Papers by Keyword: Digital Photoelasticity

Abstract: Standard experimental tests including photoelasticity in the evaluation of stress intensity factors in crack tip has been regarded widely. A least-squares method is used to determine the stress field parameters of interface crack in Aluminum/ epoxy bimaterial and its evolvement. Based on multi-parameter stress field equations and the least-squares principle, a set of over-determined nonlinear equations is established by fitting the isochromatic phase field obtained by digital phase-shifting photoelasticity in this paper. An iterative procedure based on Newton-Raphson method is utilized to estimate the unknown stress field parameters. Interface crack experiments reveal that the modulus of combined stress intensity factor increases with the applied loads and however its phase angle holds the line on the condition of same loading direction. On the other hand, the modulus of combined stress intensity factor increases with the loading direction from 30^o to 75^o under the same applied load, however, its absolute phase angle decreases and is independent of the loads.

Abstract: One of our main research areas is the trans-scale modelling of damage in composite materials, which consist of a polymer matrix and carbon or glass fibres in different material combinations and geometrical arrangements. From the local stress redistribution in the fibre-matrix interphase and in the surrounding matrix material information on the parameters of microscopic damage models for composite materials can be obtained. Owing to the difficult interface characterisation based on the properties of the single material components, a photoelastic analysis of single fibre fragmentation tests is performed. In addition to the qualitative visual interpretation in polarized light, an enhanced quantitative analysis in combination with digital photoelasticity using a four image phase shifting method will be applied [1]. As the sequential capturing of images might cause incorrect results, these four pictures are grabbed simultaneously. This allows for continuous testing. Additionally, errors due to the relaxation behaviour of the matrix material can be avoided. To this, a modular optical system consisting of a variable long distance microscope and a beam dividing module proposed by [2] was developed. It allows for the simultaneous projection of four different filtered images of one microscopic scene to the four quadrants of a CCD chip. This special equipment gives the possibility to apply quantitative photoelasticity to tensile tests performed on standard testing machines. This paper explains the measurement hardware and discusses the main problems and realised solutions from picture capturing through image processing to real-time photoelastic analysis at the present state of development. Exemplary results for the qualitative analysis of selected material combinations and different manufacturing processes are shown.

Abstract: The stress analysis for a model with initial stresses, which we term a residual stress model, is performed by digital photoelasticity. The stresses applied on the residual stress model are obtained by analyzing both the initial stresses and the resultant stresses. The method used for analyzing the stresses applies the principle of superposition of the stress to photoelasticity, which is a well-known technique in the field of elasticity. In the digital photoelasticity technique used, the principal stress direction and the relative phase retardation  are analyzed by photoelastic techniques using linearly polarized light. This technique overcomes the phase difference error associated with a quarter-wave plate by employing incident light at three different wavelengths, and using an unwrapping technique that allows and  to be determined using the arctangent function. A residual stress model produced by a disk containing frozen stresses that was subjected to a diametral compressive load at an angle of 31 was used to experimentally test this method. The values of the stresses of the loaded disk model analyzed were in good agreement with corre- sponding theoretical values at all locations far from the loading points of the residual stress model.

Abstract: Digital photoelasticity is an experimental method for determining stresses in 2D and 3D models. In digital photoelasticity one gets a wrapped isoclinic phasemap. The main issue with wrapped isoclinic phasemaps is that the isoclinic values obtained do not uniformly represent the principal stress direction of one of the principal stresses consistently over the entire domain. These zones are labelled as inconsistent zones. Such zones need to be identified and corrected to get unwrapped values of continuous isoclinic phase values. In this paper, a method is developed to plot the simulated wrapped and unwrapped isoclinic phasemap from 2D Finite Element (FE) results so that one can use this as a convenient tool for identification and correction of inconsistent zones in isoclinic phasemaps obtained experimentally for complex problems. The method is explained by using the problem of a circular disc under diametral compression. The application of this method for handling complex problems is demonstrated by solving the cantilever bending of a binocular specimen.

Abstract: Photoelasticity is one of the most widely used techniques for experimental stress and strain analysis. With the availability of low cost digital image processing systems, a separate branch of photoelasticity known as digital photoelasticity came into existence providing whole-field values of the isoclinics (θ) and isochromatics (N) in a true sense. Among the several methods available for data acquisition, phase shifting / polarization stepping techniques are most widely used for their simplicity and accuracy. In this paper a new digital photoelastic method based on phase shifting using monochromatic light source is presented. It provides full field values of θ and N. The arrangement is carefully chosen with the intention of reducing the influence of quarter wave mismatch error in the evaluation of θ and N. The methodology is validated for the benchmark problem of a disk under diametral compression.

Abstract: The main sources of error in the determination of stress intensity factors (SIFs) for an interface crack in a bi-material by conventional photoelasticity are the measurement of the positional co-ordinates of the data point and the fringe order. In the present work, use of two digital photoelasticity methods for collecting these data is discussed. SIFs are evaluated using constant radius method and a least squares approach based on the singular stress field equation. The need for developing a multi-parameter stress field solution for evaluating SIF is highlighted.