Thickness Effects on Fatigue Fracture Properties of Small Sized Materials

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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.