存在初应力时光弹性复合材料条纹值标定研究

详细研究了存在初应力时光弹性复合材料条纹值标定问题，提出了存在初应力时圆盘标定ｆＬＴ值的方法。分析用直条试件和圆盘试件进行了实例标定，两种标定结果一致，而且标定试验具有自检功能。     

光弹性复合材料对径受压圆板应力分析与材料条纹值测定

本文根据 Okubo 的各向异性分析,对纤维增强光弹性复合材料制成的对径受纹压圆板进行了研究;在此基础上提出了采用一个对径受压圆板测定光弹性复合材料三个条值(f_L,f_T,f_(LT))的方法。经与由三棵光弹性复合材料直条试样测定的相应材料条纹值数据比较,发现现在提出的方法更为简便、可靠。     

Development and assessment of a single-image fringe projection method for dynamic applications

A single-image fringe projection profiling method suitable for dynamic applications was developed by combining an accurate camera calibration procedure and improved phase extraction procedures. The improved phase extraction process used a modified Hilbert transform with Laplacian pyramid algorithms to improve measurement accuracy. The camera calibration method used an accurate pinhole camera model and pixel-by-pixel calibration of the phase-height relationship. Numerical simulations and controlled baseline experiments were performed to quantify key error sources in the measurement process and verify the accuracy of the approach. Results from numerical simulations indicate that the resulting phase error can be reduced to less than 0.02 radians provided that parameters such as fringe spacing, random measured intensity noise, fringe contrast and frequency of spatial intensity noise are carefully controlled. Experimental results show that the effects of random temporal and spatial noise in typical CCD cameras for single fringe images limits the accuracy of the method to 0.04 radians in most applications. Quantitative results from application of the fringe projection method are in very good agreement with numerical predictions, demonstrating that it is possible to design both a fringe projection system and a measurement process to achieve a prespecified accuracy and resolution in the point-to-point measurement of the spatial (X, Y, Z) positions.     

Experimental observation of stress waves propagating in laminated composites

The theoretical research on stress waves propagating in laminated composites has been reported by many authors. However, there has been little work on experimental studies of stress waves in those materials. This paper presents an experimental investigation on stress waves propagating parallel to the layers of a laminated composite. A sandwich laminated composite consisting of two aluminum facings and an epoxy core is used as a specimen. The stress wave in the specimen is observed by use of high-speed holographic interferometry with a pulsed laser. In order to obtain the relative fringe orders, the interference fringe pattern in the reconstructed image is treated as an image-processing system with a personal computer. For the calculation of the in-plane displacement, an approximate relative-fringe-order method is used. The in-plane displacements obtained at some sampling points on the surface are smoothed by using a spline function. Distributions of the in-plane displacement and the shear stress are then obtained quantitatively over the whole analyzed field.     

A texture evolution model in cubic-orthotropic polycrystalline system

A new methodology based on a conservation principle in the orientation space is developed to simulate the texture evolution in a cubic-orthotropic polycrystalline system. A least squares error method was used to improve the accuracy of the simulation results obtained from the texture evolution function. The model is applied to uniaxial tension, compression and rolling for a large deformation of more than 50% using a single evolution parameter. The validity and application range of this new model are discussed by simulating and predicting texture evolution during different loading conditions. The new methodology provides a family of texture evolution paths and streamlines which empowers the materials designer to optimize the desired microstructure.     

Shrinkage Measurement in Concrete Materials using Cure Reference Method

This paper describes the details of using Cure Reference Method (CRM) to determine the shrinkage that develops in concrete materials. The technique involves the replication of diffraction grating on the concrete specimen during curing. After demolded, the specimen is stored in a chamber where a specific drying condition is maintained for 6 days. Every day in this period, the specimen is removed from the chamber and a set of the consecutive moiré fringe patterns are recorded with the help of a specially-designed stage. An automated fringe analysis program is developed to obtain the full-field displacement and strain information. Shrinkage as a function of location, time, and drying conditions is measured. A numerical method is developed in order to obtain material properties from the complex geometry used in the tests. The test in different drying condition and the ring test are performed, and their results are compared with FEA to validate the constructed model.     

Automatic order-determination of transient in-plane displacement moire patterns

This paper presents a fringe-carrier method that eliminates sign ambiguity of transient moire fringes which can be used to automatically determine the relative orders. A fringe carrier is preset in the static state of the specimen and the dynamic in-plane displacements are recorded as the modulation to the frequency of the carrier fringes when the specimen is loaded by impact. According to a modulating criterion developed from the modulation degree, the fringes of the transient moire patterns keep monotonical in orders so that they can be automatically encoded in grey levels by a digital image system. The moire orders purely caused by dynamic loadings are evaluated by subtracting the grey-value of the unmodulated carrier image from that of the modulated carrier images encoded by their orders. With the subtracted moire orders the strain components can be obtained, and, correspondingly, the histograms of dynamic displacement moire images are shown with order variation by image-difference. The project supported by Alexander von Humboldt Foundation of Germany and the National Natural Science Foundation of China     

On the stress-optic law for orthotropic-model materials in biaxial-stress fields

The stress-optic law for othotropic-model materials, proposed by Sampson on the basis of a simple analogy to the isotropic-model materials, is examined for biaxial-stress fields. The stress-optic law is reduced to a simple form for special cases. It is also shown that the zero-order isochromatic fringe corresponds to an isotropic state of stress only in the case of balanced laminates. A glass-fiber-reinforced plastic disk with the glass fibers in only one direction is examined under diametral compression photoelastically and by means of strain-gage rosettes, with the loading direction perpendicular and at 45 deg to the reinforcement direction. The fringe order along the horizontal diameter is computed from the simplified stress-optic law making use of stress values from strain-gage readings and compared with the observed fringe order. Based on a fairly good agreement of the fringe orders, it is shown that a circular-disk specimen can be used to calibrate an orthotropic-model material. The three independent material-fringe values,f _L ,f _T ,f _LT , can be found from measurements of the fringe order and the strains at the center of the disk for the three cases of loading perpendicular, parallel and at 45 deg to the reinforcement direction.     

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.     

结合数码显微镜的数字散斑相关方法精度分析及应用

结合数字散斑相关方法和一种新型的显微镜数码显微镜,提出了一种测量多晶材料晶粒尺度面内变形的新方法,并通过零变形校准实验、重聚焦实验和平移实验等一系列验证实验分析了该方法的精度和实用性.作为应用实例,对一种镍基合金试件进行了单向拉伸和疲劳实验,得到了晶粒尺度下具有较大应变梯度的应变分布图像.结果表明,该方法能够得到精确的位移测量数据,是一种理想的测量晶粒尺度变形的光测方法.     

A new application of isodensitometry in the moiré technique

Some years ago, isodensitometry was introduced into the moiré technique in order to improve the sensitivity of this method. The application of the Sabattier effect gives now a new possibility to produce more lines of equal displacement. The obtained experimental data are the additional lines and the moiré fringes of full or half order. This method is applied on a beam under pure bending and a circular ring loaded diametrically. It will be shown in this paper that the calibration of the displacement curves can be accomplished by a calibration bar or by ‘self calibration’ which will be done on the same model to be studied. Further, the photographic process can be influenced in a way that different lines of equal displacement can be produced for a given system of moiré fringes. The additional lines are shifted here by choosing different times of exposure.     

A Shear Compression Disk Specimen with Controlled Stress Triaxiality under Quasi-Static Loading

This paper introduces a double shear axisymmetric specimen (Shear Compression Disk) and the methodology to extract flow and fracture properties of ductile materials, under various stress triaxiality levels. A thorough numerical investigation of the experimental set-up is performed, which reveals that the stresses are quite uniformly distributed in the gauge section during all the stages of the test. The attainable level of stress triaxiality (with pressures of up to 1.9 GPa) ranges from −0.1 to 1, which can be adjusted by a proper choice of geometrical parameters of the specimen. The methodology is implemented to quasi-static experiments on 4340 Steel and Aluminum 7075-T651 specimens. The flow properties are compared to those obtained by upsetting cylinders and show a very good agreement. For these materials it is observed that, contrary to the fracture strain, the flow properties are quite insensitive to the level of stress triaxiality. The fracture strain of the aluminum alloy increases with triaxiality and may be fitted with an exponential polynomial of the type suggested by [27]. These examples demonstrate the potential of the new specimen to obtain flow and fracture properties of ductile materials under controlled triaxiality.     

Development of a Hybrid Method of Reflection Photoelasticity for Crack Problems in Anisotropic Plates

Reflection type photoelastic experiment can be used more effectively than a transmission type photoelastic experiment especially in industrial fields. Moreover, composite materials have been widely used in engineering applications and structures because of their outstanding advantages which individual isotropic components do not have. The development of these materials requires a promising technique such as reflection photoelasticity to analyze their behaviors in service. Unfortunately, there are few experimental studies based on this technique. Therefore, a hybrid method based on this technique was developed in this research to analyze the fracture behavior of opaque anisotropic materials. The application of this method will help to understand the fracture behaviors of anisotropic materials used in engineering components and structures. The validity of this method was verified by comparison of the results obtained from this method with ones obtained from the hybrid methods for isotropic material on the same isotropic specimen. The reflection type photoelastic experiment for orthotropic materials was then applied to orthotropic plates with a central crack of various inclination angles. Using this hybrid method for anisotropic materials, stress intensity factors and separated stress components were obtained at the vicinity of the crack-tip in orthotropic plates from only the isochromatic fringe patterns of the isotropic coating material.     

Two-color Grating Aids Moiré-fringe Sharpening

It is well known in moiré analysis that beneficial fringe sharpening occurs when a pair of complementary gratings are employed in place of a single conventional grating to record successively the unloaded and loaded states of a model or a specimen. In this regard we propose that just a single, select, two-color grating in conjunction with suitable color filters can substitute the complementary pair. The idea is new and simple to apply.     

Application of the least-squares method to elastic and photoelastic calibration of orthotropic composites

New experimental methods continue to be proposed for determining the in-plane elastic constants of orthotropic composite materials, as no single method has been found completely satisfactory. In particular, the in-plane shear modulus is difficult to measure. In this paper, an orthotropic half-plane subjected to a concentrated edge load is proposed as a calibration specimen. Photoelastic calibration of orthotropic-birefringent composites is also difficult due to the need to conserve the commercially unavailable, difficult-to-make model materials and the absence of many closed-form solutions. In this paper, the orthotropic half-plane specimen is proposed for photoelastic calibration of birefringent-orthotropic model materials also. For elastic and photoelastic calibration of orthotropic materials, the least-squares procedure is used for processing the experimental data.     

Fracture investigation at V-notch tip using coherent gradient sensing (CGS)

Local deformation field and fracture characterization of mode I V-notch tip are studied using coherent gradient sensing (CGS). First, the governing equations that relate to the CGS measurements and the elastic solution at mode I V-notch tip are derived in terms of the stress intensity factor, material constant, notch angle and fringe order. Then, a series of CGS fringe patterns of mode I V-notch are simulated, and the effects of the notch angle on the shape and size of CGS fringe pattern are analyzed. Finally, the local deformation field and fracture characterization of mode I V-notch tip with different V-notch angles are experimentally investigated using three-point-bending specimen via CGS method. The CGS interference fringe patterns obtained from experiments and simulations show a good agreement. The stress intensity factor obtained from CGS measurements shows a good agreement with finite element results under K-dominant assumption.     

大视场双目投影条纹方法中的相机参数标定与优化研究

投影条纹法具有高精度、高分辨的特点,且实验设备简单,对实验环境要求低,适宜于不同尺度的三维形貌测量.双目投影条纹法通常采用有标准参照物的相机标定方法获取相机参数,以实现物体表面三维形貌重建.然而,在大型结构的三维形貌测量中参照物的相对尺度较小,传统的基于重投影的相机标定方法在特征检测中引入的误差会被放大,从而影响三维表...     

The micro-connection of the second-order moiré fringes and its order determination,and the determination of the boundary strains

The micro-connection method for determining the centre lines of second-order moiré fringes presented in this paper can enhance the accuracy in measuring strain fields, and generally can determine the absolute order of the second-order moiré fringes. The strain data obtained from curved beam experiment are in good accordance with theory. The relationship between the second-order moiré fringe and the strains at the specific points of the specimen is derived. Hence a reciprocate shift method is presented for determining strain distributions in the non-overlapping region of the shifted moiré patterns, (usually in the region of specimen where no second-order moiré fringes occur is the boundary region).     

Application of holography to photoelasticity

This paper discusses the exploratory application of holography to photoelasticity. It is shown that the two-beam holographic method may be used to record the birefringent pattern of a standard two-dimensional photoelastic specimen. A special type of polariscope is required. A discussion is presented of the various arrangements required to produce isochromatic, isoclinic, isopachic and combined isopachic and isochromatic fringe patterns. The three-dimensional nature of the holograph is utilized to separate the principal stresses by means of oblique incidence. Both normal- and oblique-incidence fringe patterns are recorded on a single hologram. A brief discussion of the extension to the dynamic case is presented.     

Application of birefringent coatings to glass-fiber-reinforced plastics

The application of birefringent coatings to plane-stress problems associated with orthotropic-glassreinforced plastic materials is treated. The improvement in the sensitivity of the birefringent-coating method due to the high strength and low modulus of the glassfiber-reinforced plastic materials is noted. Next, the effect of a mismatch in Poisson's ratio between the specimen material and coating is examined and a correction factor is developed which permits determination of boundary stresses even when the mismatch is large. Finally, the stress-strain relations for an orthotropic material are reviewed and an example of a nonsymmetric stress distribution associated with a symmetric fringe pattern is covered.