Gary Schajer

Residual stresses are stresses that are “locked-in” within a material, existing independently of external loads. Measuring residual stresses accurately can be beneficial for a wide range of engineering applications. The indentation method to determine residual stress is an attractive choice over current methods like Hole-Drilling due to simplicity of instrumentation and less damage to the specimen. The currently used procedure is the Force-Depth method where the indentation force is measured for constant depth of indentation on a stress-relaxed specimen vs. a specimen with residual stress. Though convenient, this method cannot distinguish the directions and separate sizes of the in-plane residual stresses. In this thesis, a novel approach is proposed using full-field optical techniques to capture rich surface displacement data from millions of pixels, in contrast to a single data source from an indentation force measurement. These data are used to determine the magnitude of residual stresses on metal surfaces for an isotropic residual stress distribution case. In this approach, finite element simulations are used to study the relationship between surface displacement and in-plane residual stresses. The studied characteristics are used to create a quantitative relationship between surface displacement and residual stress. These calibration values correlate well with corresponding experiments on steel specimens and serve as proof of concept for the proposed method to measure residual stress. Further avenues to improve the method are discussed in conclusion.

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The Hole-Drilling method is a popular technique for measuring residual stresses. It involves measuring the deformations around a drilled hole to determine the residual stresses originally within the cut hole. Because the hole drilling only partially relieves the stresses at the hole location, calibration is needed to know the correct fraction of the strain relief. The hole-drilling method is typically applied to a case where the test material is much bigger than the drilled hole, thus the area of interest conceptually has “infinite” boundaries. A conventional finite element method for hole-drilling calibration must have quasi-infinite boundaries and such a model usually becomes very large and numerically inefficient. An idea of using a more compact finite element model with a closer boundary coupled to a special ring to simulate the compliance stiffness of the far-field material is proposed here. Analytical procedures are described to specify the material properties for the needed outer ring. These are demonstrated for both hole loading and thermal loading methods. The calibration results from the simulated compact models demonstrate that the procedures work effectively for the isotropic stress state and reproduce good correspondences between the calculated deformations and the theoretical expectations. For the deviatoric stress state, some adjustments are required to create comparable matches. A procedure of forming a general stress state using the thermal loading method is also introduced with outcome performances similar to the ones in the deviatoric stress state. The research work confirms the ability to create similar “infinite” boundary responses of a special outer ring coupled to a small compact hole-drilling model. This method is useful for significantly reducing the calculation effort for hole-drilling calibration processes.

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The existence and location of knots affects the material properties and commercial value of cut lumber. Specifically, knots reduce the overall strength of lumber, with knots closer to the edge having a larger negative effect. Therefore, by determining the knot locations before cutting, and tailoring the cutting patterns to place the knots optimally within the lumber, the lumber quality and value can be increased. X-rays can image the interior of a log to detect these knots; however existing methods are either too complex and costly (Computed Tomography) or lack the ability to differentiate between knots reliably (Orthogonal Radiography). This research aims to overcome these limitations by employing a novel ‘oblique’ scanning arrangement that can determine knot orientations with both reasonable accuracy and low cost. Image processing and detection algorithms were developed to locate and orientate the knots automatically within the X-ray scans, and different methods of calculating the knot’s circumferential angle compared. Detection metrics of Precision and Recall were used to analyse the performance of the detection algorithm. Finally, purpose-built hardware was designed and constructed to conduct scans of the logs.Results indicate that the oblique scanning method is a viable way to detect and orientate knots within logs with both reasonable accuracy and low cost compared to existing methods. An average circumferential angle error of 15 degrees was achieved, with the detection algorithm being able to detect between 60% to 80% of the knots present within the log for ideal tuning parameters.

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Wood mills have used bandsaw blades for the primary breakdown of timber into lumberfor centuries. By reducing the width of the saw cutting edge, material waste can bereduced. Thin-kerf saw blades are susceptible to lateral forces during cutting. To combatthis behavior, saw lers introduce residual stresses into the blades. Classical methods formonitoring saw blade tension, such as the light-gap method, are subjective and analog.There exists a necessity to monitor the amount of residual stress induced into the bladesreliably, using a nondestructive test. A newly proposed method for stress evaluation,incremental shearography, o ers the advantage of measuring transverse plate bendingbehavior at micron resolution. Shearography, a direct measure of surface slope, promises arobust, full- eld measurement using low-cost components. By comparing measured slopesto those analytically found, a stress resultant of the tensioning process can be inferred.

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Deformation measurement is a common experimental need when testing the behaviour of mechanical devices. Such measurements are the basis for the evaluation of mechanical and thermal stresses, also for health monitoring of biological samples, study of vibrating parts of a mechanical component, and for quality testing of MEMS products. Optical methods like Electronic Speckle Pattern Interferometry (ESPI) are particularly attractive for practical applications because they are non-contact, sensitive and because they provide full-field displacement map over an extended area of the specimen. Conventional phase-stepping ESPI requires that the test object remains in the state of rest during the image acquisition, both before and after the deformation. This limits the use of ESPI to static or quasi-static measurements. Another limitation of conventional static ESPI is the measurable displacement range. Over large displacements ranging more than the size of a pixel, the speckles decorrelate and correct displacement information is lost. This project aims at developing a robust measurement technique to calculate surface displacement map for moving objects and to extend the range of measurable displacement. A newly developed algorithm that includes a technique involving camera exposure-synced phase stepper and mathematical interpolation is described. This method enables the extraction of instantaneous phase information using single interferograms. The test object is no longer required to be quasi-static during image acquisition. Every pixel in this method behaves like an independent displacement sensor. In addition, this method is computationally inexpensive. Some of the post-processing techniques are also discussed. A set of experimental results are discussed to validate the proposed technique.

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Small displacement measurements have several applications in engineering. Deformations ranging from nanometric scale to micrometric scale are very often related to mechanical stresses, materials phase transformations, residual stresses, biological processes, etc. It is common that these deformations occur in three dimensions and are not easily measured with simple equipment. Electronic speckle pattern interferometry (ESPI) systems use interference of light and laser properties to measure such small displacements in a very reliable way. The ESPI technique is a full-field non-contact technique capable of 3D measurements but the arrangement can get very complicated when it comes to number of components, size, cost, assembly complexity and operation. Among the many factors contributing for the complexity, the most important ones are the number of light beams and optical elements, and the laser itself. High quality lasers offer very high coherence – which is a measure of purity of spectrum and therefore allow for very high quality measurements with very low noise – at a very high cost, and usually are bulky, requiring many extra components to split light beams and direct the light to the region of interest. The aim of this project is to use special components to eliminate the need for an expensive laser and simplify the arrangement for a 3D ESPI measurement. The introduction of a dual diffraction grating system makes it possible to replace a high quality laser with a low coherence, compact and cheap laser diode. Moreover, with this new proposed arrangement it is possible to obtain the 3D ESPI information using two in-plane ESPI systems combined in one, reducing complexity and offering up to 6 different measurement possibilities. Based on the fact that only 3 different measurements are required for solving a complete 3D displacement field, the remaining measurements can be used for data averaging and to increase the accuracy and reduce uncertainties. The new design is presented and a portable device is developed. The arrangement is explored to test the robustness against exterior factors such as temperature and ambient light, and the accuracy of the measurement is verified with a controlled sample and finite element analysis/analytical solutions. 

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Wood grain direction is a very important quality control feature in wood industry because it is a good indicator of wood strength and uniformity. It is a three-dimensional quantity that is defined by its angles within and into the plane of the measured surface. These are respectively called the surface and dive angles. An interesting method to measure these angles involves measuring the spatial reflection from the wood surface when illuminated by concentrated light. The cellular shape of the wood microstructure causes the light reflection to be greatest perpendicular to the wood grain. This effect allows the surface and dive angles to be determined by analyzing the spatial variation of the reflected light. The conventional method for doing this involves sampling the reflection intensities around a circle above the wood surface. However, this method is effective only for small dive angles. A new method is described here where light reflection intensity variation is measured along two parallel lines on either side of the illuminated area. It is able to measure the full ranges of surface and dive angles that occur in practice. A laboratory device for making the required spatial reflection measurements is described and experimental results are presented. Based on the linear method, an equipment can be developed for industrial purpose, which consists of two parallel lines of sensors sparsely distributed along the longitudinal axis of lumber. To investigate this proposed arrangement of sensors, an interpolation study was undertaken on the associated low-resolution data, with the results compared with those from high-resolution full-field data. With the proposed sensor arrangement, grain angle measurements can be made at very high speed, which makes the equipment suitable for industrial use in sawmills and wood products factories.

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Residual stresses are self-equilibrating stresses that exist within bodies without any external loads acting on them. Monitoring residual stresses is vital to engineer safe and reliable structures. A reliable means of measuring residual stress is by drilling a small hole in the body and observing the resulting redistribution of the stress using an electronic speckle pattern interferometer (ESPI). Current ESPI systems are not suitable to measure stress on large or immovable structures because they depend on highly coherent laser sources that are bulky, delicate and not suitable for rugged field conditions. Additionally, ESPI is limited to a single direction of measurement. If the measurement axis is misaligned with the principal stress direction accuracy is reduced. This presents a practical challenge when measuring unknown residual stress states in the field. A compact ESPI device providing an isotropic residual stress measurement is presented here. The compactness of the device is possible by a novel optical arrangement that uses a diffraction grating and a miniature laser diode. This optical arrangement can be geometrically tuned to provide high quality measurements even with low coherence laser diodes. Furthermore, the device is constructed with two orthogonal measurement axes to reduce the influence of instrument alignment on measurement accuracy and to improve the overall precision by doubling the quantity of data. Experimentation with a calibrated bend specimen showed that the device has an accuracy ranging 9-12MPa and precision of 13MPa. This integration of ESPI and hole drilling modules into a compact, stand-alone unit is a significant milestone for in-field residual stress measurements.

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From a measurement standpoint structural stresses can be divided into two broad categories: stresses that can be measured straightforwardly by adjusting loads, e.g., live loads on a bridge, and those that are much more difficult, e.g., gravitational loads and loads due to static indeterminacy. This research focuses on the development of a method that combines the hole-drilling technique, a method used to measure residual stresses, and digital image correlation (DIC), an optical method for determining displacements, to measure these difficult-to-measure structural stresses. The hole-drilling technique works by relating local displacements caused by the removal of a small amount of stressed material to the material stresses. Adapting the hole-drilling technique to measure structural stresses requires scaling the hole size and modifying the calculation approach to measure deeper into a material. DIC is a robust means to measure full-field displacements and unlike other methods used to measure hole-drilling displacements, can easily be scaled to different hole sizes and corrected for measurement artifacts. There are three primary areas of investigation: the modification of the calculation method to account for the finite thickness of structural members, understanding the capabilities and limitations of DIC for measuring hole-drilling displacements, and evaluating the effects hole cutting has on the measurement. Experimental measurements are made to validate the measurement method as well as apply it to the real world problem of measuring thermally induced stresses in railroad tracks.

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The greatest cost in sawmills is contained in uncut logs. Significant increases in profit stand to be made if logs are properly processed so that the amount of defect and feature-free products is maximized. This has driven research into internal sensing such as computed tomography (CT) for use in sawmills. Traditional CT systems from the medical industry are ill-suited for industrial use and provide distorted reconstructions when used with less than perfect or partially incomplete data. Industrial CT systems in areas such quality control have embraced so-called Algebraic Reconstruction Techniques (ART) for robustness to errant data, but these systems are difficult to provide reconstructions rapidly enough for sawmill use.A system is proposed here for an ART scanning arrangement specific for log-scanning use. The image space's voxel pattern is defined to reflect the geometry of a log's internal features. This greatly reduces the number of unknowns without a loss of information and allows for faster reconstruction. The geometry of the voxels makes exact calculation and storage of the series solution basis fast and practical for multi-slice scans. Data scaling and normalization eliminate unnecessary voxels in the reconstruction. It also makes the reconstruction scheme tolerant of rigid body motion radially and circumferentially about the cone beam source. The voxel numbering scheme means that knot features are contained in voxels that are numerically close for quick registration.A camera-based detector system was implemented to collect radiographs of logs. Logs were translated and rotated in an x-ray cone beam and their position was monitored by an optical encoder. The detector was activated at the proper intervals to yield radiographic data. The series solution basis for this helical movement was constructed. An iterative solver and selective low-pass filtering was found to provide good reconstruction results. Segmentation was implemented to demonstrate use of the reconstructions for lumber processing. Eccentric spiral motion tests demonstrated the effectiveness of the data scaling and normalization process. Results are provided that point towards efficient automated registration of features.

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Digital Image Correlation (DIC) is an optical and numerical method capable of accurately providing full-field, two-dimensional (2-D) and three-dimensional (3-D) surface displacements and strains. 3-D DIC is typically done using two cameras that view the measured object from differing oblique directions. The measured images are independent and must be spatially connected using a detailed calibration procedure. This places a large demand on the practitioner, the optical equipment and the computational method. A novel approach is presented here where a single color-camera is used in place of multiple monochrome cameras. The color-camera measures three independent Red-Green-Blue (RGB) color-coded images. This feature greatly reduces the scale of the required system calibrations and spatial computations because the color images are physically aligned on the camera sensor. The in-plane surface displacements are obtained by performing traditional 2-D DIC in a single color. The out-of-plane information is obtained by a second 2-D DIC analysis and triangulation using oblique illumination from a differently colored light source. Further, the camera perspective errors associated with out-of-plane displacements can independently be measured during this second DIC analysis of the oblique illumination pattern. The 3-D Digital Image Correlation is completed by combining the 2-D correlations for each color. The design and creation of an example apparatus is described here. Experimental results show that the single-camera method can measure 3-D displacements with to within 1% error, with precision of the in-plane and out-of-plane measurements being consistently less than 0.04 and 0.12 pixels, respectively.

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Electronic Speckle Pattern Interferometry (ESPI) is typically done using a single monochromatic light source with a monochrome camera. This arrangement enables full-field measurements of a single deformation quantity according to the particular arrangement of the optical system. If a further deformation quantity is to be measured, then the associated optical arrangement must be used sequentially. Here, an alternative approach is described where multiple interferometric measurements are simultaneously made using a color camera imaging monochromatic light sources of different wavelengths. The Red-Green-Blue (RGB) sensors of a conventional Bayer type camera can be read separately, thereby providing three independent color signals and independent ESPI phase maps. An example case is demonstrated here where two Michelson interferometers are combined to form a shearography camera with surface slope sensitivity in two perpendicular directions. By the use of two colors, both surface slopes can be measured simultaneously. ESPI is a relative measuring technique and the third available color is used for absolute phase determination through the Two-Wavelength Method. The availability of the two surface slopes gives the opportunity for the data to be summed numerically to give the surface displacement shape. This application is of significant practical interest because the surface displacement measurement can be made under field conditions by taking advantage of the well-known stability of shearography measurements.

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No abstract available.

The natural frequencies and vibration mode shapes of flat plates are measured using ESPI, even when multiple modes are simultaneously present. The method involves measuring the surface shape of a vibrating plate at high frame rate using a Michelson interferometer and high-speed camera. The vibration is either excited by white (random) noise or by impact. Fourier analysis of the acquired data gives the natural frequencies and associated mode shapes. The analytical procedure used has the advantage that it simultaneously identifies all vibration modes with frequencies up to half the sampling frequency. In comparison, the ESPI Time-Averaged method and the traditional Chladni method both require that the plate be sinusoidally excited at each natural frequency to allow separate measurements of the associated mode shapes. Example measurements are presented to illustrate the use and capabilities of the proposed plate natural frequency and mode shape measurement method.

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Few experimental methods exist for evaluating all in-plane stress components in solid materials; because of the tensor nature of these quantities. Full field measurement of all three stress components is desirable, since plastic deformation or failure can result from any combination of the three. A new photoelastic stress measurement method is presented for evaluating all three in-plane stress components within a two-dimensional photoelastic material. The measurement method is based on the observation that the complex transmission factors that describe the optical phase changes due to stress-induced birefringence have a second order tensor character, similar to that of other tensor quantities such as stress and strain. The same transformation equations and Mohr’s circle construction can be applied to the rotation of optical axis. A Michelson type interferometer and phase shifting are used to quantify the phases of the complex transmission factors. Mohr’s circle calculation is applied to obtain the principal transmission factors and principal axis orientation. The principal stresses are then obtained from the principal transmission factors through the stresss optical relationship. The effectiveness of this technique is demonstrated by comparing the experimental and analytical results for a hollow circular ring under diametric compression.

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Electronic Speckle Pattern Interferometry (ESPI) provides a sensitive technique for measuringsurface deformations. The technique involves comparison of the speckle phase angles withinsurface images measured before and after material deformation. This phase angle comparisonrequires that the speckle positions be consistent in all images. A lateral shift between imagesby just one pixel substantially degrades ESPI measurements, while a shift of two or more pixelstypically causes complete speckle decorrelation and compromises the measurement entirely.To prevent such lateral motions, the specimen and the optical system must be rigidly fixed.This requirement typically prevents use of the ESPI method in applications outside laboratoriesor where it is necessary to remove the specimen from the optical setup between ESPImeasurements. Here, Digital Image Correlation (DIC) is used to track speckle motion caused byspecimen displacement between ESPI measurements. The measured images can then bemathematically shifted to restore the original speckle locations, thereby recorrelating the ESPImeasurements. Examples are presented where ESPI measurements are successfully made withspecimen shifts in excess of 60 pixels. A novel ESPI measurement technique where the specimenis removed in between ESPI measurements is also developed and validated.

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