Cyclic damage characterization of an off-axis unidirectional graphite bismaleimide composite

Cyclic tension-tension testing was performed on an off-axis (10 degrees) unidirectional graphite bismaleimide (BMI) composite to determine the effect of the matrix dominant properties on the fatigue behaviour of the material. Rosette strain gauges mounted in the major directions were used to monitor cyclic stresses and strains while acetate replicas of surface cracks were taken as the number of cycles increased allowing a correlation of the stress-strain response with the accompanying cracking mechanisms to be established. Initial rapid crack nucleation and modulus degradation in the first 10% of life were followed by crack coalescence and steady state growth in the bulk material. Final fracture ensued from the development of coalesced matrix shear cracks parallel to the fibres. Comparing the amount of cracked area with modulus degradation, the decrease in shear modulus was found to be a more sensitive and practical parameter to represent fatigue damage evolution.