: 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

General principles and perspectives Specimen preparation Tension Compression Shear Flexure Through-thickness testing Interlaminar fracture toughness Impact and damage tolerance Fatigue Environmental testing of organic matrix composites Scaling effects in laminated composites Statistical modelling and testing of data variability Development and use of standard test methods.

Mechanical testing of advanced fibre composites

Mechanical properties of carbon fiber reinforced epoxy/clay nanocomposites

Translaminar fracture toughness testing of composites: A review

A shell/3D modeling technique was developed for which a local solid finite element model is used only in the immediate vicinity of the delamination front. The goal was to combine the accuracy of the full three-dimensional solution with the computational efficiency of a shell finite element model. Multi-point constraints provided a kinematically compatible interface between the local 3D model and the global structural model which has been meshed with shell finite elements. Double Cantilever Beam, End Notched Flexure, and Single Leg Bending specimens were analyzed first using full 3D finite element models to obtain reference solutions. Mixed mode strain energy release rate distributions were computed using the virtual crack closure technique. The analyses were repeated using the shell/3D technique to study the feasibility for pure mode I, mode II and mixed mode I/II cases. Specimens with a unidirectional layup and with a multidirectional layup were simulated. For a local 3D model, extending to a minimum of about three specimen thicknesses on either side of the delamination front, the results were in good agreement with mixed mode strain energy release rates obtained from computations where the entire specimen had been modeled with solid elements. For large built-up composite structures the shell/3D modeling technique offers a great potential for reducing the model size, since only a relatively small section in the vicinity of the delamination front needs to be modeled with solid elements.

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A Shell/3D Modeling Technique for the Analysis of Delaminated Composite Laminates

This report summarizes the results of several interlaboratory “round robin” test programs for measuring the Mode I interlaminar fracture toughness of advanced fiber-reinforced composite materials. Double cantilever beam (DCB) tests were conducted by participants in ASTM Committee D-30 on High Modulus Fibers and Their Composites and by representatives of the European Group on Fracture (EGF) and the Japanese Industrial Standards Group (JIS). DCB tests were performed on three AS4 carbon fiber-reinforced composite materials: AS4/3501-6 with a brittle epoxy matrix, AS4/BP907 with a tough epoxy matrix, and AS4/PEEK with a tough thermoplastic matrix. Difficulties encountered in manufacturing panels, as well as conducting the tests, are discussed. Critical issues that developed during the course of the testing are highlighted. Results of the round robin testing used to determine the precision of the ASTM DCB test standard are summarized.

TK O'Brien

Papers

Round Robin Testing for Mode I Interlaminar Fracture Toughness of Composite Materials