中文摘要：

建立了声波监测系统，探测颗粒体系在直剪过程中声速与剪切应力的变化关系．发现声速在剪切开始后随着剪切应力的增加会迅速减小．而在应力达到屈服点之后，声速变化逐渐变缓并趋近于～个稳定值．剪切作用对声速变化的影响小于20％，而非所预测由于剪切带的形成，声波（尤其是横向波）的振动无法通过剪切带而使得探测的飞行速度变为零．这是由于直剪的应变率甚低于声波的频率，直剪的颗粒体系可被视为局部区域膨胀的准静态的弹塑固体．结合等效介质理论和颗粒弹性理论对以上结果进行了解释和讨论．

英文摘要：

Acoustic signal is used to study the mechanical properties of dense granular system under direct shear. In the process of direct shearing, it is found that the velocity of the sound wave decreases rapidly with the increase of shear stress. After the yield point of stress, with the increase of strain, the velocity of the propagating wave decreases gradually and attains a constant. The net reduction in velocity is found to be less than 20%, which differs from the fact that acoustic wave （especially the transverse wave） could not propagate through the shear band. This is because the direct shear strain rate is far smaller than the frequency of sound wave, and particles under shear can be regarded as quasi-static elastic-plastic solid. Effective medium theory and granular dlastic theory are employed for explaining these results.