In virtue of a self -invented centring clamp and load separation principle, the estimation for the fracture toughness of circumferentially-cracked round bar was carried out. On the basis of finite element analyses and compliance method, uniform formulas for crack length measurement are presented, in which the effect of elastic modulus, specimen diameter, gage length and crack depth is under consideration. In this paper, two extensometers are employed to measure axial displacement, the final crack length is determined by use of equivalent area method, and consequently, the effect of eccentricity of crack growth is eliminated. Finite element analyses confirm that the load separation principle is applicable for the circumferentially-cracked round bar, and then the detailed expression of geometry function for this specimen configuration is proposed. With normalization method, the JR curves of these specimens for 16Mn are obtained. The results show that the curves obtained by the circumferentially-cracked round bars are close to those by standard single edged notched bending (SEB) specimens.%借助自制对中夹头和载荷分离法成功实现了周向裂纹圆棒试样的断裂韧性测试.基于有限元分析和柔度法得到了考虑弹性模量、试样直径、测试标距及裂纹深度等影响的统一裂纹长度预测公式；采用双引伸计测轴向位移、等面积法计算最终裂纹长度,解决了裂纹扩展偏心所带来的影响；有限元模拟分析证明了载荷分离理论适用于周向裂纹圆棒试样,并提出了几何函数G(a/R)的具体表达式；采用规则化方法获得了16Mn周向裂纹圆棒试样的JR阻力曲线.结果表明:由周向裂纹圆棒试样测得的结果与标准三点弯曲试样得到的结果接近,分散性小.
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