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钢梁腹板纵向纤维受压区和中性轴附近纯剪区疲劳裂缝的出现很难用正截面疲劳应力幅是否超标来判别。本文对受疲劳荷载铜梁腹板中部纯剪区、上部受压区及下部受拉区的应力单元进行了分析,揭示了因轮压滚动引起单元体剪应力变号,从而引起主应力面拉、压应力突变,造成很大的疲劳应力幅。因而,对于腹板剪应力水平较高的梁,存在较大的危险性。同时,本文说明了因剪应力变号而发生的腹板疲劳裂缝,其扩展机理不同于因拉应力幅而引起的翼缘疲劳裂缝的扩展机理,前者属于渐进型,后者属于扩张型。
The occurrence of fatigue cracks in the pure fiber shear zone around the longitudinal fiber compression zone and the neutral axis of the steel web is difficult to judge by whether the fatigue stress amplitude of the normal section is exceeded or not. In this paper, the stress element of the pure shear zone, the upper compression zone and the lower tension zone in the central part of the web under fatigue load are analyzed. It is revealed that the shear stress of the element body changes due to wheel rolling, , Sudden changes in compressive stress, resulting in a large fatigue stress amplitude. Therefore, there is a big danger to beams with high shear stress on webs. At the same time, this paper illustrates the fatigue crack of the web caused by the change of shear stress, which is different from the expansion mechanism of the fatigue crack caused by the tensile stress amplitude. The former belongs to the progressive type and the latter belongs to the expanding type.