Micro-mechanics based cohesive zone modeling of full scale ductile plate tearing: From initiation to steady-state

Abstract

Ductile plate tearing, where the crack propagates multiple plate thicknesses, is targeted by the micro-mechanics based Gurson-Tvergaard-Needleman (GTN) model. The focus is on extracting detailed information on the fracture process that governs ductile crack initiation from a blunt pre-crack until the crack reaches steady-state propagation in order to enhance accuracy of the traditionally used cohesive zone traction-separation relations. The aim is to facilitate an accurate representation of the tearing process within the cohesive zone modeling framework as such simplistic models are largely exploited by engineers worldwide. Unfortunately, accuracy in the representation of crack propagation is often sacrificed for computation speed, but the present work allows correlating the cohesive zone modeling to a much more accurate, though computational expensive, micro-mechanics based (full 3D) model response. In the modeling of large-scale plate tearing, shell elements are typically employed to represent the engineering scale of the structure while the cohesive zone represents the micro-scale in terms of crack initiation and growth process. Thus, the cohesive zone essentially has to take over at the onset of the first localization (thinning far ahead of the crack tip). Calibration of the cohesive zone parameters has earlier been made in accordance with experimental observations such that the overall response of the system is well reproduced. But, the present work takes the calibration of the cohesive zone one step further and exploits details from a large-scale GTN model calculation. The goal is to match the response from the GTN model with the much less computation demanding cohesive zone modeling approach by incorporating knowledge of the loading history for individual cross-sections, in front of the pre-crack, through which the tearing crack propagates. The full 3D micro-mechanics based model set-up allows tracking of key parameters, such as peak traction and tearing energy, which goes into the cohesive traction-separation relation. The dependency on distance from the crack initiation site of the cohesive zone parameters is determined - from crack initiation to steady-state propagation - and followed up by a discussion on how to construct a traction-separation relation for ductile plate tearing.