Development of Glass/Steel Bibeam Specimen for Study of Brittle Crack Path Stability
We have developed a novel specimen for studying crack paths in glass. Under certain conditions, the specimen reaches a state where the crack must select between multiple paths satisfying the K II = 0 condition. This path selection is a simple but difficul
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Development of Glass/Steel Bibeam Specimen for Study of Brittle Crack Path Stability S.J. Grutzik and E.D. Reedy Abstract We have developed a novel specimen for studying crack paths in glass. Under certain conditions, the specimen reaches a state where the crack must select between multiple paths satisfying the KII ¼ 0 condition. This path selection is a simple but difficult benchmark case for both analytical and numerical methods of predicting crack propagation. We document the development of the specimen, using an uncracked and instrumented test case to study the effect of adhesive choice and validate the accuracy of both a simple beam theory model and a finite element model. In addition, we present preliminary fracture test results and provide a comparison to the path predicted by two numerical methods (mesh restructuring and XFEM). Keywords Brittle fracture • Glass • Stability • Crack path
8.1
Introduction
Predicting the path along which a brittle crack will propagate is an important but difficult problem. A number of criteria have been proposed since the 1960s to predict crack propagation under mixed mode loading which include maximum hoop stress (or maximum tensile stress) [1], minimum strain energy density [2], and maximum energy release rate [3]. All of these criteria predict that a crack under pure mode I loading will propagate straight ahead. Abstract here In other words, a crack will propagate in a direction such that KII ¼ 0 where KII is the mode II stress intensity factor. The issue of whether a crack path is stable, if a small deviation from the KII ¼ 0 path will bring the crack back in line with the original path or cause further deviation, is a different question. Cotterell and Rice [4] proposed that the KII ¼ 0 path is stable when T < 0, where T is uniform tensile stress parallel to the crack direction superposed over the r1/2 asymptotic crack tip field [5]. T is in fact the r0 term of the asymptotic expansion developed Williams [6]. Yang and Ravi-Chandar [7] have shown the T < 0 condition to be of limited applicability. We have developed a novel, thermally loaded specimen which achieves a condition where two independent crack growth directions satisfy the KII ¼ 0 criterion. These paths intersect at a 90∘ angle and a crack initiated along one can switch to the other with no nominal preference for the crack to turn to the left or to the right. This specimen allows for easy study of the factors controlling crack path stability and the sensitivity of the crack path to various geometrical and material parameters thus providing simple but difficult benchmark case for numerical methods of predicting crack path such as extended finite elements (XFEM)[8] and adaptive remeshing [9].
8.2
Specimen Preparation
The specimens consist of nominally 300 mm beams of borosilicate glass and 304 stainless steel with nominal cross sections 6 mm 3.17 mm and 6 mm 31.7 mm respectively joined with an adhesive. Epon 828/Epikure 3140 (100/45 pbw) and U-05FL Hysol urethane adhesive were chosen based on their extreme
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