Papers by Author: Kenichi Shimizu

Abstract: Using a fatigue testing method by which fatigue cracks can be initiated and propagated in a film adhered to cover a circular through-hole in a base plate subjected to cyclic loads, annealed copper films of 100m thickness with different crystal grain sizes were fatigued. The fatigue crack propagation in the film with large grains was often decelerated, so the crack propagation rate of the film with the large grain was lower than that of the film with the small grain. When the crack propagation was decelerated, the crack opening displacement obtained from the film with large grain size was smaller than that obtained from the film with small grain size. The relationship between the fatigue crack propagation rate and the stress intensity factor estimated from the crack opening displacement was identical for the cracks in the film with the large grain and the small grain.

Abstract: Laminated copper films of the epoxy-bonded or diffusion-bonded to the base metal were used in order to investigate and analyze film fatigue behavior depending on the inevitable bonding interfaces for electric/functional parts used in MEMS. Fatigue damage was observed using SEM and crack initiation lives were evaluated at the notch root where the bonding interfaces could be observed directly through the thickness. These observations showed that the resin interface layer caused cracks without slips in a zig-zig pattern and also decreased fatigue crack initiation lives. On the other hand, fatigue damage was observed using an optical microscope on surface of the film with resin bonding or with diffusion bonded interfaces. In this case, many cracks were caused and propagated towards the width direction on the film bonded to the base steel with resin, while slip and cracks were caused along slip lines during fatigue on the film bonded to the base steel by diffusion. There was a significant difference in crack initiation behavior of the films between resin and diffusion bonded to base metals. This finding was not only for crack initiation site but also for roughness near the crack on the film. Using Eulerian equation of motion in continuum, this difference was discussed in terms of elastic displacement field with a wave caused from the base plate subjected to cyclic deformation. Such a wave motion enables us to understand the geometric effects of bonding interfaces on the fatigue damage behavior of the bonded film to base metal.

Abstract: Fatigue crack bending and propagation behaviors were studied under mixed-mode conditions using annealed and fatigue slant precracks. The bent fatigue crack initiated from the fatigue slant precrack propagated under mixed-mode conditions with mode II stress intensity factor evaluated from the crack sliding displacement measured along the crack. On the other hand, bent fatigue cracks propagated under the mode I condition for an annealed slant precrack specimen. The forces which suppress the crack opening/sliding were calculated along the slant precrack and the bent crack by FEM (Finite Element Method) analysis. As a result, the crack opening suppress forces were generated by the compressive residual stress around the fatigue slant precrack, while the forces which promote the crack sliding were caused by the residual stress field in front of the fatigue slant precrack.

Abstract: For an actual crack growth in structures subjected to the applied stress from the various directions, it is important to study about the fatigue crack propagation behavior under mixed-mode condition. In particular under the condition, crack surfaces tend to contact when the load is applied because of the compressive residual stress distributed near the crack and the zigzag crack surface morphology. In this study, using slant cracks with compressive residual stress induced in mode I fatigue crack propagation under the stress ratio of R= –1 and 0, stress intensity factors (KI)est and (KII)est were evaluated from the measured crack opening and sliding displacements. As a result, the stress intensity factor (KII)est for the crack made under the stress ratio of R= –1 with the slant angle of 45 deg. was decreased owing to the crack surface contact, while (KI)est showed relatively large values in spite of compressive residual stress.