材料期刊网

<正> 以往的研究结果表明,晶粒尺寸不仅影响材料的门坎值,而且也与常规机械性能特别是强度指标有一定的关系。为了解材料的门坎值、晶粒尺寸和强度之间的关系,本工作以奥氏体不锈钢为研究对象试图探索这一关系,同时用浮凸法研究晶粒尺寸对门坎值和低速扩展中裂纹顶端塑性变形的影响。

Fatigue crack propagation threshold and slow propagation near it were studied at the constant amplitude of the cyclic load ratio, R=0.5 for three-point bend specimen of the austenite stainless steel. The effect of grain size on the plastic deformation of crack tip at the threshold and at slow propagation near it were investigated using the relief method. The results show that the threshold value of crack propagation increases linearly with the square root of grain size and the rate was slower for larger grain sized material. The proportionality factors of plastic deformation zone size of crack tip and (K_(max)/σ_z)~2 for various grain size are different at the threshold. The larger the grain size, the larger is the proportionality factor. At the same time, the distinction of the plastic deformation development is the uncontinuous and unsymmetric as da/dN is from 2×10~(-8)-10(-7)mm/cyc. A dimensionless criterion of crack starting propagation, n(△K_(th)/2σ_sd~(1/2)) which is constant for various grain size was obtained after the results of the test were put in order. Its physical significance is that fatigue crack starts to propagate when the square root of the ratio of the cyclic plastic zone size and grain size equals 0.6. The metallographic and SEM observations show that the crack propagation near the threshold of austenite stainless steel proceeds on the transgranular shear mode cyclic cleavage and plasticity tear.