Fatigue mechanisms in unidirectional glass-fibre-reinforced polypropylene

Polypropylene (PP) and polypropylene modified with maleic anhydride (MA-PP) reinforced by continuous longitudinal glass fibres have been investigated. The most prominent effect of the modification with maleic anhydride in the composite is a stronger fibre/matrix interface. The effects of interfacial strength on fatigue performance and on the underlying micromechanisms have been studied for these composite systems. Tension-tension fatigue tests (R = 0.1) were carried out on 0 degrees glass-fibre/PP and glass-fibre/ MA-PP coupons. The macroscopic fatigue behaviour was characterized in terms of s tiffness reduction and fatigue-life curves. The results showed that the lon gitudinal Young's modulus degraded more rapidly for glass-fibre/PP, which w as caused by a higher degree of damage growth and accumulation. The improve ment in monotonic strength was negligible, but the fatigue life was prolong ed by about one decade for the composite with the stronger interface by use of the maleic-anhydride grafted polypropylene matrix. During the fatigue t esting, the microscopic mechanisms were monitored intermittently by a surfa ce replication technique. From microscopic observations, it could be conclu ded that the better fatigue resistance of glass-fibre/MA-PP can be attribut ed to the greater interfacial strength and the resistance to debond propaga tion. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.