0013-7944/87 53.00 + 0.00 Pergamon Journals Ltd. zyxwvuts THE NT-CRITERION FOR PREDICTING CRACK GROWTH INCREMENTS NABIL A. B. YEHIA Assistant Professor, Civil Engineering Department, University of Pittsburgh, Pittsburgh, PA 15261, U.S.A. and MARK S. SHEPHARD Associate Director, CICG and Associate Professor, Civil Engineering, Rensselaer Polytechnic Institute, Troy, NY 12181, U.S.A. Abstract-A new approach is presented to determine the crack propagation increment after the direction of crack propagation has been predicted. The maximum dilatational strain energy density (NT-criterion) is employed in the derivation for predicting both direction and increment of the propagating crack. The crack propagation path predicted by the NT-criterion is compared to the one predicted by the S-criterion and to some available experimental data. zyxwvutsrqponmlkjihgfedcbaZ INTRODUCTION THE LOCAL instability or failure at the crack tip in precracked material, has been observed not to be synonymous with the global instability or failure except in very brittle materials and for some configurations[l]. The crack may be, temporarily, unstable and therefore grows a small amount. Due to the release in energy and the stress redistribution accompanying the crack propagation, its stress intensity factors may decrease resulting in the arrest of crack growth. The crack stops and remains stationary until a further load increase occurs or a change in geometry, like initiation of another crack, takes place[2]. A complete study of crack propagation history in such precracked materials can be carried out if the following questions can be answered: (1) When will the crack propagate? (2) In which direction will the crack propagate? (3) How far the crack will propagate under the current applied load? Several criteria for mixed mode fracture had been proposed in the literature to answer the first two questions[3-81. The prediction of crack propagation increment in mixed mode problems, which is the answer of the third question above, has also been considered by a number of researchers using different approaches with varying success[9-12, 141. An energy balance approach was used by Ingraffea zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPO et af.[9] to study the crack propagation in a plate containing a central elliptical notch. The energy balance equation is used to check whether the trial crack extension area (Au) is correct or not by using (Aa)r = A(V- v) (1) where, y = FE the surface energy for elliptic cavity, K,, = the material fracture toughness, E = Young’s modulus of the material, and A( I/- U) is the change in the difference between potential energy due to applied loads and the strain energy in the plate. At each increment of load resulting in fracture extension, the program iterates on trial fracture lengths until satisfactory agreement of the energy balance is reached. Another energy balance approach was used by Ingraffea and others[l&121. According to this approach, the crack will keep propagating under the applied load as long as the energy release 371