Uniaxial compression test is a useful method of determining the mechanism of rock failure. However, the direct observation of microcracks during the propagation of stress-induced cracks is difficult, since it is usually difficult to distinguish these microcracks from those by other sources during microscopic examination. In this study, the microcracks were visualized using a new fluorescent approach combined with image processing, and then the propagation of stress-induced microcracks was analyzed. The following results were obtained: (1) Many hidden microcracks which were not detected under natural light, were visualized clearly via ultraviolet light. Most hidden microcracks were composed of intercrystalline cracks. (2) Actual microcracks in the specimens can be represented realistically through several stages of image processing, such as Laplacian operator (matrix size = 9x9) and line detection operator. After this operation, the quantitative analysis was performed. (3) Microcracks found in the damaged specimens can be divided into three categories
microcracks parallel to the axial stress, microcracks normal to the axial stress, and microcracks inclined to the axial stress, based on the microcrack direction measured on image analysis. As rock failure proceeded, the microcrack length in all directions decreased gradually. At the ultimate stage of rock failure, the microcracks parallel to the axial stress increased rapidly, while those normal or inclined to the compressive axis showed no remarkable change. The occurrence of microcracks demonstrated by image analysis agreed with the P-wave velocity determined by the usual method. It was concluded that the reduction of P-wave velocity is mainly caused by an increase in microcracks parallel and inclined to the axial stress observed under microscopy using fluorescent resins. © 1995, The Society of Materials Science, Japan. All rights reserved.