Positive and negative bending experiment for notched specimens of low alloy hot rolled steel 16MnR with different pre-loads were carried out at room temperature to introduce strip micro-crakes with different dimensions in front of notch. And then the residual stress and work hardening were eliminated through high temperature tempering. Four points bending fracture experiment was carried out at --196 ~C. And effect of stripe micro-crake dimension on cleavage fracture toughness of notched specimens was investigated. Results show that when the pre-load ratio Po/Pgy~O. 861, the pre-load had little effect on the cleavage fracture notch toughness; when P0/Pgy~0, 861, the strip defect was torn and formed the micro cracks under applied loads. With the increase of P0/ Pgy, the number of strip micro-cracks increased, and the size as well as the distribution region was enlarged, too. Fracture load and fracture absorbed energy rapidly decreased with pre-load ratio increased. The increase of high stress-strain region caused by the strip micro-cracks made the distribution of cleavage initiation increase, as the result, numeric values of material notch toughness Pf and W were discreted.%在常温下对热轧16MnR低合金钢缺口试样进行了不同预加载荷的正反弯曲试验，以在缺口前引入不同尺寸的条形微裂纹，然后通过高温回火消除残余应力和加工硬化，随后在-196C下进行4点弯曲断裂试验，研究了条形微裂纹尺寸对缺口试样解理断裂韧性的影响。结果表明：当预载荷比（P0／P）小于0．861时，预载荷对解理断裂韧性影响较小；当预载荷比大于0．861后，条形缺陷在外加载荷作用下撕裂而形成的条形微裂纹，并且随预载荷比的增大条形微裂纹数量增多、尺寸增大、分布区域扩大，断裂载荷、断裂吸收功随预载荷比的增加而迅速下降；条形微裂纹所引起的局部高应力应变范围增大，使解理起裂源分布范围增大，也造成了缺口韧性参数P1和W数值的分散。
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