Study on the Stress Relaxation Characteristics and Constitutive Model of Unloading Damage Mudstone

The influence of the damage caused by excavation and unloading of soft rock mass on the stress relaxation characteristics of its support reinforcement cannot be ignored. Taking the red layer mudstone as the research object, the uniaxial stress relaxation failure test of unloading damaged mudstone was carried out, and the influence of different strain levels and unloading damage degrees on the relaxation characteristics of mudstone was studied. The results show that: The stress curves of the graded stress relaxation test of the unloading damaged mudstone before and after failure are all incomplete relaxation, and the stress relaxation curve mainly includes two stages: deceleration and stable relaxation, and the relaxation amount and relaxation time at all levels of strain level show a linear increase trend with the increase of the degree of unloading damage. Based on the fractional calculus theory, the R-L fractional order elastic viscous element (F element) is selected, and an elastic viscous time-varying element (FT element) considering the time-varying characteristics of viscosity coefficient is proposed, and then the unloading damage degree factor is introduced, and a fractional order elastic viscous time-varying element (UFT element) considering the initial unloading damage degree is proposed. Finally, the component is introduced into the traditional Burgers model to establish a nonlinear relaxation constitutive model based on fractional order. The comparison and discussion of the calculation values and experimental data of the theoretical constitutive model based on the three fractional order elements shows that the relaxation constitutive model based on UFT elements has higher accuracy, and the theoretical calculation results of the model under different stress levels and unloading damage degrees are consistent with the experimental data, indicating that the relaxation constitutive model can better and accurately describe the relaxation characteristics of unloading damage mudstone.