岩样失稳回跳与直剪试验机-岩样系统失稳回跳关系研究

研究了岩样及直剪试验机-岩样系统的弹性回跳不稳定性。将试验机简化为具有一定高度和剪切模量的钢块，利用梯度塑性理论得到了直剪试验机．岩样系统的剪应力．剪应变的理论关系。当不考虑钢块的高度时，这一关系便蜕化为岩样的剪应力-平均剪应变的理论关系。如果试验机-岩样系统回跳，那么岩样可能失稳回跳，也可能不失稳回跳；当钢块高度及剪切模量比值越大，试验机-岩样系统越容易发生失稳回跳。当这一比值较大时，纵然岩样不回跳，系统也会回跳：当这一比值较小时，只有当岩样回跳，系统才会回跳；当系统不回跳时，试样必不回跳。试样回跳，必然导致系统回跳：如果试样不回跳，则系统可能回跳，也可能不回跳。理论结果可解释若干常见的实验现象。

ANALYSIS ON RELATION BETWEEN SNAP-BACK OF SPECIMEN AND SNAP-BACK OF SYSTEM COMPOSED OF DIRECT SHEAR TESTING MACHINE AND SPECIMEN

Snap-back instability of rock specimen and testing machinespecimen system are investigated. The direct shear testing machine is simplified as a steel block with certain length and elastic shear modulus. Based on gradient-dependent plasticity,the localized shear band is initiated at peak shear stress and a closed-form analytical solution on shear stress-shear strain curve in strain-softening stage is proposed. If the length of block is zero,the simplified solution can be applied to rock specimen subjected to direct shear. If snap-back of the system occurs, the structural response of the specimen is uncertain (namely,both snap-back and snap-through are possible). When the ratio of length-to-shear elastic modulus increases,the system can lose its stability easily. For the larger ratio, though snap-back of the specimen does not occur,the system will be unstable certainly. But,for the lower ratio, only if the specimen exhibits snap-back,snap-back of the system is possible. Besides,the snap-back of a specimen must lead to snap-back instability of the system. If snap-back is impossible for the specimen,both snap-back instability and snap-through are possible for the system. Some experimental results on rock failure are explained based on the presented theoretical analysis,though the experimental results seem difficult in understanding.