Abstract:

The electric potential field calculations under all useful current boundary conditions for compact tension specimen show that varying current input point on boundary evidently affects the electric potential drop-crack extension calibration function. Therefore, although Johnson's formula can be applied to three-point bend specimen, it can't be adapted to compact tension specimen as the current input location is at different point on its boundary.The initial electric potential drop V0 is constant for a given geometry and current boundary condition, but for different current boundary conditions V0 may change greatly, so the relative sensitivity for the latter situation must be based on by directly and not on the normalized exporssion V/V0, then we can get the preferential order for choosing the current input point, i e. from crack mouth along front face, tip/bottom face and finally to back face. This order also agrees with the order of current density intensity near the notch root or crack tip.Of course, when making one's option in practice it is also necessary to consider technical feasibility and convenience. Since both geometry and current boundary condition influence the calibration function, the applied function had better be offered by finite element method individually after making the preferential option mentioned above.