Long-term durability of glass-fiber-reinforced composites under operation in pulp and reactant pipelines

Composite pipes 215 mm in diameter with a 6-mm wall, 3-mm functional layer, and +/-60degrees glass-fiber-reinforced plastic (GFRP) layers were examined by means of microstructural analysis and mechanical testing. Three pipes of pipes were considered: unused, after a five-year operation at a pressure of 0.6 Wa and temperature of 70degreesC, and after a five-year operation at 0.6 MPa and 20degreesC The GFRP load-carrying layer and the inter,faces were investigated by a metallographic optical microscope, a computerized microscanning equipment, and a software image processing package. The initial and accumulated damages in the microstructure of the composite and interfaces were examined. The mechanical properties of the, pipes after a long-term operation were examined on two-layer specimens in three-point bending. The stiffness, and strength characteristics were measured in the axial and circumferential directions under tensile and compressive loads at elevated temperatures. Composite pipes, 8 m in length and 215 mm in diameter, were simulated numerically as shells of revolution. The shell model was corrected by a refined Timoshenko theory and a dual-modulus temperature-dependent model of stiffness for the multilayer composite structure. The strength margin of the composite pipes was determined based on real strength properties.