Date of Award

1-1-2013

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Xiaomin Deng

Abstract

Numerical analysis of stable tearing crack growth events plays an important role in assessing the structural integrity and residual strength of critical engineering structures. The cohesive zone model (CZM) has been widely applied to simulate fracture processes in a variety of material systems. However, its application to the study of elastic-plastic stable tearing crack growth events in ductile materials, especially under mixed-mode loading conditions, has been limited. The current study is aimed at investigating the applicability of the CZM based approach in simulating mixed-mode stable tearing crack growth events in aluminum alloys. In the simulations, which are carried out using the 3D finite element method, the material is treated as elastic-plastic following the J2 flow theory of plasticity, and the triangular cohesive law is employed to describe the traction-separation relation in the cohesive zone ahead of the crack front. CZM parameter values of 2024-T3 aluminum alloy are chosen by trial & error through matching simulation predictions with experimental data of the load-crack extension curve for a Mode I stable tearing crack growth. With the same set of CZM parameter values, simulations are performed for mixed-mode I/II stable tearing crack growth events. Predictions of the load-crack extension curve show a good agreement with experimental results. It is also found that CZM simulation predictions of the CTOD variation with crack extension agree well with measurements, which provide a connection between the CZM approach and a simulation approach based on the crack tip opening displacement (CTOD) at a fixed distance behind the current crack front. In order to automate the process of selecting cohesive parameter values, an inverse analysis procedure based on a modified Levenberg-Marquardt method has been developed and applied to the simulations of Mode I and mixed-mode I/II crack growth events in Arcan specimens made of 2024-T3 aluminum alloy. From three different initial values, similar cohesive parameter value sets are reached. Using these sets of values, the predictions are well validated by experimental measurements. The CZM approach is also used to simulate mixed-mode I/III crack growth events in ductile materials, 6061-T6 aluminum alloy and GM 6028 steel, under combined in-plane and out-of-plane loading and large deformation conditions. A hybrid numerical/experimental approach is employed in the simulations using 3D finite element method. For each material, CZM parameter values are estimated by matching simulation prediction with experimental measurement of crack extension-time curve for a 30° mixed-mode I/III stable tearing crack growth test. With the same sets of CZM parameter values, simulations are performed for 60° loading cases. Good agreements are reached between simulation predictions of the crack extension-time curve and experimental results.

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