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Investigation of the creep failure behavior of polyamide pipes

 

Hiroto Taguchi, Kaori Numata

Papers # 2018 Las-Vegas

We elucidated the creep failure behavior of polyamide pipes. We conducted the hydrostatic test and Notched Pipe Test (NPT) at 80 and 90 °C for up to 10000 h for PA pipes whose creep failure behavior has not been fully elucidated yet. In the tests, the change in the failure mode from ductile failure to brittle failure, i.e. SCG, seen in polyethylene pipes, was not observed. Additionally, the PA pipes fractured after more than 6000 h of testing at 90 °C and showed the brittle failure mode, in which the ductile failure originates; subsequently, the cracks propagate in the brittle mode. The observation and analysis of the fractured pipes suggest that hardening by increase in crystallinity could cause the brittle failure. It is assumed that the ductile deformation was inhibited because of the hardening and subsequently the brittle failure occurred.

To reduce installation and maintenance costs, plastic pipes expected to be used more widely under high-pressure conditions. Recently, multilayer polyethylene (PE) pipes and polyamide (PA) pipes have been used at pressures above 1.0 MPa as an alternative for steel pipes in several countries. When plastic pipes are under higher pressure, the evaluation of their creep properties is important. However, only a few studies have been reported regarding the creep failure behavior of high-strength pipes such as polyamide pipes. We performed the hydrostatic test and the notched pipe test (NPT) of PA pipes at 80 °C and 90 °C.

The failure mode of the PA pipes was ductile failure when the pipes was fractured in the entire test time of the hydrostatic test and the NPT at 80 °C and until 4826 h of the tests at 90 °C. The transition of the failure mode from ductile failure to SCG was not observed. Furthermore, NPT method did not shorten the failure time. Hence, we conclude that the PA pipes have extremely high resistance to crack propagation compared with PE pipes. 

The PA pipes fractured after more than approximately 6000 h in the tests at 90 °C showed the brittle failure mode. The degradation behavior such as oxidation was not observed in the brittle failure pipes. The fractography showed their failure mode changed from ductile to brittle while proceeding. FT-IR analysis and indentation test suggested that the increase in crystallinity proceeded at 90 °C and it caused hardening. The hardening resulted in inhibiting deformation and the change in failure mode from ductile to brittle.

Related keywords : Polyamide, creep failure, brittle failure mode..
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