Three-Dimensional and K-Dominance Effects in Crack Tip Fields
Santosh V. Prabhu, Doctoral Candidate
University of Delaware
Dept. of Mechanical Engineering
Newark, DE
Sponsored by the Dept. of Engineering Mechanics
Date: Friday, March 10, 2000
Time: 3:30 p.m.
Place: W131 Nebraska Hall
It is being increasingly recognized that the stress and deformation fields in the vicinity of crack tips are highly three-dimensional in nature. However, most failure theories are based on the concepts of the critical stress intensity factors, Kc, critical strain energy release rate, Gc or the J-integral, Jc. From the very nature of their definition these are two-dimensional quantities. Since a fracture criterion is external to any continuum theory, critical values of these fracture quantities can only be obtained experimentally. Their successful measurement, therefore, requires that experimental data be taken under two-dimensional conditions. Further, for an accurate measurement of the stress intensity factors, in addition to two-dimensional conditions, K-dominance is also required. This necessitates a detailed knowledge of the two and three-dimensional deformation regions in the specimen under investigation. This work focuses on the use of optical techniques in the determination of reliable values of fracture parameters, such as the stress intensity factors and the extent of mode mixity, from the regions of K-dominance.
The lateral shearing interferometer of Coherent Gradient Sensing (CGS) is used in conjunction with finite element analysis to study the effects of mode mixity on crack tip stress fields. It was noted that, in isotropic materials, increasing mode mixity leads to an increased rotation of the three-dimensional zone while keeping its shape and size unchanged. In contrast, the region of K-dominance is seen to dramatically depend on mode mixity, both in shape and size. A least squares fit data analysis technique was used to obtain fracture parameters, namely KI, KII and the crack tip phase angle, j, from the CGS interferograms. The data points used in the least square fitting were obtained from the determined regions of K-dominance. The same procedure was also used to investigate the issue of three-dimensionality and K-dominance in unidirectional composite materials. It was noted that the shapes of the three-dimensional zones and the regions of K-dominance vary markedly with fiber orientation. This information was then utilized to extract meaningful values of the fracture parameters from the interferometric data. Crack tips stress and deformation fields were also investigated using an experimental technique that combines the optical method of caustics and the interferometer of CGS.
