PSI - Issue 19

Masanobu Kubota et al. / Procedia Structural Integrity 19 (2019) 520–527 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

522

3

Table 1. Chemical composition of the material. C Si Mn P

S

Cr

Mo

Ni

Cu

0.35

0.18

0.76

0.014

0.015

1.1

0.16

0.02

0.01

2.2. Slow strain rate testing

A slow strain rate test (SSRT) was carried out in air and in a low-pressure hydrogen gas (0.1 MPa in gauge pressure) in order to confirm the susceptibility of the material to hydrogen embrittlement. Figure 2 (a) shows the specimen for the SSRT. The strain rate was 6.67 × 10 -5 1/s.

a)

b)

c)

30 95

95

15 45

12.5

12.5

60 ゜

R20

R8

R20

R20

R8

2-M8

 3

 5

 6

 5

 13

 13

 19

 19

 0.2

Fig. 2. (a) SSRT specimen; (b) Smooth specimen; (c) Deep-notched specimen.

2.3. Fatigue testing

High-cycle fatigue tests were carried out using a smooth specimen [Fig. 2 (b)] and a deep-notched one [Fig. 2 (c)]. The smooth specimen had a blunt and shallow circumferential notch to determine the crack initiation site. Since the stress concentration factor, K t , of the notch was 1.06, the specimen was regarded as a smooth specimen. The value of K t for the deep notch was 4.84. The deep notch was used to produce non-propagating cracks at the fatigue limit. In addition, the deep notch may have another role in this test. Cyclic plastic deformation locally occurred at the notch root. This might enhance the interaction between deformation and hydrogen. The loading type was tension and compression. The stress ratio was -1. The loading frequency was 20 Hz. The fatigue test was terminated at 10 7 cycles if the specimen did not break. A staircase method, which was a statistical method to obtain the fatigue limit provided by JSME S002 (1994), was adopted to clarify the effect of hydrogen on the fatigue limit. Macadre et al. (2011) showed that the loading frequency used during the fatigue crack propagation test impacts the sensitivity to hydrogen embrittlement. The effect of hydrogen was more enhanced by a slower loading frequency during the crack propagation test. However, this study aimed at the discussion of the effect of hydrogen on the fatigue strength at 10 7 cycles using a statistical fatigue testing method. Therefore, a normal loading frequency was used for the fatigue test. Note that a 20 Hz loading frequency is allowed for the fatigue test to determine the hydrogen compatibility of materials in the high-cycle regime by the ANSI/CSACHMC1 standard (2014). The environments were air and hydrogen gas. Part of the experiment was also carried out in nitrogen gas. The purity of the gases was 99.9999%. As shown by Komoda et al. (2019), the purity of hydrogen gas stored in a hydrogen chamber degraded during the testing. Considering the time for 10 7 cycles, we needed to avoid such a degradation of the environment. Therefore, an open gas system (flow of gases was maintained throughout the fatigue test) was adopted in this experiment. The purity of the gases was monitored by an oxygen meter throughout the experiment. The measured O 2 amount contained in the gases was less than 0.1 vppm. The pressure of the hydrogen and nitrogen gases was 0.1 MPa in gauge pressure. The temperature of the gases was ambient.