FATIGUE FRACTURE PROCESS IN A MODEL COMPOSITE

This paper reports certain aspects of fatigue crack propagation in com posite specimens consisting of epoxy matrix and one row of long aligne d glass fibers. The specimens were prepared in such a way that fiber s pacing was approximately the same in each specimen and the fibers were sufficiently stronger than the matrix. Fatigue crack propagation show ed three distinguishable phases: two transient ones separated by a ste ady phase. The first transient phase is associated with the interactio n between the crack and the fibers ahead of the crack. A simple analys is indicated that the third phase was due to an effect of the fiber di stance to the specimen edge. Changes in the matrix material during fra cture were qualitatively revealed using light polarizers during testin g. 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 zon e and the corresponding crack opening displacement rates reached stead y values, i.e. independent of the crack length. The crack opening disp lacement at a fiber location and the corresponding debonding were Line arly related in the steady phase. The crack speed showed relatively la rge fluctuations between and around the fibers. Namely, larger crack s peeds were recorded when the crack tip was between two consecutive fib ers and lower speeds when near a fiber. Within the resolution of the o bservations, no fiber fracture was observed in the bridging zone. Simu lations for the total stress intensity factor were carried out using f racture mechanics principles. When the fibers in the bridging zone wer e under uniform load, the total stress intensity factor K-t, was found to be approximately constant at the steady state and proportional to sigma(infinity)root lambda (where sigma(infinity) is the remote stress and lambda is the fiber spacing). (C) 1997 Published by Elsevier Scie nce Limited.