In-Plane Plane Strain Testing of Sheet Materials for Multi-Stage Processes

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Abstract

This thesis examines the development of a small-scale in-plane tensile test capable of replicating strain paths on the left hand side of a forming limit diagram (FLD), from uniaxial tension to plane strain. The process, termed in-plane plane strain (IPPS) tensile testing, can also be employed to evaluate the formability of a sheet material for a multi-stage process. In the current study, tube bending and hydroforming strain paths are examined and replicated using this IPPS setup. Two steel sheet materials are considered herein, a highly formable aluminum killed drawing quality (DQ) steel and a high strength dual phase (DP600) steel.

The research methodology included a parametric finite element method (FEM) study to design the specimen geometry needed to acquire the "pre-strain" bending and "final strain" hydroforming strain paths being replicated at the center of flat sheet specimens. For each of the steel sheets, a series of blanks were pre-strained to varying degrees in the rolling direction (RD) and subsequently strained to failure in the transverse sheet direction (TD).

Effective strain diagrams (ESDs) and forming limit stress diagrams (FLSDs) were constructed for both the DQ and DP600 sheet materials to present the multistage IPPS forming limit data in a form that is suitable for a comparison to full-scale test data.

A new objective forming limit criterion is also proposed. All IPPS tests were digitally recorded in real-time and post-processed to determine the strains from a noded 3 mm square grid applied to the surface of undeformed sheet specimens. Individual strain measurements were found to have a two-sided 95% confidence interval of ± 0.02 engineering strain. Using the criterion, a forming limit strain is reached when a difference of 0.03 engineering strain develops in the major loading direction between neighboring rows of three grids. A repeatability study, carried out on single-path tests revealed that the 95% confidence interval of this objective forming limit method is ± 0.06 engineering strain, which is higher than expected and warrants further refinement.

Cited Works (2)

Year	Entry
2004	Normani FV. Analytical Modeling of Tube Bending With Hydroforming [Master's thesis]. University of Waterloo; 2004.
2004	Dyment JN. Effect of the Bending Process on Hydroformability of Steel Tubes [Master's thesis]. University of Waterloo; 2004.

Cited By (3)

Year	Entry
2007	Kilfoil LJ. In-Plane Plane Strain Testing to Evaluate Formability of Sheet Steels Used in Tubular Products [Master's thesis]. Kingston, ON: Queen's University; September 2007.
2009	Hamelin CJ. Multi-Scale Modelling of Texture Evolution and Surface Roughening of BCC Metals During Sheet Forming [PhD thesis]. Queen's University; April 2009.
2011	Sloan ADC. The Role of Non-Ferritic Phase in the Micro-Void Damage Accumulation and Failure of Dual-Phase Steels [Master's thesis]. Kingston, ON: Queen's University; September 2011.