Kensuke Ichinose

An optical interferometric band as an indicator of plastic deformation front

The Luders’ front and a previously discovered optical interferometric band structure were observed simultaneously in steel specimens under tensile loading. The observed Luders’ front and optical band structure show the same propagation characteristics, confirming our previous interpretation that the optical band structure represents the plastic deformation front. Analysis shows that the stress at which the optical band structure begins to appear is approximately 10% lower than the corresponding Luders’ front, indicating that the optical band structure reveals the plastic deformation front with higher sensitivity than the Luders’ front.

Prebreakdown Phenomena and Formation Processes of Low-Pressure Glow Discharges in Ar, Ar/N2 and Ar/O2 Mixtures

In this research, both the prebreakdown phenomena and the transient processes from the prebreakdown phenomena to the glow discharge in low-pressure Ar gas, Ar/N2 and Ar/O2 mixtures were investigated by electrical and optical methods. The prebreakdown phenomenon in the Ar/N2 mixture was similar to that of Ar gas, and a pulseless component has been observed. The prebreakdown phenomenon in the Ar/O2 mixture differed from those in the Ar gas and Ar/N2 mixture, and formed a pulse discharge. Thus, the prebreakdown phenomenon in the Ar gas has been changed to pulse discharge by mixing a small amount of O2 gas which played an important role in the formation of pulse discharge.

Detection of Crack Initiation by Observations of Free Surface Condition

A simple method to detect crack initiation is presented. By polishing the surface of carbon steel structure properly and using the Luders bands and orange peel as indicators for damage, it is possible to predict crack initiation with the naked eye. The initiation of the Luders bands is the yellow light of the crack initiation, for instance, in the traffic signal, and orange peel is the red light. The relationship between the observed surface condition and the cyclic stress-strain characteristics is discussed.

A New Measurement Technique of Low-Level Strain Retardation in Optoelectronic Materials

This paper introduces the principles and execution of a new technique for low-level strain retardation measurement, which requires only three phase-stepping images, with precision. It then discusses the potential of this new technique for improving automated photoelasticity. To verify the new technique experimentally, a precise crystal wave plate of having low-level retardation was used as a specimen. The tolerance in retardation was 10.0plusmn4.7 nanometers. The measurements of the retardation with standard deviation using the new technique were also found to be 10.0plusmn4.7 nanometers, which agreed well in spite of low-level amount of retardation. Moreover, the measured offsets of the angular orientation were found to be -0.05plusmn4.46 degrees with standard deviation for angular position of the rotation stage for the specimen. The instrument described in this paper includes beam splitters to measure the influence on these measurements to develop the instrument into an automated one in future research. Finally, it is suggested that an automated photoelasticity will now be possible within an error of around plusmn3nm in retardation and around plusmn5degrees in its related orientation

Yield Strength in Relation to Cyclic Loading

The yielding phenomenon under cyclic loading has been investigated for low-carbon steel. Under various loading ratios, the surface state of the specimen was observed with the naked eye, using the Luders band as an indicator for the yield point elongation. The stress-strain curve was measured simultaneously. The results show that both the surface states and the cyclic stress-strain characteristics vary depending on the load ratio. This makes it questionable to use the yield strength evaluated under monotonic loading to describe the mechanical properties of the specimen material for fatigue tests, though it is a prevailing practice recommended by ASTM. The result of this study also indicates that the Luders band can be used as a smart sensor to investigate the yield phenomenon.

Observation of Optical Interferometric Band Structure Representing Plastic Deformation Front under Cyclic Loading

An interferometric band structure previously observed under monotonic tensile loading has been observed under cyclic loading as well. Like the previous monotonic loading case, the band structure is found to represent the plastic deformation front, indicating that it can be used for visualization of stress concentration. Observed correlation between the motions of the plastic deformation front and the test machines crosshead indicates that the plastic deformation front moves in proportion to the strength of the displacement field induced by the test machine. This is consistent with a previous report that the propagation speed of the plastic deformation front varies in proportion to the monotonic loading speed.

Prebreakdown phenomena and formation processes of low pressure glow discharges in N2, O2, and N2/O2 mixtures

The prebreakdown phenomena and the resulting transient processes from the prebreakdown phenomena to the glow discharge in low-pressure N2, O2 gases, and N2/O2 mixtures were investigated. When the applied voltage was a lower overvoltage ratio less than about 2%, nonprebreakdown phenomenon was observed, and the overvoltage ratio was independent of the gas materials. This phenomenon, however, was observed at the overvoltage ratio of higher than about 3%. The prebreakdown phenomenon in N2 gas differed from those in O2 gas, and formed pulseless components. On the other hand, those in O2 gas formed pulsed components. The prebreakdown phenomenon in N2 gas changed to a pulsed discharge by mixing it with a small amount of more than 0.3% O2 gas. These transient processes from the prebreakdown phenomena to the glow discharges were also observed by a high-speed video camera. As a result, the formation processes of the glow discharge became clear.

Development of Scanning Stress Measurement Method Using Laser Photoelasticity.

We have developed an optical equipment which possesses high detection sensitivity for measuring small optical retardation induced by stress by means of laser photoelasticity. A He-Ne laser is used as a light source to measure small stress in transparent materials. We explain the theory and process of the measurement of optical retardation in the materials. The magnitude of principal stress difference as well as the direction of the principal stress are obtained simultaneously and quantitatively using our equipment. In order to evaluate the performance of the optical retardation measurement equipment, the stress distribution of a pulled rectangular glass plate with notches at both sides was measured using the equipment. The results of stress distribution agreed with the analytical results. Stress at many points can be determined quickly using the equipment and scanning stress distribution measurement has been realized.

Scale Independent Fracture Mechanics

Fracture mechanics is considered from the viewpoint of a field theoretical approach based on the physical principle known as gauge invariance. The advantage of this approach is scale independent and universal. All stages of deformation, from the elastic stage to fracturing stage can be treated on the same theoretical foundation. A quantity identified as the deformation charge is found to play a significant role in transition from plastic deformation to fracture. Theoretical details along with supporting experimental results are discussed.

Residual stress estimation in SiC wafer using IR polariscope

Silicon carbide (SiC) single crystals have got into the spotlight as a material for power devices [1]. For quality control of SiC wafers, it is important that the estimation of the optical retardation in the wafers corresponds to the residual stress. For this estimation, photoelastic stress-strain measurement is suitable. Because, photoelasticity has the distinct advantages of being non-destructive, convenient, real time, precise, and quantitative compared with other stress-strain measurement techniques. Thus, an optical birefringence measurement system is made with a photoelastic modulator and polarized laser. A helium-neon (He-Ne) infrared laser is utilized as the light source for measuring the birefringence in SiC wafers. In this paper we will explain the principles behind the system and the process of estimation the stress in SiC wafers. The magnitudes of the retardations correspond to principal strain differences as well as the orientations of them are obtained simultaneously and quantitatively by using the system. We compared the distribution of the magnitudes and orientations in two different finishing wafers of SiC.