Stress-concentration factors for fatigue configurations

Stress concentrations in thin-plate configurations commonly used in fatigue bending tests were determined by photoelastic means. The test configurations were rectangular and tapered plates with a centrally located hole. These configurations were machined from plates constructed by cementing together two sheets of similar photoelastic material. A reflective-type cement was used. In this manner, photoelastic measurements for the bending case were indicative of the average between maximum fiber stress and the stress at the reflective surface. The maximum fiber stress was then computed assuming that plane sections remain plane. Although bending was of primary concern, tension tests were also performed. The measured stress concentrations are compared with available analytical solutions. In the case of bending, the results are compared with infinite-plate solutions since the perforated finite-width plate bending problem has not been solved.     

一种动光弹模型材料的制作方法及其应用

基于#618环氧树脂、甲基六氢苯酐、促进剂DMP-30、环氧树脂消泡剂四种原料,提出制作光弹性模型的新配方和新方法,并利用单轴压缩实验、电测法和动态光弹法分别测定了制作的光弹模型的动态力学参数。新方法工艺简单,制作周期短,对人体无害。制作的光弹模型初始应力小,表面光洁,质地均匀,透光性好,光学灵敏度高,具有良好的机械加工和切削性能。通过三点弯曲梁冲击实验,得到了清晰的光弹等差条纹图像,验证了该配方和方法制作的模型可以应用于动态光弹性实验。     

Stress concentrations around multiple windows in a high-pressure vessel

A pressure vessel with four radial windows was designed in which to conduct equation-of-state experiments at pressures up to 1 GPa (145,000 psi). To determine the relative safety of the vessel, the stress distribution in the cylindrical sidewalls, particularly the stress concentration near the window openings, was studied. Conventional analysis was used for the initial evaluation but was found to give somewhat higher stress values than were actually present. Better determinations of the stress distribution were obtained by photoelastic analysis of a model of the pressurevessel linear and by proof tests with strain gages of the actual liner and the assembled pressure vessel. Apparently, the tapered sapphire windows, loaded in compression by the internal pressure in the vessel, modify the stress distribution sufficiently to allow operation at higher pressures than otherwise would be attainable. Design features of the pressure vessel are presented, together with a synoptic stress analysis, a photoelastic analysis, and results of the strain-gage studies.     

Use of electrical-resistance strain elements in three-dimensional stress analysis

A method for constructing resistance-wire strain gages which may be imbedded in a plastic model without materially altering the stress pattern in the model is presented. The methods used to calibrate the gages and the photoelastic tests made to investigate the effect of the gages on the stress pattern are described. The application of this new three-dimensional technique to evaluate the stress distribution in a thick-walled cylindrical pressure vessel is discussed. Correlation between experimental data and calculated values is given.     

Stress-intensity distributions for corner cracks emanating from open holes in plates of finite width

A series of frozen stress photoelastic tests is carried out in the present investigation. Linear Elastic Fracture Mechanics is used to analyze the problem of a part-through corner crack at an open hole in a plate of finite width loaded in tension. The photoelastic modeling capability of three-dimensional subcritical crack growth problems is assessed. This is achieved by comparing monotonically grown flaw shapes in epoxy models with crack profiles relative to fatigue crack propagation tests in a different material. Photoelastic stress-intensity factor distributions are checked against numerical results obtained for quarter elliptical geometries. The dependence of stress intensity factors on flaw shape is also discussed.     

复合材料光弹性分析的工程方法

本文在推导均衡光弹性复合材料应力(应变)—光定律的基础上,导出了均衡光弹性复合材料的近似应变—光定律。然后加以推广,提出适用于一般光弹性复合材料的近似应变—光定律.精度分析及实验验证表明:作为工程计算,在采用本文推荐的参数时,光弹性复合材料可视作各向同性材料进行光弹性分析。     

Measurement of thermal stresses within an experimental nuclear-reactor vessel head

This paper describes certain tests and techniques employed in measuring stresses within an experimental nuclear-reactor head of unusual design. The incorporation of certain desired design features necessitated that the head be extremely thick. Due to the thickness and its complex geometry, it was considered desirable to determine stress distribution within the head under conditions of steady-state pressure combined with rapid heating and cooling transients within the reactor, in order to determine safe limits for the operation of the head. A photoelastic study of a three-dimensional model of the reactor head was completed in 1956; this study permitted prediction of the stress distribution throughout the head as a function of internal pressure, but it was not possible to assimilate the head thermal stresses by photoelastic means. It is the purpose of this paper to describe the technique employed in measuring thermal stresses in the interior of the head, under simulated operating conditions of steady-state pressure and temperature transients.     

Wave propagation in layered model due to point-source loading in low-impedance medium

Dynamic photoelastic and moiré techniques were used to study the incident, reflected and refracted waves in a layered model. The model, consisting of two birefringent layers with an impedance ratio of approximately 2∶1, was subjected to point-source explosive loading in the low-impedance medium. The isochromatic and moiré-fringe patterns were photographed with a Cranz-Schardin multiplespark camera operating at a rate of 200,000 frames per second. Propagation and attenuation characteristics of waves in both layers were studied. The formation of head-waves at the interface was clearly evident. Stress determinations were made in some instances using photoelastic and moiré data.     

复合材料光弹性分析的近似方法

本文提出了一种用于光弹性复合材料的简化应变——光学定律。按照这一简化定律。模型材料的主应变差和主应变方向只要利用光弹性实验测出的等差线与等倾线即可求得。些是一种正交异性光弹性分析的近似方法,这一方法所得结果与实验数据比较,最大误差在10％左右。由于采用简化应变——光学定律使得正交异性光弹性分析工作大为简便,因此它是一种适合于工程应用的近似方法。     

Bonding of birefringent coatings to composite flat-belt-drive materials

The bonding of birefringent coatings to composite flat-belt-drive materials is described. The belts were subsequently subjected to tension tests without apparent disassociation of the coating from the belt. Additionally, lengths of photoelastic coating bonded to endless flat belts run at speeds approaching 50 m/s exhibited no evidence of disassociation.     

Photoelastic assessment of finite-element analysis of a pipe tee

A three-dimensional photoelastic analysis and a finite-element analysis of a pressurized pipe tee are compared. The pipe tee is a 152-mm typical commercial straight buttwelding tee, having a nominal wall thickness of 11.0 mm. The finite-element program employs a doubly curved shell element in which stresses vary linearly through the thickness. The three-dimensional photoelastic model was cast from the pattern of an actual pipe tee. The model was pressurized and stress-frozen. Its principal planes were analyzed for in-plane surface stresses, then subsliced and analyzed for transverse stresses. The photoelastic stresses are graphically compared to those from finite elements. For large regions of the tee there is substantial agreement in the stresses from the two methods. Considerable disagreement is revealed in the sharply curved corners between the main pipe and the stem. Paper was presented at the 1986 SEM Spring Conference on Experimental Mechanics held in New Orleans, LA on June 8–13.     

应用新型动光弹系统研究某设备受冲击载荷作用

结构受到冲击时,其应力分布与在静荷载作用下完全不同.本丈应用新型动光弹实验系统研究了某设备在冲击载荷作用下其内部动态应力分布情况.该系统由部分传统的动光弹实验装置、高速摄影系统和数字光弹图像处理系统组成,并且采用激光光源、落锤冲击载荷加载装置.加工制作了该设备模型的光弹性模型,应用到冲击载荷作用下的实验研究中,得到了该...     

发光光弹性涂层荧光信号强度的影响因素

为提高发光光弹性涂层方法中涂层的荧光信号强度,以液态E-44型环氧树脂为光弹性材料、罗丹明B为荧光染料,制作了新、旧两种不同结构的发光光弹性涂层.通过对新、旧两种结构发光光弹性涂层受465nm光波激发时荧光发射光谱的比较,说明新的发光光弹性涂层结构比原有发光光弹性涂层在提高测量信号强度方面的优势,测量结果还表明:发光光弹性涂层测量信号的强度提高还与合理选择涂层中荧光染料浓度、荧光信号发射波长有直接关系.     

Photoelasticity for stress analysis under high hydrostatic pressure

Experimental research relative to pressure effects on the mechanical behavior of materials is frequently handicapped by the difficulties associated with making load and deformation measurements in a hostile environment. The application of photoelasticity in high-pressure experiments provides a means for studying the effect of hydrostatic stress on varying stress fields. The purpose of this paper is to examine the feasibility of using the photoelastic method of stress analysis in a high-pressure environment. The unusual feature of this application is the finite elastic deformations suffered by the photoelastic model under high pressure. As a result, the mechanical and stress-optical properties of the model materials are functions of pressure. Another important feature in this study is the selection of a suitable model material. Since the model must come into contact with the liquid pressure media, chemical and absorption resistance are essential considerations. Although it was found that photoelastic investigations can be carried out at high pressure, limitations are imposed by the presence of the optical vessel and pressurized fluid.     

Ultimate tests of thick shells

Experimental investigation of the elastoplastic and ultimate load response of thick hemispheres with cylindrical skirts is presented. A shell with a hinged boundary condition, loaded symmetrically, and one having a fixed edge, with unsymmetric lateral load, were tested. Electrical-resistance bonded foil strain gages and photoelastic surface coatings were used in monitoring the strain response while spring-activated dial gages were used in recording displacements. Both panchromatic and color still photographs were utilized in observing the photoelastic patterns, while the failure mode of one model was recorded by means of high-speed motion pictures.     

Linear limit stresses of some photoelastic and mechanical model materials

Certain laws of similarity must be observed in structural-model analyses. In this paper, one aspect of model similarity—that of linearity—is examined quite extensively. Most model analyses assume that both prototype and model materials obey Hooke's law. But the plastics often used for structural or photoelastic models are viscoelastic or photoviscoelastic. The stress-strain and stress-birefringence relations are time dependent and may be nonlinear. Through careful calibration of model materials and proper design of model tests, potential errors due to the time dependence of material properties can usually be avoided. If the results of the test are to be interpreted conveniently and accurately, the stresses in the model material must be within the linear range. This range is limited and time dependent for most plastics. The linear range may extend only to stresses considerably below the ultimate or fracture strength of the material. Hence, analyses based don linearity may be in error if the initial stresses are too high and/or if given stresses are sustained too long before desired information is collected. The stresses which limit the linear range, called linear limit stresses, were determined for both stress-strain and stress-birefringence relations for four commonly used plastics: CR-39 (Cast Optics Co.), PS-1 and PS-2 (Photolastic, Inc.) and P6-K (B.A.S.F., Germany). A graphical presentation of the time-dependent photoelastic and mechanical properties is employed. It was concluded that linear limit stresses for birefringence are approximately equal to those based on strain and can therefore be used to establish, within reasonable bounds of accuracy, the linear range of behavior of the material.     

Photoelastic model measurement with rotated principal axes by scattered-light photoelasticity

The method of measurement of a three-dimensional photoelastic model with rotated principal axes has not yet been fully experimentally established. It is known that a three-dimensional photoelastic model can be reduced to an optically equivalent model. In this paper, the optically equivalent model is realized from a stratified model consisting of two layers of the frozen stress model. The secondary principal stress direction and the relative phase retardation of the frozen disk model in the stratified frozen stress model are determined for the entire field from Stokes parameters obtained by scattered-light photoelasticity using unpolarized light. The accuracy of these values is confirmed by a comparison with results only from the frozen stress disk model.     

Experimental stress analysis of a nuclear-reactor pressure vessel

The stresses from internal pressure in a nuclear-reactor pressure vessel were investigated using both strain-gaged and photoelastic models. The methods used in obtaining complete stress distributions along the nozzles from strain-gage data are described, and comparisons are made between the distributions obtained from the photoelastic model and those derived from strain-gage data. The effects of nonradial attachment of a nozzle to a spherical shell upon the stresses in the nozzle and in the shell are shown. Finally, comprisons are made between the experimental results and those for simplified analytical models.     

Gelatin models for photoelastic analysis of gravity structures

The potential of gelatin as a highly sensitive photoelastic material has long been known but seldom utilized. This paper describes a series of tests to show that body-force stress distributions can be conveniently found with gelatin models. Problems arising from the extreme variability of gelatin, including edge dehydration and bacterial attack which have plagued investigators in the past, may be overcome or even turned to advantage by careful control of the mixture and simultaneous calibration. Previous work with gelatin in this country and abroad has been reviewed in order to indicate the available information on the instantaneous physical and optical properties. Data from the calibration test performed at Princeton were used to obtain both the instantaneous and the linearly viscoelastic creep behavior of the gelatin mixture chosen in terms of constant moduli which can be compared with other time-dependent, prototype materials. Apparatus and procedure for both calibration and model tests are outlined, and test results for one of the wedge-shaped gravity structures investigated are compared with analytic predictions.     

Three-dimensional photoelastic and finite-element analysis of a propellant grain

The results of a three-dimensional photoelastic analysis of a propellant-grain model are compared with the results obtained from a three-dimensional finite-element analysis of the same model. The loading considered was differential thermal expansion between the case and the grain model. The model analyzed was the Stage I Minuteman propellant grain which consists of a six-point-star configuration with a head-end web. Stresses and strains determined from the two analyses in the star valley are compared. The excellent correlation between the stresses determined from the photoelastic analysis and the stresses calculated by the three-dimensional finite-element computer program established the validity of the computer program.