Showing papers on "Photoelasticity published in 1992"

Two‐dimensional state of stress in a silicon wafer

Abstract: Two‐dimensional state of stress in (001) and (111) silicon wafer is studied with infrared photoelasticity. In two widely used groups of coordinate systems, the silicon piezo‐optical coefficient tensors due to photoelastic anisotropy of silicon crystal are derived. The relation between stress ellipsoid and refractive index ellipsoid is analyzed with infrared polarized light transmitting through the silicon crystal in certain directions. The applicability of the stress‐optical law in (001) and (111) silicon wafers is presented. A three direction observation method is developed to decide the magnitude and direction of principal stresses in silicon wafer. The stress states in (100) and (111) silicon wafers after certain device processes are also measured and calculated. Comparisons of experimental and calculated results are made.

The determination of principal stresses from photoelastic data

Abstract: Current developments in the automation of photoelastic analysis have enabled the fast and accurate collection of the isochromatic and isoclinic parametersl from photoelastic specimens. Since the isochromatic parameter yields the difference in the principal stresses then a suitable procedure has to be used for their separation. This paper provides a survey of the stress separation techniques available with a view to incorporating the most suitable method into an automated full field polariscope.

Optical tomography of the stress tensor field

Abstract: Integrated photoelasticity, a nondestructive method of three-dimensional stress analysis, is treated as optical tomography of the stress tensor field. Distinguishing features of the optical tensor field tomography are considered. Since in the general case the measurement data is in a nonlinear way related to the stress field, two particular cases are considered in detail: (1) weak birefringence, (2) constant principal stress directions. In these cases it is possible to measure two line integrals of the components of the stress tensor. It is shown that the general problem of stress tensor field tomography can be reduced to a problem of scalar field tomography for a single component of the stress tensor. The other stress components can in some cases be determined using equations of the theory of elasticity. The paper is illustrated by some examples of application of the method.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

On the dominance of the singular dynamic crack tip stress field under high rate loading

Abstract: The dynamic stress field near a running crack tip is investigated using photoelasticity and caustics. Both quasistatic and dynamic impact loads were considered. Under impact loading, it was found that the range of dominance of the singular term in the asymptotic crack tip stress field expansion was very small. The need for considering higher order transient terms in interpreting the isochromatic fringes is demonstrated. Also, the importance of considering the effect of the higher order transient terms in the characterization of failure criteria is discussed.

Photoelastic and finite element analysis of different size spheres in contact

Abstract: The photoelastic stress-freezing technique is applied to observe the stress distribution inside two spheres of different sizes compressed together elastically. After the stress is frozen in, thin slices of the material containing the symmetry axis are prepared for observation through a polariscope. The stress distribution is compared with both the finite element numerical analysis and the Hertz analytical theory which is limited to small deformations. Among the three, the agreement between the experimental results and the finite element analysis is the best. The deviation from the Hertz theory is less in the larger sphere contacting a smaller one than in the smaller sphere contacting a larger one.

An assessment of frozen stress photoelasticity

Abstract: The frozen stress technique is compared with other experimental and numerical techniques. The frozen stress technique gives elastic stresses due to steady loading, but is the only experimental technique which can give interior stresses; the others are limited to free surfaces, but can be used to give plastic and creep strains. The requirements for frozen stress work are set out and the types of results which can be obtained are stated.Finite element analysis is the only real alternative for the interior of components and surfaces which are inaccessible due to the proximity of other components.

Holographic tomography in three-dimensional photoelasticity within the Born approximation

Abstract: In this paper, a holographic tomography method for the reconstruction of the applied stress field in a three-dimensional (3-D) photoelastic object is presented based on the weak scattering approximation. The optical inhomogeneity and anisotropy in the object are caused by the applied stress field through the photoelastic effect. The method uses angular-diversity illumination with monochromatic plane waves and planar recording screens. Polarizations of wave fields are considered and used together with the equilibrium equations for uncoupling the components of the dyadic scattering function. In this method, the low-pass version of the 3-D stress dyadic field in a model is reconstructed in a nondestructive manner.

Optical tensor field tomography: the Kerr effect and axisymmetric integrated photoelasticity

Abstract: The tomographic problem of the optical tensor field is solved for the Kerr effect and for axisymmetric integrated photoelasticity.

Experimental determination of the dynamic stress-intensity factor using caustics and photoelasticity

Abstract: The methods of photoelasticity and caustics were used in conjunction with high-speed photography to determine the dynamic stress field near a moving crack. The photographs were analyzed to extract information on crack speed and the dynamic stress-intensity factor. The stress-intensity-factor histories obtained from both techniques were compared to determine the reliability of the two techniques.

Tomographic problem of photoelasticity

Abstract: A problem of determination of three-dimensional stress state from optical measurements by the method of integrated photoelasticity is considered. The stress medium is contained in the cylinder domain, the element of the cylinder is parallel to z-axis. Only the equilibrium equations are satisfied and strains are not considered. The weak optical anisotropy is supposed. The optical measurements are carried out along all horizontal straight lines which are parallel to the plane z equals 0. It is proved under this assumption that the longitudinal component (sigma) zz of the stress tensor can be determined. The method of numerical determination is offered. It is proved that no information about stress tensor, except (sigma) zz, can be determined.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Dynamic stresses in granular assemblies with microstructural defects

Abstract: An experimental study was conducted to investigate the effects of microstructural defects, such as inclusions and voids, on the wave propagation phenomena in granular materials. The granular materials as well as the defects were simulated using circular disks in both one‐ and two‐dimensional experiments, The defects were of two types, namely, inclusions and voids. Dynamic photoelasticity and high‐speed photography were used to study the effects of these defects on the local stressfield as the stress wave passed by. The dynamic loading was achieved by detonating a small amount of an explosive on top of the model assembly. The experimental results indicate that both inclusions and voids produce local wave scattering through various reflection mechanisms. Inclusions increase the wavelength of the loading pulse and produce more local attenuation and voids can change the local energy‐transfer paths.

Novel Photoelastic Approach in Analysis of Elliptical Holes in Thick Plates

Abstract: Nondestructive photoelastic stress analysis of elliptical holes in thick plates was carried out. Using Plexiglas as a model material, the scattered photoelastic technique was used. In a novel approach, the residing birefringence in unstressed commercially available polymer plates was introduced as an information carrier. Pairs of monochromatic coplanar rays of polarized light were sent, in two perpendicular directions. The experimental data captured from each pair, for points at the load‐free boundaries of the holes, were combined to achieve a complete determination of the stress state at the point of a pair's intersection. The nondestructive technique used allows optical scanning of stresses through the thickness. The photoelastic data obtained was utilized in determining stress‐concentration factors at the ends of the major and minor axes of the elliptical holes. The experimentally obtained distributions of the stress concentration through the thickness were compared with those cited in the literature. ...

Evaluation of automated full field analysis using phase stepping

Abstract: A full field automated polariscope for photoelastic analysis has been developed in the author's laboratory and has been described in detail elsewhere Briefly, the system uses phase-stepping to determine both the fractional isochromatic fringes and isoclinic parameter at all points in the field of view independently of their neighbouring points A wrapping algorithm is then employed to produce continuous ischromatic and isoclinic data, which can be subsequently be used in stress separation procedures The idea of using phase stepping in photoelasticity is a fairly recent innovation and can be described as changing incremently the absolute phase of the reference wave by rotating the output elements of the polariscope and measuring the local light intensity after each step In the apparatus described here, the output elements are rotated to six different positions providing six images of the specimen Maps of the periodic values of the isoclinic and isochromatic parameters are subsequently obtained by combining, mathematically, these six images A number of full field techniques have been developed Poloshin and Redner have developed half fringe photoelasticity, and two laboratories in Japan are working on the technique of phase stepping It appears, however, that no detailed evaluation has been made of the accuracy and reliability of the results generated by the technique The objective of the work described in this paper has been provided such an evaluation Five different models were selected for analysis using the automated system and manually using the Tardy compensation method: (a) a disk in diametral compression: (b) a constrained beam subject to a point load: (c) a tensile plate with a central hole: (d) a turbine blade; and (e) a turbine disk slot These models provided a range of different fringe patterns, orders and stress gradients to test the performance of the system

Wave Propagation and Dynamic Load Transfer due to Explosive Loading in Heterogenous Granular Media with Microstructure

Abstract: : An experimental and numerical investigation has been conducted to study wave propagation and load transfer due to dynamic loading in granular materials. Systematic as well as random aggregates of particles of different material properties as well as shapes were used to simulate the granular media. These assemblies were loaded either with explosives or projectiles driven by a gas gun to generate short duration pulses. The resulting dynamic phenomenon was experimentally studied by using the optical technique of photoelasticity and dynamic strain gage equipment. Of particular attention was the effect of local microstructure on the wave propagation process. The effect of the loading pulse wavelength as well as the material heterogeniety on the wave propagation phenomenon was evaluated. The effect of the particle shape and size on the load transfer process was also studied. Initial experiments were also conducted to investigate the effect of prestress as well as interstitial moisture on the wave propagation process. The experiment al results elucidated the basic mechanisms of load transfer, provided the duration and magnitude of the dynamic contact stresses, wave propagation velocities, envelope of load transfer and wave attenuation.

Evaluating method of internal stress of ic element

Abstract: PURPOSE:To enable simple and accurate evaluation of a stress inside an element by a method wherein a bonded sample is cut at a right angle to the bonding face of a base and a glass face, a light is applied perpendicularly to the cut glass face and the internal stress is measured. CONSTITUTION:A transparent glass plate having about the same thermal expansion coefficient as an IC element to be bonded on a base 7 with a die-attach material 8 interlaid is bonded on the base 7 with the material 8 interlaid, and a bonded sample 4 is cut 0.5 to 1 mm wide at a right angle to the bonded face of the base 7 and glass 9. A light is applied perpendicularly to the cut glass face, an internal stress is measured by a method of photoelasticity and the stress thus obtained is taken for a stress generated inside the IC element. The glass plate needs only to have about the same thermal expansion coefficient as the element within a range of about -30 to 350 deg.C. As for the glass sample, the presence or absence of a residual stress inside is inspected by the method of photoelasticity before the sample is bonded on the base 7, the internal stress is removed by heat treatment or the like when it is present, and the sample made free from the internal stress is employed.

Ultrasonic and laser measurements in structural contact problems

Abstract: This paper deals with two experimental techniques recently set up by the authors which enable the study of contact problems between elastic bodies. The more widely used experimental techniques for determining the response of elastic bodies in unilateral contact include moire fringes, holography and photoelasticity. The experimental techniques proposed here utilize either the ultrasonic pulse transit time method or the laser beam intensity method. Both techniques are applied to measure the relative displacements and the contact area of beam and plate models in unilateral contact with an elastic half-space model. The experimental results, compared with those obtained by numerical analysis, show the efficiency and validity of the measurement techniques proposed.

Crack growth kinetics under impulse loading

Abstract: The dynamic photoelasticity method has been used to study the effect of the acting wave, and the size and orientation of the incident wave front on crack growth kinetics (under short-term pulse loading). It has also been used to study the mechanism of dynamic stress formation at the crack tip. It has been shown that when compression or tensile waves are incident on the crack at an angle of 0<α<80 and 100<α<180°, the field of dynamic stress which arises is determined by the transverse shear strain while at an angle of 90±10°, stress formation is caused by strain tearing. When transverse waves act on a crack, there is a considerable amount of stress concentration at the crack tip irrespective of the incident angle. It has been established that one-, two-, or threefold crack growth occurs depending on the pulses and the crack depth.

Combining Photoelasticity and BEM for Stress Analysis

Abstract: This paper presents a new method which combines the boundary element method(BEM) and the photoelastic fringe pattern for stress analysis over the region of interest bounded by isochromatics and the body surface. Using this hybrid numerical-experimental method, we can obtain stress solution at selected locations without isoclinic data. According to the isochromatics, boundary stress information can be obtained, which is used in BEM. The result by the method is more accurate than that by regular BEM analysis where unreal boundary conditions are often presumed, such as displacement boundary conditions. The isochromatic fringe pattern supplies the actual specimen stress information, and therefore this method improves the calculated accuracy, besides also decreasing man work hours. The results presented compare well with theoretical or purely experimental solutions.

Transient thermal stress analysis of interacting surface defects by digital photoelasticity

Abstract: The digital photoelastic technique was used to obtain an experimental solution of the transient thermal stresses of two interacting semielliptic surface defects due to a step temperature change at the free surface. A series of computer programs were developed on the digital photoelastic system to extract the maximum transient thermal stress on the defect boundary. By using a statistical analysis package, the variation of the maximum transient thermal stresses and their locations were correlated with the time, temperature difference, and geometrical parameters of the defects.

Time dependent principal stress analysis by the hybrid method of photoviscoelasticity and boundary element

Abstract: In this paper the basic relations in linear isotropic photoviscoelasticity have been discussed theoretically in detail. A new routine to solve the time dependent principal stress without the measurement of isoclinics has been found. As a proof of the method, examples are illustrated at the end of this paper.

Effect of Photoelasticity on Photothermal Beam Deflection in Transparent Solids

Abstract: The photothermal beam deflection(PTBD) signals were measured as a function of polarization angle of the probe beam. The PTBD signal within the transparent solids strongly depends on the polarization angle of the probe beam and orientation of the samples. The photoelastic effect in the solid was successfully introduced to analyze the experimental results.