: 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

PVC foam (Divnycell© H200) was tested under shear loading to mimic the fracture processes which have been observed in mode II delamination growth in carbon fibre reinforced polymer (CFRP) laminates. By varying the cross-section geometry of the foam, different load-displacement behaviours and fracture morphologies were produced. The cross-section geometries considered included variations in the size and shape of longitudinal side grooves in the foam. The influence of pre-cracking was also investigated. Cusp formation was observed to begin with the initiation of a series of inclined cracks distributed along the length of the specimen. Propagation of these cracks resulted in S-shaped ligands and the subsequent deformation of these ligands and coalescence of the cracks resulted in cusp features in the fracture surfaces of the foam. Cusp size was found to be influenced by the height and width of the side grooves and these effects were comparable to those observed in mode II delamination of CFRP due to inter-fibre spacing. The results of this study provide understanding and data which can be used in the development and validation of numerical models of the delamination processes in fibre reinforced laminates. Such models may eventually provide the means for optimisation of the interlaminar fracture toughness of CFRP laminates.

Developing a Mode II Fracture Model for Composite Laminates

The 4-point end notched flexure test (4-ENF) has been proposed as a test method for measuring the Mode II interlaminar fracture toughness of laminated composites. However, like other Mode II test methods, there are compressive stresses acting across the delaminated interface and so the test results may be affected by friction. In this paper a strategy for experimentally establishing the significance of friction in the 4-ENF test is proposed and details of the fixture designed to implement this strategy are presented.

A Concept for Experimentally Evaluating the Effect of Friction in the 4-ENF Interlaminar Toughness Test

This research investigates the post-impact behaviour of composite fuselage panels subjected to multi-site low-velocity impacts. Large curved stiffened panels (1.2 m × 0.8 m, with composite skins/stiffeners and aluminium frames) of two different skin thicknesses were subjected to sequential drop-weight impacts at locations previously determined to be critical in FE simulations. After assessment of the impact damage, each panel was tested in compression. High-speed video, strain gauges, digital image correlation and acoustic emission were used to monitor the failure development and to provide data for comparison with the FE simulations. The FE models, which were based on a mesomechanical approach, showed a good agreement with both the impact damage and the subsequent compression performance.

Compression After Multiple Impacts: Modelling and Experimental Validation on Composite Curved Stiffened Panels

Models describing longitudinal compressive failure of carbon fibre reinforced polymers (CFRP) have been attempted for decades. Despite many developments, no single model has surfaced to provide simultaneously a definitive explanation for the micromechanics of failure and validated predictions for a generic stress state. This chapter explores why, by presenting new experimental data (including scanning electron microscope (SEM) observations of loaded kink bands during propagation, and brittle shear fracture at 45 ° to the fibres) and proposing new micromechanics analytical and numerical models. The paper focuses on unidirectional (UD) composites, but studies of woven and recycled composites are also presented, highlighting similarities and differences with UD.

Fibre-dominated compressive failure in polymer matrix composites

Erratum to “Mode I DCB testing of composite laminates reinforced with z-direction pins: a simple model for the investigation of data reduction strategies” [Engineering Fracture Mechanics 2004 (71) 345–364]

Paul Robinson

Papers

Developing a Mode II Fracture Model for Composite Laminates

A Concept for Experimentally Evaluating the Effect of Friction in the 4-ENF Interlaminar Toughness Test

Compression After Multiple Impacts: Modelling and Experimental Validation on Composite Curved Stiffened Panels

Fibre-dominated compressive failure in polymer matrix composites

Erratum to “Mode I DCB testing of composite laminates reinforced with z-direction pins: a simple model for the investigation of data reduction strategies” [Engineering Fracture Mechanics 2004 (71) 345–364]