| Summary: | 碩士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 91 === This study aims to explore the effects of sampling direction, artificial aging and stretch treatment on the resonant vibration properties of the commercial 7075 alloy. The influence of crack propagation characteristics on the vibration life was also investigated.
Experimental results indicate that different sampling direction caused the difference in crack propagation rate. The RD (rolling direction) samples which show the lowest crack growth rate exhibit the greatest vibration life. The crack propagation path of the specimens is almost transgranular regardless of the sampling direction.
The results also show that under constant force conditions the initial deflection in a decreasing order and the vibration life in an increasing order are the peak-aged (PA), solution-treated (ST), under-aged (UA) and over-aged (OA) specimens. That is, the specimen with a greater damping capacity possesses a higher vibration fracture resistance under constant-force conditions. On the other hand, under constant initial deflection conditions the vibration life is proportional to the yield strength of the specimen. In addition, the major cracks of the ST specimens propagate mainly transgranularly, and the crack growth along the persistent slip bands can also be observed occasionally. As for other samples, main crack propagates transgranularly but the slip band cracking feature is absent. Notably, grain boundaries don’t affected the growth of the major crack of the ST and UA specimens, but slightly change the propagation direction of the cracks of the PA and OA samples.
Worthy of notice is that, the vibration life of the 7075-T651 samples undergoing stretch and peak aging treatments possess a lower vibration life in comparison with aforementioned artificially aged samples without the pre-strain under constant force conditions. This result suggests that the stretched 7075 alloys are not suitable to be applied under the vibration environment.
|
|---|
