Double-layer composite structures with one load-bearing high-strength layer made of metal, reinforced composite or rigid plastic and other one made of high deformability polymer or elastomer providing high tightness, chemical and corrosion protection are widespread. Due to differences in the mechanical properties and linear thermal expansion coefficient, high level of stresses can occur in the layers of such structures during temperature changes. Considering the long-term nature of loading, the defects probability, the availability of complex stress state in the layers and temperature dependence of the physical and mechanical properties of polymers, the issues of the providing of long-term mechanical strength and failure-free operability of such structures are highly relevant. Object of study in this paper is polymeric pipe combined of two layers made of thermoplastic (HDPE) and fiberglass. The purpose of research is to identify the causes of brittle fracture of thermoplastic layer at low temperatures. The experimental results of cold resistance of HDPE specimens and pipe samples at temperatures down to -50°С are presented in this paper. A comparison with the previous results of calculations of technological residual stresses in these pipes was done. The test method for fragility of HDPE under complex stress state at low temperatures was proposed. As a result of pipe samples testing the process steps which can effect on cold resistance of plastic pipes were identified.

stress-strain state, fracture, HDPE, polyethylene, fiberglass, cold-resistance, test, bi-axial loading, combined pipe, long-term strength, technological stress.