Development of non-linearity of stress-strain dependencies in hysteresis loops during repeated uni-axial quasi-static tensile testing of unidirectional CFRP

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The purpose of this study is to investigate the degradation of the mechanical properties of carbon fiber-reinforced plastic (CFRP) under repeated loading, while simultaneously monitoring damages accumulation using the acoustic emission method. In the first part, the issues of the development of the non-linearity of stress-strain diagrams in the hysteresis loops of unidirectional CFRP under repeated uni-axial quasi-static tensile loading in the direction of highest stiffness are considered. Hysteresis loops were identified and the equations of the upper and lower branches were compiled. The linear and nonlinear components of these equations were determined. Non-linear elasticity is disrupted in the sections of active deformation due to intense damages. The preservation of non-linear elasticity occurs in the sections of passive deformation with minor damages. A sign of the transition of the material to a state of pre-destruction is the formation of a second extremum in the non-linear component of the upper branch.