Failure Analysis of High Density Polyethylene Butt Weld Joint

This article discusses a detailed failure analysis conducted on high density polyethylene which acts as the liner of composite compresses natural gas cylinder. Leakage from the cylinder was observed after about 2000 cycles of hydrostatic pressure testing at 250 bars. Visual inspection revealed that the leakage occurred from the circumferential fusion joint between the cylinder and dome section. The cylinder and dome sections were produced from different techniques and joined together by using a heated tool butt welding process conducted by a local manufacturer. The joined components work as an integral part. The investigation was carried out using various techniques including mechanical, thermal and metallurgical examination. Fractography of the failed joint surface showed stepwise marks typical of a fatigue failure. Mechanical testing results showed that the strength of dome section was significantly lower than that of the cylinder section. Moreover, both the tensile and fatigue strength of joint was also almost half that of the cylinder. The fracture surface of the broken welded joint showed brittle failure. The melting points of both the cylinder and the dome sections were found almost same by differential scanning calorimeter but large difference was found between the melt point indexes of both sections. The results suggested that the failure was actually a stress cracking via a process of slow crack growth, which occurred due to use of a dome material having inferior properties and very high melt flow index. These properties inhibited proper fusion and resulted in a poor weld joint. Consequently, the weld joint of lower strength eventually failed in macroscopically brittle manner upon cyclic loading.