砂岩三轴加载过程中力学特征与能量耗散特征

Mechanical characteristics and energy dissipation characteristics of sandstone under triaxial stress conditions

为了获得岩石加载过程力学特性与能量演化特征,开展了不同围压下砂岩力学特性试验。基于能量平衡理论,分析不同围压下砂岩加载过程能量转化规律,研究不同围压下砂岩特征应力、裂纹演化与能量耗散之间的关系。结果表明:砂岩三轴压缩加载过程中,试样的裂纹闭合应力、起裂应力、扩容应力及峰值应力均随围压增大呈线性增加;起裂应力和扩容应力可以较好的描述岩石稳定状态,起裂应力可以看作为岩石出现新生微破裂的初始应力,而岩石扩容应力可以认为是进入塑性屈服状态的标志。岩石加载过程中能量演化特征与应力-应变曲线和特征应力呈现较好的对应关系,压密阶段对应的原生裂纹压密过程能量转化率低;弹性变形及微裂纹稳定扩展阶段,外力做功转化的应变能大部分储存为弹性应变能,岩石内部损伤和塑性变形耗散的能量较小;扩容应力后的裂纹非稳定扩展阶段,岩石内部损伤和塑性变形耗散能量明显增大;峰值应力附近,积聚弹性应变能迅速转化为用于岩石破坏的耗散能。耗散比(Ud/U)随轴向应变的增加,呈现增大-减小-再增大的规律,耗散比趋势变化的转折点与裂纹闭合应力和扩容应力对应。耗散能随着轴向裂纹应变的累计逐渐增大,扩容应力前,耗散能随着轴向裂纹应变增大在低耗能水平缓慢增加;扩容应力后,伴随岩石内部裂纹的不稳定扩展、贯通,能量耗散与轴向裂纹应变近似呈线性增加趋势;峰后阶段虽然岩石承受应力降低,但裂纹持续扩展和层间滑动,能量耗散仍保持在较高水平。岩石受载变形破坏过程的能量演化可以反映岩石的损伤状态,从能量角度描述岩石的力学特性和损伤特征,对于认识岩石损伤演化具有重要意义。

In order to obtain the rock characteristics of mechanical properties and energy evolution under loading condition,the conventional triaxial compression tests are conducted under different confining pressures.Based on the energy balance theory,the energy conversion law of sandstone during loading under different confining pressures is analyzed.The relationship between characteristic stresses,crack evolution and energy dissipation of sandstone are discussed under different confining pressures.The results show that the crack closure stress,the crack initiation stress,the dilatation stress and the peak stress all increase linearly with the increase of confining pressure during the conventional triaxial compression loading of sandstone specimen.The crack initiation stress and the dilatation stress can better describe the stable state of rock.The crack initiation stress can be regarded as the initial stress for the occurrence of new micro fracture,while the dilatation stress can be regarded as the sign of entering the plastic yield state.The energy characteristics have a good correspondence with the stress-strain curve and the characteristic stresses under different confining pressures.In the stage of the micro-crack compressed,the rate of energy conversion is very low.In the stages of elastic deformation and micro-crack steady growth,most of the strain energy converted from work done of external forces is stored as elastic strain energy,and less dissipation of energy is caused by internal damage and plastic deformation of rock.In the stage of crack unsteady growth after volume dilatation stress,the dissipated energy increases significantly due to the internal damage and plastic deformation of rock.Near the peak stress,the accumulated elastic strain energy is rapidly converted into the dissipated energy for rock failure.The dissipated energy ratio(U^d/U)shows the rule of increasing-decreasing-increasing again with the increase of the axial strain.The turning points caused by the variation tendency of dissipated energy ratio correspond to crack closure stress and volume dilatation stress.Dissipated energy increases gradually with the accumulation of axial crack strain.Before the volume dilatation stress,the dissipated energy increases slowly at the low level with the increase of axial crack strain.After the volume dilatation stress,the energy dissipation increase linearly with the increase of axial crack strain caused by the unstable propagation and transfixion of cracks in rock.In the post-peak stage,although the stress decreases,the energy dissipation remains at a high level resulting from the crack continuing to expand and sliding.The energy evolution can reflect the damage state of rock in the process of deformation and failure,and it is very important to describe the mechanical properties and damage characteristics of rock from energy theory for understanding the damage evolution of rock.