DYNAMIC CRACK-PROPAGATION IN ELASTIC - PERFECTLY PLASTIC SOLIDS UNDER PLANE-STRESS CONDITIONS

THE phenomenon of steady-state dynamic crack propagation in elastic-perfectly plastic solids under mode I plane stress, small-scale yielding conditions is investigated numerically. An Eulerian finite element scheme is employed. The materials are assumed to obey the von Mises yield criterion and the associated flow rule. The ratio of the crack tip plastic zone size to that of the element nearest to the crack tip is of the order of 1.6 x 10(4). Two subjects of general interest are discussed. These are the asymptotic structure of the crack tip stress and deformation fields, and the appropriateness of a crack growth fracture criterion based on the far-field dynamic stress intensity factor. The crack-line solution by ACHENBACH and LI (Report NU-SML-TR-No. 84-1, Dept. of Civil Engineering, Northwestern University, Evanston, IL 60201, 1984a; in Fundamentals of Deformation and Fracture (edited by B. A. BRILBY et al.). Cambridge University Press, 1984b) is discussed and compared to the numerical solution. The results of this study strongly indicate that the crack tip strain and velocity fields possess logarithmic singularities, which is consistent with the assumptions in the asymptotic analysis by GAO (Int. J. Fracture 34, 111, 1987). However, it is revealed that the crack tip field variations in Gao's solution present features often contrary to the numerical findings. To this end, a preliminary asymptotic analysis is performed in an effort to resolve certain issues. Finally, the critical plastic strain criterion (MCCLINTOCK and IRWIN, in Fracture Toughness Testing and Its Applications, ASTM STP 381, p. 84, 1964) is adopted to obtain theoretical relations between the critical dynamic stress intensity factor and the crack propagation speed. These relations are found to agree well with experimental measurements by ROSAKIS et al. (J. Mech. Phys. Solids 32, 443, 1984) and by ZEHNDER and ROSAKIS (Int. J. Fracture, to appear 1990), performed on thin 4340 steel plates whose material characteristics match those of the calculation. The results seem to support the existence of a one-to-one relationship between the dynamic fracture toughness of the material and the crack propagation speed, for materials which fail in a locally ductile manner.