Low cycle fatigue crack growth and life prediction of short-fibre reinforced aluminium matrix composites

Abstract

A model has been developed to predict the low cycle fatigue crack growth and life of short-fibre reinforced aluminium-matrix composites. The proposed model is based on the assumption that there is a fatigue-damaged zone ahead of the crack tip in which the local cyclic stress level approaches the ultimate tensile strength of the composite and the actual degradation process of the composite takes place. The cyclic plastic deformation in the fatigue-damaged zone is suggested as the main mechanical driving force for the crack growth. The cyclic J integral is calculated quantitatively from the cyclic plastic deformation characterisation in the fatigue-damaged zone. The range of crack tip opening displacement is correlated to the cyclic J integral in terms of the result of fracture mechanics. The low cycle fatigue crack growth per cycle is taken by an amount equal to one half of the range of crack tip opening displacement. The low cycle fatigue life is determined by integrating the crack growth rate equation from the initial crack length to a critical crack length. The empirical Coffin–Manson and Basquin laws and total strain amplitude vs. life relationship have been derived theoretically and compared with total-strain controlled low cycle fatigue data of pure aluminium Al99.85 reinforced with Al 2O 3 Saffil fibres of a volume fraction of 20 vol.% over a wide test temperature range from −100 °C to 150 °C. The results indicate that the model gives an excellent description of the crack growth process of short-fibre reinforced aluminium-matrix composites under low cycle fatigue conditions.