Internal stresses in planar random fibre aluminium composites—II. mean stress hardening and relaxation

The hardening rate of the matrix mean stress in planar random fibre aluminium composites with a range of fibre volume fractions is measured in cyclic Bauschinger experiments at room temperature and 77 K. A particular problem with predicting the mean stress hardening rate in these composites is that the plastic strain is not isotropic in the transverse directions. A method for quantifying the experimentally determined plastic flow in a form useful for this analysis is proposed. The difference between the predicted and measured mean stress hardening rates is smallest for low fibre volume fractions at 77 K. A model for relaxation of the plastic mean stress in the matrix is developed. It is based on the assumption that the rate controlling mechanism in these composites is cross slip of screw dislocations. The measured activation energy is independent of fibre volume fraction but the activation volume decreases with increasing fibre content. The magnitudes of activation energy and activation volume support the assumptions of the model.