Design of a focal-plane array thermographic system for stress analysis

A state-of-the-art thermographic system has been developed based on a 512×512 focal-plane array thermal imager. The system, dubbed FAST (for focal-plane array for synchronous thermography) is able to deliver high-resolution full-field thermoelastic stress scans in minutes rather than hours. The paper itemizes the hardware components together with their functions.The digital signal processing required is described in detail as well as some of the problems encountered and how they were solved. Finally a thermoelastic image is displayed illustrating the capability of this system.Permission is granted toExperimental Mechanics to publish the paper on a nonexclusive one-time basis in the journal. This agreement is subject to the Commonwealth of Australia retaining Copyright of the paper, the author and source of the paper has been acknowledged. No meaningful changes have been made to the paper without the prior consent of the author.     

Thermoelastic stress analysis under nonadiabatic conditions

Thermoelastic stress analysis is a full-field stress measurement technique complementary to local techniques like strain gages. Generally, the heat transfer inside the material is neglected with respect to the frequency of the cyclical loading. An adiabaticity criterion is established to assert this simplification as a function of the thermal diffusion length and the spatial stress gradients. Under nonadiabatic conditions, heat diffusion attenuates the spatial temperature gradients, which leads to an underestimation of stress concentrations. Analytical and numerical considerations allow for the quantification of the spatial resolution. Finally, several inverse techniques can restore the thermally attenuated contrasts.     

Path dependency in thermoelastic stress analysis

In this paper we review the basis of the technique for thermoelastic stress analysis and, in particular, we examine the relationship between the theory and the technique in common practice. The theory of thermoelastic stress analysis is based on the thermomechanical behavior of bodies, which takes strain and temperature as state variables that are path-independent, whereas the conventional instrumentation used in thermoelastic stress analysis involves an integration of photon flux derived from a body's surface temperature, and hence is time- and path-dependent. This inconsistency might be negligible for some, or perhaps most, applications. However, in those cases where the waveform of the loading is irregular, experiments have shown that the difference can be significant. The nature of most apparatus for thermoelastic stress analysis implies that this results is important when conducting experiments in which the forcing signal is unknown or not sinusoidal. J.R. Estrada Estrada was a Research Student and E.A. Patterson (SEM Member) was a Professor, Department of Mechanical Engineering, University of Sheffield, Mappin Street, Sheffield, S 3JD, UK.     

Retardation of a crack with connections between the faces using an induced thermoelastic stress field

This paper considers local temperature variations near the tip of a crack in the presence of regions in which the crack faces interact. It is assumed that these regions are adjacent to the crack tip and are comparable in size to the crack size. The problem of local temperature variations consists of delay or retardation of crack growth. For a crack with connections between the crack faces subjected to external tensile loads, an induced thermoelastic stress field, and the stresses at the connections preventing crack opening, the boundary-value problem of the equilibrium of the crack reduces to a system of nonlinear singular integrodifferential equations with a Cauchy kernel. The normal and tangential stresses at the connections are found by solving this system of equations. The stress intensity factors are calculated. The energy characteristics of cracks with tip regions are considered. The limiting equilibrium condition for cracks with tip regions is formulated using the criterion of limiting stretching of the connections.Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 46, No. 1, pp. 133–143, January–February, 2005     

疲劳特性的红外热像定量分析方法研究进展

定量红外热像法, 作为一种无损、全场、实时、非接触的测试手段, 不仅能够用于对材料内部缺陷的无损检测, 而且能够对在役结构的疲劳损伤演化状态进行识别. 定量红外热像法还能够快速预测材料的疲劳极限和S-N (stress-number of cycles) 曲线, 实验周期短, 成本低. 文中较为系统地综述了定量红外热像法的发展现状及应用, 讨论了定量红外热像法应用过程中的几个重点问题. 最后总结展望了定量红外热像法的未来发展方向及应用前景.     

CCD automated polariscope system and the stress analysis methods

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Image enhancement and stress separation of thermoelastically measured data under random loading

Traditional thermoelastic stress analysis (TSA) presupposes that the structure being analyzed is cyclically loaded at a constant amplitude and frequency. This approach typically has been used to satisfy the adiabatic reversible assumptions. The authors employ an alternative signal analysis technique that enables one to evaluate the magnitude of the individual components of stress in a component subjected to a loading that is random in both frequency and magnitude. However, the nature of the measured information does not change; i.e., data are inherently noisy, and edge information is unreliable. The latter two aspects have caused many thermoelastic stress analyses to be more qualitative than quantitative. The present paper emphasizes developing the TSA technique into a practical, noncontacting quantitative method for stress analyzing actual engineering structures that are randomly loaded. In particular, ability to determine the individual stresses thermoelastically under random loading is demonstrated.     

Assembly stress for the reduction of stress concentration

A systematic study utilizing the experimental method of reflected photoelasticity was undertaken to determine the effects of assembly stress on the stresses around circular holes under uniaxial tension. The assembly stresses are the result of the contact and bearing stresses between the bolts and the member. It has been concluded that assembly stress contributes to reducing the stresses around the hole.     

Reproducibility and reliability of the response from four SPATE systems

The SPATE (stress pattern analysis by thermal emissions) system is currently the standard equipment for thermoelastic stress analysis (TSA). A carefully designed test program that studied the behavior of four independent SPATE systems over an 8-month period is described. The response of each system is compared with the response of the other systems in the study.     

Numerical simulation of stress intensity factors via the thermoelastic technique

The purpose of this study is to investigate the accuracy of the least squares method for finding the in-plane stress intensity factorsK _I andK _II using thermoelastic data from isotropic materials. To fully understand the idealized condition ofK _I andK _II calculated from thermoelastic experiments, the total stress field calculated from finite element analysis is used to take the place of data obtained from real thermoelastic experiments. In the finite element analysis, theJ-integral is also calculated to compare with (K _I ² +K _II ² )/E evaluated by the least squares method. The stress fields near the crack tip are dominated by the two stress intensity factors; however, the edge effect will cause inaccuracy of the thermoelastic data near the crack tip. Furthermore, the scan area of thermoelastic experiments cannot be too small. Therefore, we suggest that three or four terms of stress function be included in the least squares method for evaluating stress intensity factors via the thermoelastic technique. In the idealized condition, the error can be smaller than 3 percent from our numerical simulations. If only ther ^–1/2 term (K _I andK _II ) is included in the least squares method, even in the idealized case the error can be up to 20 percent.     

Thermoelastic stress analysis applied to fully reversed bending fatigue

The thermoelastic effect has been used to study stress distributions in a number of in-plane loading problems. Analysis of the temperature distribution has been largely limited to isotropic one-dimensional approximations with heat transfer through the thickness of the specimen. In sonic fatigue, specimens undergo fully reversed bending with a stress gradient along the length of the specimen as well as through the thickness. This has also been modeled as a one-dimensional heat transfer problem with negligible heat transfer along the specimen length. The authors solve this as a two-dimensional problem for an isotropic material to determine the effect of heat transfer.     

Strain-gradient stress analysis in saint-venant bending

Light beam deflections caused by stress or strain gradients are investigated analytically and experimentally in homogeneous beam specimens which are subjected to a particular case of flexure with shear. This study is a generalization of the prior analytical-experimental examination of strain-gradient light deflections produced in stressed plates, which had concentrated on the simplest case where information of interest is collected along a line of symmetry of the stress field. Main purpose of the present investigation is to document the efficacy of the strain-gradient method in analysis of the general case of stress state. The most interesting stress state is that in a beam subjected to the Saint-Venant bending, where the transversal and the longitudinal axes of the beam are in pure shear. The obtained results are compared with the predictions of the developed analytical models and with the predictions of Filon's stress function. The procedures of evaluating the photoelastic and material coefficients using strain-gradient techniques were tested positively. Dedicated to Prof. Dr. Horst Lippmann, Muenchen, on the occasion of the 65th anniversary of his birthday The project was supported by the Natural Sciences and Engineering Research Council of Canada and the NATO Scientific Affairs Division     

Stress separation of thermoelastically measured isopachics

Knowledge of individual stresses is necessary for the quantitative evaluation of engineering components. A finite element type scheme is described for accurately and efficiently separating measured isopachic data into individual in-plane stress components. Although the governing equations require complete boundary conditions to be well posed, illustrative examples demonstrate the method's ability to provide accurate individual stresses throughout a member using incomplete boundary conditions.     

Determining stress intensity factors in orthotropic composites from far-field measured temperatures

We demonstrate the ability to determine stress intensity factors in orthotropic materials directly from measured temperatures away from the crack and using far-field expressions for the stresses. This is advantageous, recognizing that recorded thermoelastic data can be very unreliable near the tip of a crack. In addition to singular terms that govern in the immediate vicinity of the crack tip, the present series expressions for the stresses contain higher-order finite terms. Little measured input information is needed and data acquisition positions can be selected largely at the user's discretion.     

A new method for stress analysis of a cyclically symmetric structure

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应力三轴度的有限元计算修正

对某高强度钢制成的光滑圆棒和缺口圆棒进行了系列准静态拉伸实验,采用ABAQUS对每个试 件进行了数值模拟,得到了该材料的真实应力应变曲线,拟合出了J-C本构模型和失效模型的部分材料常数。 最后,对该高强度钢制成的平板进行了撞击实验,并用得到的J-C模型对平板撞击实验进行了数值模拟,计算 结果与实验结果吻合很好,证明利用数值模拟并修正应力三轴度的方法是可行的。     

Photoelastic analysis of the singular stress field in a bimaterial wedge

This paper presents an experimental investigation of the singular stress field near the vertex of a bimaterial wedge using a digital photoelastic technique. Special attention is given to the casting of bimaterial wedge specimens and analysis technique for extracting stress intensity factors from photoelastic samples. Different bimaterial wedge specimens are made of two different photoelastic materials bonded through a special casting procedure and loaded in simple tension. A new multiple-parameter method is developed to obtain the stress intensity factor reliably from the isochromatic fringe patterns and the series representation of the stress field at the vertex of the wedge. Experimental results are compared with finite element predictions, and good agreement is observed.     

承压薄壁容器应力分析的讨论

通过讲述与引伸承压薄壁圆筒的应力分析, 引导学生关联几种承力构件 的受力特征, 并展示其在日常生活中的应用实例, 可作为该部分课堂教学的补充材料.     

Hybrid stress analysis of perforated composites using strain gages

A strain gage hybrid method is described for determining individual stresses on the boundary and in the neighborhood of cutouts in orthotropic composites. Results agree with independent measurements and finite element analysis. Few measured strain data are needed, and the measured strains originate away from the hole. Ability to determine the stresses on the edge of a cutout from nonboundary measurements recognizes the difficulties in obtaining reliable measurements very near an edge while circumventing the challenge of attempting to bond gages to the transverse curved surface of a small hole or notch. The method also alleviates the problem of not knowing a priori where the most serious stress will occur on the geometric boundary and, hence, where to locate strain gages.     

Three-dimensional closed-form analysis of the stress field at rectangular corners of layered plates

Summary A refined and expanded closed-form analytic tool for the calculation of the stress field at free corners of thermally loaded cross-ply and angle-ply laminate structures is developed. Based on adequate approximations for the inplane stresses, the integrated equilibrium conditions yield a full-scale 3D stress field in the vicinity of a free corner. After adjusting the stress field to stress-free conditions at the free edges and facings and obeying conditions of continuity at the interfaces, the remaining free parameters in the stress representation are calculated by minimization of the complementary potential of the laminate. Comparison of the closed-form results with finite element computations shows a good agreement.