低碳钢经拉压循环后的剩余强度、延性与表面损伤

Study of residual strength,ductility and surface damage of low-carbon steel after cyclic tension and compression action

对0.5%恒应变幅下事先经历了不同次数疲劳循环的Q235钢圆棒试样进行单轴拉破坏试验,观察到与未经疲劳循环试样相同的杯锥形断口;延伸率和断面收缩率随循环次数增加有所下降,但剩余强度反而提高。经历循环数N/Nf达到一定数值后,试样拉伸颈缩段表面出现表面损伤导致的分布裂隙。经历循环数愈多,裂隙愈密集,而未经疲劳循环试样则没有这样的裂隙。将试样进行表层磨削和抛光,再进行单轴拉断,颈缩区域表层分布裂隙不再出现,但延伸率和收缩率并不因此有明显改变。本文研究表明：Q235钢疲劳循环试样的表层疲劳损伤甚于内部疲劳损伤,其疲劳损伤不适合用剩余强度来描述,但可用经拉伸后出现于颈缩部位的表面裂隙来表征。

Uniaxial tensile failure experiment was carried out for Q235 steel round bar specimens,which are in advance subjected to different cyclic numbers N/Nf（5%~75%）fatigue cyclic action and under the condition of 0.5% constant strain amplitude.The same cup cone fracture was observed compared with the specimen without in advance fatigue cyclic action.The elongation rate and the reduction rate of cross-section area decrease with the increase of cycle numbers,but the residual strength increases.After the cycle number reaches a certain value,the distributed cracks due to surface damage appear on the tensile necking section of specimen.The more the cycles number is,the denser the cracks,and the specimens without fatigue cyclic action does not have such cracks.If uniaxial tensile failure experiment was carried after grinding and polishing the specimen surface,the distributed cracks on the tensile necking section area are no longer appeared,but the elongation rate and the reduction rate of cross-section area do not change obviously.Above results show that Q235 steel specimen surface fatigue damage is more severer than the internal fatigue damage.The residual strength is not suitable to describe fatigue damage.But it can be characterized by the surface cracks appearing in the necking part after tension action.