FATIGUE-CRACK GROWTH-BEHAVIOR OF COMPOSITES

Fatigue crack growth data of discontinuously reinforced composites pub lished in the literature has been re-evaluated using the two-parametri c approach developed by the authors. The use of these two parameters i nvolves Delta K and K-max as the driving forces, which are required si multaneously for fatigue crack growth to occur. These two parameters a re intrinsic to fatigue deformation process. The first parameter is re lated to the degree of cyclic plasticity that results in fatigue damag e near the crack tip, and the second (the peak stress) is required for initiating the local fracture processes in the fatigue-damaged region . Both driving forces are required simultaneously and have to exceed s ome critical minima for crack advancement. Thus, there are two fatigue thresholds instead of one as is normally assumed. However, in a given region, depending on the material and its crack-tip environment, one or the other parameter controls the growth behavior. Thus, normally fo r all materials (including composites) below a certain critical R rati o, fatigue crack growth is K-max controlled. Above the critical R rati o, it is Delta K controlled. Although one parameter is the controlling factor in a given regime, both driving forces are needed to complete the fatigue description. This two-parametric approach is valid not onl y at the thresholds, but also at the higher crack growth rates. An und erstanding of fatigue process, then, requires a systematic evaluation of how these two driving forces vary with the reinforcement size, shap e, volume fraction, and distribution, along with other material proper ties of the constituent phases, such as the interfaces. Finally, this article discusses some of the possible mechanisms and their effects on the two driving forces, using the limited available data in the liter ature.