Modelling lamina longitudinal compression strength of carbon fibre composite laminates

Abstract

The determination of the lamina longitudinal compression strength remains an unsolved problem. Non-valid failure modes are commonly observed in compression tests of unidirectional specimens. Micromechanical modelling has also proven to be a difficult task. It is believed that failure is initiated by fibre microbuckling, and that non-linear matrix behaviour and fibre misalignments play a major role. The main obstacles are thus the non-linear nature of the phenomenon and the scarcity of material data. Here 2D and 3D Finite Element models are presented. The Finite Element Method can easily deal with non-linear problems. The models are computationally inexpensive and can be easily implemented on a commercial code. A sensitivity analysis was performed using realistic input data. The model predictions are well above the unreliable experimental values so far available. Further, the most important variables were found to be the fibre misalignment angle and the matrix yield stress. The fibre spatial arrangement also affects the predicted results. Reliable strength data are essential to validate the models here presented. Nevertheless this study shows that accurate predictions should be expected to prove difficult, mainly because of the complex matrix stress-strain behaviour and the statistical nature of fibre misalignments and spatial distribution. In addition, the important role of the matrix in model results raises the question of the existence of a unique lamina longitudinal compression strength (LLCS). Lay-up dependent transverse and shear stresses acting on the load aligned laminae should accordingly yield lay-up dependent LLCSs, which is confirmed by experimental data.