PSI - Issue 13

Yasuyuki Furuta et al. / Procedia Structural Integrity 13 (2018) 110–115 Furuta / Structural Integrity Procedia 00 (2018) 000 – 000

112

3

Fig. 2 Crack propagation behavior, helium gas test, 0 = 0.006 MPa. Figure 3 summarized the experimental results in terms of crack velocity, 0 , versus initial pressure, 0 , in which the crack velocity was measured at tube diameter distance from crack initiation point, where almost steady state crack opening profile was realized. The Figure clearly shows the dependence of 0 on 0 . Interesting point is that the gas medium does not change this relationship, considerably. This result might be quite natural because the pressure just after the burst is expected not influenced by gas medium. However, crack velocity at 500mm from crack initiation point is influenced by the gas medium, as shown in Fig.4. Note that a crack did not reach this distance in the two low pressure tests. Figure 5 shows histories of crack velocity in the air and helium gas test at approximately the same initial pressure, 0 = 0.0067 MPa and 0.006 MPa for the air and helium gas test, respectively. Fast drop in the crack velocity in the helium gas test is evident. Note that the crack velocity was smaller than the sound velocity of the air at ambient temperature, 340 m/s, as compared with 970 m/s for helium gas. Figure 6 shows histories of crack velocity in the air and helium gas test at initial pressure 0 = 0.012 MPa, in which case crack velocity was close to the sound velocity of the air. The difference of the crack velocity was smaller than that in Fig. 5.

Fig.3 Dependence of initial crack velocity on initial pressure.

Fig.4 Dependence of crack velocity at 500 mm on initial pressure.

3. Discussion

As is inferred from the knowledge of crack propagation analysis of high-pressure gas pipelines, crack propagation behavior is greatly influenced by gas decompression behavior, especially when the crack velocity is close to the gas decompression velocity, as in the present air test. The pressure change in the present study is much smaller than that of the burst tests of high-pressure gas pipelines due to low pressure. Therefore, linear wave equation can be applied, in which case pressure propagates at sound velocity, 0 .