Fatigue sensitivity of GFRP under proportional cyclic tension with torsion

The constructions made of composite materials are subjected to cyclic, dynamic, vibration and other loads in operations related to damage accumulation and degradation of progressive mechanical characteristics. Thus it is important to conduct experimental and theoretical studies of the combined impact effects on the change in the mechanical characteristics of the material. In this case, it is necessary to take into account the complex stress state realized in the structures. This work deals with an experimental study of the degradation patterns of the stiffness characteristics of fiberglass tubular specimens obtained by continuous winding as fatigue damage accumulates due to biaxial proportional cyclic loading. The methodological aspects of realization of biaxial loading are considered. Quasi-static and fatigue tests were performed on specimens with different winding angles under uniaxial tension, torsion, and proportional tension, i.e. torsion with three different ratios of the normal and shear stress tensor components. The presence of a decreasing region in the torsional load diagrams has been revealed. Strength surfaces are constructed. By using the approximation of the fatigue sensitivity curves previously developed by the authors, we process the experimental data on the decrease in the dynamic modulus of elasticity as the number of cycles of exposure increases. The high descriptive ability of the developed model and the low values of the variation coefficients of the calculated parameters were noted. Non-monotonic dependences of model parameters on the type of stress state are revealed. A significant influence of the winding angle on the fatigue sensitivity of the composite has been found. We have made the conclusion about necessity of taking into account the decrease of mechanical characteristics of materials in calculations of constructions and rationality of further experimental researches for verification of earlier developed models.