The fatigue behavior of two different batches of an alumina fiber reinforced aluminum composite (FP/Al) was investigated. Both batches had a nominal 55% fiber volume fraction with fibers uniaxially oriented in the test direction. Tension-tension fatigue tests were conducted on flat, untabbed, contoured specimens in a 90 KN MTS axial fatigue test machine at R = 0.1. The cyclic frequency of the fatigue tests was 30 Hz except for a limited number conducted at various frequencies to determine if fatigue behavior was frequency dependent. Some specimens were strain gaged so that load-strain response could be monitored during fatigue tests. Metallographic and fractographic examination of specimens was conducted to evaluate failure modes and damage mechanisms.
Preliminary static and fatigue tests showed a significant difference in mechanical properties of the two batches of FP/Al investigated. The first batch had an endurance limit of 410 MPa compared to 330 MPa for the second batch. Static strength and modulus were correspondingly higher for the first batch than the second. Microstructural differences were also observed, such as a significantly higher number of debonded fibers in the second batch of FP/Al, that could account for the difference in mechanical properties. There was no decrease in secant modulus with fatigue cycling for either batch of material, contrary to reported behavior for boron/aluminum composites. In spite of observed fabrication defects, the ratio of endurance limit and residual strength to ultimate tensile strength was greater than 60% for both batches tested. Fiber failure was found to dominate the fatigue life of FP/Al, and failure of the composite generally occurred after a sufficient number of fibers fractured at a given cross section.