Strain behavior of glass fiber reinforced polyamide 66 during fatigue and creep

In order to investigate the effect of hysteric temperature rise on the stress-strain behavior during stress-controlled fatigue of glass fiber reinforced polyamide 66 (GF/PA66); both surface temperature and strain of the specimen were measured under various cyclic stresses. Steady state temperature rise and strain increase were observed in a fairly wide time scale except for the vicinity of the start point of fatigue and fracture. The creep tests were carried out under the same stress condition as fatigue test. Creep strain increased with elapsed time without temperature rise. This strain increase is similar to that of fatigue strain except for diffrence in their strain levels and temperature rise. Therefore, we tried to find out relationship between fatigue and creep with regard to strain. The strain variable of Leaderman's equation applied for creep of polyamide fibers was used to study relationship between fatigue and creep. This equation was extended to fatigue as follows: Delta epsilon=epsilon-epsilon(i), where Delta epsilon=strain variable under fatigue and creep, epsilon=mean strain at elapsed time, and epsilon(i)=instantaneous mean strain. Delta epsilon of fatigue shifts to a shorter time region than that of creep. This reveals that Delta epsilon is accelerated by temperature rise of fatigue. It is found that Delta epsilon between fatigue and creep can be superposed to the unique curve under the same stress conditions if considering temperature rise during fatigue. This result suggests that stress-strain behavior during fatigue and creep obeys the same viscoelastic mechanism from the viewpoint of strain. Further, applying Catsiff's procedure to stress-superposition of creep strain, Delta epsilon under various fatigue mean stresses is superposed to the universal curve.