基于虚拟裂缝模型，针对钢筋混凝土试件在三点弯曲作用下开裂截面的受力特征，在合理假定的前提下，给出了一种计算钢筋混凝土三点弯曲梁的失稳断裂韧度的解析方法。然后，应用该方法计算了初始缝高比 α₀（初始裂缝长度与试件高度的比值）分别为0.2，0.3，0.4，0.5的三点弯曲试件的最大荷载和临界有效裂缝长度，进而求得了钢筋混凝土三点弯曲梁的失稳断裂韧度。通过对计算结果与试验数据的误差分析，发现失稳断裂韧度最大误差为4.915%，说明这种方法可以较为准确地预测三点弯曲梁的失稳断裂韧度。在此基础上研究了初始缝高比 α₀对失稳断裂韧度的影响，发现失稳断裂韧度基本上不随 α₀ 变化，失稳断裂韧度可以作为材料常数，应用于裂缝扩展状态的判断。

On the basis of fictitious crack model, an analytical method to calculate the fracture toughness of reinforced concrete was proposed under reasonable hypothesis by analyzing the mechanical characteristics at the cracked section of a specimen in three-point bending. The method was used to calculate the maximum load, critical effective crack length and in turn the fracture toughness of reinforced concrete beams in three-point bending whose  α₀ (ratio of initial crack length to specimen height) were 0.2, 0.3, 0.4 and 0.5 respectively. The maximum error of unstable fracture toughness was 4.951% by comparing calculation results with test data. The results indicated that the method could predict the unstable fracture toughness precisely. Furthermore, the impact of  α₀ on the unstable fracture toughness was studied. It was found that the unstable fracture toughness generally did not vary with α₀, thus could be regarded as material constant to determine whether a crack will propagate.