Photoelastic Technique Research Articles

Stress Analysis in Automobile Components Using Reflexive Photoelasticity Technique

The main objective of this work was to introduce Reflexive Photoelasticity Technique in qualifying automotive components at Fiat Automoveis S.A in Brazil. Actual stresses were determined in a simple geometry plate and also in nodes of a body shell of a passenger vehicle by using reflexive photoelasticity methodology. Initially, tests were performed in welded steel plates submitted to traction loads. These plates were previously coated with birrefringent plastic material. External loads were applied through hydraulic MTS actuators. Stress and strain distribution in welded points neighboring areas using reflexive polariscope were quantitative determined. Simultaneously, these stress and strains acting on the welded plate were determined using strain gauges. The results from both experimental techniques (photoelasticity and strain gauges) were compared with those obtained from a numerical model using finite element method. The results were correlated and analyzed. Finally, reflexive photoelasticity technique was used to determine the stresses in nodes of a body shell of a passenger vehicle. Through this analysis it was possible to determine critical points in the structure with peak stresses.

Analysis of the interface between a thermoplastic fiber and a thermosetting matrix using photoelasticity

Abstract The photoelastic images of the isochromatic fringes for a pull-out specimen were used to measure and to analyze the maximum interfacial shear strength for a thermoplastic–thermoset interface (PET/Epoxy). The photoelastic technique allows the loading process to be followed in-situ, as well as the determination of the shear stress distribution on a tested sample. A description of the stress transfer process and the measurement of the shear stress level along the embedded fiber were done. The maximum shear strength was located along the fiber surface at a distance of 2.5 times the diameter from the specimen border. The maximum shear stress values obtained from the photoelastic analysis were corrected to include the tri-dimensional axisymmetric distribution of the shear stress around the fiber. The corrected shear stress measured τcMax ranged from 0.27 to 1.17 MPa.

Uncovering the secrets to relieving stress: discrete element analysis of force chains in particulate media

The traditional analyses for structures that are either built upon (for example, foundations), or are used for storage and handling of particulate solids (for example, bins, silos, hoppers, and bunkers) cannot account for important processes originating in the evolution of microscale particle kinematics and kinetics. Until recently, the complexity of these processes could only be perceived through macroscale experiments, but information derived from these experiments lacks the necessary level of detail that is needed for sound theoretical development. Limited information on the evolution of microstructural properties in deforming particulate solids emerged through ingenious experiments using photoelastic techniques. These techniques revealed the complex microstructural fabric and force propagation, the evolution of which governs what is observed as the constitutive response of the material. More recently, numerical simulations using the discrete element method have also proven to be powerful tools --- effectively providing a virtual laboratory for studying microstructural evolution. We use the discrete element technique to examine the propagation of forces within a particulate solid that is subject to an indenting rigid flat punch on its surface. Particular attention is given to the formation, evolution and collapse of ``force chains'' (that is, quasi-linear chains of load-bearing particles). These force chains are surrounded by a weak network of particles that, despite carrying only a small amount of the load, provides the necessary stability for such chains to persist. A mathematical framework for the characterisation of these chains is used to investigate the evolution of the force chain network with indentation, with a focus on microstructure, force chain lengths, and length distributions. Finally, the first steps toward establishing correlations across multiple length scales are made by linking the macroscopic load-penetration response to the underlying evolution of contact forces and force chains.

Stress distribution in a post‐restored tooth using the three‐dimensional finite element method

Clinicians are opting ever more frequently for restorative materials which have an elastic modulus similar to that of dentin when reconstructing endodontically treated teeth. Metallic posts, which are capable of causing dangerous and non-homogenous stresses in root dentin, are slowly being abandoned. Ideal posts may be those made of various types of fibre (carbon, mineral and glass) and which are adhesively luted into the canal. Among the different methods for evaluating the mechanical behaviour of posts in root canals (progressive loads and photo-elastic technique) the finite element method (FEM) presents many advantages. The aim of this paper is to evaluate, utilizing three-dimensional analysis of the finite elements, what the effect of material rigidity, depth of insertion and post diameter could be on the stress distribution in the different components of the single tooth-post-core reconstruction unit. The results of the FEM analyses, expressed as the distribution of Von Mises stress values, has allowed us to conclude that (i) fibreglass-reinforced composite distributes stress better than titanium alloy or stainless steel; (ii) fibreglass-reinforced composite posts should be inserted as deeply as possible (but maintaining 5-6 mm of gutta-percha apical seal); (iii) fibreglass-reinforced composite post diameter does not affect stress distribution, therefore, as much radicular dentin as possible should be preserved.

Dynamic sliding of frictionally held bimaterial interfaces subjected to impact shear loading

The fast frictional sliding along an incoherent interface of a bimaterial system composed of a Homalite and a steel plate is studied experimentally in a microsecond time-scale. The plates are held together by a static uniform compressive pre-stress while dynamic sliding is initiated by asymmetric impact. The full-field technique of dynamic photoelasticity is simultaneously used with a local technique of velocimetry based on laser interferometry. In the case where the impact loading is applied to the Homalite plate, a shear Mach line originates from a disturbance propagating along the interface supersonically with respect to the dilatational wave speed of Homalite and it crosses the P-wave front. The sliding starts well behind the P-wave front in Homalite and it propagates with a supershear speed with respect to Homalite. A fast interface wave and a wrinkle-like opening pulse (detachment wave) travelling along the interface are observed. When the impact loading is applied to the steel plate, the local sliding velocity measurement reveals that sliding initiates with the arrival of the P-wave front in the steel plate.

Particle Velocimetry and Photoelasticity Applied to the Study of Dynamic Sliding Along Frictionally-Held Bimaterial Interfaces: Techniques and Feasibility

A laser interferometry-based technique was developed to locally measure the in-plane components of particle velocity in dynamic experiments. This technique was applied in the experimental investigation of dynamic sliding along the incoherent (frictional) interface of a Homalite–steel bimaterial structure. The bimaterial specimen was subjected to uniform compressive stress and impact-induced shear loading. The evolution of the dynamic stress field was recorded by high-speed photography in conjunction with dynamic photoelasticity. The combination of the full-field technique of photoelasticity with the local technique of velocimetry was proven to be a very powerful tool in the investigation of dynamic sliding. A relatively broad loading wave with an eye-like structure emanated from the interface. The particle velocity measurements established that sliding started behind the eye-like fringe pattern. It propagated with supershear speed with respect to Homalite. A shear Mach line originating from the sliding tip is visible in the photoelastic images. A vertical particle velocity measurement revealed the existence of a wrinkle-like pulse traveling along the bimaterial interface. The wrinkle-like pulse followed the initial shear rupture tip and propagated at a specific subshear speed.

Deformation and failure of a film/substrate system subjected to spherical indentation: Part I. Experimental validation of stresses and strains derived using Hankel transform technique in an elastic film/substrate system

Our concern here is to rationalize experimental observations of failure modes brought about by indentation of hard thin ceramic films deposited on metallic substrates. By undertaking this exercise, we would like to evolve an analytical framework that can be used for designs of coatings. In Part I of the paper we develop an algorithm and test it for a model system. Using this analytical framework we address the issue of failure of columnar TiN films in Part II [J. Mater. Res.21, 783 (2006)] of the paper. In this part, we used a previously derived Hankel transform procedure to derive stress and strain in a birefringent polymer film glued to a strong substrate and subjected to spherical indentation. We measure surface radial strains using strain gauges and bulk film stresses using photo elastic technique (stress freezing). For a boundary condition based on Hertzian traction with no film interface constraint and assuming the substrate constraint to be a function of the imposed strain, the theory describes the stress distributions well. The variation in peak stresses also demonstrates the usefulness of depositing even a soft film to protect an underlying substrate.

An Integrated Approach to the Separation of Principal Surface Stresses Using Combined Thermo-Photo-Elasticity

A new instrument capable of the full-field separation of principal stresses on the surface of a component is presented. The instrument combines the techniques of thermoelastic stress analysis and reflection photoelasticity in a single optical head, permitting the simultaneous capture of both data from the same point of view. A single strain witness coating is employed for the acquisition of both the thermoelastic and photoelastic data, which is both birefringent under applied stress conditions and opaque at the infrared wavelengths to which the thermoelastic analysis system is sensitive. This enables the combined technique to be performed continuously from the same surface during loading. The performance of the new instrument is validated in the analysis of a classical laboratory specimen of known geometry. Separated stress data from the experiment is compared to simulated data, demonstrating that the accuracy of the stress separation technique is comparable to that of the individual thermoelastic and photoelastic techniques, and it is concluded that combined thermo-photo-elasticity is a powerful tool for the experimental separation of principal surface stresses.

Photoelastic characterization of residual stress in GaAs-wafers

Residual stress in GaAs-wafers was investigated on different length scales by rapid full wafer imaging and by microscopic imaging of dislocation cells, using photoelastic homodyne techniques. These non-contact and non-destructive defect and stress imaging methods will be described in detail. In connection with in situ calibration of the photoelastic SIRD™ measurement system absolute shear stress values can be extracted. Local stress fields imaged in μm-scale by the photoelastic instrument SIREX™ show the defect arrangement in cell patterns and allow to characterize the local stress enhancement. The advantages and limits of the vertically integrated photoelasticity measurement employed in both systems will be discussed.

Photoelastic methods to determine K I, K II and K III of internal cracks subjected to mixed mode loading

The ever increasing research of performances and lightness leads engineers and manufactures toward the development of heavily loaded components, subjected to multiaxial fatigue loads, made of materials with improved surface mechanical properties. In these cases it is not rare the development of internal cracks, whose propagation involves mixed-mode conditions; where an accurate prediction of the growth rate and of the crack path requires the knowledge of the stress intensity factors concerning modes I, II and III. In this paper, an original method for the experimental determination of these quantities is described. It is based on the photoelastic technique and allows to obtain internal cracks with regular shape without using any cutting device.

A photoelastic clinical study of the static load distributionat the stump/socket interface of PTB sockets

It is recognized that the assessment of prosthetic socket fit is based largely on the subjective clinical judgement of the prosthetist. This study assesses a novel technique, photoelasticity, for use as a tool for the qualitative and quantitative assessment of socket fit. Photoelasticity is a visual technique that produces contours of principal stress or strain differences. The colour and/or distance between the contours can be qualitatively or quantitatively assessed, using a polariscope, to give a full-field analysis of the stresses on the sockets's surface. This paper presents qualitative photoelastic socket surface contour data gathered during several prosthesis fitting sessions for two male trans-tibial amputees. Results are compared with the actual known contact regions at the stump/socket interface to determine if a relationship exists. This comparison of results has then been used to conclude the suitability of photoelasticity as a tool for the assessment of socket fit and recommendations are made as to the future developments of the technique. A direct relationship between the stump/socket contact regions and the qualitative photoelastic contours was demonstrated. Given further development this photoelastic technique may therefore be suitable for qualitative analysis of the interactions between the stump and prosthetic socket.

Phase-stepping photoelasticity for quantifying the interfacial response in fibre composites at fibre-breaks

A phase-stepping photoelastic technique has been used to study the fragmentation of E-glass fibre and evaluate quantitatively the effect of an interphase region on interfacial shear strength at a fibre-break and hence the efficiency of the interfacial bond. To investigate the interfacial response of an E-glass fibre in the presence of a soft interphase, it was coated with an epoxy resin of lower Young's modulus and yield strength than that of the matrix resin. This enables the effect of a sizing or modified matrix region to be quantified. The micro-mechanical response in the matrix at the interface/interphase has been described in detail using contour maps of fringe order. From these, the profile of the interfacial shear stress at fibre-breaks has been calculated, which demonstrated that in this cold-cured epoxy resin a good interface formed with uncoupled and unsized E-glass. In the presence of a modified matrix or interphase region the stress transfer process was modified and the stress concentration associated with a transverse crack reduced.

A robust approach to demodulating and unwrapping phase-stepped photoelastic data

New algorithms are proposed for processing wrapped isochromatic and isoclinic fringe data generated by the digital photoelastic technique of phase-stepping. The new algorithms are based on processing areas rather than individual pixels to demodulate the data and employing quality assessments of the data to guide the unwrapping procedure. Comparisons with an existing algorithm, manual analysis, and finite element modeling have been used to demonstrate that the new approach is more robust without any reduction in speed or accuracy. This represents a significant advance for processing complex fringe patterns, which are often encountered in the aerospace industry from which the exemplars are drawn.

Complete Isochromatic Fringe‐order Analysis in Digital Photoelasticity by Fourier Transform and Load Stepping

Abstract: In this paper, a new fully automated photoelastic technique based on both the Fourier transform and the envelope of the fringe centres, is presented. Using two images of the isochromatics acquired with different loads and a simple calculation routine, the method allows the user the complete determination of the isochromatic fringe retardation on any arbitrary line chosen over the model domain, without the need for supplementary external information or calibration. Various experiments have corroborated the accuracy of the method, estimated theoretically to about 0.07 fringe orders. As a result of its simplicity and sufficient accuracy, the method can be used in the industrial field also by a non‐expert user.

Clinical Applications of Angled Abutments-A Literature Review

Dental implants have been proven to be an effective way of restoring the masticatory ability to completely or partially edentulous patients. But when clinicians deal with undesirable implant orientation caused by poor bone conditions, an angled abutment is often the treatment of choice for prosthodontic restorations. A search of articles related to the success rate, stress analysis, and bone response to loading of angled abutments was performed in order to understand the rationale for their current clinical applications. Evidence shows that the clinical performance of angled abutments is comparable to that of straight abutments with respect to both soft-tissue responses and general survival rates. However, in vitro studies of stress/strain analyses of angled abutments can only agree that stress/strain levels increase as abutment angulation increases. Three analytical techniques including the photo elastic technique, strain gauge analysis, and finite element analysis have been applied for analyzing angled abutments. Further comparative studies are required using these three techniques because some quantitative aspects are still inconclusive such as how much stress is introduced upon a certain degree of abutment angulation.

Probing Process-Induced Defects in Si Using Infrared Photoelastic Stress Measurement Technique

AbstractThe stress depth distribution in the silicon substrate under thin thermal oxide layers has been measured using the photoelastic technique. With the high sensitivity and improved spatial resolution of a newly developed low level birefringence detection system, it is confirmed that the stress distribution deviates significantly from the linear distribution predicted by classical bi-metallic strip theory. The deviation can be attributed to arising from Si self-interstitial injection during the thermal oxidation process. Long term changes in the stress distribution with time have also been observed and the changes can be understood in terms of point defect movements under the stress field in the substrate. Our preliminary results have demonstrated how we can probe microscopic point defect processes in thin film SiO2/Si substrate systems using a macroscopic photoelastic stress measurement technique.

Measurement of residual stress in multicrystalline silicon ribbons by a self-calibrating infrared photoelastic method

This article reports on a method for the measurement of residual stress in multicrystalline silicon ribbons, based on the infrared photoelastic technique. This self-calibrating method allows the in situ determination of the photoelastic coefficients and can thus be used for any crystal orientation. The method was validated by the experimental determination of the photoelastic coefficient of monocrystalline (100) silicon wafers and by comparison with strain measurements using asymmetrical x-ray diffraction. The distribution of residual stress in multicrystalline silicon ribbons was also measured. The results showed strong evidence for tensile stress in the central region and compressive stress near the edges of the ribbons. Both the measured residual stress and the photoelastic coefficient distributions are correlated to grain boundaries.

Slip initiation on frictional fractures

Direct shear tests and biaxial compression tests are conducted to investigate the onset of slip along a non-homogeneous frictional surface and to determine the effect of specimen thickness and confining stress on slip initiation and propagation. The specimens are made of two and three acrylic blocks with the contact surfaces between blocks having on their upper half a frictional strength smaller than on their lower half. This creates a “weak” surface on the upper half and a “strong” surface on the lower half. The specimens are then loaded in direct shear or biaxial compression with confining pressures ranging from 0.7 to 3.5 MPa. The onset of slip, slip propagation, and the stress field generated at the front and center of the blocks interfaces are monitored using a photoelastic technique where a thin photoelastic film is placed at the location where observations are made. The onset of slip at the weak–strong zone interface is treated as propagation of a frictional crack under Mode II loading. The critical stress intensity factor, K IIC, at the onset of slip is obtained from photoelastic techniques. The results show a weak dependency of K IIC on the normal stress applied and no influence of the specimen size for specimens thicker than 25.4 mm; for thinner specimens the K IIC values are smaller because the boundaries of the specimen prevent the full development of the stress field ahead of the crack tip. The experiments show a linear increase of the critical energy release rate with normal stress which is explained with linear elastic fracture mechanics theories.

The influence on the distribution of interfacial stresses of embedded inclusions and voids in the adherends of bimaterial structures

In this paper, the digital photoelastic technique and finite element method (FEM) were combined to investigate the influence of embedded inclusions and voids on the distribution of interfacial stresses of bimaterial structures. A special casting procedure was employed to manufacture the bimaterial structures with and without voids and inclusions in the adherends. As indicated from the results of this paper, after introducing inclusions or voids in the adherends, the interfacial peeling stresses may become most critical.

Plastic Collapse Behavior of a SUS316 Single-edge Notched Member Subjected to Combined Tension and Bending Using Photoelastic Coating Technique

Plastic Collapse Behavior of a SUS316 Single-edge Notched Member Subjected to Combined Tension and Bending Using Photoelastic Coating Technique