Characterization and Modelling of 3rd Generation Advanced High Strength Steel Automotive B-pillars from Forming to Crash

Abstract

This thesis investigates the application of 3rd Generation Advanced High-Strength Steels (3rd Gen AHSS) with ultimate tensile strengths of 980 and 1180 MPa to the forming and crash performance of an automotive B-pillar. The two primary steels considered were a 3rd Gen 980 and 3rd Gen 1180 V1. A third steel, 3rd Gen 1180 V2, was made available later in the project to enable select comparisons with the other two steels. The steel sheets were formed into a full-sized automotive B-pillar with the assistance of Bowman Precision Tooling and then subjected to dynamic impact testing.

The B-pillar tool was designed was by Bowman Precision Tooling using preliminary material data generated early on in the project using AutoForm R7 software. As improved plasticity and formability data became available, the correlation of the AutoForm predictions with the stamping trials could be revisited to investigate formability and springback prediction. Forming simulations were also performed using LS-DYNA to support mapping of the forming predictions to the secondary impact simulations. The 3rd Gen 980 B-pillar was successfully formed in the stamping trials as predicted by both LS-DYNA and AutoForm. The 3rd Gen 1180 V1 simulations predicted multiple false-positives for fracture in regions of bending while occasional splitting in the forming trials only occurred at one location near in-plane uniaxial tension.

Springback prediction was investigated by varying the constitutive models and element settings and evaluated with white-light scanned B-pillars. Overall, springback predictions were deemed successful by the industry partners since deviation of the final shape over the B-pillar was on the order of the sheet thickness. Springback predictions were found to be sensitive to the kinematic hardening parameters and is a direction for future work to consider more advanced models.

Impact testing of the B-pillars involved laser cutting of the formed B-pillars and spot welding of 590R steel backing plates. A modified three-point bend test fixture was designed to approximate the boundary conditions for a side impact scenario. Deformation and force evolutions from impact tests were compared and correlated under different friction conditions with LS-DYNA simulations. It was shown that different friction conditions had a marked effect on the forces and the deformation of the side wall, in particular for the 3rd Gen 980. No fractures occurred in the crash testing of the 3rd Gen 980 while minimal cracking was observed in the 3rd Gen 1180 V1, highlighting the potential of this emerging class of steels for automotive lightweighting.