Investigation of subcritical crack growth using load relaxation tests on fiber bundles Original Research Article

Many inorganic fibers are sensitive to subcritical crack growth activated by the environment. Failure occurs even though the applied stress is much smaller than the fracture stress. This mechanism has been extensively investigated on ceramics, glasses and glass fibers, essentially at room temperature, and has recently been shown in SiC-based fibers at high temperatures. The present paper proposes a very powerful approach to static fatigue. It is based on tests performed on multifilament tows under deformation-controlled conditions (the load relaxation technique). This technique, which has not been used previously owing to practical difficulties of deformation control during long-term tests, permits the application of identical constant stresses on all the fibers during a single test. Thus it provides a statistically significant rupture time database containing just as many data as there are fibers in the tows. Bundles of ∼2000 E-glass filaments were used in the present paper. The samples were immersed in water during the static fatigue tests. Crack velocity–stress intensity factor diagrams for single filaments were derived from experimental stress–rupture time data. A closed form expression for statistical distributions of fiber lifetimes was established, and was found to compare fairly well with the experimental results, which assessed the approach and validated new findings.