通过对两种钢热处理后得到的三种材料组织的缺口试样在常温下进行了不同预加载荷的四点弯曲正反弯实验，以在缺口前引入不同的微孔洞损伤量，而后通过高温回火处理消除残余应力和加工硬化。随后在-196℃低温下进行弯曲断裂实验，通过力学参数测量和微观观察就初始损伤对不同钢组织缺口试样解理断裂韧性的影响进行了实验研究。研究表明，对于材料A和C，随预加载荷比P0/Pgy的增加，材料中的初始损伤量增加，从而使材料的缺口解理断裂韧性降低。其原因是初始损伤的长条形孔洞附近产生了局部高应力应变集中，促使了解理断裂过程的进行。细晶粒A 材料的初始损伤量大，韧性下降幅度大。对于材料B，初始损伤主要为小尺寸的球形孔洞，并且其损伤量远小于材料A和C，故B材料的缺口解理断裂韧性几乎不随预加载荷比变化。

In this paper, a series of four point normal and reverse bending experiments are carried out on the two heat-treat microstructures of three steels, which introduce different levels of micro-cavity damage. The residual stress and work hardening are removed by tempering the specimens at high temperature. Then the specimens are fractured by four point bending at a low temperature of -196℃. By measurement of mechanical parameters and microscopic observation, the effect of initial damage on the cleavage fracture toughness of notched specimens with different microstructures of steels are experimentally investigated. The experimental results show that for material A and C, the level of damage increases with the increase of the pre-load ratio P0/Pgy, which makes the notched cleavage fracture toughness decreasing. The reason for this is that there is local high stress and strain concentration around the big string voids, which promote the cleavage fracture process. The material A with fine grains has a large amount of initial damage and a large decreasing amount of toughness. For material B, the initial damage voids are mainly little spherical shape voids, and its damage amount is largely less then that of the material A and C. So the cleavage fracture toughness of the material B does almost not change with the P0/Pgy.