Stress Corrosion Cracking and Fracture Toughness of Materials with Short Cracks

研制了一种带锁相放大器的交流电气系统，用于测量高强度钢缺口试样中的裂纹试样。该系统成功地用于测量SCC过程中的裂纹长度和断裂韧性。在空气和3.5% NaCl溶液中进行了裂纹扩展试验。用x射线衍射法测量了断口附近的电阻应力。所得结果总结如下：(1)虽然短裂纹的扩展速率高于长裂纹的速率-<DELTA>K关系预测的扩展速率，但它与有效应力强度因子范围的关系是唯一的，长裂纹的关系也是相同的。(2)在空气疲劳和腐蚀疲劳状态下，断口表面的残余应力均表现为张力。根据残余应力分布确定了塑性区的最大深度。深度y与K_<max>的关系式为：alpha) (K_<max>/<delta> y) ^2(omega) y = <alpha> (K_<max>/<delta> y) ^2，其中<delta> y为拉伸试验获得的屈服强度。< α >空气疲劳为0.19，腐蚀疲劳为0.06。腐蚀疲劳< α >值较小，表明材料在塑性区由于环境影响而发生硬化。

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.