Photoelasticity has been used for decades in the experimental measurement of stresses and strains. As data-acquisition techniques only identify the differences between principal stresses and strains, stress-separation techniques exist to identify each of their values separately. A variety of techniques have been developed since the early days of photoelasticity, aiming to automate the measurement process, to exploit the full potential of photoelastic methods, and to facilitate its application. The purpose of this paper is to provide a general overview of the different stress-separation techniques, and their current state of development and usage. The paper presents the publications and the authors who have contributed to the development of new techniques.

Stress-separation techniques in photoelasticity: A review

A shortcoming of the conventional holographic interferometer used in photoelasticity is that, for the double-exposure cases, the resulting fringe patterns are a complex combination of conventional isochromatic and isopachic fringes. This paper describes a holographic interferometer that may be used to obtain separate but simultaneous isochromatic- and isopachic-fringe patterns for photoelastic models in states of plane stress. The method requires a model with a partially reflecting front surface. Isopachics, which are proportional to the thickness change, are recorded using holographic interferometry from the transmitted light. The isochromatics are obtained from the transmitted light by conventional means. General equations relating the surface displacement of the specimen to the observed fringe patterns are developed, and examples of static and dynamic loadings are shown.

https://link.springer.com/content/pdf/10.1007/BF02328816.pdf

A neoteric interferometer for use in holographic photoelasticity

A physical explanation of photoelastic interferometry and holography is presented. Reasons for special properties and specific restrictions are argued in relation to this physical picture. The controlling intensity relationships for interferometry and holography are derived by simple physical analysis. The relationship between absolute retardation, isochromatic- and isopachic-fringe systems is clarified. Model material requirements for photoelastic interferometry and holography are shown to be identical.

Holography and interferometry in photoelasticity

In a general view, a comparison is made of different optical methods where either photoelastic models, opaque bodies with transparent coatings or noncontacted objects are examined with coherent light. Further, it is tried to show that some common features are contained by the three groups. These aspects concern the separation, by controlled determination, of displacement, rotation and strain (or of certain of their components), the multiplication of the optical effects in the detection and the elimination of secondary influences. In particular, the noncontacting method of wavefront reconstruction with double-exposure holograms is discussed and the role of the localization condition for the fringes in space is analyzed. On the other hand, it is illustrated that, besides the generally known birefringence and diffraction. use could be made of nonorthodox effects like rotation of the plane of polarization, second-harmonic generation in nonlinear optics, etc. This may, for instance, contribute to the optical instrumentation when transparent models and bodies with photoelastic coatings are investigated under the above aspects.

Some aspects of the optical techniques for strain determination

For a long while the so called Rayleigh limit posed severe constraints to the application of optical techniques beyond the range of λ/2 of the wavelength of light, In 1952, Toraldo di Francia [1] pointed out that the Rayleigh limit came from historical period in which the existence of evanescent fields was not known. On the basis of the possibility of taking advantage of evanescent fields, he postulated the possibility of going beyond the Rayleigh limit practically within the limitation posed by energy one can go to much smaller fractions of the wavelength of light, n the present paper this argument is pursued both on the theoretical ground and experimental results showing the possibility of reaching 1/20 of the wavelength of light, the limit for eyes from the energy and the availability of sensors with enough spatial resolution. Examples of particle dimensions, analysis of the phase of grating patterns and moire patterns (Fig. 1) are provided. The properties of the speckle fields are investigated by carrying out a statistical analysis of the experimental data.

Observation of Fundamental Variables of Optical Techniques in the Nanometric Range

The purpose of this paper is to give an interpretation to the fringes observed in holographic interferometry when plane-polarized light or circularly polarized light is utilized. It is shown that, when plane-polarized light is utilized and both the loaded and the unloaded states are considered, the obtained patterns are formed by the superposition of three families of fringes: the two families of absolute optical retardation and the family of relative retardation. The intensity distribution is also a function of the orientation of the plane of polarization, and along the points where the plane of polarization is parallel to one of the principal directions, only one of the families of absolute retardation is observed. By utilizing circularly polarized light, the dependence on the orientation of the principal axis is eliminated and patterns consisting of the superposition of the three above-mentioned families are obtained. If only the loaded state is considered, the holographic interferometer behaves as an ordinary polariscope with the reference beam playing the role of the analyzer. The relationships between the observed families are discussed. Examples of application to the disk and ring under diametral compression are also given.

C. A. Sciammarella

Papers

Techniques for the determination of absolute retardation in photoelasticity