Fatigue behavior of long fiber reinforced thermoplastic (LFT) composites

Long fiber reinforced thermoplastic (LFT) composites are finding increasing use in structural applications. An understanding of the fatigue behavior of LFT is of considerable importance since the structural parts made from LFT are subjected to cyclic loading conditions. In the present work, we studied the fatigue behavior in terms of stress (S) - Number of cycles to failure (N) of LFT (20 vol. % E-glass fiber/polypropylene), made by extrusion-compression molding process. We report the results of the effect of frequency and the effect of processing induced preferential fiber orientation on the fatigue life of LFT. Fatigue tests were conducted at 10, 15 and 20 Hz. The R-ratio was kept constant at 0.2. Fatigue life decreased with an increase in frequency. This result was attributed to the hysteretic heating phenomenon taking place in the composite at higher frequency. Such heating can cause the polypropylene matrix to soften and reduce its capacity to transfer load to the reinforcing fiber causing premature failure of the composite. Preferential orientation of long fibers also develops in the LFT during processing. Fatigue life of samples tested along longitudinal direction showed a higher fatigue life than the transverse samples. This was a direct result of the contribution of better load transfer to long fibers in the longitudinal direction.