Experimental investigation of the effect of residual stresses and interfacial adhesion on rapid crack propagation in bilayered polypropylene/polyethylene (PP/PE100) pipes

Rapid crack propagation (RCP) resistance of bilayered pipes consisting of a polyethylene core and a polypropylene skin was investigated using the ISO 13477 Small Scale Steady State “S4” test. It was found that bilayered pipes met all the RCP requirements, but the addition of a thin (0.4–1.5 mm) skin slightly increased the S4 critical temperature. This embrittlement effect is attributed to increased constraint, which is mainly influenced by the elastic moduli of the skin and core and the adhesion between them. Because the elastic moduli of the skins were very similar, the investigation focused on the effect of adhesion and residual stresses. It was found that higher adhesion leads to higher constraint and higher S4 critical temperature. However, when both adhesion and residual stresses were modified during annealing, the effect of residual stress relaxation was more significant on RCP performance than that of increasing adhesion. Annealed pipes in which residual stresses had relaxed by ～40%, showed better RCP resistance even though adhesion was almost doubled. It was found that in some cases, bilayered pipes may even present better RCP resistance than monolayered ones when the adhesion falls below a threshold at which the skin no longer constrains the pipe. POLYM. ENG. SCI., 57:458–463, 2017. © 2016 Society of Plastics Engineers