Compressive Fatigue Coaxing in Composites

Compressive fatigue experiments on unidirectional fiber reinforced composites, of both fiberglass and graphite fiber, reveal that compressive fatigue damage takes place by crack extension directly transverse to the fibers. It is also found that specimens which are first subjected to a few hundred thousand cycles of compressive fatigue at low stress levels, and then subjected to compressive fatigue at high stress levels, will survive longer at high stress levels than specimens which are placed under high compressive fatigue stresses from the outset. The increase of fatigue life at high compressive stress levels can be quite large; increases of fatigue life by as much as a factor of five were observed. The phenomenon of prolonging fatigue life at high stress levels by first "running in" a specimen at low stress levels is called coaxing, and is directly opposite to the often reported cumulative damage phenomena observed in tensile fatigue of composites. A possible mechanism for coaxing in compressive fatigue of composites is offered, and practical applications of the effect are suggested.