Thickness Effects on Fatigue Fracture Properties of Small Sized Materials

提出了一种新的疲劳试验方法，该方法可以在推拉循环载荷作用下，使附着在椭圆孔和圆孔上的薄膜产生疲劳裂纹扩展。利用边界元法和有限元法对该试验进行了分析，结果表明，即使考虑了晶粒的结晶取向，应力也不会因为在薄膜中的位置而发生很大的变化，并且在裂纹扩展的大部分时间内应力强度因子几乎是恒定的。在使用100 μ m厚的工业级铁膜的试验结果中，疲劳裂纹扩展速率用应力强度因子范围的幂律表示。然而，在薄膜上留下了相当大的残余塑性层。也就是说，有效应力强度因子范围是控制膜状和体状试样疲劳裂纹扩展的良好参数。此外，使用相同厚度为100、50和40-mum的铁膜，不同应力幅值下的疲劳裂纹扩展速率数据集中在一个窄带内，显示出应力强度因子范围的幂律，但严格地说，较薄的铁膜略高。结果表明，在相同的有效应力强度因子范围内，疲劳裂纹扩展速率较大，膜越薄，滑移/裂纹起裂寿命越短。在相同膜厚t_f=100mum的情况下，膜的疲劳寿命随晶粒尺寸的减小而减小。

A new fatigue testing method was proposed by which fatigue crack propagation could be caused in a film attached to elliptical and circular holes on a base plate subjected to push-pull cyclic loads. Using a BEM and FEM,analytical results for this testing showed that the stress has not a great change due to the place in a film even though a crystallographic orientation of grain was considered, and the stress intensity factor was almost constant during the greater part of crack propagation. In the experimental results with the use of commercial-grade iron films of 100-mum thickness, the fatigue crack propagation rate was expressed by a power law of stress intensity factor range. However, a considerable residual plastic layr was left in the wake of propagation cracks on the film. In other words, effective stress intensity factor range was a good parameter controlling the fatigue crack growth in both film and bulk specimens. In addtion, using the same iron films of 100,50 and 40-mum thickness, the data of fatigue crack propagation rate under various stress amplitudes gathered together in a narrow band showing a power law of stress intensity factor range, but strictly speaking a little higher side for the thinner film. It was found that the fatigue crack propagation rate was larger under the same effective stress intensity factor range and the slip/crack initiation life was shorter for the thinner film. Also, the fatigue life was apt to decrease on the film with the smaller grain size under the same film thickness of t_f=100mum.