4 Consideration of gas system in fracture toughness test using open gas system and closes gas
4.4 Discussion
4.4.2 Effect of crosshead speed on fracture toughness in air
Strain rate or loading rate sometimes affects strength properties [20, 21]. Therefore, the effect of crosshead speed on the fracture toughness in this material was investigated prior to the characterization of the effect of hydrogen and addition of oxygen. The crosshead speed was varied as 2.0×10-5 mm/s, 2.0×10-3 mm/s and 2.0×10-2 mm/s. Figure 4.13 shows the J-a curves in air with different crosshead speeds. The J-a curves for the fastest crosshead speed were separated from that of other slower crosshead speeds. Based on these results, it is reasonable that the inhibitory effect of oxygen was separately discussed between V = 2.0×10-2 mm/s and other crosshead speeds in order to remove the effect of the strain rate dependency of this material.
0 0.5 1 1.5 2
0 100 200 300 400 500
Crack extension, a (mm)
J (N/mm)
2.0×10-2 2.0×10-3 2.0×10-5 A333 grade 6 Crosshead speed
(mm/s) In air
Fig. 4.13 Effect of crosshead speed on J-a curve in air
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4.4.3 Effect of oxygen content and crosshead speed on inhibitory effect of oxygen
Figure 4.14 shows the J-a curve in the high-purity hydrogen, 10 vol. ppm oxygen-hydrogen mixture, 100 vol. ppm oxygen-hydrogen mixture and air. The crosshead speed was 2.0 × 10-3, 2.0×10-4 and 2.0×10-5 mm/s. No effect of the crosshead speed on the J-a curves was found regardless of the environment within these crosshead speeds. The J-a curves in the high-purity hydrogen were significantly lower than that in air, in other words, severe hydrogen-assisted degradation occurred in these test conditions. On the other hand, the J-a curve in the 100 vol. ppm oxygen-hydrogen mixture was equivalent to that in air. The addition of 100 vol.
ppm oxygen to hydrogen environment completely inhibited the hydrogen-assisted degradation of the fracture toughness. The J-a curves in the 10 vol. ppm oxygen-hydrogen mixture were higher than that in the high-purity hydrogen, but lower than that in air. The addition of 10 vol. ppm oxygen to hydrogen environment partially inhibited the hydrogen-assisted degradation of the fracture toughness.
Fig. 4.14 Inhibitory effect of oxygen on J-a curve in hydrogen (V ≦ 2.0 × 10-3 mm/s)
0 0.5 1 1.5 2
0 100 200 300 400 500
Crack extension, a (mm)
J (N/mm)
2.0×10-3 2.0×10-4 2.0×10-5 Crosshead speed
(mm/s) Oxygen content (vol.ppm)
< 0.1 10 100 air ASTM A333 grade 6
In hydrogen containing oxygen Gas pressure: 0.6 MPa gauge Temperature: 293 K
0 0.5 1 1.5 2
0 100 200 300 400 500
Crack extension, a (mm)
J (N/mm)
2.0×10-3 2.0×10-4 2.0×10-5 Crosshead speed
(mm/s) Oxygen content (vol.ppm)
< 0.1 10 100 air ASTM A333 grade 6
In hydrogen containing oxygen Gas pressure: 0.6 MPa gauge Temperature: 293 K
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Figure 4.15 shows the J-a curve in each environment at the crosshead speed, V = 2.0 × 10-2 mm/s. The most significant difference of the results between Fig. 4.14 and Fig. 4.15 was the inhibitory effect of 10 vol. ppm oxygen. 10 vol. ppm oxygen partially inhibited the hydrogen-assisted degradation at the slower crosshead speed as shown in Fig. 4.14. However, 10 vol.
ppm oxygen did not inhibit the hydrogen-assisted degradation at V = 2.0 × 10-2 mm/s. The inhibitory effect of 10 vol. ppm oxygen disappeared by the increase in the crosshead speed. Otherwise, the J-a curve in 100 vol. ppm oxygen-hydrogen mixture agreed with that in air and the J-a curve in high-purity hydrogen showed significant reduction, similarly to the results in Fig. 4.14.
0 0.5 1 1.5 2
0 100 200 300 400 500
Crack extension, a (mm)
J (N/mm)
2.0×10-2 Crosshead speed
(mm/s) Oxygen content (vol.ppm)
< 0.1 10 100 air ASTM A333 grade 6
In hydrogen containing oxygen Gas pressure: 0.6 MPa gauge Temperature: 293 K
0 0.5 1 1.5 2
0 100 200 300 400 500
Crack extension, a (mm)
J (N/mm)
2.0×10-2 Crosshead speed
(mm/s) Oxygen content (vol.ppm)
< 0.1 10 100 air ASTM A333 grade 6
In hydrogen containing oxygen Gas pressure: 0.6 MPa gauge Temperature: 293 K
Fig. 4.15 Inhibitory effect of oxygen on J-a curve in hydrogen (V = 2.0 × 10-2 mm/s)
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4.4.4 Effects of oxygen content and loading rate on inhibitory effect of oxygen on
hydrogen-assisted degradation of fracture toughness
Figure 4.16 shows a map showing the degree of degradation of fracture toughness due to hydrogen correlated to the loading rate and oxygen content contained in hydrogen gas. When the oxygen content was low and loading rate was high, the inhibitory effect of oxygen did not appear.
On the other hand, when the oxygen content was high and the loading rate was low, oxygen completely inhibited the hydrogen-assisted degradation of fracture toughness. These results are well in agreement with the prediction by the formula proposed by Somerday et al. shown in Fig. 4.12. It means that the inhibitory effect of oxygen in this study is also governed by the competition of the rate of bare surface creation by the crack extension and the rate of the repassivation of the bare surface by oxygen as explained by Somerday et al.
Furthermore, as shown in Fig. 4.16, the result of 1 vol. ppm oxygen was at the transition region between severe hydrogen-assisted degradation and no hydrogen-assisted degradation.
Therefore, the result that 1 vol. ppm oxygen inhibited the hydrogen-assisted degradation of fracture toughness test is considered to be valid. However, we must be careful that the results in this study were obtained at 0.6 MPa of gas pressure and 293 K of the temperature using one material. The result may change depending on these test conditions. According to the ANSI/CSA CHMC 1-2014 standard [7], which determines the test method for evaluating material compatibility in compressed hydrogen applications, this standard allows hydrogen gas used for the test to contain 2 vol. ppm oxygen. It means that there is a possibility that the effect of hydrogen cannot be correctly evaluated depending on the test conditions even if the test is conducted according to the standard.
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Fig. 4.16 Oxygen content and crosshead speed dependency on inhibitory effect of oxygen 100
10
<0.1
Oxygen content (vol. ppm)
Crosshead speed (mm/s) 2×10-2
2×10-3 2×10-4 2×10-5
In air 1
Non-inhibitory region
Inhibitory region Transition
region
Severe hydrogen-assisted degradation Partial hydrogen-assisted degradation No hydrogen-assisted degradation
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Matsumoto et al. reported a loading rate dependency on fracture toughness of carbon steel in 0.6 MPa hydrogen gas [22]. They showed that the fracture toughness in hydrogen environment increased with a decrease in the crosshead speed of the fracture toughness test from 2.0 × 10-3 to 2.0
× 10-5 mm/s. They concluded that the mitigation of the hydrogen-assisted degradation by the decrease in the crosshead speed can be explained by the gradient of the hydrogen concentration in the material proposed by Yoshikawa et al. [19].
The trend of the results reported by Matsumoto et al. that hydrogen-assisted degradation of fracture toughness was mitigated by the decreasing in the loading rate is corresponds to the fracture toughness test result in the 10 vol. ppm oxygen-hydrogen mixture in this study. Therefore, it can be considered that the mitigation of the hydrogen-assisted degradation with the decrease in the crosshead speed in this study was due to the gradient of the hydrogen concentration. However, in the high-purity hydrogen at 2.0 × 10-5 mm/s of the crosshead speed, where the amount of hydrogen uptake was the largest and the gradient of hydrogen concentration was the mildest in this study, the mitigation of the hydrogen-assisted degradation of the fracture toughness did not appear. Thus, it can be considered that the gradient of the hydrogen concentration did not affect the fracture toughness test in this study. Therefore, the mitigation of the hydrogen-assisted degradation of the fracture toughness in the 10 vol. ppm oxygen-hydrogen mixture with the decreasing in the loading rate in this study was due to the addition of oxygen, but not due to the gradient of the hydrogen concentration.
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