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作为对海浪引起的随机载荷下船舶结构部件的冲击疲劳的基础研究,对平均应力对低碳钢冲击疲劳强度的影响进行了试验性研究。在系统地改变应力比的交变轴向载荷条件下,对四种具有应力集中系数K_t=1～4.7的圆棒进行了低周冲击疲劳试验。还对这些同样试样进行了普通疲劳试验,以便与上述试验结果进行比较。试验结果归纳如下: (1)光滑试样的断裂过程是蠕变型的,而具有大K_t值的缺口试样的断裂是开裂型的,与应力比R(最小应力■与最大应力■之比)无关。具有小K_t值的试样的断裂随应力大小和应力比的不同而不同。 (2)在蠕变型情况下,永久变形速率■和断裂循环数N_f之间的关系与R无关,但应力幅度■(=(■(1-R)/2)和N_f之间的关系明显地取决于R。在其它情况下,■和裂纹发生循环数N_c以及断裂循环数N也明显取决于R。 (3)由上述结果可以发现,除具有很大K_t值的缺口试样以外,对每种试样应力幅度■和平均应力■之间关系都可写成如下形式:式中:■应力比R=-O.8的应力幅度;■:每种试样的最大应力 (4)由普通疲劳试验获得的疲劳性状几乎和由冲击疲劳试验获得的疲劳性状相同,但应力比对疲劳性状的影响程度低于冲击疲劳试验的情形。
As a basic research on the impact fatigue of structural parts of ships under random loading caused by waves, the impact of average stress on the impact fatigue strength of mild steel has been studied. Low-cycle impact fatigue tests were carried out on four round bars with stress concentration factor K_t = 1 ~ 4.7 under alternating axial load with systematically changing stress ratio. The same samples were also subjected to normal fatigue tests to compare with the above test results. The test results are summarized as follows: (1) The fracture process of the smooth specimen is creep-type, while the fracture of the notched specimen with large K_t value is cracking type and the ratio of stress ratio R (minimum stress 与 and maximum stress ■ Has nothing to do. The fracture of a specimen with a small K_t value will vary with stress magnitude and stress ratio. (2) In the case of creep, the relationship between the permanent deformation rate and the number of fracture cycles, N_f, has nothing to do with R, but the relationship between the stress amplitude, (= (■ (1-R) / 2) and N_f is significant Depends on R. In other cases, the number of cycles of occurrence of cracks and the number of cycles N_c and the number of cycles of fracture N also depend significantly on R. From the above results, it can be found that in addition to notched samples having a large K_t value, The relationship between the stress amplitude of each sample and the average stress can be written as follows: where: ■ stress ratio R = stress amplitude of -0.8 ■: maximum stress per specimen (4) The fatigue properties obtained by the fatigue test are almost the same as the fatigue properties obtained by the impact fatigue test but the impact of the stress ratio on the fatigue properties is lower than that of the impact fatigue test.