Stress intensity factors for near edge cracks by digital image analysis (original) (raw)
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This study presented a type of image processing code which is used for sharpening photoelastic fringe patterns of transparent materials in photoelastic experiences to determine the stress distribution. C-Sharp software was utilized for coding the algorithm of this image processing method. For evaluation of this code, the results of a photoelastic experience of a sample contact problem between a half-plane with an oblique edge crack and a tilted wedge using this image processing method was compared with the FEM results of the same problem in order to obtain the stress intensity factors (SIF) of the specimen. A good agreement between experimental results extracted from this method of image processing and computational results was observed.
Stress intensity factors for interacting cracks
Engineering Fracture Mechanics, 1987
Experimental stress intensity factors (SIFs) for two interacting straight cracks in planehomogeneous regions were determined. Photoelastic data were collected from digitally sharpened isochromatic fringe patterns by using a digital image analysis system. SIFs were extracted by using the field equations derived from Williams' stress function. Numerical SIFs were also obtained by the boundary integral equation method. Good agreement was observed between experimental and numerical results. NOTATION crack tips as shown in Fig. 4 one-half crack length one-half horizontal distance between crack tips A and D one-half vertical distance between crack tips B and C one-half length of specimens specimen thickness one-half width of specimens orientation of crack AB with respect to the long direction of the specimens polar coordinates as shown in Fig. 9 applied far-field tensile stress stress intensity factor mode I SIF mode II SIF term used to normalize SIFs (= o/;;;; in this study) Young's Modulus Poisson's Ratio material fringe value
The Journal of Strain Analysis for Engineering Design, 2009
Transient thermal stresses of a bimaterial specimen with interface edge cracks subjected to heating along an edge is analysed by refined three-fringe photoelasticity (RTFP). Whole-field, noise-free, fringe order estimation using a single colour image is made possible using RTFP combined with colour adaptation. The stress intensity factors (SIFs) of the interface crack are determined through a multiparameter overdeterministic system of equations by a least-squares approach using experimental data collected automatically. The transient SIFs are found to peak to a higher value than in steady state, and the opening mode is found to be dominant. An increase in thermal load causes the crack to propagate, and this is easily visualized on the basis of quantitative fringe order data available for the whole field. The SIFs of a propagating crack are found to be low. The study shows that the crack propagates easily when the opening mode is dominant.
Experimental investigations of three-dimensional effects near a crack tip using computer vision
An experimental study of the near tip deformation fields for a Single Edge-Cracked specimen (SEC) has been completed. The surface deformation fields for a thin SEC plexiglas specimen have been obtained in the region 0.15 ~< r/t ~ 0.60, where r is radial distance from the crack tip and t is the specimen thickness, by using a novel computer vision method. The results of the study indicate that the value of the J-Integral obtained from the measured surface deformation, and under the assumption of plane stress linearly elastic behavior, is essentially path independent over a region that is considered to be within the crack tip three-dimensional zone by researchers who have performed tests on other materials.
High magnification crack_tip field characterisation under biaxial conditions.PDF
This work presents a novel methodology for characterising fatigue cracks under biaxial conditions. The methodology uses high magnification Digital Image Correlation (DIC) technique for measuring displacement and strain crack-tip fields. By applying micro-speckle pattern on the metal surface it is possible to achieve high magnification for DIC technique. The speckles were created by electro-spray technique. The validity of this novel technique is demonstrated by direct comparison with standard extensometer measurements, under tension-compression and torsion conditions. In order to image the correct region, the notch effect on the fatigue life was also evaluated.
Journal of Physics D: Applied Physics, 2009
Digital image correlation is a measurement technique that allows one to retrieve displacement fields "separating" two digital images of the same sample at different stages of loading. Because of its remarkable sensitivity, it is not only possible to detect cracks with sub-pixel opening, which would not be visible, but also to provide accurate estimates of stress intensity factors. For this purpose suitable tools have been devised to minimize the sensitivity to noise. Working with digital images allows the experimentalist to deal with a wide range of scales from atomistic to geophysical one with the same tools. Various examples are shown at different scales, as well as some recent extensions to three dimensional cracks based on X-ray Computed micro-tomographic images.
Strojnícky casopis – Journal of Mechanical Engineering
In the article we show the possibility of distorting the results of the experiment by photoelasticity path, in case the isochromatic fringe patterns are recorded by the camera. The recording of the color spectrum may be distorted by inappropriate selection of the recording medium, which is reflected by different types of aberrations. Transferring a record to a computer allows you to automate the experiment process, thus saving time to work with isochromatic fringes. As an example, we present the experimental determination of the stresses of the rotor with a symmetrical arm. The analysis was performed using software developed by the authors. This software enables full automation of the experiment process. The rotor is investigated by the method of reflection photoelasticity. The experimental solution is realized as a case of periodically repeating dynamic effects, in which a stroboscopic source of white light is needed. The results of the experiment are compared with the numerical so...
Half-fringe photoelasticity: A new approach to whole-field stress analysis
Experimental Mechanics, 1983
This paper presents a new method for whole-field stress analysis based on a symbiosis of two techniques—classical photoelasticity and modern digital image analysis. The resulting method is called ‘half-fringe photoelasticity (HFP)’. Classical photoelasticity demands materials with high birefringence, which leads to extensive use of plastics as model materials. Since the behavior of these materials is often different from that of the prototype materials, their use distorts the similitude relationships. In many contemporary problems this distortion is untenable. HFP offers a way out of this dilemma. It permits materials and loads to be chosen so that no more than one half of a fringe order appears in the area of interest. Thus, for example, glass, which behaves linearly up to high stress levels and over a wide range of temperatures, could be used as model material. Alternatively, models from polymeric materials could be used under very low load in order to stay within the linear part of the stress-strain diagram and to prevent large deformations. The half-fringe-photoelasticity system, which is described here, utilizes the resulting low levels of birefringence for effective stress analysis. This paper describes the system. It outlines a calibration routine and illustrates its application to two simple problems using glass models.
1991
Photoelasticity in stress analysis has long been a powerful full-field technique that helps researchers and designers find solutions to complex problems. In most applications of this technique, fringe patterns are recorded on photographic film for data analysis. The resolution of fringe orders recorded on a photographic negative is, in general, limited to whole-or half-order fringes which, in turn, requires a combination of high loads and optically sensitive birefringent materials to obtain enough data points to reliably define the stress field. This limitation in resolution is due to the fact that the light intensities recorded on the photographic negative are affected by film grain noise, exposure time, sensitivity of film and developing procedures. 1,2 Consequently, recorded intensities do not exhibit the well-behaved sinusoidal trend as would be e~p e c t e d .~ Unless there are whole-or halforder fringes present in the stress field, the recording does not yield a light intensity distribution that results from the birefringence of the material only. Measurement of partial fringe orders in order to increase the resolution in the photoelastic data has been achieved by automated polari~cope~,~ and by half-fringe photoelasticity.6 Both of these methods require 'live' models and they cannot be applied to photographic negatives.
Towards a new model of crack tip stress fields
2007
This work introduces a novel mathematical model of the stresses around the tip of a fatigue crack, which considers the effects of plasticity through an analysis of their shielding effects on the applied elastic field. The ability of the model to characterize plasticity-induced effects of cyclic loading on the elastic stress fields is assessed and demonstrated using fullfield photoelasticity. The focus is on determining the form of the shielding stress components (induced by compatibility requirements at the elastic-plastic interface along the crack flank and via the crack tip plastic zone) and how they influence the crack tip elastic stress fields during a load cycle. The model is successfully applied to the analysis of a fatigue crack growing in a polycarbonate CT specimen.