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In numerous cases, metals and alloys traditionally used as structural materials in the process equipment can now be successfully replaced by nonmetallic composite materials. We study the influence of service solutions on the state of stress and tensile properties of glass-polyester pipes in the longitudinal and circumferential directions. These analyses are of great importance for the use of examined pipes in chemical industry. The pipes were produced by 'Poliester' Corporation (Priboj). The influence of two solutions, methanol and ammonia, was studied. The samples were treated in solutions for three, seven and ten days. After this, the samples were subjected to tensile testing by the standard procedure. The stresses and strengths were determined both in the longitudinal direction (testing on flat test specimens) and in the circumferential direction (ring tests). The analysis of test results, according to the type of solution and the period of holding, was realized in comparison with the results obtained by testing intact pipes (not subjected to the influence of the solution). The micromechanical analysis of broken samples helped to determine the influence of the solution on the structure of composite pipes and determine the models and mechanisms responsible for the decrease in strength. [ABSTRACT FROM AUTHOR]