Showing papers on "Photoelasticity published in 1982"

Photoelastic Investigation of Dynamic Load Transfer in Granular Media

Abstract: Dynamic photoelasticity in conjunction with high-speed photography is utilized to study impact wave propagation and dynamic load transfer in granular soil. Series of sequentially recorded isochromatic fringe patterns provide full field information of the dynamic event. Experimental results show that the wave propagation process is governed by a contact wave which increases the density of contacts in propagation direction. The study of dynamic load transfer along specific representative load chains provides basic information about wave velocities, contact duration, and directional stability necessary for a rigorous analysis of the general complex dynamic wave-soil interaction problem.

Reflection Moire Methods For Plate Bending Studies

Abstract: The term "isodynes" has been proposed by Pindera and Mazurkiewicz to denote a new family of characteristic lines of plane stress fields. These lines carry information on total normal forces acting on related cross sections and yield the distribution and values of related normal and shear stress components. Two families of isodynes related to two characteristic directions yield the values of all three components of a plane stress field and additional redundant information. Isodyne photoelasticity methods can be applied to determine all three stress components in photoelastic models and in original machine or structural parts using isodyne coatings. The term "gradient photoelasticity" has been proposed by Pindera and Hecker to denote a new method of photoelasticity which utilizes relationships between the curvature of light paths in a photoelastic object and the gradients of symmetrical and distortional parts of stress/strain tensors. Utilizing a basic mathematical model of photoelastic effect presented by Ramachandran and Ramaseshan, gradient photoelasticity yields the momentary values of absolute and relative photoelastic coefficients. Both methods can be applied to determine the values of stress intensity factors for arbitrary cracks and all stress components in composite structures.

Fracture initiation and stress wave diffraction at cracked interfaces in layered media I. brittle/brittle transition

Abstract: Fracture Initiation and Stress Wave Diffraction at Cracked Interfaces in Layered Media — I. Brittle/Brittle Transition Stress wave scattering about the tips of stationary interface cracks at a brittle-brittle junction in a layered medium and the associated stress-wave induced fracture were investigated. Dynamic photoelasticity was employed to visualize the highly complex interaction process between stress waves and cracks. Methods of linear elastic fracture mechanics were used to analyze regions of high stress intensity as possible sources for crack initiation. The phenomenon of partial load transmission across closed crack walls and imperfect joints and its effect on fracture initiation was also studied.

Apparatus for determination of elastic isodynes and of the general state of birefringence whole field-wise using the device for birefringence measurements in a scanning mode (isodyne polariscope)

Abstract: An apparatus has been developed to experimentally determine the state and amount of birefringence in an arbitrary plane within a birefringent body using an entirely new method. Such information can be used to determine the optical anisotropy in solid and liquid bodies, residual and induced stress fields, etc. This apparatus uses a collimated and polarized light beam which scans linearly the plane of measurement within a transparent birefringent or photoelastic object, and a device which collects particular sheets of light scattered along the path of the light beam, the intensity of which is directly related to the state of birefringence at each scattering point at the plane of measurement within the birefringent body. An imaging device produces in the image plane of the apparatus the lines of constant light intensities which are the lines of constant values of accumulated birefringence. For plane stress states in engineering objects such lines are identical with the elastic isodynes and are therefore called photoelastic isodynes. Two independent fields of such isodynes allow the direct determination of all three stress components of the plane stress state and give an additional relation between the strees components. This is the only experimental method which supplies directly four or more independent data on the stress state at each point within the photoelastic object.

Determination of the stress distribution in assemblies of photoelastic particles: Theoretical and experimental description of the application of the photoelasticity method to three-dimensional assemblies of glass particles

Abstract: An optical measuring method and a calculation procedure for determining the distribution of the stress tensor in a plane-strained three-dimensional assembly of random-shaped photoelastic particles are described. The stress tensor at an arbitrary point of the model is determined by an integration procedure, based on the equations of equilibrium of stresses. The distribution of the principal-stress direction and the relative principal-stress difference and at least two normal stresses in a plane have to be known to perform the integration. The distribution of the principal-stress directions and their difference are measured optically by scanning the model with an optical filter system with a single rotating polarizer.

Quasi-Square Hole with Optimum Shape in an Infinite Plate Subjected to In-Plane Loading. Optimization of Inner and Outer Boundaries of Beams and Plates with Holes,

Abstract: : This paper (Report no. 58) deals with the optimization of the shape of the corners and sides of a square hole, located in a large plate and subjected to in-plane loads, with the object of minimizing stress concentrations. Appreciable disagreement has been found between the results obtained previously by other investigators. In this paper new tests have been conducted and discrepancies have been corrected. Using an optimization technique, the authors have developed a quasi square shape which introduces a stress concentration of only 2.54 in a uniaxial field, the comparable value for the circular hole being 3. The efficiency factor of the proposed optimum shape is 0.90 whereas the efficiency factor of the best shape developed previously was 0.71. The shape also is developed that minimizes the stress concentration in the case of biaxial loading when the ratio of biaxiality is 1:-1. This paper (Report no. 59) presents optimized shapes of inner and outer boundaries for three specific problems: a long rectangular plate with a central hole subjected to uniaxial tension, a simply-supported slotted beam subjected to a load uniformly distributed over a small area at the centre, and a square plate with a central hole under uniaxial uniform pressure. The two-dimensional photoelastic method is used for optimization. The results indicate a significant reduction in stress concentration factor or in weight, or in both.

New Development In Photoelastic Studies: Isodyne And Gradient Photoelasticity

Abstract: The term "isodynes" has been proposed by Pindera and Mazurkiewicz to denote a new family of characteristic lines of plane stress fields. These lines carry information on total normal forces acting on related cross sections and yield the distribution and values of related normal and shear stress components. Two families of isodynes related to two characteristic directions yield the values of all three components of a plane stress field and additional redundant information. Isodyne photoelasticity methods can be applied to determine all three stress components in photoelastic models and in original machine or structural parts using isodyne coatings. The term "gradient photoelasticity" has been proposed by Pindera and Hecker to denote a new method of photoelasticity which utilizes relationships between the curvature of light paths in a photoelastic object and the gradients of symmetrical and distortional parts of stress/strain tensors. Utilizing a basic mathematical model of photoelastic effect presented by Ramachandran and Ramaseshan, gradient photoelasticity yields the momentary values of absolute and relative photoelastic coefficients. Both methods can be applied to determine the values of stress intensity factors for arbitrary cracks and all stress components in composite structures.

Photo-orthotropic elasticity - A new technique for stress analysis of composites

Abstract: This paper deals with the application of photoelastic methods to glass fiber reinforced plastics. The basic principles governing the photoelastic response in birefringent orthotropic composites are briefly reviewed. Then, three important aspects of photo-orthotropic elasticity, namely, photoelastic calibration, separation of principal stresses, and application to problems of engineering importance, are considered in detail. A new calibration specimen is proposed for determining the three independent photoelastic constants: an orthotropic half-plane subjected to a concentrated edge load. From the isochromatic response, the three stress-fringe values can be obtained by a least-squares procedure based on the theoretical stress distribution. Next, four methods of separating the principal stresses in birefringent composite models are proposed: the method of drilling small holes at the points of interest, the method of oblique incidence, the method of combining the transmitted photoelastic response with the reflected photoelastic response, and the method of birefringence dispersion.

Bending Stresses of Internal Spur Gear

Abstract: The influences of rim thickness of an internal spur gear on tensile and compressive bending stresses at tooth fillets and bottoms are discussed. The stress measurements by strain gages, the photoelasticity experiments, and stress analyses by use of the finite element method showed that the position where the maximum stress appears in an internal gear is different from that of an external gear. Especially when the rim thickness of an internal gear is thin, the maximum tensile stress appears at the center of the bottom of the tooth space, which is apart from the loaded tooth. As the rim thickness becomes thinner, the maximum compressive stress at the fillet of the loaded tooth increases sharply, while the maximum tensile stress hardly changes. As meshing proceeds, the tooth fillet and bottom stresses of a thin-rimmed internal gear alternate more strongly than those of a solid or thicker rimmed gear.

Photoelasticity in the Assessment of Structural Integrity

Abstract: The linear-elastic fracture mechanics concept of a stress-intensity factor considered as a parameter which may be used to assess the severity of a defect in a component. The photoelastic technique is presented as a convenient method of determining stress-intensity values in complex configurations. The limitations of the experimental technique are separated from those inherent in the stress-intensity-factor concept and the required accuracy discussed. Results of a number of two- and three-dimensional problems are reviewed.

Dynamic photoelasticity as an aid in developing new ultrasonic-test methods

Abstract: A prerequisite for the development of quantitative ultrasonic-inspection techniques for surface flaws is a thorough understanding of the ways in which elastic waves interact with defects. Analytical and numerical approaches are presently inadequate. Experimental methods are needed for a better understanding of wave interactions with real geometries. This paper describes how dynamic photoelasticity was used to study the interaction between Rayleigh waves and slots. To fully interpret the interactions between an incident Rayleigh wave and a surface slot, the problem was subdivided as follows: first, the reflections and mode conversions of a Rayleigh wave at a corner were studied. This simulated the Rayleigh-wave interaction with a slot opening. Then, the interaction when a Rayleigh wave ran off the tip of a slot was observed, and, finally, the total interaction with slots perpendicular to the surface was studied. The results for these three cases are presented. It is suggested that the most important property of a Rayleigh wave that can be used to size surface and near-surface defects is the subsurface particle motions. These motions persist up to a depth of the order of a wavelength. The shape (that is, the frequency spectrum of the transmitted wave) should, therefore, be affected by the depth of the slot. Spectroscopic analysis is applied to the photoelastic data to develop a simple method for sizing slots. Results from ultrasonic tests on slots in steel confirm the validity of the suggested method. By applying contemporary concepts of signal processing to photoelastic data, a powerful new area of experimental investigation is introduced. It promises to overcome the current inability of scatter theories to predict the interactions between real-life defects and acoustic waves as used in ultrasonic testing. Applications of this approach will improve the quantitative ability of ultrasonic-inspection methods.

Determination of stress profiles in optical-fibre preforms

Abstract: A computational technique is presented for the nondestructive determination of the radial, axial and tangential stress profiles in optical-fibre preforms. Optical retardation which arises as a consequence of the photoelastic effect is the basis for the technique. The stress profiles are obtained by first evaluating and then numerically differentiating indefinite integrals defined on the optical retardation profiles.

Separation of isochromatics and isopachics using a Faraday rotator in dynamic-holographic photoelasticity

Abstract: A method is presented that allows the simultaneous separation of isochromatic- and isopachic-fringe patterns for transient-plane stress problems. Isopachic fringes are obtained by means of holography with a Faraday cell and a pulsed ruby laser flashing dual pulses. As usual isochromatic whole-order fringes are recorded in a circular-light polariscope. The shock generator (air-gun) and its synchronizing system with the ruby laser is described. The procedure is applied to the recording of the isochromatic- and isopachic-fringe patterns in a disk under radial dynamic loads.

Measurement of mode I stress-intensity factors by scattered-light photoelasticity

Abstract: The feasibility of using the scattered-light technique to determine Mode I stress-intensity factors is demonstrated for the case of an edge-cracked beam subjected to pure bending. Photoelastic-fringe data were utilized to obtain an expression for the fringe gradient in the singular region surrounding the crack tip. Mode I stress-intensity factors were then determined by relating the fringe gradient to the local stresses in the singular region and extrapolating these results to the crack tip. Experimental and analytical results showed good agreement and the technique is suggested for application to three-dimensional fracture-mechanics problems.

Linear electro‐optic effect in tellurium dioxide

Abstract: The only non‐zero dc electro‐optic (or Pockel’s) coefficient in tellurium dioxide was measured at constant stress. The result was rT41 = −0.76 (±0.19)×10−12m/V. The secondary electro‐optic effect, due to the converse piezoelectric and photoelastic effects, is approximately twice the observed electro‐optic response.

Integrated Photoelasticity Of Axisymmetric Bodies

Abstract: The paper reviews integrated photoelastic methods for stress determination in axisymmetric bodies. The specimens are put in an immersion bath and viewed in a common transmission polariscope. If birefringence of the specimen is high, integral fringe pattern as well as quasi-isoclinics may be photographed. By point-by-point technique the characteristic directions as well as the characteristic phase retardation on a number of light rays are measured. This experimental data, often combined with the application of the equilibrium equations and macrostatic equilibrium conditions, permit one to determine stress distribution in the specimen. Investigation of stresses in isotropic speci-mens as well as in cubic single crystals is considered. A new method of determining stress in cylinders, including measurement of the angular deflec-tion of light rays, is described.

A non‐destructive method to determine stresses in a three‐dimensional photoelastic model using a transmission polariscope

Abstract: The determination of stresses within a loaded three dimensional photoelastic model without cutting it into slices has been the ultimate aim of many investigators. The standard methods using a transmission polariscope do not yield much information. Scattered light methods no doubt enable one to completely determine the state of stress at a point within the body. However, the methods proposed up to now necessitate the use of an arrangement where both the body under test and the sensing unit which picks up the scattered light or both will have to be rotated. This leads to a complicated experimental set-up and increases the possibility of making errors, especially while rotating the model. In the proposed method, most of the information needed to determine the state of stress at a point within the body is obtained using the transmission polariscope set-up. The model and the direction of observation of scattered light are fixed. This allows observations to be made in test models which have at least one flat face without using a liquid with matched refractive index and increases the accuracy of measurements. The method is an almost purely transmission polariscope type of method and where space permits tests can be done using ‘live’ loading without having to use the stress freezing technique.

Use Of Optical Methods In Stress Analysis Of Three-Dimensional Cracked Body Problems

Abstract: A major unsolved problem in fracture mechanics concerns the subcritical growth of cracks under fatigue loading to a critical size at which catastrophic fracture occurs. Such growth is the precursor to most service fractures and often involves non-self-similar flaw growth, nonplanar flaws, and nonuniform stress intensity distributions along flaw borders. These geometric complexities coupled with notch geometries and stress gradients which precede and accompany the flaw growth remove the problem from the realm of mathematical tractability. Beginning a decade ago, the author and his associates have been involved in a continuing effort to develop optical methods for extracting stress intensity factor estimates and, more recently, crack shapes and displacement fields from photoelastic models of such bodies. Methods developed involve a marriage between the field equations of linear elastic fracture mechanics with the optical techniques of frozen stress photoelasticity and moire interferometry. After briefly reviewing the analytical foundations of the methods and describing the techniques, this paper presents results obtained from applying the technique to problems of current technological interest in the aerospace and nuclear fields.

Photoelastic analysis of contact stresses in the presence of machining irregularities

Abstract: Three-dimensional stress-freezing photoelasticity is used to analyze the stress distributions in a shaft with a shrink-fit ring. Of particular concern is the stress redistribution when machining irregularities are present at the contact surface. Details of the model manufacture, data reduction, and analysis are given.

Extension of the hole-drilling method to birefringent composites

Abstract: A complete stress analysis and reliable failure criteria are essential for important structural applications of composites in order to fully utilize their unique properties. The inhomogeneity, anisotropy and inelasticity of many composites make the use of experimental methods indispensable. Among the experimental techniques, transmission photoelasticity has been extended to birefringent composites in recent years. The extension is not straight-forward, in view of the complex nature of the photoelastic response of such model materials. This paper very briefly reviews the important developments in the subject and then describes the theoretical basis for a new method of determining the individual values of principal stresses in composite models. The method consists in drilling very small holes at points where the state of stress has to be determined. Experiments are then described which verify the theoretical predictions. The limitations of the method are pointed out and it is concluded that valuable information concerning the state of stress in a composite model can be obtained through the suggested method.

Exact and approximate strain‐optic laws for photoelastic composites

Abstract: An exact strain-optic law for othotropic materials has been derived using Pockels' theory of crystalline photoelasticity. The law requires three experimentally determined photoelastic constants. Photoelastic response of materials such as glass-fiber-reinforced epoxy or polyester composites subjected to any state of strain can be predicted through this law. However, due to practical difficulties, this law is not sufficient to solve for principal strain differences or the direction of principal strain from conventional measurements in a polariscope. An approximate strain-optic law, which has been derived by minimizing square of the error between predictions of exact and approximate laws, can be employed to estimate the difference between principal strains and their directions. The error in using the approximate law appears to be small. The approximate strain-optic law has been found to be preferable over an approximate stress-optic law.

Experimental comparison of solution methods for the axisymmetric photoelastic problem

Abstract: A method of component separation in axisymmetric photoelastic problems based on the use of the compatibility condition is experimentally tested against the more common methods of oblique incidence and subslicing The results show the method to be comparable in accuracy, but to have the distinct advantage of not requiring additional measurements other than those in the meridional plane

Electric-field-induced ionic contributions to photoelasticity of diamond

Abstract: It is shown from lattice dynamical calculations that an externally applied field can induce measurable changes in the photoelastic constants and the piezo-optical coefficients of diamond, in the region of the long-wavelength optical phonon at 1332 cm-1. This in turn may lead to narrow resonances or antiresonances of the Brillouin scattering efficiency when the incident radiation is chosen to be in the vicinity of the phonon frequency. The field may also induce Brillouin scattering in configurations which are normally forbidden. In all these calculations the phonon linewidth is taken into account explicitly. Assuming a value of 103 esu for the electric field, the complete dispersion curves are plotted in the region of 1332 cm-1, for the photoelasticity constants, the elasto-optical coefficients and the Brillouin scattering efficiency in allowed and forbidden configurations.

凍結光弾性法による曲げモーメントを受けるマイタ管の応力解析 : 第1報,交差角が90°の場合

Abstract: 2本の同直径円筒がたがいに接合する形状のマイタ形屈曲管が二中心軸を含む面内曲げモーメントを受ける場合の応力集中率および応力分布状態を光弾性凍結法をもちいて解析した.集中応力に影響をおよぼす形状ファクタは種々あるが,ここでは交さ角が90゜の場合について肉厚および交さ部すみ肉曲率半径を順次変えたときの接合部の応力分布状態および応力集中率の変化を調べた.

Stress-strain state in a high-pressure electromagnetic valve

Abstract: Ignorance of stress distribution patterns in the different elements of high-pressure armatures frequently results in an unjustified safety factor at some places and the development of dangerous strains at others. The authors studied stress distribution in an electromagnetic valve, which is complex in design with high gradients of stress zones. The stress-strain state was studied by a polarization-optical method and by holographic interferometry together with the numerical potential method. The maximum stress state was found in the zone of a vertical section. Nearly axisymmetric stress distribution was noticed in the valve body. A change in the sign of stresses was noticed directly along the welded joint. Also under study was the thermal stress state in this valve design under conditions of settled flow of the working medium. The magnitude of maximum thermal stresses was not much and ranged from 8 to 10% of the maximum stresses arising in the apparatus under the influence of the pressure of the working medium. Based on the data obtained, several alterations were made in the design of the electromagnetic valve, optimizing the stress distribution and reducing the wall thickness in places.

Stress analysis of spherical mirror panels

Abstract: An experimental analysis is reported of the stresses that occur in elastically deformed, spherically curved glass mirrors for solar energy applications. Bending and membrance stresses generated in forming the glass and the effects of springback are analyzed. In addition, thermal stresses caused by focusing the sun's rays on a small region of the mirrors are analyzed. Methods used in the analysis included the use of grids, photoelasticity, and strain gages. Results of the analysis are compared with a theoretical analysis performed by Shelltech Associates in a parallel effort.