| There are some problems in the test of the fracture mechanics of metal material. Although many investigators tried to solve them, the diversed fracture criteria are not able to solve all fracture problelms of materials. This paper investigated the nature of the problems in the test of fracture toughness by the theoritical foundation and test results, and obtained that the fracture toughness KIC only reflects the performance of traction fracture, while JIC represents the performance of shear fracture of small size specimen of ductile material, and that the conversion of JIC to KIC is not suitable and that the thickness B≥2.5(KIC/σY)2 in the codes shouldn't be the basis of specimen size.The fracture criteria based on stress triaxicity Rσcould be used to differentiate fracture the modes and mechanism of materials under variable stress. This paper designed a new experimental method to measure the size of the necking segment of specimen, then analyzed several typical stress results. The dividing value R2 of tress triaxicity between traction fracture and shear fracture of three materials were calculated firstly.The experimental method for fracture toughness of planar strain was also studied in the paper. The traction fracture criteria index is determined quantitively which is 80% and verified by the existent experimental data. Thus a reasonable forcasting method for selection of B in determing fracture toughness KIC is put forward..
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