Influence of interfacial strength on micro- and macroscopic fatigue behavior of longitudinal glass fiber reinforced polypropylene

In this study the thermoplastic polypropylene (PP) and maleic anhydride modified polypropylene (MA-PP) reinforced by continuous longitudinal glass fibers (GF) have been investigated. The most protruding effect of the modification with maleic anhydride in the composite is a stronger fiber/matrix interface. The effects of interfacial strength on fatigue performance and on the underlying micromechanisms were studied for these composite systems. Tension-tension fatigue tests (R = 0.1) were carried out on 0° GF/PP and GF/MA-PP coupons. The macroscopic fatigue behavior was characterized in terms of stiffness reduction and fatigue life curves. The longitudinal Young modulus degraded more rapidly for GF/PP which was caused by a higher degree of damage growth and accumulation. The S-N curves show that the improvement in static strength is negligible, and the fatigue life is prolonged by about decade with the stronger interface by addition of maleic anhydride to the polypropylene matrix. The static strength is controlled by the strain to failure of the fibers, hence the superposition. The better fatigue resistance of GF/MA-PP is attributed to the greater interfacial strength and the restistance to debond propagation. During the course of the fatigue testing, the microscopic mechanisms were monitored intermittently using a surface replication technique. The observed differences in fatigue micromechanisms (debonding, fiber breakage, matrix cracking etc.) are presented and discussed in reference with the macroscopic fatigue behavior.