2D-displacement measurement with real time image processing is developed. The target of camera image is a lattice pattern drawn on the paper sheet. The range of measurement depends only on the size of target sheet, so that high resolution power as well as large measuring range are realized with proposal method. The histogram distribution of the image of the lattice pattern is calculated, and the position of the grid lines are detected from the peak positions of the distribution. By continuously performing this detection, it is possible to calculate the movement of the image. From the target grid lines spacing (in mm) and the grid lines spacing on the image (in pixels), the calculation of the displacement is done automatically. The features of this method are that real-time measurement can be performed with high-speed processing, measurement can be easily performed simply by setting the grid line interval (mm), and measurement can be performed with sub-pixel accuracy. In this research, the measurement accuracy and the tolerance to poor image quality of the proposed method were evaluated. As a result, it has been shown that the position of the grid line can be detected with sub-pixel accuracy even in a poor-quality image. As an application of this method, the relative displacement between the base and tested building was obtained with the measurement during 3-dimensional vibration tests.
Birefringence measuring equipment currently used has either a high-sampling-rate or large-area measurement without high-spatial resolution. A birefringence distribution measurement of 10 kilopixels or more with high-spatial resolution at a sampling rate of 10 kHz or more has yet to be achieved. To develop the elemental technology to achieve this, this study proposes preliminary equipment, namely, a circular polariscope with a polarized laser, high-speed camera, and photoelastic modulator (PEM). The light source used was a 5-mW He–Ne laser with a wavelength of 632.8 nm; the light-receiving element was a high-speed camera with a photographing speed of 200 kHz, and harmonic analysis of the light intensity was achieved with a PEM. To demonstrate the possibility of high-speed measurement with the proposed equipment, the birefringence of a variable-wave plate was measured using one pixel of the high-speed camera. As a result, the maximum error of the measured value was 5.3% in the birefringence range of 10–20 nm of the specimen, 7.9% between 20–40 nm, and 4.0% between 40–60 nm. To show that measurement is also possible when the imaging range is expanded, the birefringence distribution in the area of 60 × 60 µm of the 40-nm wave plate was measured using 8 × 8 pixels of the high-speed camera. This method returned a smooth monotonously varying birefringence distribution close to 40 nm. The result shows that expanding the 8×8 pixels to 10 kilopixels can achieve the aforementioned research goals.
This paper introduces the principle and execution of a new method for measuring of distributed minute birefringence
based on simple polarimetry with phase-shifting method. The new method requires only three stepped photoelastic data
although conventional phase-stepping methods require four or more. To evaluate the new method experimentally, two
precise crystal wave plates having nominal retardation ± tolerance of 79.1±3.5 and 10.0±4.7 nanometers were used as
specimens. The experimental averages of the distributed retardation in the specimens with standard deviations were
found to be 80.2±15.0 and 18.8±7.06 nanometers. To estimate the measurement accuracy of the angular orientations of
the distributed birefringence in the specimens, the angular positions of the rotation stage for the specimens were rotated
intermittently 45 or 30 degrees at a time during the experiment. As a result, the averages of measured offsets of the
angular orientations were found to be 30.1±8.14 for the specimen of 79.1 nanometers with standard deviations. It is
concluded that the new method has potential of measuring for distributed minute birefringence.
A previously observed optical interferometric band pattern interpreted as representing localized strain is compared with a mechanical band structure known as the Luders' line. The interferometric band pattern is observed in fringe systems formed with the subtraction method of electronic speckle-pattern interferometry applied to in-plane displacement of plastically deforming metal specimens. The Luders' line is known to be representing localized strain. Experiments have been carried out to compare the locations of the optical band pattern and the Luders' line under the same stress condition. The experimental results indicate clear coincidence of the optical band pattern and the Luders' line, confirming the previous interpretation that the optical band pattern represents localized strain. It has been observed that the optical band pattern begins to appear at 11% lower stress than the Luders' line, indicating that strain localization occurs prior to the appearance of the first Luders' line.
Under the cyclic loading, stress concentration takes place in the member with circular hole, and it affected to fatigue fracture. Therefore, it is important to obtain the information on the stress concentration of circular hole in the study of fatigue fracture. In the carbon steel strip, it is well known under static loading that Luders' lines which arose by yield stress in the member surface and observed easily by naked eyes exists. The same phenomenon takes place by application of cyclic loading at load ratio: R=0. The direct observation using Luders' lines becomes visible smart sensor that discerns the yielding region. This method is using the property of material itself, and it is a simple method. And, the continuous change of the member surface can discern real-time. The purpose of this study is to obtain basic data on the stress concentation of the vicinity of circular hole by observing the continuous change of the specimen surface in R=0, -1, using the carbon steel strip with a circular hole. And, the stress concentration factor required by FEM analysis and experiment was compared, when yield criterion was changed.
In this paper, plastic region growing process which appeared in crack tip was visualized by stretcher strain, and it was observed using high-speed camera. Then, the fracture toughness value was calculated from the largest plastic region size. It was assumed that the relation equal to the case in which it is static under dynamic load was established, and we carried out dynamic experiment. The dynamic load was measured using piezo load-cell which is difficult to receive the effect of stress wave, the fracture toughness value was decided by the strain gauge method. For comparison, we also carried out static experiment comply ASTM E399-90. Then, the relationship between fracture toughness value calculated from the maximum load and it calculated from the largest plastic region size was investigated.
The direct observation was made of the stress concentration around circular holes as a cause of fatigue failure by cyclic loading. The change of surface conditions of carbon steel plates with 3 circular holes subjected to cyclic loading with the load ratio 0 was observed. In this time, Lueders' bands were used as a visible smart sensor. Generation and propagation of the yielding zone that appeared around circular holes by cyclic loading were clarified. A clear difference was demonstrated between stress concentration factors and experimental values of stress concentration by cyclic loading. It was confirmed that the interference of stress concentration generated in monotonic loading was similar to that of cyclic loading. Comparison of the yielding zone that appeared around circular holes with the stress distribution that was obtained by the FEM analysis, confirmed a difference between monotonic and cyclic loading.
The prevention of fracture in a structure is one of the most important problems of mechanical engineering. The fracture of a structure occurs at mechanical failure locations. It would be possible to prevent fracture if it were possible to discern failure locations. One possible method of detection might be to make use of the visible properties of the material. Yield point elongation is accompanied by the formation of visible bands known as Lueders' lines or stretcher strains. These Lueders' lines result not only from monotonic loading, but also from cyclic loading. Utilization of the appearance conditions of Lueders' lines enables the naked eye to detect yielding failure locations of ductile machine members. The appearance of those striped patterns indicates where a fracture is about to occur. It is possible to monitor the position where fracture may occur, over time, if changing striped patterns can be detected by digital signals. It is possible to detect the failure locations of a structure from a remote location. Change of striped patterns indicating maximum stress during cyclic loading is examined. Direct observation was made of change in surface conditions at the vicinity of a hole and round corners where cyclic loading was applied.
There is a very easy technique for detecting where the part of a structure undergoes failure due to fatigue loading; this technique if of great practical value in its employment to prevent fracture. It is well known that when yield stress is applied to a low-carbon steel plate, Lueders' lines become observable on its surface to the naked eye. It is a plausible hypothesis that the locations on the surface of the plate where the Lueders' lines appear are in a state of failure. The correlation of cyclic loading and Lueders' lines should confirm whether or not Lueders' lines reliably indicate where failure results from fatigue loading. The Ra value of the surface roughness of the specimen before and after the appearance of Lueders' lines is used to ascertain this correlation. The present paper takes up cyclic tensile loading with shoulder fillets as a concrete example of the correlation of failure and stress.
High-speed event capturing was conducted to determine the fracture initiation load of a hot-rolled steel under rapid loading conditions. The loading tests were carried out on compact specimens which were a single edge-notched and fatigue cracked plate loaded in tension. The impact velocities in the tests were 0.1 - 5.0 m/s. The influences of the impact velocity on the fracture initiation load were confirmed. The new application of a high-speed camera to the fracture initiation experiments has been confirmed.
Yield point is one of the most important mechanical properties of materials, however, there have not been any rules for obtaining the yield point caused by cyclic loading. Also in the case of analyzing the fatigue phenomena, the mechanical properties obtained by tension testing are normally used. It is widely known that when a tensile load is applied to a plane carbon steel specimen, Lueders' lines corresponding to yield stress occur on the surface. We decided to utilize Lueders' lines, in order to obtain the yield point under cyclic loading, therefore we investigated the state of their surface. Then it was clarified that we can utilize Lueders' lines in order to detect the yield point caused by cyclic loading. And that is used to detect the failure place of structural members. This method is very easy, namely only to have a smooth finish at the fixed portion, we can observe an occurrence of the striped pattern by the naked eye.
We developed an optical birefringence measurement equipment by using a photoelastic modulator and a polarized laser. A He-Ne infrared laser is used as a light source to measure the optical birefringence in silicon wafers. We explain the theory and process of the measurement of stress in silicon wafers. The magnitude of principal stress difference and also the direction of the principal stress are obtained simultaneously and quantitatively using our experiment. The optical birefringence of (100), (111) and (110) face silicon stressed specimens were measured. From the experimental results, the photoelastic constant depends on the crystalline orientation. By the stress-strain analysis of silicon single crystal, it was found that the relation between the principal strain difference and the retardation was independent of crystalline orientation.
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