拉压、扭转载荷下铸铝合金的亿周疲劳性能研究

应用超声疲劳试验机对铸铝合金2-AS5U3G-Y35在扭转和拉压循环载荷下进行了超高周疲劳性能测试.介绍了超声扭转疲劳试验装置的设计.应用35 Hz常规疲劳试验机和20 kHz的超声疲劳试验机完成应力比R=-1的拉压、扭转疲劳试验,进而研究不同载荷条件、加载频率对铸铝合金超高周疲劳性能的影响.S-N曲线显示,铝合金在105～1010疲劳周次间仍发生疲劳断裂,不存在疲劳极限.断口分析表明,在超高周循环拉压载荷下,疲劳裂纹常萌生于试样次表面材料内部缩孔.与循环拉压载荷下的疲劳断裂机理不同,在循环扭转载荷下疲劳裂纹主要萌生于试样表面,疲劳断裂面为一种典型的沿试样轴向45°的螺旋面,即沿最大主应力平面断裂.扭转疲劳断面清晰的剪切条带表明扭转疲劳断裂实质上是剪切断裂.

GIGACYCLE FATIGUE BEHAVIOR OF CAST ALUMINUM IN TENSION AND TORSION LOADING

An improved understanding of fatigue behavior of a cast aluminum alloy (2-AS5U3G-Y35) in very high cycle regime is developed through the ultrasonic fatigue test in axial and torsion loading. The new developed torsion fatigue system is presented. The effects of loading condition and frequency on the very high cycle fatigue (VHCF) are investigated. The cyclic loading in axial and torsion at 35 Hz and 20 kHz with stress ratio R=-1 is used respectively to demonstrate the effect of loading condition. S-N curves show that the fatigue failure occurs in the range of 105-1010 cycles in axial or torsion loading and the asymptote of S-N curve is inclined, but no fatigue limit exists under the torsion and axial loading condition. The fatigue fracture surface shows that the fatigue crack initiates from the specimen surface subjected to the cyclic torsion loading. It is different from the fatigue fracture characteristic in axial loading in which fatigue crack initiates from subsurface defect in very high cycle regime. The fatigue initiation is on the maximum shear plane, the overall crack orientation is on a typical spiral 45° to the fracture plane and it is the maximum principle stress plane. The clear shear strip in the torsion fatigue fracture surface shows that the torsion fracture is the shear fracture.