超声振动载荷下LY12合金的超高周疲劳性能研究

简述了超声疲劳试验原理，通过有限元计算及动态模态分析方法，研究了试件在振动过程中应力和振动位移的分布。应用超声疲劳试验技术研究了LY12合金在超声振动载荷(f=20kHz，R=-1)下的超高周疲劳性能。与常规疲劳载荷(f=30Hz)的疲劳性能做了对比分析。结果表明，在10^5～10^7周次之间，超声疲劳与常规疲劳载荷下的试验结果一致，在10^7～10^9周次之间，曲线仍有下降趋势，而未呈现水平。从扫描电镜分析10^7～10^9周之间发生疲劳断裂的试件发现：(1)多数试件的疲劳破坏源于试件表面缺陷；(2)对于一些无明显表面缺陷的试件，在超高周循环下，疲劳破坏会从试件次表面处发生。超高周循环下，表面缺陷和材料内部缺陷都会引起疲劳破坏。

On Fatigue Life in Very High Cycle Range(107～109) of Chinese LY12 Aluminum Alloy

Our experimental study made use of special equipment in Southwest Jiaotong University; the main part of this equipment had been imported from France and can perform rapid fatigue testing through applying ultrasonic vibratory loading (tension compression) on specimens. Rapid testing allows us to study fatigue life of Chinese LY12 aluminum alloy (approximately equivalent to ASTM 24ST aluminum alloy) specimens in very high cycle range (10 7~10 9); such exceedingly high number of cycles should be investigated according to the opinion of many researchers. We studied the fatigue capability of LY12 aluminum alloy with ultrasonic fatigue testing technique at 20 kHz. S-N curves show that the results for 20 kHz testing agree quite well with those for conventional 30 Hz testing in the 10 5~10 7 cycle range; in 10 7~10 9 cycle range, S-N curve is not a horizontal line as quite often assumed; instead, S drops very slowly with increasing N . Analysis of fatigue surfaces of specimens that failed in 10 7~10 9 cycle range reveals that: (1) most cracks initiate at surface defects of specimens; (2) for some specimens without surface defects, failures initiate at defects just under surface. So, for Chinese LY12 aluminum alloy, fatigue is sensitive to defect in the specimen, either on surface or just under surface.