On Measuring the Near-Tip Plastic Strain Singularity

A recently developed experimental method, stress intensity factor tracer, is extended to measure the strength, J, of the HRR singularity for near-tip plastic deformation. Focal-plane mapping of the HRR field shows that the light intensity, I, collected on a finite area of the focal plane has a simple relationship with J as I = βJ^2n/(2n+1). The constant, β, is a product of several experimental parameters and “n” is the hardening parameter of a power-law hardening material. The focal-plane mapping technique is also capable of estimating the shape and size of the HRR-field dominant region for a relatively thin (<10mm) metallic specimen. In addition, a continuous trace of the J variation can be monitored using a single, stationary photodetector. Because the measurement value of this method is independent of crack-tip motion, the transition of HRR singularity from stationary to moving can also be studied. In this paper, the theoretical analysis of the method is presented.