| Under quasi-static condition, rock and concrete materials present an anomalous nonlinear elastic behavior which accompanied with hysteresis. Numerous experimental results have shown that, there are bound system(crack, grain contact, etc) in these materials which control the characteristic of nonlinear elastic behavior. Thus, one phenomenological model which named Preisach-Mayergoyz model is proposed to describe nonlinear elastic especially the hysteresis of the rock and concrete materials. However, with the PM model, there is some difficulty in assigning a set of model hysteretic mesoscopic elements to real physical processes. It is difficult to take into account the effect of loading rate on the stress-strain relations, as well as the nonequilibrium phenomenon during the loading and unloading process. Therefore, one modified PM model is proposed in this thesis by introducing the viscoelastic mesoscopic unit possessing time-dependent peculiarity. And then, loading rate-dependent of the stress-strain curves, nonequilibrium phenomenon of the rock and concrete samples during the loading and unloading process are investigated, feasibility and forecasting performance of the modified PM model proposed in this thesis are verified. The contents of the paper are summarized as follows:(1) Theory of the classic PM space model is introduced in brief. Then, effect of number of the classic mesoscopic units, density distribution of the classic mesoscopic units and load step on the stress-strain curves are investigated. In addition, inadequacies of the classic PM model are presented.(2) Based on the classic Preisach-Mayergoyz space theoretical model, viscoelastic mesoscopic unit(VMU) possessing time-dependent peculiarity is introduced to obtain the modified PM theoretical model. By adopting the modified PM space model, effect of the VMUs characteristic and density distribution of the VMUs in the modified PM space on the stress-strain curves are studied. Then, effect of the loading rate on the stress-strain curves, nonequilibrium phenomenon during the loading and unloading process under quasi-static condition is investigated. In addition, experimental approach to obtain the strain-time curve of the VMU and reconfiguration of the modified PM space is proposed.(3) Nonlinear elasticity of the concrete and Xi’an sandstone are studied experimentally by adopting the MTS815 testing system. Complete stress-strain curves are obtained to determine the nonlinear elastic range of the samples. After that, features of the stress-strain relations for the concrete and Xi’an sandstone with different loading rates, nonequilibrium phenomenon during the loading and unloading process under quasi-static are investigated. And then, creep-like tests are carried out, and strain-time curves of the samples are obtained.(4) Based on the theory of neural network, three layers BP neural network model used to inversion of the density distribution of the VMUs in the modified PM is established. By combining with the experimental result, density distribution of the VMUs in the modified PM is obtained. And stress-strain curves of the concrete sample with different loading rate are obtained from rule of density distribution of the VMUs. Stress-strain curves between theoretical and experimental result are agreed well with each other under different loading rate, which means feasibility and forecasting performance of the modified PM space model proposed in this thesis are verified. |
