Half-fringe photoelasticity: A new approach to whole-field stress analysis

This paper presents a new method for whole-field stress analysis based on a symbiosis of two techniques—classical photoelasticity and modern digital image analysis. The resulting method is called ‘half-fringe photoelasticity (HFP)’. Classical photoelasticity demands materials with high birefringence, which leads to extensive use of plastics as model materials. Since the behavior of these materials is often different from that of the prototype materials, their use distorts the similitude relationships. In many contemporary problems this distortion is untenable. HFP offers a way out of this dilemma. It permits materials and loads to be chosen so that no more than one half of a fringe order appears in the area of interest. Thus, for example, glass, which behaves linearly up to high stress levels and over a wide range of temperatures, could be used as model material. Alternatively, models from polymeric materials could be used under very low load in order to stay within the linear part of the stress-strain diagram and to prevent large deformations. The half-fringe-photoelasticity system, which is described here, utilizes the resulting low levels of birefringence for effective stress analysis. This paper describes the system. It outlines a calibration routine and illustrates its application to two simple problems using glass models.     

Colour Adaptation in Three Fringe Photoelasticity

Three fringe photoelasticity (TFP) can give the total fringe order from a single colour isochromatic fringe field by suitably comparing the colour with a calibration specimen. The fringe order evaluation can be erroneous when the materials for the calibration specimen and the application specimen are different. This is because of the colour variation between the two materials. This is conventionally handled by preparing individual calibration tables for each application. A new methodology to tune the calibration table obtained for a single material to accommodate the tint variation in TFP is proposed for the use of different specimen materials. Discontinuities in fringe order variation are smoothed using the refined TFP (RTFP) procedure. The elegance of the new methodology for solving a multi-material system is bought out by solving the problem of a bi-material Brazilian disc. The results obtained are compared with the phase shifting technique.     

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.     

数字光弹性法综述

将光弹性法与计算机图像处理技术相结合 ,来自动采集光弹性数据和分析应力的方法 ,称为数字光弹性法 ,与传统光弹性法相比 ,它可以进一步提高实验速度和精度。本文详细讨论了以下两个方面 :一是光弹性条纹的细化和倍增处理技术 ;二是自动确定光弹性参数的技术 ,包括相移法、傅立叶变换法、逐步载荷法、广谱分析法和RGB光弹性法等。通过对近二十年来国内外在这些方面的研究、应用和进展作了综述 ,认为采用白光的彩色域相移技术计算光弹性等倾角 ,结合采用白光源或三色光源的相移法来确定光弹性等色线级数 ,有望成为解决静态二维和三维冻结模型薄切片应力分析的最佳方法 ;另外 ,设计一种能实时和同步采集多幅条纹图的实验装置 ,通过相移法来自动获取动态光弹性数据 ,是数字动态光弹性法很有前景的发展方向     

A novel instrument for transient photoelasticity

A new instrument has been developed for the photoelastic analysis of transient events. The instrument is based on the Phase-Stepped Images Obtained Simultaneously (PSIOS) system developed by Patterson and Wang, which enables four phase-stepped photoelastic images to be collected simultaneously. Where the new instrument differs is that the original instrument requires four cameras to collect the four images, whereas the new instrument requires only one camera. This makes the use of phase-stepping viable for events, in which the fringe order varies with time. Three examples are given of the use of the instrument in static and dynamic photoelasticity to generate full field maps of isochromatic fringes. The instrument has been found to work well and significantly increases the potential for the use of photoelasticity to study transient and possibly dynamic events. J.W. Hobbs was a Research Associate, R.J. Greene was a Research Associate     

Three-dimensional photoelasticity by stress freezing

Three-dimensional photoelasticity by the stress-freezing method has been considered to be one of the most powerful methods of experimental stress analysis. Its principles were well established more than thirty years ago, but its use outside the academic world has steadily declined. The reasons are cost and time needed to generate the desired information. This presentation summarizes fifteen year's effort to develop-stress-freezing photoelasticity into a responsive and inexpensive tool for stress analysis in the industrial environment. The whole procedure of stress-freezing photoelasticity is reviewed and evaluated from a cost-effectiveness point of view. The basic properties of model materials, the method of model casting, and the method of mold preparation are discussed in detail. The methods of model slicing, measurement of fringe order, and evaluation of stresses in models are reviewed together with available equipment. Finally, several applications of developed procedures are described in detail.     

压缩剪切测试中载荷传递行为的数字光弹性研究

结合光弹性四步彩色相移法,提出改进的六步相移法来计算全场剪应力,对径压缩圆盘实验证实了该方法的正确性,并进一步研究了同质材料连接结构在压缩剪切测试中的层间剪切行为。实验结果表明:全场等色线条纹级数和剪应力连续分布且随载荷而增加;中线附近剪应力趋于零,而条纹级数为峰谷。在连接界面上的条纹级数和剪应力出现峰值,特别是在连接界面端部为最高,在最大载荷下可分别达3.5级和2.4MPa,是导致连接界面脱粘开裂的危险区域。连接结构通过层间剪应力进行应力传递,在连接界面上剪应力与外载基本保持平衡。     

Computerized fringe analysis in photomechanics

A generalized method is presented for analyzing the fringe patterns frequently encountered in experimental mechanics. By utilizing digital-image-processing and computer-graphics techniques, a set of menu-driven software is developed for interactively implementing the fringe processing. Tests of this software on the images obtained experimentally by photoelasticity, holographic interferometry and speckle interferometry demonstrate its usefulness. Good agreement between the experimental and theoretical results is established.     

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.     

正交异性动态光弹性方法的几个基本问题的研究

文章对适用于动态研究的正交异性光弹性复合材料进行了分析，详细说明了光弹性复合材料中残余双折射的确定方法；基于静态下Ｈｙｅｒ和Ｌｉｕ应力－光性定律，提出了正交异性动态应力－光性定律，并对正交异性材料的动态力学参数及动态光弹性常数给出了实用的标定方法；最后，利用三个单轴压缩试件（０°，９０°及４５°），采用动态应变测量方法，证实了单轴应力状态下正交异性动态应力－光性定律的正确性。     

Separation of principal stresses in dynamic photoelasticity

A new and effective method used to separate the transient principal stresses for dynamic photoelasticity is proposed. This is a hybrid method combining the optical method of dynamic caustics and the boundary element numerical method. Firstly, a modified Cranz-Schardin spark camera is used to record simultaneously the isochromatic fringe patterns of photoelasticity and the shadow spot patterns in the dynamic process. By means of the isochromatic fringe patterns, the difference between transient principal stresses in the whole domain and the principal stresses along the free boundary can be solved. In addition, the method of caustics is a very powerful technique for measuring the concentrative load. Then, the sum of the principal stresses is calculated by the boundary integral equation obtained from the Laplace integral transform of the wave equation. So, the transient principal stresses can be determined from the experimental and numerical results. As an example, the transient principal stresses in a polycarbonate disk under an impact load are resolved. Concurrently published in the Chinese Edition of Acta Mechanica Sinica, Vol. 26, No. 1, 1994     

电子散斑和全息光弹法相结合测量透明材料的应力强度因子K1

全息光弹性中等和线是获得断裂力学中应力强度因子的一种有效方法之一。但用传统的全息光弹性方法获取等和线需暗房，要经过显影定影及再现，而且不能直接数字化使其应用受到限制。本文提出一种将全息光弹性与相移电子散斑干涉（Electric Speckle Pattern Interferometry，简称ESPI）有机结合的方法，不但克服传统全息光弹性的不足，而且使全息光弹性实现了数字化。文中将这一方法成功应用于有预制裂纹的三点弯曲试件上，定量求得其应力强度因子（Stress Intensity Factor，简称SIF）K1，而且实验值和理论值具有相一致的结果。     

A new holographic interferometer for stress analysis

In holographic interferometry with a photoelastic model, two families of fringes are generated simultaneously when the model is stressed. One family represents the isochromatic-fringe pattern normally associated with photoelasticity which yields the difference between the principal stresses. The other family represents the isopachic-fringe pattern associated with interferometry which yields the sum of the principal stresses. From these complementary patterns, the magnitudes of the principal stresses can readily be determined throughout the field of observation. Unfortunately, these fringe patterns are not completely independent but interact in such a way as to make interpretation difficult in critical regions of the model. A new system has been developed which readily permits simultaneous acquisition of these fringe patterns without their undesirable mutual interaction, as well as providing increased sensitivity. This new interferometer uses a double-pass object beam and an optical rotator to eliminate the isochromatic-fringe pattern and its effect from the isopachic interferogram. Such a system has considerable value in experimental mechanics for applications to both static and dynamic model studies and to materials investigation.     

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.     

混合光载波法分离动静态主应力

本文将等差载波位相调制原理与夹层全息术结合形成的混合光载波法,可同时获得等差载波条纹图与混合载波条纹图.对混合载波纹图进行光学付里叶变换,利用空间滤波技术可将等和载波条纹从中分离出来.利用图象处理技术实现了对两种载波条纹图从预处理到应力计算的自动化处理.将混合光载波法应用于动态全息光弹性,分离了动态主应力. 文中还提出了一个经济实用的大尺寸载波片制作技术.     

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.     

Doppler picture velocimetry applied to hypersonics: automated DPV fringe pattern analysis using the FFT method

Doppler picture velocimetry (DPV) is a tool for visualizing and measuring the flow velocity distribution of tracer particles in a laser light sheet. A frequency sensitive Michelson interferometer, tuned for detecting the velocity distribution by the Doppler effect, visualizes the velocity information of tracer particles crossing an illuminating laser light sheet as interference fringe patterns. Many efforts have been done to evaluate best these DPV patterns, in order to obtain the frequency distribution and, by applying the Doppler formula, the velocity profile of the tracers. The first processing method, developed in 1982, relied on manual processing of the pictures by the user, due to the unavailability of suitable high performance picture processing algorithms. This drawback made DPV being considered as a rather time-consuming measurement technique with limited accuracy, compared to existing commercial velocity measurement systems (e.g. PIV). This is no more the state of the art: The new DPV analysis software, presented in this paper, allows automated processing of the interference fringe samples obtained by two images, a reference picture without frequency shift and a Doppler picture containing the frequency shift, using single beam velocimetry. Based on Fast Fourier transformation (FFT), the presented algorithm determines the corresponding velocity profile (in pseudo colours) within only a few seconds on a standard personal computer without user intervention.     

Computer-aided fringe-pattern analyzer—A case of photoelastic fringe

A real-time “Computer-Aided FRinge-pattern AN-alyzer” (CAFRAN) system for the analysis of photoelastic fringe is developed. The pattern-analyzing process which utilizes image analysis or pattern-recognition technique and discretized-computation process in terms of finite-element method play an important role in this system. The former makes it possible to readout the fringe position and the fringe order of both isochromatic and isoclinic informations, and the latter analyzes the sum of the principal stresses which constitutes the Laplace field. Both results are combined to give the distributions of stress components over the domain under consideration. As far as the results of classical problems solved by this technique are concerned, the CAFRAN is promising in reading the fractional fringe order and encouraging the use of this technique to such problems as stress concentration.