AN INVESTIGATION INTO PROPERTY VARIANCES BETWEEN OUTER AND INNER HDPE PIPE LAYERS

Many studies devoted to plastic piping investigated mechanical properties as suggested by technical standards; however very few of them were designed for surface roughness and hardness analyses in relation to microstructure. The objective of this research is to establish property differences between inner and outer surface layers of a polyethylene pipe. Machined specimens are prepared from both sides of a pipe and subjected to mechanical testing, roughness and hardness measurements. It is found that local mechanical properties are in favor of the inner layer since the fabrication process produces residual stresses and structural variations. On the other hand, the outer die-hardened pipe face develops high roughness and Shore hardness values. All methods used to obtain crystallinity provided acceptable measurements and they indicated that the inner pipe layer is more crystalline because of slow cooling. Finally, it is shown that oxidation induction time results are 14% in favor of the outer surface layer. This indicates that the inner layer is more vulnerable to thermal degradation compared to the outer one. Such conclusions are meant to improve the understanding of plastic pipe behavior under mutual external and internal frictional environmental effects during service conditions.