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前面已经提到,实际的机件总不是理想的连续体,存在着初始裂纹。由于外力的作用,在裂纹的尖端产生应力应变场,在线弹性断裂力学中,应力场用应力强度因子K来衡量。对于交变载荷,用应力强度因子振程△K来衡量。裂纹在交变载荷的作用下不断扩展,扩展的速率除了与材料的特性有关外,主要取决于应力强度因子振程。因此,在寿命估算时,应用应力强度因子振程和裂纹扩展速率的关系式来进行,由材料抗力指标的研究中已知三种关系式: 对于等幅对称循环的载荷,其关系式为:
Mentioned earlier, the actual mechanical parts are not always the ideal continuum, there are initial cracks. Due to the external force, stress and strain fields are generated at the tip of the crack. In linear elastic fracture mechanics, the stress field is measured by the stress intensity factor K. For alternating loads, the stress intensity factor vibration range △ K is used to measure. Crack under the action of alternating load continues to expand, the rate of expansion in addition to the material properties, mainly depends on the stress intensity factor vibration range. Therefore, in the estimation of life span, the relationship between the vibration amplitude of the stress intensity factor and the crack growth rate is applied. Three kinds of relations are known from the study of the material resistance index: for the load of the symmetrical symmetrical cycle, the relation is: