Fatigue and reliability analysis of unidirectional GFRP composites under rotating bending loads

Rotating bending fatigue tests have been conducted on unidirectional glass fiber reinforced polyester (GFRP) composites. Standard test specimens were manufactured in form of circular rods with various fiber volume fraction (V-f) ratios. Failure modes of the composite rods have been examined using scanning electron microscope. The two-parameter Weibull distribution function was used to investigate the statistical analysis of the experimental fatigue life results. Safe design life based on time to first failure (TTFF) concept was calculated at high confidence level gamma=0.99 and at two values of reliability, R=0.368 and 0.99. S-N diagrams of mean life, 50% survival life, lower bound life and safe design fatigue life have been constructed for GFRP rods with various values of Vf ratios. These diagrams are of considerable value to the designer specially when the structure contains a critical component where any failure is catastrophic. No rigorous fatigue limit was observed within 10(7) cycles in these S-N diagrams. The fiber volume fractions have insignificant effect on the slope of the power function that fits the mean fatigue life. At the same number of cycles the stress amplitude required to fracture the specimens with V-f=44.7% was increased by an order equal to 1.58 and 1.25 than specimens with V-f=15.8 and 31.8% respectively. The widest scatter was observed at the life range of 10(5) and 10(6) cycles for GFRP rode with different Vf ratios. This tendency in the dispersion of fatigue life at varying stress levels is extremely important and deserves much attention for the design and application of GFRP composites.