon-fibre-reinforced-polyamide-12-5sdms5rbe4.pdf

Unidirectional carbon fibre reinforced polyamide-12 composites with enhanced strain to tensile failure by introducing fibre waviness

Failure mechanisms in polymer matrix composites

Exploring the potential of interleaving to delay catastrophic failure in unidirectional composites under tensile loading

https://spiral.imperial.ac.uk/bitstream/10044/1/49550/2/Shape%20memory%20paper%20CST%204%20with%20reviewer%20revisions.pdf

Deployable, shape memory carbon fibre composites without shape memory constituents

This paper addresses the characterisation of the mode I interlaminar fracture toughness of a carbon fibre/epoxy composite material, toughened with thermoplastic particles in the ply interlayers. The characterisation is undertaken at −55 °C, 19 °C, and 90 °C, on both dry and fully moisture saturated coupons. Fractographic observations of the delamination surfaces allows identification of the failure mechanisms. The mode I propagation fracture toughness tested at wet/90 °C exhibits a 176% increase compared to the dry/19 °C specimens, due to enhanced plastic deformation of the interlayers and more prominent fibre bridging. Moisture-saturated coupons tested at −55 °C suffered a 57% reduction of mode I fracture toughness compared to those under dry/19 °C conditions. This is due to the dis-bond and consequent plucking of the thermoplastic particles from the surrounding matrix. This observation points to the fact that wet/cold conditions may represent the worst-case scenario for the interlaminar fracture performance of composite systems toughened with thermoplastic interleaves.

Paul Robinson

Papers

Unidirectional carbon fibre reinforced polyamide-12 composites with enhanced strain to tensile failure by introducing fibre waviness

Failure mechanisms in polymer matrix composites

Exploring the potential of interleaving to delay catastrophic failure in unidirectional composites under tensile loading

Deployable, shape memory carbon fibre composites without shape memory constituents

The influence of temperature and moisture on the mode I fracture toughness and associated fracture morphology of a highly toughened aerospace CFRP