Session: MF-02-06 Materials for Hydrogen Service-Test Methods 1

Paper Number: 122165

122165 - Effect of the Loading Rate on Fracture Toughness of an Ultra-High Strength Steel Sheet in High-Pressure Hydrogen Environment 

To attain low CO₂ emissions from automobiles, ultra-high strength steel with a tensile strength (TS) of 1.5 GPa class has come to be applied to structural components of car bodies, aiming to improve fuel efficiency via weight saving. It is well known that metallic materials become sensitive to hydrogen with an increase in their TS. Moreover, the fracture toughness of high-strength steel is significantly reduced by hydrogen. Therefore, the expansion of this effective means to low CO₂ emission depends on the success in the safety assurance of the fracture toughness under the influence of hydrogen. In our previous study, a novel fracture toughness evaluation method suitable for ultra-high strength steel sheets was developed, and the non-negligible impact of the crack closure on the fracture toughness was demonstrated at the slow loading rate. In the present study, aiming to reveal the effect of loading rate on the fracture toughness and crack opening behavior under the hydrogen environment, fracture toughness tests of a 1.7 GPa class steel were carried out in 90 MPa hydrogen gas under the different loading rates ranging from 0.004 to 0.39 MPa∙m^1/2/s using a 1.6 mm-thick, middle-crack tension (MT) specimen. The results showed the fracture toughness and the degree of the crack closure depended on the loading rate. As the loading rate becomes slower, the impact of crack closure gets smaller. However, on the contrary, the essential fracture toughness becomes higher with a decrease in the loading rate. The reason for this contradiction will be discussed in the presentation.

Presenting Author: Yuya Tanaka Department of Mechanical Engineering, Fukuoka University

Presenting Author Biography: Yuya Tanaka is an assistant professor of Mechanical Engineering at Fukuoka University. His work focuses on the fatigue limit, fatigue crack-growth threshold, and fracture toughness of metallic materials.

Authors:

Yuya Tanaka Department of Mechanical Engineering, Fukuoka University
Naoki Hirakawa Graduate School of Engineering, Kyushu University
Akinobu Shibata Research Center for Structural Materials, National Institute for Materials Science (NIMS)
Hisao Matsunaga Department of Mechanical Engineering, Kyushu University