Loss-energy to determine on-line the state of damage of composites under fatigue loadings

This paper is dealing with a new method based on determining on-line loss energy as measure of the state of damage of composite materials under tension-tension fatigue loadings. Experimentally, cross-ply glass-fibre epoxy composites with a fibre volume fraction of 0.4 and differences in fibre-matrix adhesion and matrix ductility were tested by a stress-ratio of 0.1 and frequency of 10 Hz. The paper shows, that the online measured loss energy is very sensitive to evaluate the damage state of composite materials. Number of load cycles between a new defined state of damage (damage state 'A' in that paper) and final failure for a giver n/N ratio is shown to be sensitive to characterise differences in composite morphology such as level of interfacial strength and matrix ductility. For brittle composites, this damage state 'A' can be used to predict fatigue life of composite materials.