Thickness Effects on Fatigue Fracture Properties of Small Sized Materials

提出了一种新的疲劳试验方法，在推拉循环载荷作用下，可以在底板上的椭圆孔和圆孔上形成疲劳裂纹扩展的薄膜。用边界元和有限元方法对试验结果进行了分析，结果表明，即使考虑了晶粒的结晶取向，应力也不会因为薄膜中的位置而发生很大的变化，在裂纹扩展的大部分过程中应力强度因子几乎是恒定的。在使用100微米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函数来表示。然而，薄膜上的扩展裂纹留下了相当大的塑料残留层。换言之，有效应力强度因子范围是控制薄膜和块体疲劳裂纹扩展的良好参数。此外，采用相同厚度的100、50和40微米的铁膜，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，呈现出应力强度因子范围的幂规律，但严格地说，较薄的铁膜的应力强度因子范围略高。结果表明，在相同的有效应力强度因子范围内，薄膜越薄，疲劳裂纹扩展速率越大，滑移/裂纹萌生寿命越短。当薄膜厚度tf=100um时，晶粒尺寸越小的薄膜，疲劳寿命越低。

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.