低频循环荷载下盐岩轴向蠕变的Burgers模型分析

Analysis of axial creep properties of salt rock under low frequency cyclic loading using Burgers model

为了研究盐岩在低频循环荷载作用下的蠕变特性,采用恒轴压、循环围压的应力加载方式对盐岩开展了低频循环荷载蠕变试验。试验结果显示,当蠕变时间较短时,盐岩轴向蠕变曲线近似为平滑曲线,波动性不大;当时间增加到一定程度以后,盐岩变形随围压变化而发生明显的波动。总体来看,循环荷载下盐岩轴向蠕变曲线与Burgers模型蠕变曲线较为相似。将Burgers模型分解为Maxwell模型和Kelvin模型,忽略材料的疲劳效应,分别推导了恒轴压、循环围压应力加载方式下Maxwell模型和Kelvin模型的轴向蠕变方程,分析了荷载循环周期对模型蠕变性的影响规律;将循环荷载下Maxwell模型和Kelvin模型的轴向蠕变方程进行叠加,得到了循环荷载下Burgers模型的轴向蠕变方程,并利用盐岩蠕变试验结果对其合理性进行了验证。结果表明:拟合曲线和试验曲线吻合良好,该模型能够较好地反映盐岩在循环荷载作用下的蠕变特性和荷载变化对盐岩蠕变性的重要影响,特别是能够反映出不同荷载循环周期下盐岩变形随荷载变化而发生的明显的波动现象。

In order to study the creep properties of salt rock under low frequency cyclic loading, creep tests on salt rock samples under low frequency cyclic loading are carried out by adopting the loading mode of cyclic confining pressure with a constant axial stress. The test results show that up to a certain time, the axial creep curves of salt rock are smooth; while after that point, the axial creep curves clearly fluctuate with the confining pressure. Overall, the axial creep curves of salt rock under low frequency cyclic loading are similar to that of Burgers model. According to the connection relation of component creep model, the Burgers model is divided into Maxwell model and Kelvin model. The axial creep equations of Maxwell model and Kelvin model under the loading mode of cyclic confining pressure with a constant axial stress are deduced respectively in the superposition condition that the fatigue failure of material is neglected, and the effect of loading cycle on the models are also analyzed. Then, by combining the axial creep equations of Maxwell model and Kelvin model, the axial creep equation of Burgers model under cyclic loading is obtained; and the reasonability of which is verified by using the creep test data of salt rock. The result indicates that fitting curves and test curves conform well each other; and the proposed model can well reflect the creep properties of salt rock under cyclic loading, especially the fluctuation of creep deformation with loading change.