Cracking Failure Analysis of a Steel Wire-Reinforced Thermoplastic Composite Pipe Used in an Oily Sewage Conveying System

A steel wire-reinforced thermoplastic composite pipe used in an oily sewage conveying system cracked after an 8-year service. Visual inspection and microscopic observation, FTIR, density and hardness measurements, differential scanning calorimeter, gel permeation chromatography (GPC), Vicat softening temperature, and the stability of no splitting for pressed composite pipes were carried out to analyze the causes and prevent the recurrence of such cases. The results show that the size of the pipe and the wire winding met the requirements of the standard of CJ/T 189-2007, but the cracks along the axial direction of the pipe appeared on the outside surface of the pipe during the pressing test, which is like the cracks in the failed pipe, indicating that the pressure-bearing performance of the pipe decreased. The direct cause of the crack was found that due to the performance degradation of the hot glue wrapped around reinforced steel wire, cracks induced by stress first appeared in the hot glue matrix and developed in both the inner and outer layers of the PE matrix. The crack of the outer matrix allowed the corrosion and even fracture of the reinforced steel wire, resulting in a decrease in the pressure-bearing capacity of the pipe. The pipe expanded due to wire corrosion or even fracture under pressure conditions, which resulted in the further development of cracks in the inner layer matrix and, eventually, penetrated the inner layer matrix. Finally, leakage occurred.