Synchronization problems in dynamic holographic photoelasticity

An analysis of synchronization problems encountered when recording transient-fringe patterns which are dependent on the light source and the method of shock generation is presented. The analysis is specifically designed to be valid on a photoelastic material having either a high or low Young's modulus when the shock generator is an air gun and the light source is a Q-switched ruby laser. Synchronization is performed using integrated circuits in T.T.L. logic which give a triggering order to the ruby laser under the control of the projectile velocity.     

A neoteric interferometer for use in holographic photoelasticity

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.     

Application of faraday's effect in static and dynamic holographic photoelasticity

The basic principle of applying Faraday's effect to achieve the separation of fringes in static and dynamic holographic photoelasticity, and a study and application of Faraday's light rotator are described in this paper. It is proposed that Faraday's light rotator be used for automating photoelastic instrumentation for measuring isoclinics and the decimal orders of isochromatic fringes.     

Separation of the isochromatics-isopachics patterns by use of retarders in holographic photoelasticity

By double passing the object beam in photoelastic-holographic interferometry, separation of the isochromatic-isopachic patterns can be achieved. The object beam must then interact with a suitable polarizing element between the first and second pass. This problem is analyzed using Jones' matrices. This analysis shows that, apart from active and passive rotators, half-wave and quarter-wave retarders can also be used as polarizing elements. Possible experimental arrangements of the method are reviewed and experimental verifications are presented.     

Towards RGB photoelasticity: Full-field automated photoelasticity in white light

In this paper a new full-field method for the automatic analysis of isochromatic fringes in white light is presented. The method, named RGB photoelasticity, eliminates the typical drawbacks of the classical approach to photoelasticity in white light which requires a subjective analysis of colors and an experienced analyst to acquire and interpret the results. The proposed method makes it possible to determine retardations uniquely in the range of 0–3 fringe orders. For this purpose the isochromatics are acquired by means of a color video camera and the colors are decomposed in the three primary colors (red, green and blue) and compared to those stored in a calibration array in the system. Furthermore, the influence of various spurious effects on the accuracy of the proposed method is experimentally evaluated.     

New methods in photoelasticity

The principle of polarization of scattered light is applied to determine the principal stresses in the interior of a model. On the basis of the theorem which states that “a series of birefringents is equivalent to a unique birefringent, followed by a medium endowed with rotational power,” it can be assumed that, if the characteristics of a series of birefringents are known, it is possible to find the characteristics of an interior section. The measurement of the characteristics of a birefringent (eventually following a medium endowed with rotational power) can be accomplished by means of the new methods, making use of a photomultiplier, a constant-speed rotating analyzer and a servomechanism These new methods of measurement are applicable to two-dimensional photoelasticity.     

High-precision stress determination in photoelasticity

Stress separation is usually achieved by solving differential equations of equilibrium after parameter determination from isochromatics and isoclinics. The numerical error resulting from the stress determination is a main concern as it is always a function of parameters in discretization. To improve the accuracy of stress calculation,a novel meshless barycentric rational interpolation collocation method(BRICM) is proposed. The derivatives of the shear stress on the calculation path are determine...     

New compensation method in photoelasticity

Proposed compensation method eliminates the necessity to align the principal directions in the model with compensator for determination isoclinic parameter and fringe order. The advantage and limitation of the proposed method are discussed from the point of view of its potential use in the automatic data collection.     

Data analysis in dynamic photoelasticity

A method to separate the principal stresses and to obtain displacements from dynamic photoelastic fringe patterns is presented. The method utilizes geometric characteristics of either the model or the stress wave to provide the additional equation necessary for separation. The method is illustrated in ten specific cases.     

Holography and interferometry in 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.1971 William M. Murray Lecture was presented at 1971 SESA Fall Meeting held in Milwaukee, Wisc., On October 19–22.     

A scattered-light method in photoelasticity

The birefringence and, thereby, the stresses in a photoelastic model are investigated utilizing the light scattered from a beam of light propagating through the model. The retardance from the entry point of the beam into the model to a certain point along the beam is expressed in terms of the intensity of the scattered light. The retardance for a short distance along the light path within the model is determined as a function of the total retardances from the entry point of the model to the two end points of the distance investigated. The effects of retarders and polarizers on the state of polarization of the light beam are treated by Mueller calculus. It is not necessary to make other assumptions than those made in the usual stress-freezing and slicing method.     

Basic relations of photoelasticity

The polarization-optical method is used to study stresses and to measure the optical quantities (the isochromatic fringe orders and the isoclinic parameters) and the mechanical quantities (stresses and strains). The dependencies between these quantities are considered. The problem of disk compression along its diameter is solved exactly. These dependencies are analyzed on the basis of this exact solution and the experimental data. The most preferred form to express the basic relations of photoelasticity is discussed.     

Corneal studies by photoelasticity

Freshly enucleated andin vivo eyes of cats were analyzed to determine the detailed distribution of birefringence across the cornea and through its thickness, and to determine the change of birefringence with intraocular pressure. Scattered light and oblique-incidence photoelasticity were used. The experiments are summarized and special considerations are discussed. These include the necessity for a laser-light source; diffraction limitation in producing a narrow ribbon of light; rotation of the plane of polarization; inequalities in reflected components of polarized light.     

Equidensometry applied to photoelasticity

Agfacontour film has simplified photographic procedures so that equidensity techniques can now be applied in photoelastic practice and other work requiring evaluation of optical fringes for fractional orders. Direct photography and copying from conventional negatives are described and practical considerations indicated.     

Analysis of scattered-light methods in photoelasticity

The use of scattered light for nondestructive analysis of general photoelastic models is becoming quite common and, during the last few years, several methods have appeared in the literature to determine the directions of the secondary principal stresses and their differences at any general point. Among the methods suggested, some are mentioned as “exact” and some “approximate”. Even the exact methods have limitations in their applicability for a generally stressed model. The present discussion attempts to bring out the inaccuracies involved in the various exact methods. Also, a few modifications to improve the accuracy and a new method called the Mini-max method are proposed. References 1, 2 and 3 give the general methods and the derivations of equations used in the present analysis.     

Simulation of errors in automated photoelasticity

In automated photoelasticity, there are some advantages in using white light; however, the theory underlying many such systems, namely phase stepping, is based on monochromatic light. The effect of using white light has been investigated both by experiment and by a simulation of a polariscope. The simulation has been validated and used to explore the use of bandwidth filters to control the errors caused by using white light. These errors were found to be dependent on the form of the spectrum of the light and to be large for high fringe orders.