Stress Corrosion Cracking and Fracture Toughness of Materials with Short Cracks

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An AC electrical system with a lock-in amplifier was developed to measure the speciemen of crack in notched specimens of a high strength steel. This system was succesfully used to measure the crack length during SCC and fracture toughnesstests. Crack propagation tests were conducted both in air and in 3.5% NaCl solution. Resisual stress near the fracture surface were measured by X-Ray diffraction method. The results obtained are surmarized as follows;(1) Although the growth rate of short cracks was higher than that predicted from the rate-<DELTA>K relation for long cracks, it was uniquely related to the effective stress intensity factor range and the relation was identical for long cracks.(2) The residual stress measured on the fracture surface was tention both in air-fatigue and corrosion fatigue. The maximum depth of the plastic zone was evaluated on the basis of the residual stress distribution. The depth y is related to K _<max>by the following equation:alpha) ( K_<max>/<delta> y )^2(omega) y = <alpha> (K_<max>/<delta> y) ^2where <delta> y is the yield strength obtained in tension tests. <alpha> is 0.19 for air fatigue and 0.06 for corrosion fatigue. The small value of <alpha> in corrosion fatigue suggests the hardening of the material in the plastic zone due to the environmental effect.