An Experimental Method to Measure Q-Parameter for Two Parameter Theory of Ductile Fracture

In the two parameter approaches, one of the second parameters, T-stress, Q and A₂ is used together with J-integral to describe the constraint loss near the crack tip under large scale yielding states. Among the second parameters, Q seems to be most promising from a practical point of view because it applies for three-dimensional geometries. In order to determine the Q for a given geometry and load, however, three-dimensional elastic-plastic finite element analysis, which is time consuming and costly, has had to be performed so far. In this work an experimental method to measure Q-parameter insitu is described. The basic idea comes from the fact that side necking near a crack tip indicates the loss of stress triaxiality, which can be scaled by Q. From the out-of-plane displacement and the in-plane strain measured on the surface of side necking near the crack tip, stress field averaged through thickness is calculated and then Q is determined from the difference between the stress filed and the HRR field corresponding to the identical J-integral. To prove the validity, three-dimensional finite element simulation has been performed for a CT configuration with side-grooves. Q-value which was calculated directly from the near-tip stress field is compared with that determined by simulating the experimental procedure according to the proposed method, that is, the Q-value determined from the lateral displacement and the in-plane strain. Also, the effect of location where the displacement and strain are measured is explored. Moreover, an easy way for measuring the displacement and strain simultaneously is described. That is based on Stereoscopic Digital Photography and high resolution Digital Image Correlation (DIC) software, and can be performed along with conventional fracture tests. A case study for a CT specimen of ferritic steel is presented.