The state of stress around crack tip under different loadings was studied. To judge what fracture mode will occur under given loading is vital important. A prerequisite of possible occurrence of mode I or mode II fracture is proposed. The study shows that only pure mode I fracture could occur under tensile, shear and mixed loading. The prediction of K11/K1C by maximum circumferential tensile theory should not be interpreted as K11C/K1C but as . The predicted K11c/K1c < 1 is incorrect.
Classical fracture mechanics always distinguish three basic models from point of crack surface displacement view: mode 1 (Opening mode), mode II (sliding mode) and mode III (tearing mode). The stress intensity factors and its critical values (as called fracture toughness) associated with each mode are labeled by K1, K2, K3 and K1c, K2c, K3c respectively. In addition, fracture mechanics always relates three basic modes of fracture to loading mode as shown in Fig.l.(Lawn & Wilshaw, 1975) when researchers investigate mode 1 fracture under tensile load, they achieved fruitful results. Numbers or study works and papers are related to mode 1 fracture. There are standard methods for determining mode 1 fracture toughness for different materials. Considering mode II fracture, the picture is completely different. Since 60ty to 90ty years the shear fracture tests of different materials, such as plexiglas (Erdogan & Shi, 1963), rock (granite, limestone, shale, basalt, marble and syenite) (Ingraffea, 1981, Huang & Wang, 1985), concrete (Swartz et al, 1988), ceramic (Li & Sakai, 1996), PMMA (Ewing & Williams, 1974) and 4340 steel (Shah, 1974), were conducted with different specimens configuration, such as a large plate with central crack (Erdogan & Shi, 1963), antisymmetric four point bending specimens, edge cracked Arcan specimen (Banks-Sills, et al, 1984), short beam compression specimen (Watkin & Lin, 1985), and compact tension-shear specimen (Richard. 1984). All test results showed that the fracture angles measured were in range of 60- 70 deg. The ratio of K11c I K1C was less than 1. Present fracture theories of mixed loading, for example, the maximum circumferential tensile stress theory (Erdogan & Shi, 1963), the maximum energy release rate theory (Hussain, et al, 1974, Ichikawa & Tanaka, 1982) and the minimum strain energy density theory (Shi, 1974), predicted crack fracture angle under shear loading being -70.5, -75.6 and -70--- 80(v=0.2–0.3) degree apart from original crack plane and predicted ratio ofK11c/K1c, equal to 0.866, 0.82(Awaji, 1998) and 0.96 respectively. All facture theories aimed at prediction of direction of crack extension (fracture angle) and offering criterion for initiation of crack extension. But they did not show any concern for mode of fracture under different load conditions. Thus mode II fracture tests lasted nearly 40 years off and on, the experiments agreed well with present theories, but a series of questions have not been solved, regarding crack branching and ratio of K11c/K1c being less than 1. Maximum circumferential stress theory considered the fracture mode under mixed loading as mode I fracture.
