13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Small Crack Effect on Threshold Stress Intensity KTH for High Strength Steel with Internal Hydrogen Yukitaka Murakami1*, Hisao Matsunaga1, 2, 3, Arezou Abyazi4, Yoshihiro Fukushima1, 3 1 International Institute for Carbon-Neutral Energy Research (I2CNER), Kyushu University, Fukuoka 819-0395, Japan 2 Institute of Materials Science and Technology, Fukuoka University, Fukuoka 814-0180, Japan 3 Department of Mechanical Engineering, Kyushu University, Fukuoka 819-0395, Japan 4 Department of Materials Engineering, Sahand University of Technology, Tabriz, Iran * Corresponding author: murakami.yukitaka.600@m.kyushu-u.ac.jp Abstract The size effect of the threshold stress intensity KTH for hydrogen-precharged specimens of SAE52100 were investigated. Four types of tensile specimens were used: (i) smooth specimens and (ii) specimens having various shapes of artificial defects with sizes of about 35 ~ 500 μm. In the smooth specimens, fracture origins were nonmetallic inclusions with the size of 10 ~ 30 μm (e.g. Al2O3·(CaO)x, TiN and TiC). The fracture toughness determined for the small defects at fracture origin, i.e. the KTH calculated from the defect size area and the fracture tensile stress σf, showed a defect size dependence, where, area denotes the area of the domain defined by projecting the defect on a plane normal to the cylindrical axis of the specimen. The values of KTH for both the nonmetallic inclusions and artificial defects were much smaller than those for large cracks measured by the standard WOL and CT specimens. Keywords Threshold stress intensity, Defect size, Nonmetallic inclusion, Hydrogen embrittlement, High strength steel 1. Introduction Hydrogen causes degradation of various strength properties, such as tensile strength, ductility, fatigue strength, fracture toughness and etc. [1-3] Among them, the fracture toughness such as KIC and JIC is recognized as one of the most important property for the structural integrity of high-pressure hydrogen storage vessels and hydrogen supply systems. Moody et al. [4] showed how the fracture toughness KTH of low alloy steels varies with yield strength and hydrogen pressure. Figure 1 schematically shows the relationship between KTH of a steel in hydrogen gas as a function of the yield strength σYS [1]. Once σYS exceeds about 1500 MPa, KTH reachs a lower limit between 10 and 20 MPa m. Similar results on the hydrogen effect on the fracture toughness have been reported also by the other researchers [5 and others]. In most cases, KTH for various materials has been measured for large cracks by using standard specimens, e.g. WOL and CT. On the other hand, in order to evaluate the effect of small defects on the fracture toughness of high strength steels, Murakami et al. [6] carried out a series of tensile tests using hydrogen-precharged (H-precharged) specimens. They determined the KTH values from the fracture stresses and sizes of the nonmetallic inclusions at fracture origins. They reported the followings: (i) KTH was decreased with an increase in the residual hydrogen content in the material. (ii) KTH was decreased with a decrease in the defect size. This crack size dependence was similar to that of ΔKth for the small fatigue crack or defect [7, 8]. Above experimental facts caution that the use of the threshold values measured for the large cracks leads to an unconservative estimation in the fracture toughness design against small cracks or small defects. However, the significance of the size effect has not been well recognized despite the presence
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