A Novel Grip Design for High-Accuracy Thermo-Mechanical Tensile Testing of Boron Steel under Hot Stamping Conditions

M Ganapathy,M Abspoel,D Bhattacharjee,N Li,J Lin

doi:10.1007/s11340-017-0333-8

Abstract

Achieving uniform temperature within the effective gauge length in thermo-mechanical testing is crucial for obtaining accurate material data under hot stamping conditions. A new grip design for the Gleeble Materials-Simulator has been developed to reduce the long-standing problem of temperature gradient along a test-piece during thermo-mechanical tensile testing. The grip design process comprised two parts. For the first part, the new design concept was analysed with the help of Abaqus coupled Thermal-Electric Finite element simulation through the user defined feedback control subroutine. The second part was Gleeble thermo-mechanical experiments using a dog-bone test-piece with both new and conventional grips. The temperature and strain distributions of the new design were compared with those obtained using the conventional system within the effective gauge length of 40 mm. Temperature difference from centre to edge of effective gauge length (temperature gradient) was reduced by 56% during soaking and reduced by 100% at 700 °C. Consequently, the strain gradient also reduced by 95%, and thus facilitated homogeneous deformation. Finally to correlate the design parameters of the electrical conductor used in the new grip design with the geometry and material of test-piece, an analytical relationship has been derived between the test-piece and electrical conductor.

Highlights

Current global emission regulations are driving automotive industries to improve fuel efficiency and reduce CO2 emissions of their vehicles, while passenger safety guidelines are forcing the car manufacturers to increase the level of passenger safety by increasing crashworthiness
Based on Finite element simulation, a new design of grip was fabricated for Gleeble uniaxial thermo-mechanical testing
The design was practically tested using boron steel testpieces for test in which test-pieces were heated to the austenite range and air cooled to deformation temperature and the results were compared with those obtained using conventional grips

Summary

Introduction

Current global emission regulations are driving automotive industries to improve fuel efficiency and reduce CO2 emissions of their vehicles, while passenger safety guidelines are forcing the car manufacturers to increase the level of passenger safety by increasing crashworthiness Achieving both environmental and safety norms simultaneously is a major challenge for car manufacturers, since the two goals impose conflicting requirements, reduced weight and increased strength, on vehicle design [1]. Manufacture of lightweight components made of UHSS in traditional cold stamping is limited by poor formability and excessive spring-back of the workpiece material. To overcome these drawbacks, hot stamping technology has been developed in the recent decade and is widely used [2, 3]. A novel biaxial testing system for use in Gleeble testing machine has been developed for practical hot stamping conditions [11]

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