Access Type

Open Access Dissertation

Date of Award

January 2015

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Mechanical Engineering

First Advisor

Xin Wu

Abstract

In recent years, Advanced High Strength Steels (AHSS) have been used for the lightweight structural design and manufacturing in automotive industry. This class of sheet metals are prone to edge fracture during stamping production, and the fracture often occurs at much lower strain than that predicted based on the forming limit curves. The uncertainty in predicting edge fracture represents a great challenge in the application of AHSS. This dissertation is focused on the better understanding of edge fracture phenomenon through experimental observation and computer modeling with the consideration of microstructure effect.

In this dissertation, Hole Expansion (HE) test was used to investigate the mechanisms of edge fracture of two AHSS, Dual Phase DP780 and DP980. The HE test includes two processing steps: hole punching and hole expanding. The punching process creates holes of various sheared edge morphologies, which are the input to the hole expanding process, along with the intrinsic property of material, produce a joint effect on the crack initiation and propagation on the sheared edges during HE test.

The hole punching process was first investigated. The sheared edges were analyzed with image processing, and the relative heights of sheared four edge zones were measured, which were used to determine the strain distributions of sheared edges. The punching process was further simulated with FEA, and referenced with the experimental observations. Fracture mechanism of hole punching process was deduced.

The process of crack initiation and propagation in hole expanding process was studied the next. Scanning Electron Microscope (SEM) was used to observe cracks on the sheared edges. FEA simulation was applied to investigate the effect of sheared edge geometry on the stress evolution in the hole expanding process. The interaction of two neighboring parallel cracks in the crack propagation process was analyzed to explain the effect of crack population on the main crack propagation.

To investigate the microstructure effect on the edge fracture, the dual phase grain structure and its effect on mechanical properties and fracture behavior of DP steels were analyzed. Based on HE test results and observed DP steel grain structures, a criterion to predict edge fracture was proposed. This criterion includes mesoscale grain structure features of DP steels, and it can be applied to predict the HE test results of DP steels. The heterogeneous microstructures of DP steels were built into the Representative Volume Element (RVE) models to study the effects of microstructure parameters and fracture mechanisms on macroscale mechanical properties of DP steels, with damage based failure criteria applied to constituent ferritic and martensitic phases.

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