Further applications of holography to photoelasticity

A holographic method for reconstructing the polarization of light emitted by a photoelastic model is presented. Isochromatic and the entire family of isoclinic fringe patterns may be obtained by examining the holographic image, a posteriori. This is accomplished by using two orthogonally polarized reference beams so arranged that the reconstructed crosstalk images do not overlap. An analysis is preseted crosstalk images do not overlap. An analysis is presented which shows that, to reconstruct polarization, the phase relationship between the two reference beams must be maintained. This requirement is more stringent than that normally required in holography. Experimental verification is presented.The use of double-exposure holography to obtain isopachic-isochromatic fringe patterns in the reconstructed image is discussed. An analytic treatment of the method is presented. Expressions relating the isopachic-isochromatic fringe pattern in the reconstructed virtual image to the principal stresses are developed. Differences between this analysis and that presented in a recent paper are discussed. Experimental results are obtained and compared to theory and to other experimental results with good agreement.Paper was presented at 1969 SESA Spring Meeting held in Philadelphia, Pa., on May 13–16.The study reported herein was supported in part by NASA Grant No. NsG-401.     

Application of photoelasticity to a weld-penetration problem

The need for more information on the “initiation period” in fatigue tests of weld specimens with penetration defects is discussed and the literature which relates the elasticity stress-concentration factor and Irwin's stress-intensity factor is reviewed. A series of photoelasticity tests on two-dimensional plane-stress models of typical penetration defects is described. In particular a method for casting “ready to use” very narrow defects is explained. The results are presented in a graph of stress-concentration factor against defect length. This graph has a “knee” at defect length-to-plate thickness ratios around 0.2. Below the “knee”, the stress-concentration factor changes very little with changes in defect length but, for lengths beyond the knee, i.e., ratios larger than 0.2, the stress concentrations increase linearly with defect length. It is concluded that such a critical defect length should have a strong effect on fatigue life of defective welds and that it may constitute a first approach to the specification of an “acceptable” level of penetration defects for production processes.     

An experimental study of thermal stress in porous materials by methods of holography and photoelasticity

In heat-loaded structures of energy equipment at the moment of crisis of heat exchange in working elements, the ultimate state of the material occurs. Thermal stresses were studied experimentally in a bulk porous material with a groove on the surface with a view to finding out the distribution of deformations and thermal stresses due to the location of the heat source. It appeared efficient to apply optical methods. Using holographic interferometry, a pattern of distribution of thermal deformations over the surface of the specimens was obtained. Using the photoelastic method to investigate a grooved porous structure, a physical pattern of the distribution of thermal stresses inside the block and in the groove was obtained. Ways are outlined for designing and reducing the probability of occurrence of destructive cracks. A similarity is observed in the distribution of strains and stresses that indicates the interrelatedness of surface and internal processes. The application of optical methods made it possible to discover a physical pattern of destruction. The results are expected to be used in the future to investigate other porous materials.     

Application of holography to anisotropic composite plates

The application of holography to stress analysis of opaque, anisotropic composite plates subjected to static or dynamic transverse and in-plane loads is presented. The equations of motion show that, for flexed, anisotropic, laminated composite plates, the complete state of stress at a generic point in any lamina, plus the moments and shear forces, are related to the temporal and spatial partial derivatives of the transverse displacement of the middle surface. Interferometric holography is shown to be well suited for obtaining the transverse displacement. Methods of determining the required spatial and temporal partial derivatives are discussed, an example being demonstrated. The holographic determination of anisotropic-material properties, stress and strain concentrations and the nondestructive evaluation of critical buckling loads for composite structures, plus the use of holographically obtained isopachics to supplement photoelastic analyses of composites, are also discussed. Experimental results for statically and dynamically loaded composite plates and beams with and without geometric discontinuities are presented to illustrate the concepts and techniques.     

Application of scattered-light photoelasticity to doubly connected tapered torsion bars

This paper reports the use of scattered-light photoelasticity to solve the doubly connected tapered-shaft problem. Some techniques are presented which help realize more fully the potential of scattered-light photoelasticity. These include the use of a continuousemission gas laser as a light source for the polariscope, the use of a photometer arrangement to read fringe spacings, and the use of curve-fitting techniques to analyze the data. Also, some design features for constructing a scattered-light polariscope are presented.     

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.     

Application of reflection holography to deformation measurement problems

The advantages of using reflection holography for measuring the deformation of structures in a workshop environment are outlined, and practical examples of its application are given. These include (1) a study of crack propagation in an aluminum sheet, showing the yielding of the metal in the vicinity of the crack tip, (2) measurement of the deformation of the platen in a compression testing machine when subjected to the localized force of a load cell, and (3) monitoring of long-term deformation of fiber composites using a real-time version of the holographic technique. Particular attention is paid to the design of the plate-holder mounting and to the photographic processing techniques that are required to achieve optimum results.     

Applications of lasers to photoelasticity

This paper discusses briefly the principles of gaseous and ruby lasers and makes comparisons of pertinent properties of lasers and conventional light sources. Since the output from a laser is a highly collimated beam of monochromatic light that can be made extremely intense and plane polarized, the eminent suitability of lasers as light sources for photoelasticity is suggested. Lasers are shown to be superior to conventional light sources, especially for the scattered-light method and for high-speed dynamic work. Experiments are described and results are given for using lasers for (1) the scattered-light method, (2) a conventional transmission polariscope with static loads and (3) dynamic photoelasticity.Paper was presented at 1965 SESA Spring Meeting held in Denver, Colo., on May 5–7.     

An introduction to dynamic photoelasticity

The general area of elastodynamics is divided into four more limited topics including, stress-wave propagation, vibration and impact, fracture propagation and quasi-static transients. The application of dynamic photoelasticity to each topic is discussed. Recording methods used in dynamic photoelasticity which are reviewed include the high-speed framing camera, the Cranz-Schardin system, Q-switched ruby lasers, and a stopaction strobe system. Advantages and disadvantages of each method of recording are covered. Analysis procedures used in interpreting the dynamic isochromatic-fringe patterns are described. Examples are illustrated where separation of the principal stresses is possible and a calibration method for determining the dynamic material-fringe valuef _σ ^* is reviewed. Finally, four applications of dynamic photoelasticity to problems arising in geophysics, fracture mechanics, flaw detection and mining are briefly reviewed to show the versatility of the dynamic photoelastic method are described.     

同轴数字全息技术在高速射流粒子测量中的应用

讨论了数字全息的理论和数字全息在爆轰实验中的应用。在解决了系统分辨率、动态模糊、安全和防护、精确同步的基础上,使用数字全息技术获得了中心刻有锥形小孔的铝飞片在爆轰加载下的喷射物的全息图,并提供了该全息图的数字再现图像。在照相视场内喷射粒子的速度约2.7~4 km/s,粒子粒径从几微米至几十微米。     

Application of two-dimensional photoelasticity to the study of acceleration stresses under quasi-static conditions

A laboratory method has been developed to simulate the state of stress in various structures under rapid-acceleration loading conditions. This method subjects a two-dimensional photoelastic model to impact loading, using a commercially purchased drop tester. The particular approach selected for studying this dynamic situation was a static technique utilizing a 4×5 studio camera and a high-intensity Xenon-light source. The light was flashed by the maximum pulse from an accelerator mounted on the model. This static treatment provided a significant improvement in the optical quality of the fringe patterns over that obtained by dynamic high-speed photographic methods used in earlier impact studies. It permitted evaluation of the reproducibility of the loading system and associated photographic synchronization and triggering circuits, since double exposures could be taken of separate individual tests of the same loading condition with little degradation of the optical fringe data. A composite picture made of four separate photographs of a series of individual tests, with the camera viewing a different portion of the sample in each picture, showed continuity of fringe pattern and highlighted the regions of critical stress within the model.     

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.     

Contribution to the analysis of errors in photoelasticity

Photoelasticity data are expressed in terms of the direction and the differences of the principal stresses. To determine the principal stresses, separation is necessary. The author deals with some of the numerical methods of separating principal stresses, permitting evaluation of the stresses along straight sections. In the first part of this paper the author presents methods of separation and in the second part the errors involved.     

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.     

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.