AN INVESTIGATION INTO PROPERTY VARIANCES BETWEEN OUTER AND INNER HDPE PIPE LAYERS
Keywords:polyethylene pipe, surface roughness, hardness, crystallinity, oxidation induction time.
Many studies devoted to plastic piping investigated mechanical properties as suggested by many standards; however very few of them were designed for surface roughness and hardness analyses in relation to microstructure. The objective of this research work is to establish property differences between inner and outer surface layers of a high density 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 as a result of the fabrication process which produces residual stresses and imposes structural variations. On the other hand, the outer die-hardened pipe face develops higher roughness and higher shore hardness values. All methods used to obtain crystallinity provided acceptable measurements and indicated that the inner pipe layer is more crystalline because of slow cooling. Finally, it is shown that OIT results are in favor of the outer surface layer with 14% difference. This indicates that the inner surface layer is more vulnerable to thermal degradation compared to the outer one. Such results should improve the understanding of HDPE pipe behavior under combined outer and inner frictional effects during service operations.