Date of Award

2014

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

Anisotropic yield function, DP600, TRIP780, AA5182, Electrohydraulic Forming, Implicit inetgration, Plasticity, Rate-dependent

Supervisor

Green, Daniel

Rights

info:eu-repo/semantics/openAccess

Abstract

Electrohydraulic forming (EHF) is a pulsed forming process in which two or more electrodes are positioned in a chamber filled with a liquid and a high-voltage discharge between the electrodes generates a high-pressure to form the sheet. Deformation history of a sheet material in EHF process shows substantial changes in the strain rate of the material during the forming process. In this research, the mechanical properties of DP600, TRIP780, and AA5182-O were obtained at different strain rates. Uniaxial tensile tests showed significant strain-rate sensitivity in all three material orientations (RD, DD, and TD) for DP600 and TRIP780. In contrast, AA5182-O exhibits almost near-zero strain-rate sensitivity. Several anisotropic yield functions were calibrated at various strain rates to evaluate the effect of strain rate on the flow surface shape. By comparing the quasistatic and updated flow surfaces of DP600 and TRIP780 predicted by Yld2000-2d, results show a relatively considerable effect of updating anisotropy coefficients for higher strain rates. Several rate-dependent anisotropic material models (plane stress and general) were developed, by combining updated anisotropic yield functions and a rate-dependent hardening model (KHL). The developed models were implemented as user-defined material subroutines (VUMATs) based on implicit stress integration algorithm for ABAQUS/Explicit code to simulate electrohydraulic free-forming (EHFF) and die-forming (EHDF) processes. EHF simulations were completed, using Eulerian elements and ignition-and-growth model. The EHFF process was simulated for four different geometries (representing four different strain paths). Also, the EHDF process was simulated using a conical die The EHFF simulation results for the DP600 biaxial specimen showed that von Mises predicts a maximum effective plastic strain around 11% greater than Yld2000-2d for the same amount of applied energy. The EHDF simulation result for DP600 showed that with the same applied energy magnitude, von Mises overpredicts major, minor and through-thickness shear strains and consequently effective plastic strain (14% higher) compared to Yld2004-18p. Results showed that 82% of the effective plastic strain occurs under a proportional biaxial strain path before contacting the die. Also, results showed that von Mises overpredicts maximum absolute compressive through-thickness stress and shear strain compared to the values predicted by Yld2004-18p.

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