Fatigue behaviour of ribbon-reinforced composites

The fatigue behaviour of aluminium-based composites reinforced with high-strength steel ribbons has been examined. In the directions both parallel (longitudinal composites) and perpendicular (transverse composites) to the principal ribbon axis the development of the fatigue deformation is controlled by the matrix. The steel-matrix bond does not deteriorate during the initial stages of fatigue. The fatigue limit at 10⁷ cycles was arbitrarily defined in the case of the longitudinal composites as corresponding to approximately the half-destruction of the composite. In this way the limit for a composite with a volume fraction,V_r of 0.25 was around 25% of the fracture strength while with aV_r of 0.34, the limit was raised to about 33% of the fracture strength. The use of an alloy matrix increases the fatigue strength of the composite. The fatigue mechanism in both longitudinal and transverse composites initially involves matrix cracking; this leads to steel-aluminium or aluminium-aluminium debonding parallel to the principal stress axis in specimens fabricated from pre-coated steel ribbons, followed by ribbon fracture. In specimens fabricated by simply rolling together the steel ribbons and aluminium foils, little debonding is evident. The debonding results in a high fracture resistance.