PSI - Issue 13

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

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Fig.8 Histories of crack velocity with ′ , 0 = 0.006, 0.0067 MPa.

Fig.9 Histories of crack velocity with ′ , 0 = 0.012 MPa.

Another interesting result is shown in Fig.2, where the crack was decelerated and finally arrested at 680mm. The crack direction changed from the axial to circumferential direction, leading to ring-off and final crack arrest. Similar crack arrest was observed in the other helium gas tests. This phenomenon is very similar to the crack arrest often observed in full scale burst tests of natural gas pipelines (Kobayashi et al, 1988). Although, reason to the crack deviation before crack arrest is not fully understood, the authors presented a hypothesis that a crack begins to deviate if circumferential stress becomes lower than axial stress (Miyamoto et al, 2015). In the present study, axial force is expected to be maintained at a certain level even after the decompression wave passed through while the circumferential stress drops along with shrinkage of the tube section. The present test might support the above hypothesis. The present experimental method has been developed in the last few years used as project-based learning educational courses in Brazilian and Japanese universities, successfully. For attracting the students, the courses are run as a competition style. Example themes of the competitions are shown in the Appendices. The present experiment is expected to be utilized for educating engineers and researchers as well as students relevant to the pipeline engineering. Dynamic frack propagation experiment using rubber tubes has been developed which is intended to simulate running ductile fracture in high-pressure gas pipelines. Crack velocity was measured by high-speed camera. It ranged 100 to 500m/s, depending on pressure. The crack velocity decreased more rapidly in helium gas test than in the air test. This result was explained by a comparison of crack propagation velocity and sound velocity of the gas media. Crack opening profile was found similar to that of full-scale burst tests of steel pipes. Also, crack deviation leading to ring-off and crack arrest took place, which was similar to full-scale burst tests. Bi-axial stress state was suggested as a factor controlling the crack deviation. Acknowledgements The present experiments were conducted as a part of educational programs, (i) Human Resource Development in Shipbuilding Technology / Brazil-Japan Collaborative Program on Naval Architecture and Offshore Engineering, Federal University of Pernambuco, Brazil, 2017-2018, (ii) Seminar of First-Year Education, The University of Tokyo, 2018 and (iii) Project-Based Learning course, Dept. Systems Innovation, The University of Tokyo, 2016-2017. The program conducted in Brazil was financially supported by Petrobras. The authors are grateful to the students conducted the experiments in their educational course. 4. Conclusions