Monotonic and cyclic fatigue behavior of high-performance ceramic fibers

Monotonic and cyclic fatigue behavior of single fibers or fiber fabrics are of significant interest, since fiber assemblies or fiber-reinforced composite materials in structural applications are often subjected to cyclic loading. Studying the cyclic fatigue behavior of fibers is particularly difficult because of their small diameter ( ∼ 10 μm) and high aspect ratio. In this paper, we report results of monotonic tension and tension-tension fatigue behavior of two sol-gel-derived ceramic fibers: Al ₂O ₃-SiO ₂-B ₂O ₃ (Nextel 312) and Al ₂O ₃ (Nextel 610). Nextel 312 exhibited a great deal of variability in tensile strength, reflected by a Weibull modulus of 4.6, versus Nextel 610, which had a Weibull modulus of 10.5. Our experiments showed clearly that cyclic loading was more damaging than static loading and, thus, resulted in a lower cyclic fatigue life compared with static loading. The fracture behavior under fatigue loading was distinctly different from that under monotonic loading. It is believed that processing-induced flaws acted as crack initiation sites, and that the cyclic loading induced subcritical cracking, followed by coalescence of cracks immediately prior to failure.