本文用三点弯曲和四点弯曲试样,在K_Ⅱ/K_Ⅰ=0—14的范围内,对GC-4超高强度钢、30Cr2MoV中强度转子钢和稀土球墨铸铁进行了线弹性平面应变复合型脆断试验。结果表明:复合型裂纹的脆断开裂方向与现有的三种复合型理论符合较好;但是,复合型裂纹扩展阻力随K_Ⅱ/K_Ⅰ比值增加而增大,与现有复合型理论差别较大。 本文从塑性区的形状、大小和裂纹顶端在开裂方向的应力状态等方面对此问题进行了分析并指出,现有理论关于复合型裂纹扩展阻力与变形特征无关的假设,对于具有一定塑性变形能力的金属材料,是不合适的。文章还对简单且偏于安全的处理复合型问题的工程分析方法作了讨论。
The extention of the K_Ⅰ-K_Ⅱ composite mode (K_Ⅱ/K_Ⅰ=0-14) cracks has been inves-tigated by means of three-point and four-point bend specimens of high or mediumstrength steels as well as of nodular cast iron. All specimens were broken withbrittle fractures under linear elastic plane strain conditions, and the extending di-rection of the cracks appeared to agree well with the three existing theories, e.g.,the theory of maximum stress, the theory of strain energy and the theory of ener-gy releasing rate. The crack extension resistance, however, was found to increasewith the increase of K_Ⅱ-K_Ⅰ ratio, thus deviating obviously from the theoreticalprediction. This finding has been investigated with regard to the following aspects,e.g., the configuration and the size of the plastic zone, the state of stress at cracktips, etc. and in addition, the assumption that the crack extension resistance isindependent of the deformation characteristics was found to be untenable for speci-mens undergoing plastic deformation. Attempts have also been made to give a ten-tative discussion on a simple and reliable analytical method to deal with composi-te mode cracks for practical purpose.
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