Internal crack initiation and growth behavior and the influence of shape of specimen on crack damage of EPDM for sealing high-pressure hydrogen gas

When a cylindrical specimen (φ29.0mm × 12.5mm) made from sealing rubbers was exposed in high-pressure hydrogen gas, it was clarified that cracks initiated in the specimen after decompression in previous studies. However, it was not clear whether crack damage obtained from the cylindrical specimen was the same as that obtained from an O-ring specimen used for practical sealing or not, because the shapes and dimensions of these specimens were different. From this viewpoint, seven cylindrical specimens (φ13.0mm × 2.0, 4.0, 6.0, 8.0, 10.0, 12.0 mm, and φ29.0mm × 12.5mm) made from an unfilled peroxide-crosslinked EPDM composite were exposed to high-pressure hydrogen gas at a maximum pressure of 10MPa ; then, the influence of the shape of specimen on crack initiation and growth behavior was investigated. Micrometer-sized defects, facets, and notch-like regions were observed at fracture origins of internal cracks by SEM observation. It was inferred that micrometer-sized bubbles were formed at these sites by the coalescence of submicrometer-sized bubbles, and internal cracks initiated due to stress concentration of the micrometer-sized bubbles. Measurement of AE (acoustic emission) was also conducted to confirm the existence of the sub-micrometer-sized bubbles. The crack damage became more serious with an increase in the thickness of specimen as well as an increase in hydrogen pressure. Although the critical hydrogen pressures at crack initiation of the specimens with thicknesses from 2.0 to 6.0mm decreased with an increase in the thickness of specimen, those of the specimens with thicknesses from 6.0 to 12.0mm were the same. The influence of the shape of specimen on the crack damage and the critical hydrogen pressure was successfully explained in terms of retained hydrogen content and fracture mechanics.