This paper presents a pressure-dependent three-dimensional constitutive law to predict failure for laminated composites. The nonlinear constitutive response in shear and in the transverse and through-the-thickness directions, which is measured experimentally, is incorporated directly into the model. In addition, secant stiffnesses are dependent on the state of hydrostatic pressure and on the general state of strain. The failure criteria distinguish between matrix failure, fibre kinking and fibre tensile failure. In-situ strengths are used for matrix failure. Propagation of failure takes into consideration the fracture energy associated with each failure mode and, for matrix failure, the accumulation of cracks in the plies. A detailed discussion is undertaken of the mismatch between the available experimental data and the physical properties required to characterise the constitutive response up to final failure. The model is employed to make blind predictions of the triaxial failure envelopes and stress-strain curves of all 12 test cases provided by the organisers of the second World-Wide Failure Exercise.

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Results are presented from a study on the effect of friction on the Mode II delamination toughness as obtained by three- and four-point bend end-notched flexure tests. Finite-element analyses are used to assess the effect of friction on the compliance and energy release rate of the two types of test specimens. Energy release rates are first obtained by a virtual crack closure technique, which can be used to separate the energy lost by the system into that dissipated by friction and that used for crack advance. Energy release rates are then obtained by a simulated compliance calibration procedure. By modeling this commonly used method of data reduction and comparing results with those obtained by crack closure, the ratio of the material's intrinsic toughness to the toughness perceived by users of the tests can be assessed. For both tests, this assessment is made for physically realistic coefficients of friction and, for the four-point end-notched flexure test, as a function of the ratio of inner to outer span length. It is shown that frictional effects on the perceived toughness are considerably larger in the four-point than in the three-point bend end-notched flexure geometry, but that an appropriate choice of the four-point test geometry can make the influence of friction quite small.

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