INTERNAL-STRESSES IN PLANAR RANDOM FIBER ALUMINUM COMPOSITES .2. MEANSTRESS HARDENING AND RELAXATION

The hardening rate of the matrix mean stress in planar random fibre al uminium composites with a range of fibre volume fractions is measured in cyclic Bauschinger experiments at room temperature and 77 K. A part icular problem with predicting the mean stress hardening rate in these composites is that the plastic strain is not isotropic in the transve rse directions. A method for quantifying the experimentally determined plastic flow in a form useful for this analysis is proposed. The diff erence between the predicted and measured mean stress hardening rates is smallest for low fibre volume fractions at 77 K. A model for relaxa tion of the plastic mean stress in the matrix is developed. It is base d on the assumption that the rate controlling mechanism in these compo sites is cross slip of screw dislocations. The measured activation ene rgy 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.