MICROMECHANICAL CHARACTERIZATION OF FIBER-MATRIX INTERFACES

Theoretical analyses for the single fibre pull-out an push-out models under monotonic loading are given which are based on a shear-lag analy sis in a fracture mechanics approach considering non-constant friction at the debonded interface as a result of fibre Poisson contraction (o r expansion). The solutions allow the determination of typical fibre/m atrix interfacial properties such as the interfacial fracture toughnes s, G(ic), the coefficient of friction, mu, and the residual clamping s tress, q0. Under cyclic loading the interfacial properties are expecte d to degrade as a result of repetitive abrasion, and a power law funct ion is assumed between mu and the number of elapsed cycles, N. However , G(ic) is assumed to be unaffected and a fracture mechanics based deb ond criterion is derived for the relationship between the external app lied stress, the debond length and the reduced friction coefficient fo r both fibre pull-out and fibre push-out. In addition, the relative di splacements between the free fibre end and the matrix top are obtained for cyclic fatigue when the fibre is loaded and unloaded. A relations hip obtained for the protrusion (or intrusion) length in fibre pull-ou t (or push-out) experiments allows the severity of the interface frict ional degradation to be evaluated and characterised. Similarities and differences in the frictional degradation behaviour between fibre pull -out and push-out are also identified.