Computer aided photoelasticity by an optimum phase stepping method

In this paper an automated photoelastic method based on the phase stepping technique is described. It provides full-field maps of the isoclinic parameter and the relative retardation. The technique is based on processing six images of a photoelastic specimen acquired using plane and circularly polarized light. The number of acquisitions and the type of polariscope used in this approach have been chosen with the aim at reducing the influence of quarter wave plate errors and obtaining raw photoelastic data in a periodic form suitable for easy applications of automatic unwrapping routines.     

The application of fourier transform in regularized speckle photography

Two speckle patterns are recorded using a video camera corresponding to undisplaced and displaced states into the memory of a computer. After one pattern is added to another, the displacement of the measured points can be calculated by using the Fourier transform. The expression of displacement field is deduced for non-uniform displacement, and as an example, a rotatanle disc is tested for illustration. The results prove that the present method is promising.     

界面对应力波作用的动光弹试验研究

用动光弹法分析研究了在冲击载荷作用下界面上应力波的传播过程,利用应力波理论分析了界面上应力波的作用机理,证实了界面上存在着能量累积和阻滞能量传递的作用,并结合弹性波动理论和应力 光学定律导出了波动方程与条纹级数之间的关系式。     

2D and 3D separation of stresses using automated photoelasticity

A procedure for the separation of full-field photoelastic images for use with an automated polariscope is described. Regions of background in the image are identified thus producing the boundary of the model. The shear difference method is used to calculate the components of stress along all raster lines in the image using photoelastic parameters at the boundary points to calculate the initial values of stress. Algorithms were also used to evaluate the stress components along raster lines which did not contain boundary points. A plastic template was used to evaluate the efficiency of the boundary routine. It was found that it was able to identify edges to within approximately one pixel on screen. The complete procedure for stress separation was evaluated using a stress frozen disc in compression and a turbine slot. The values of stress found using the automated polariscope with the stress-separation procedure were found to agree well with theory and with results determined using the method of Tardy compensation and manual analysis. The automated polariscope was also used to analyze three-dimensional stress components along arbitrary lines of a 3D model. A two-model slicing regime was used to analyze a strut subjected to a vertical load. This work was compared to results obtained by Frocht and Guernsey on an identical model machined from Fosterite and subjected to a higher load. Good agreement was found between the results for points away from the region of loading. Significant differences were found near to the load point, however. A finite element analysis of the same problem suggested that this was due to the effects of plasticity.     

数字图像处理技术用于等倾线条纹的处理

本文利用数字图像处理技术,对光弹性条纹进行处理,实现了等倾线与等差线的分离,提高了等倾线的测试精度。同时利用微机系统进行主应力迹线的绘制,提高了主应力迹线绘制的准确性.     

Real-time analysis of isochromatics and isoclinics using the phase-shifting method

This paper studies a new real-time phase-shifting method for the analysis of isochromatic and isoclinic parameters in photoelasticity. By rotating an analyzer at a constant rate and an output quarter-wave plate at a double rate of the analyzer and recording images by a CCD camera continuously, sequential images which brightness is integrated by sensors in a CCD camera during phase-shifting are obtained. Then, the distributions of the isochromatic and isoclinic parameters are obtained immediately and quantitatively using the proposed phase-shifting algorithm. The proposed method can be applied to high-speed inspection of optical elements or glass products. Also, it is expected that slowly varying time-dependent problems can be analyzed by the proposed method.     

Determination of resudual stress fields beneath a Vickers indentation using photoelasticity

Static Vickers indentation tests were performed on Homalite specimens with an intent to obtain the residual stress distribution beneath the indentation. The indented specimens were placed in a circular polariscope to view the fringe patterns corresponding to the induced residual stress. Similitude analysis was later employed to identify the functional relationship between the various parameters related to an indentation test. The analysis resulted in a unified relationship that can assist in the determination of residual stress in nontransparent materials subjected to similar geometric and loading conditions. The shear stress contours provided here can also be used as guidelines to verify constitutive models under complex three-dimensional loads.     

应力波和动光弹等差条纹的分析与判读

介绍了动光弹的基本原理，并从理论上回答了动光弹应用中的一些问题，如惯性力可否忽略等．并在总结回顾动光弹等差条纹级次判读方法的基础上，提出了把应力波理论与等差条纹分析判读相结合的方法，这样不仅使条纹判读的概念更清晰，而且可提高条纹分析和级次判读的准确性．     

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     

Absolute determination of the isochromatic parameter by load-stepping photoelasticity

A new approach to full-field automated photoelasticity is presented in which a circular polariscope is used to enable the isochromatic phase value () to be determined unambiguously and without input of a known isochromatic value obtained using an auxiliary technique. Values of cos are obtained from light-field and dark-field images for three loads of small incremental steps. Using a relatively straight-forward procedure, ramped phase maps for are produced which can be unwrapped using conventional techniques. The resulting distribution of is then found absolutely using information provided by which is the incremental change in the isochromatic phase value between the load steps. The results obtained for disk-in-compression tests presented here in comparison with theoretical solutions demonstrate that the technique is both simple to use and very accurate. A similar approach may be adopted using three wavelengths instead of three load steps.     

计算机辅助光弹性应力分析

本文提出一种新的计算机辅助光弹性应力分析方法,它具有全电子记录、全自动应力场分析、光路简单等优点。相位技术的采用使数据精度大大提高。文中给出了几种实验光路以及提高图象质量的方法,用对经受压圆盘光弹实验证明了方法的可行性。     

The variational principles of coupled systems in photoelasticity

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The use of phase-stepping for the measurement of characteristic parameters in integrated photoelasticity

A three-dimensional photoelastic body can be represented by an optically equivalent model, which consists of a linear retarder, δ, at a certain angle, θ, and a pure rotator, χ. These have been described as the characteristic retardation, δ, and the primary and secondary characteristic directions, θ and θ+χ. Until now these characteristic parameters have only been determined using manual, point-by-point collection methods which are involved and time consuming. Therefore an automated phase-stepping method has been developed to enable the determination of the three characteristic parameters for three-dimensional or integrated photoelasticity. Expressions have been derived to obtain δ, θ and θ+χ from six phase-stepped images. These images are collected using a CCD camera and the full-field data is processed using a standard personal computer. This novel method allows accurate, full-field maps of all three characteristic parameters to be obtained in a relatively short time, which makes full-field tomographic reconstruction of photoelastic data a real possibility. Both are SEM Members.     

提取等倾线的数字图象处理法

本文详细介绍了应用数字图象处理技术进行图象相除和图象相减,获取没有等色线干扰的、精确定位的和倍增信息的等倾线的全场分析方法,并作了典型验证实验,为光测弹性力学的改革提供了途径和理论依据。     

Extracting inter-particle forces in opaque granular materials: Beyond photoelasticity

This paper presents the first example of inter-particle force inference in real granular materials using an improved version of the methodology known as the Granular Element Method (GEM). GEM combines experimental imaging techniques with equations governing particle behavior to allow force inference in cohesionless materials with grains of arbitrary shape, texture, and opacity. This novel capability serves as a useful tool for experimentally characterizing granular materials, and provides a new means for investigating force networks. In addition to an experimental example, this paper presents a precise mathematical formulation of the inverse problem involving the governing equations and illustrates solution strategies.     

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 the Fourier transform in electronic speckle photography

In speckle photograph technology, to determine the displacement of the points on the surface of the measured body, the conventional method is to put the film which has recorded the speckle patterns before and after displacement into a system of optical Fourier transforms. After filtering on the spectrum plane, the experimentalist can obtain the displacement information from the interference pattern on the image plane. Instead of setting up a complex optical Fourier transform system, we consider the speckle field as a light intensity function of 2 dimensions, which will change with different positioning of the points. After working on the function's discrete Fourier transform (DFT), according to one of the properties of Fourier transformation, the displacement of the measured point is involved in the phase of its spectrum. Having extracted the displacement information from the phase, we obtain the distribution of the displacement field. In this paper, we deduce the expression for the displacement field by using Fourier transformation under conditions of both equal and unequal displacement and show their applications.     

斜拉桥塔锚固区光弹性应力分析与等倾线图像处理

本文采用光弹性冻结应力法，对独塔斜拉桥塔锚固区进行了应力分析，给出了有关截面的边界应力分布及主应力迹线，从而为优化工程设计提供了重要的参考依据。本文还提出了一种新的提取等倾线的方法──图像对数增强相减法，实现了等倾线与等差线条纹的分离，同时使条纹得到了细化，提高了处理等倾线的速度与准确度。编制了自动绘制主应力迹线的程序，利用它绘制了桥塔截面的次主应力迹线。     

结构局部应力场全景光弹测量方法研究

针对大型结构局部复杂应力场直观测量的需求,提出了全景光弹测试技术。在经典光弹实验系统中,偏振系统的大小限制了试件尺寸;同时理想光源系统的复杂性降低了实验测量的机动性。由于这两方面的限制导致经典光弹方法在测量大型复杂结构时存在较多的非确定性。为此建立了一个等效偏振系统,通过在试件表面镀制偏振膜的办法将偏振系统依附于试件表面,以解决试件尺寸限制问题。同时放弃了理想光源而采用自然光源,以提高测量的机动性。通过四点弯曲实验来检验本文方法的有效性。结果表明提出的测量技术是有效可靠的。利用本文提出的方法,对实际桥梁中缩尺试件的局部应力场进行了测量,得到了可用于结构模型修正的应力中心线信息。     

细观力学实验技术综述

本文以应用较多的光测技术为主,综述了细观力学测量中光、声、磁、射线等试验技术.可望在细观研究中发挥重要作用的光学方法有全息干涉、激光或非相干光散斑法、云纹和云纹干涉法、格子法、各种显微镜技术、衍射法和射线技术.在各种方法中,图像处理系统和技术都将发挥很大的作用。