Evolution of Crack Density in Cross-Ply Laminates - Application of a Coupled Stress and Energy Criterion

The first damage mode to appear in continuous fibre-reinforced composite laminates subjected to in-plane loading is usually transverse cracking, i.e. matrix cracking in the off-axis plies of the laminate. Since the density of transverse cracks has a great influence on the subsequent failure steps like delaminations, it is important to be able to predict it accurately. In this paper, the evolution of crack density with increasing external load is predicted using a combination of the Coupled Criterion of Finite Fracture Mechanics and the Equivalent Constraint Model.

Modelling of stiffness degradation due to cracking in laminates subjected to multi-axial loading

The paper presents an analytical approach to predicting the effect of intra- and interlaminar cracking on residual stiffness properties of the laminate, which can be used in the post-initial failure analysis, taking full account of damage mode interaction. The approach is based on a two-dimensional shear lag stress analysis and the equivalent constraint model of the laminate with multiple damaged plies. The application of the approach to predicting degraded stiffness properties of multidirectional laminates under multi-axial loading is demonstrated on cross-ply glass/epoxy and carbon/epoxy laminates with transverse and longitudinal matrix cracks and crack-induced transverse and longitudinal delaminations. This article is part of the themed issue ‘Multiscale modelling of the structural integrity of composite materials’.

Analysis of Fatigue Damage Mechanisms and Residual Properties of Polymer Matrix Composites

Resin dominated damage modes such as matrix cracking in the off-axis plies and matrix crack-induced local and edge delaminations are common failure mechanisms in composite laminates under tensile or thermal fatigue. Accurate prediction of the laminate stiffness and strength must consider all the above-mentioned damage modes. In the present paper, an approach is developed for the analysis of cross-ply laminates damaged by transverse and longitudinal cracks and transverse and longitudinal delaminations that initiate and grow along these cracks. It is based on the Equivalent Constraint Model (ECM) of the damaged ply and employs an improved 2-D shear lag method to determine the stress field in the cracked and locally delaminated ply. The method is applied to predict residual stiffness properties of cross-ply graphite/epoxy laminates using experimentally observed damage patterns.

Application of the Equivalent Constraint Model to Investigate Stiffness Properties of Transversally Cracked and Split Frp Laminates

A new approach based on the Equivalent Constraint Model (ECM) [ 1 ] of the damaged lamina is applied to investigate the stiffness degradation in [0m/90n]s laminates due to matrix cracking both in the 90° (transverse cracking) and 0° (splitting) plies. The advantage of the approach is that it avoids cumbersome consideration of a repeated laminate element defined by the intersecting pairs of transverse cracks and splits, intrinsic to the earlier developed models [ 2 – 6 ]. Instead, two coupled problems for ECM laminates are solved. The stress field in the damaged lamina is determined by means of an improved 2-D shear lag analysis, and the reduced stiffness properties are described with the help of Insitu Damage Effective Functions, for which closed form expressions are obtained. Comparison of the new ECM/2-D shear lag model with the earlier developed models shows a reasonable agreement.

A Study of Matrix Crack Tip Delaminations and their Influence on Composite Laminate Stiffness

Reduction of the stiffness properties of cross-ply [0m/90n]s laminates due to delaminations, growing at the 0/90 interface from the tips of transverse cracks in the 90° plies and splits in the 0° plies, is analysed by means of a theoretical approach based on the Equivalent Constraint Model (ECM). Reduced stiffness properties of the damaged lamina are derived as explicit functions of the crack density and relative delamination area associated with that lamina and implicit functions of the two damage parameters associated with the neighbouring laminae. Transverse crack tip delaminations are found to cause significant reduction in the laminate shear modulus and Poisson's ratio. Contribution of each damage mode (transverse cracking, transverse crack tip delaminations, splitting and split tip delaminations) into stiffness loss is established.