Three-Dimensional Cohesive Crack Analysis of Short-Rod Specimens

The use of short-rod specimens with chevron notches for determination of fracture toughness is becoming more common. Standards for such evaluations are available for a variety of materials, including some ceramics. The size of the specimens can be substantially reduced with the introduction of the inelastic correction factor. Although the size requirements for linear elastic fracture mechanics application are violated, accurate predictions of fracture toughness can still be made with the application of this correction. This is extremely important because the reduced size of specimens allows for reduction of costs in the experimental procedure. However, the inelastic correction factor is based on the assumption of elasto-plastic material behavior, and might not be applicable to the types of inelasticity associated with fracture in ceramics. The objective of this paper is to discuss the applicability of a similar correction for ceramic like materials. It is known that ceramic materials in general exhibit strong size effects with respect to fracture resistance. This is mainly due to the presence of inelastic effects on the crack surface. These effects become more significant with the reduction of the size of the specimen. A generalization of the Dugdale-Barenblatt cohesive crack model to three dimensions is introduced to analyze the short-rod problem for natural and man-made ceramics. The model is capable of simulating the evolution of both stress free crack and softening process zones in the case of planar propagation. The influence of specimen size is investigated for the mentioned specimen configuration. Insight into crack evolution and load-displacement curves is also provided. Finally, schemes for inelastic or size effect corrections of fracture toughness values are discussed.