“The past and present or a history of photoelasticity and its future image of becoming automated”, this is first of all, a history of photoelasticity and its present status based upon an extensive current over view, whereupon with due consideration of the methods already attempted in automating the process of photoelasticity, it is an attempt to predict the most likely future image of such automation along the lines it is presently following in such development.
Recent experiments in photoelasticity seem to no longer predominantly employ methods of stress analysis of machinery and structures that are subjected to complicated stress conditions. This is because of it being exceedingly difficult to conduct any experiment on photoelasticity alone when undertaking stress analysis inside the mass of matter, in addition to the inconvenience of requiring a great deal of time. Furthermore under certain stress conditions, there is the chance that any such analysis is impossible. We can therefore consider the parallel adoption of some sort of analysis of values. This may lead the way to automation of stress analysis in photoelasticity. Automation is at present roughly proceeding in 2 directions, the 1st being in the measurement of the differences arising in main stresses and their directions and the 2nd alternative being in analysis of such stresses. Any attempt at predicting the most likely future image of such automation should pay due consideration to these 2 alternative directions currently being followed.
However it is extremely difficult to give a fair and lucid overview of studies in photoelasticity. It is inconceivable to even generalize in any such an over view. This is due to the fact that the outcome of the specifically separate studies of each and everyone of the current persons conducting such research shall each contribute to the structure of the future image of photoelasticity.

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Digital photoelasticity is a full-field method for obtaining photoelastic parameters, isochromatic fringes and isoclinic angles, and for analyzing stresses in a model using digital image processing combined with computers. There have been several different attempts made to automate the acquisition of photoelastic parameters in the last two decades. Most of these use the approach of phase shifting, enabling the photoelastic parameters to be calculated automatically and exactly. In this paper, a survey of recent methods of phase stepping methods developed in the last 10 years is provided.

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A phase-measuring method is proposed for stress separation in two-dimensional interferometric photoelasticity. A Mach-Zehnder type interferometer combined with a circular polariscope is used for the observation of isochromatics and isoclinics as well as isopachics. Phase-shifting is performed for the determination of the phase values of isochromatics and isoclinics by rotating both of a quarter-wave plate and an analyzer. On the other hand, the phase of the isopachics is analyzed from images obtained by shifting a mirror in the interferometer. Then, the phases of isochromatics and isoclinics without ambiguity are obtained by introducing a load-stepping technique. The phases obtained by the load-stepping are also used for unwrapping the phases of isochromatics and isopachics. In the present paper, light intensity equations for the proposed optical setup are derived using Jones calculus. Then, the effectiveness is demonstrated by applying the proposed method to a simple problem.

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The measurement method of scattered light photoelasticity using unpolarized light, which the secondary principal stress directions Ψj and the relative phase retardation ρj in 3-D stressed model having the rotation of the secondary principal stress directions can measure, is described by the Mueller calculus. One is the method by obtaining characteristic parameters of the optically equivalent model. The other is the proposed method using the Stokes parameters, which the Ψj and ρj can be directly measured from the scattered light. In the application of this method, the Ψj and ρj in a frozen stress sphere model are nondestructively measured by using a computer combined with a CCD camera. The experimental results for the proposed method are qualitatively compared with the mechanical slicing method and the usefulness of the method are presented.

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Inclusions often become the cause of stress concentration and failures. As there are many cases in which high stress arises around an inclusion and causes cracks and exfoliations from the matrix leading to a fracture, it is important to analyze the deformation behaviors around the inclusion. Although there have been many researches through theoretical analysis, research examples of inclusions through experimental analysis are very few. The analyses on the elliptic inclusions at a tensile load were conducted in this study. Polycaibonate was used as a test specimen model, and three different kinds of materials, epoxy resin, PMMA and polyethylene, were used as elliptic inclusions; 39 specimens were prepared in total. The experiment was conducted by the photoelastic method, method of caustics and finite element method. As a result of this experiment, the experimental equations relating to the principal stress difference, equivalent stress and stress intensity factor in the matrixes around the elliptic inclusions were obtained.

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In the x-ray stress measurement, diffraction lines have to be corrected for the LPA (Lorentz-polarization and absorption) factor to accurately determine the stress. The absorption factors for the fixed-ψ and ψ₀ methods using the iso-inclination method, and the fixed-η and η₀ methods using the side-inclination method were given. In the usual x-ray stress measurement, an area of x-ray irradiation on the surface of a specimen increases with increasing yr angle, The absorption factors for the measurement with restricted irradiation area at all ψ angles are also given. Using quenched and tempered steel specimens having various diffraction line widths, the effect of the LPA factor on the stress value was investigated. The LPA factor has stronger influence on the stress value as the line width increases. In the side-inclination method, the correction for the LPA factor has little influence on the stress and it can be omitted. The correction for the LPA factor has no influence on the diffraction line width. The equation for calculating the standard deviation of the difference between stresses with and without correction for LPA factor was derived.

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The impact erosion tests were carried out in mercury by using the direct tension test apparatus converted the Split Hopkinson Bar method to investigate the formation behavior of the pits. The stainless steel specimen that was screwed at the end of the elastic bar was immersed in mercury. The polished surface of the specimen was eroded by tensile waves propagating through the interference of the solid and liquid metals. The relationship between the mass loss due to the pits formation and the shot number was described with a linear function in logarithm graph, and the mass loss depends on the applied stress level. The maximum pits were present around the center of the specimen and the pit was developed with the increase of the shot number. The distribution of the relatively large pits was very dependent on the position of specimen in the mercury container.

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The shape of surface crack is estimated using infrared stress analysis system. The length of the surface crack, 2c, is measured directly using this method, but it is difficult to estimate the depth of the surface crack. For this purpose, tests are conducted changing the frequency of the cyclic load from 20 [Hz] to 5 [Hz] in four steps. By the cyclic loading, the ellipsoidal area appears between two crack tips of a surface crack. It means the change of the summation of the normal stress. By multiplying the stress value to the ellipsoidal area the depth of surface crack, a can be evaluated. For 5 [Hz] cyclic loading, it is found that the volume changes systematically with the change of the aspect ratio, a/c, of the surface crack. And for a/c=0.57 specimen, it is observed that this volume changes linearly with the change of 2c value. Based on these experimental results, a basic method to estimate the shape of the surface crack is proposed.

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The authors investigate plastic collapse patterns in a SUS316 single-edge notched member subjected to combined tension and bending using photoelastic coating technique. Conclusions are as follows: (1) The slip-line field of rigid-perfect plasticity body assumption which gives upper-bound solution is correspondent to the locus of the tip of plastic region which grows to the ligament direction in actual elastic-plastic material, (2) The plastic collapse pattern is almost decided in plotted position, when the collapsing load of the member is plotted on the plastic collapse limit diagram. The plastic collapse pattern is divided into three, Green, Rice and Hundy types and (3) From conclusion (1) and (2), the collapsing load of the member follows upper-bound solution regardless of load history.

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