Fracture initiation at sharp notches: Correlation using critical stress intensities

the context of linear elasticity, a stress singularity of the type K”# (6 < 0) may exist at sharp re-entrant corners, with an intensity R. The magnitude of the intensity fully characterizes the stress state in the region of the comer. A critical value of K” may be appropriate as a failure criterion in situations where the region around the comer dominated by the singular field is large compared to intrinsic flaw sizes, inelastic zones, and process zone sizes. We determined K” for notched mode I three-point flexure specimens using a combination of the Williams (Williams, M. L. (1952) Stress singularities resulting from various boundary conditions in angular comers of plates in extension. Journal of Appkd Mechanics 74, 526-528) asymptotic method, dimensional considerations, and detailed finite element analysis. We carried out an experimental study of the feasibility of using a critical value of K” to correlate failure with a series-of notched polymethyl methacrvlate (PMMA) three-noint flexure snecimens with notch anales of 60”. 90”. and 120”. Using the measured failure loads and the finite element solutions for K”, we infer the critical notch stress intensity K, for sharp-notched PMMA. The data show that excellent failure correlation is obtained through the use of the single parameter cr. Furthermore, simple estimates of the size of the inelastic zone for the notched PMMA specimens show it to be small relative to the singularity-dominated zone. This supports the applicability of linear elastic notch mechanics (LENM), and the idea that a critical value of K” can be used to correlate fracture initiation. 0 1997 Elsevier Science Ltd.