Abstract
This paper reports certain aspects of fatigue crack propagation in composite specimens consisting of epoxy matrix and one row of long aligned glass fibers. The specimens were prepared in such a way that fiber spacing was approximately the same in each specimen and the fibers were sufficiently stronger than the matrix. Fatigue crack propagation showed three distinguishable phases: two transient ones separated by a steady phase. The first transient phase is associated with the interaction between the crack and the fibers ahead of the crack. A simple analysis indicated that the third phase was due to an effect of the fiber distance to the specimen edge. Changes in the matrix material during fracture were qualitatively revealed using light polarizers during testing. Whereas at the first transient phase and during steady growth most activities were confined within the bridging zone, in the third phase, damage was also spread in the matrix material in the form of discrete elements. In the steady phase of propagation, the average crack speed, energy dissipation rate, debonding of the fibers in the bridging zone and the corresponding crack opening displacement rates reached steady values, i.e. independent of the crack length. The crack opening displacement at a fiber location and the corresponding debonding were linearly related in the steady phase. The crack speed showed relatively large fluctuations between and around the fibers. Namely, larger crack speeds were recorded when the crack tip was between two consecutive fibers and lower speeds when near a fiber. Within the resolution of the observations, no fiber fracture was observed in the bridging zone. Simulations for the total stress intensity factor were carried out using fracture mechanics principles. When the fibers in the bridging zone were under uniform load, the total stress intensity factor Kt, was found to be approximately constant at the steady state and proportional toσ∞ √λ (where σ∞ is the remote stress and λ is the fiber spacing).
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