The objective of this study aims at increasing the understandings of the effects of bridge length and material on surface crack growth and coalescence. Both granite and PMMA specimens containing two parallel pre-existing surface cracks were tested under uniaxial compression. Two observation systems were used including CCD camera and acoustic emission (AE) system. Results are summarized as follows: (i) shorter bridge length facilitated the growth and coalescence of surface cracks; (ii) in granite material, two pre-existing surface cracks were coalesced together by a trust movement of the rock at the bridge area when the bridge length was 16.5 mm, and by a shear displacement at the bridge area when the bridge length was 26 mm; (iii) in granite material white patches appeared as the sign of the emanation of macrocracks, while cracks in PMMA material occurred just in a sudden form; (iv) in granite material petal cracks initiated first inside the specimen, and in PMMA material wing cracks emanated first on the specimen surface; (v) petal cracks inside the granite specimens propagated from the anti-wing crack side towards the wing crack side, whereas petal cracks in the PMMA specimens extended from the wing crack side to the other tip of the pre-existing surface crack; and (vi) petal crack coalescence appeared in both granite material and PMMA material representing as a special character of 3-D cracks.
The failure of material with pre-existing fractures, such as jointed rock masses, is usually induced by the coalescence of fractures [1]. Fractures are classified into two types: two-dimensional (2-D) fractures that penetrate the whole thickness of the object and threedimensional (3-D) fractures that either fully embed inside the specimen (named 3-D internal fracture) or semi-embed inside the specimen (named 3-D surface crack).
