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-An investigation of the relationship between the resistances & and 6, during crack growth in several medium strength steels is reported in this paper. The results show that we can find out some reasonable rules governing their relationship only after analysing the plastic components of the resistances Jp and Sp. As can be seen from the tests, the &/,lss -S, relationship is linear within the range of crack growth amount investigated and is probably affected by the material property and the specimen geometry. A comparison between P -V and P -h curves made by the plastic hinge model shows that the two kinds of records are essentially equi~~alent. Therefore, the compatibility between I-integral and COD is proved by extensive experiments. NOTATION yield strength (kg/mm') tensile strength (~~rnrn') true stress at fracture (kg/mm2), which is corrected for necking work-hardening exponent up to necking work-hardening exponent from necking to fracture specimen thickness (mm) specimen width (mm) total crack length (mm) loading span in three-point bend test (mm) applied load (kg) as subscript, plastic value clip gauge opening displacement (mm) load point displacement (mm) value of COD (mm) resistance represented with J (kg/mm) resistance represented with S (mm) model I stress intensity factor (kg/mm"? = (E/l -2) = 23,MO kg/mm' for tested steels in present work work of deformation (kg-mm) coefficient of the dimensionless stress intensity factor as subscript, elastic value rotational factor duricg plastic deformation in a bend test height of clip gauge mounted on abutments (mm) stress cYY at crack tip A (kg/mm*) limit load = 1.456(8( W-a)'/S)u3 (kg)