Search

The objective of this work was to develop the capability to perform 3D crack propagation computations for penetrator cases during impact and penetration into hard concrete targets. The first step was to develop a new 2D algorithm that did not require rezoning (as rezoning would be very complex in 3D geometry), the second step was to extend the new technique to 3D geometry, and the third step was to apply the 3D algorithm to actual hard target penetrator impact events. The numerical algorithms described in this report have been incorporated into the 1999 version of the EPIC code. A finite element algorithm is presented for dynamic crack calculations in general directions. The algorithm avoids node splitting and the associated remeshing and redefinition of contact surfaces. and does not rely on failure mechanisms embedded in the constitutive model. The T* energy integral is employed as the dynamic fracture parameter, guiding the crack tip through the mesh. Elements through which the crack tip passes lose the ability to sustain deviatoric and tensile volumetric Stresses, and their interfaces with the nest of the mesh model fractured surfaces that can sustain only compressive normal tractions (when the crack is closed). This technique is therefore referred to as the element failure algorithm.

A dynamic fracture mechanics methodology has been implemented into the Lagrangian finite element hydrocode EPIC. The generalized energy release rate T* was assumed to be a measure of the crack driving force. This representation for the crack driving force was selected because its generality includes elastic plastic behavior and inertial effect. In addition to the introduction of the crack driving force, a nodal release algorithm to model the formation of traction free surfaces produced by a propagating crack was incorporated into EPIC. The dynamic fracture mechanics enhanced EPIC code was used in the generation phase mode to deduce be dynamic fracture toughness from measurements made during high loading rate fracture tests of AISI 4340 and AP 1410 steel specimens. The internal consistency of the approach and algorithms was demonstrated by performing application phase analyses of these fracture tests to predict the crack growth history. A second set of experiments was devised. using a different experimental arrangement and specimen geometry, to demonstrate the predictive capability of the methodology. A new dynamic test method was developed referred to as a dynamic three point bend test-in which an instrumented, precracked three-point tend specimen is dynamically loaded through a transmitter bar that is struck by a striker bar fired from a gas gun. The length and slated of the striker bar control the duration and intensity of the loading pulse. A finite element model of the experiment was developed. Numerical predictions, using the fracture toughness determined from the independent coupled pressure bar tests, are in relatively good agreement with measured crack growth and strain gage histories, thereby validating the dynamic fracture mechanics computational methodology implemented into EPIC.