Growth and coalescence of small crack on glass surface immersed in water

Not only the coalescence of two coplanar cracks and the growth of single crack but also the coalescence of three coplanar and two noncoplanar cracks were investigated here by the method of increasing K₁. The glass samples used were 3.0mm in thickness, 20mm in width, and their composition (wt-%) were SiO₂ 71.6, Al₂O₃ 1.0, CaO9.8, MgO4.3, Na₂O13.3, SO₃0.2. The results show: (1) For one, two, three coplanar or noncoplanar cracks, it is always true that crack propagation rate da/dt first drops to a low point and then increases with increasing K. (2) There are important differences between coplanar and noncoplanar cracks. For noncoplanar cracks, not only stress superposition but also stress relaxation can occur.