Tashiyuki Torii

Observation of Microscopic Plastic Deformation of Polycrystalline Aluminum during Uniaxial Tension by Confocal Laser-Scanning Microscope

Microscopic plastic deformation behavior of aluminum sheet during uniaxial tension is experimentally investigated by a confocal laser-scanning microscope. The surface roughening is closely related to the inhomogeneity of polycrystalline metals, that is, to the inhomogeneous plastic deformation of respective grains. Image measuring method of rigid rotation of grain is proposed. It is shown that the averaged grain rotation on the surface plane is much less than the averaged grain rotation out of the surface plane. The averaged grain rotation, the grain-area strain and the mean size strain and the standard deviations of these parameters increase statistically in proportion to the applied strain. The grain rotation in the material is less than that on the surface, thus it is considered that deformation inhomogeneity of grains in the material is less than that of grains on the surface of the material. This fact may indicate that the surface roughness is mainly affected by inhomogeneous deformation of the surface layer. Discussions are made on the grain rotation, the inhomogeneous deformation of the grains and the free surface roughening mechanism. Introduction Almost all metals used in the industries are polycrystalline, and microscopic deformation behaviour of polycrystalline metals is important for estimating macroscopic deformation behavior and strength. Free surface of polycrystalline metal becomes roughened after plastic deformation. It is known that the surface roughening is closely related to the inhomogeneity of polycrystalline metals, that is, to the inhomogeneous plastic deformation of respective grains [1-6] as well as that in the grains [5-10]. Therefore, the experimental study of the surface roughening is also important for the understanding of microscopic deformation behavior of polycrystalline metals. In the present paper, the inhomogeneous deformation on the free surface of polycrystalline aluminum specimen during uniaxial tension is studied. The inhomogeneous deformation of grains in the central area of the free surface of specimen is observed by a confocal laser-scanning microscope (CLSM). The confocal laser-scanning microscope is convenient, which can be used to observe in the atmosphere, and the scanning distance in z-direction is larger than that of the normal optical microscope. The relation between the strain in respective grains and the shape of free surface roughness curve caused by tensile plastic strain is investigated. Discussions are also made on the relation among the variations of strain in each grain, the wavelength of the profile and the surface roughness due to the applied strain. Experimental Method Specimen. The material used in this work is polycrystalline pure aluminum for industrial use (Al 99.5% by weight). The shape of tensile specimen is shown in Fig. 1. The z-axis is the normal Key Engineering Materials Online: 2004-10-15 ISSN: 1662-9795, Vols. 274-276, pp 337-342 doi:10.4028/www.scientific.net/KEM.274-276.337

Crack Opening/Sliding Morphology and Stress Intensity Factor of Slant Fatigue Crack

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.

Effects of Grain Size on Fatigue Crack Propagation in Copper Film

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.

Fatigue Damage Behavior Depending on the Bonding Interface Layer in Copper Film Bonded to Base Metals

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.

Effect of Residual Stress Field in Front of the Slant Precrack Tip on Bent Fatigue Crack Propagation

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.

Effects of Interlayer Resin Bonding on Fatigue Crack Initiation/Propagation in Laminate Materials With a Surface Layer of Copper Film

As model specimens to examine the effects of interlayer resin bonding on fatigue properties in laminate materials with a surface film, pure copper films with a thickness of 100μm and 50μm were bonded with epoxy resin to steel base plates. The fatigue crack propagation from the notch root of the specimen was slower for the epoxy-bonded film than for the base specimen, because the epoxy bonding interlayer restricted crack propagation from the surface film to the inner base plate. On the epoxy-bonded film, however, many fatigue cracks initiated at multiple sites sufficiently away from the notch root, and propagated only on the surface copper film. In addition, the number of these multiple fatigue cracks, caused mostly at the site of the annealing twin boundaries, was larger on the surface copper film with a thickness of 100μm than 50μm.Copyright

Study of Fatigue Crack Propagation Behavior Based on Crack Opening Displacement Distributions (Nothing the Residual Stress Distribution Near a Notch).

A method was proposed which evaluates crack opening stress distribution and stress intensity factor from the crack opening displacement distribution near a circular-hole. The fatigue crack and the low-temperature annealed crack emanating from the hole were prepared and the crack opening displacements were measured along their faces. The difference in the crcak opening stress distribution between the fatigue and the annealed cracks was used to estimate the residual stress distribution along the fatigue crack. They showed the compressive residual stresses near the hole and the crack tip, but the tensile ones between them, which were caused by cyclic deformation near the hole and subsequently redistributed due to fatigue crack propagation. The stress intensity factors estimated from the fatigue crack opening displacements were smaller than those calculated from the applied stress and were closely related to the change in fatigue crack propagation rate.

Behavior of fatigue crack propagation from precrack in thermal residual stress field (In the case of longitudinal tensile residual stress field)

The purpose of this paper was to investigate quantitatively the effect of tensile residual stress on fatigue crack propagation based on the experimental facts and the linear fracture mechanics. In the experiment, the X-ray oscillating stress measurement method was applied to examine the residual stress distribution around a precrack in the plate specimens which were made so as to have longitudinal residual stress due to water cooling from the temperature below 873K. By using these specimens, it was confirmed that the acceleration of the fatigue crack propagation from the precrack in tensile residual stress field was induced by the crack opening due to the tensile residual stress. On the other hand, it was found that the initial stress intensity factor of precrack in tensile residual stress field could be calculated by substituting the measured value of tensile residual stress for the crack opening stress on the ideal crack surface in a linear fracture mechanics model. The effective stress intensity factor range calculated from the initial stress intensity factor of precrack was useful for understanding of the fatigue crack propagation from the precrack in tensile residual stress field.

Influence of matrix micro-structure on the relation between applied stress and fatigue crack propagating rate

Three kinds of low carbon steel were examined to study the relation between the applied stress condition and the crack propagating rate. The respectively chosen specimens were those initially annealed, those stretched and those recovered. These were all subjected to alternating push-pull load.The study was made from the view point of micro-structural change at the crack tip observed by the X-ray microbeam diffraction technique.