破裂带发展的扫描电镜实验研究及其对地震前兆的启示

SEM STUDY OF FRACTURE DEVELOPMENT IN COMPRESSED MARBLE SPECIMEN AND IMPLICATIONS FOR EARTHQUAKE PRECURSORS

在扫描电镜下对带割缝的大理岩试件进行了单轴压缩实验.在加载的全过程中,观察了试件表面裂纹的产生、扩展、平面分布以及断裂扩展前缘处的微裂纹在主断裂扩展前后的开、闭变化,并分析了它们与外载之间的关系.在显微照片上量测了主断裂的宽度随外载水平及测点所在部位的变化规律.在实验测定和结果分析的基础上,解释了脆性岩石材料在宏观裂纹扩展时的断裂能明显偏离线弹性断裂力学预测值的原因,提出了以标量作为岩石损伤力学中的损伤量的设想,探讨了实验结果对了解地震前兆(如大地形变和地下水位异常)可能起到的作用.将实验观察到的微破裂活动与1975年海城7.3级地震前地下水位异常做了对比分析,发现两者在时空分布方面有许多相似之处.

Uniaxial compression tests of marble plates each containing a slot were conducted under direct SEM observation. The specimen was 20× 10×2.5 mm3 in size. The slat was 7 mm long and 0.44 mm wide and cut through the specimen normally at an inclination of 45° to the loading axis. The initiation, propagation, plan distribution and coalescence of cracks on the specimen surface were monitored throughout the whole test process. From the microphotog-raghs taken at different stages of loading, the opening and closure of microcracks in the frontier area of a propagating fracture were studied, and the variations of fracture width with the loading level and with the position of measuring point were analysed. Based upon the test results, the significant deviation of the fracture energy needed to propagate a macrocrack in brittle rock materials from the value estimated by the theory of linear elastic fracture mechanics was explained, the concept of using a scalar as the damage parameter in rock damage mechanics was put forward and the possible significance of the test results for understanding various earthquake precursors, such as crustal deformation and groundwater anomalies, etc., was investigated. By comparing the test results with the precursory anomalies of groundwater level prior to the 1975 Haicheng earthquake (Ms = 7.3), it was found that the temporal-spatial distribution of groundwater level anomalies had showed many features closely resembling the sequence of crack activities observed in our test.