Measurement of crack tip displacement field using laser speckle photography

It is shown how double exposure laser speckle photography, when used with an automatic image processing system, can provide detailed information on both in-plane displacement components around the tip of a crack. This paper describes the results of the analysis of six speckle photographs of sharp cracks in polymethylmethacrylate (PMMA), each on a mesh of 16 × 16 datapoints. Two numerical methods for calculation of the mode I stress intensity factor (K_I) from the measured displacement field have been investigated. Least-squares fitting of the theoretical displacement field gave good estimates of K_I, with low systematic and random errors (˜1%). Evaluation of the J-integral around square paths enclosing the crack tip was found to result in larger errors (up to 5%). It is thought that laser speckle photography together with J-integral calculations may find applications in nondestructive testing, as a method of automatically detecting the presence of a crack.     

Electronic speckle photography: measurement of in-plane strain fields through the use of defocused laser speckle

High-accuracy, noncontact measurements of in-plane strain fields have been performed through the use of an electronic-speckle-photography system. The strain fields are extracted from the displacement of defocused laser speckle in a telecentric imaging system. Two different illumination configurations have been suggested, both of which use four illumination directions. Both configurations produce results of an accuracy according to Me/ΔL, where M is the demagnification of the telecentric imaging system, e is the random error in the speckle-displacement fields, and ΔL is the magnitude of the defocusing distance. The maximum defocusing distance possible was found to be restricted by the spatial resolution, especially at high magnifications. In experiments on a semicircularly and a rectangularly notched aluminum sheet, the principal strain field around the notch was measured with a random error in the strain field of less than 10 μstrain (μm/m).     

Accuracy in electronic speckle photography

Electronic speckle photography is an accurate, easy-to-use, video-based technique for the analysis of two- and three-dimensional deformation fields and in-plane strain fields, based on numerical cross correlation. Through the use of statistical optics, simulated speckle patterns, and experiments the accuracy in electronic speckle photography was found to depend on correlation, speckle size, window size, and correlation filter. The estimated correlation was found to be the combined effect of three mutually competing factors because of classical speckle correlation, subimage overlap, and displacement gradients. In many applications white-light speckle patterns provide a more accurate estimate of the displacement field than do laser speckle patterns.     

Adaptive aperture defocused digital speckle photography

Speckle photography can be used to monitor deformations of solid surfaces. Its measuring characteristics, such as range or lateral resolution, depend heavily on the optical recording and illumination setup. I show how, by the addition of two suitably perforated masks, the effective optical aperture of the system may vary from point to point of the surface, accordingly adapting the range and resolution to local requirements. Furthermore, by illuminating narrow areas, speckle size can be chosen independently from the optical aperture, thus lifting an important constraint on the choice of the latter. The technique, which I believe to be new, is described within the framework of digital defocused speckle photography under normal collimated illumination. Mutually limiting relations between the range of measurement and the spatial frequency resolution turn up both locally and when the whole surface under study is considered. They are deduced and discussed in detail. Finally, experimental results are presented.     

Automatic analysis of speckle photography fringes

Speckle interferometry is a technique adequate to metrological problems such as the measurement of object deformation. An automatic system of analysis of such measurements is given; it consists of a motorized x-y plate positioner controlled by computer, a CCD video camera, and software for image analysis. A fringe-recognition algorithm determines the spacing and orientation of the fringes and permits the calculation of the magnitude and direction of the displacement of the analyzed object point in images with variable degrees of illumination. For a 256 x 256 pixel image resolution, the procedure allows one to analyze from three fringes to a number of fringes that corresponds to 3 pixels/fringe.     

Systematic and random errors in electronic speckle photography

Electronic speckle photography offers a simple and fast technique for measuring in-plane displacement fields in solid and fluid mechanics. Errors from undersampling, illumination divergence, and displacement magnitude have been analyzed and measured. The nature of the systematic error is such that a drift toward the closest integral pixel value is introduced. Because of the finite extent of the sensor area, considerable undersampling is tolerable before systematic errors occur. The random errors are mainly dependent on the effective ?-number of the imaging system and speckle decorrelation introduced by object displacement. When sampling at a rate of ~ 70% of the Nyquist frequency, we avoided systematic errors and minimized random errors.     

Diffraction halo function of partially coherent speckle photography

We studied the diffraction halo function of partially coherent speckle photography, using the theory of partial coherence of light and speckle statistics. Numerical calculations were made on the basis of the derived expression. It was shown that the anisotropic coherence of the illumination induced by the source could lead to the inhomogeneity of the halo distribution. An experiment was performed to prove the theoretical predictions.     

Multidirectional speckle photography of density fields in gasdynamic flows

Double-exposure speckle photography is employed experimentally to determine the fields of local density (temperature) values in a freely burning flame, as well as in the region of natural convection near a heated cylinder.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 56, No. 4, pp. 540–547, April, 1989.     

Multiple rotation assessment through isothetic fringes in speckle photography

The use of different pupils for storing each speckled image in speckle photography is employed to determine multiple in-plane rotations. The method consists of recording a four-exposure specklegram where the rotations are done between exposures. This specklegram is then optically processed in a whole field approach rendering isothetic fringes, which give detailed information about the multiple rotations. It is experimentally demonstrated that the proposed arrangement permits the depiction of six isothetics in order to measure either six different angles or three nonparallel components for two local general in-plane displacements.     

Fringe visibility analysis with different scale apertures in speckle photography

Abstract

The use of different scale aperture pupils for image recording in speckle photography is analysed. In particular a double-exposure specklegram is considered. The ensemble-average intensity in the Fourier plane is analytically derived and fringe visibility is investigated. The theoretical results are verified by in-plane displacement translation experiments.     

Electronic speckle photography: increased accuracy by nonintegral pixel shifting

Electronic speckle photography offers a simple and fast technique for measuring in-plane displacement fields in solid and fluid mechanics. An improved algorithm is presented and analyzed by use of both computer-simulated speckle patterns and real experiments. The idea of the improved algorithm is to maximize the correlation between correlated subimages from different images by shifting one of them by nonintegral pixel values. The improved algorithm was found to determine displacement components with an uncertainty of less than 1% of a pixel and with negligible systematic errors in ideal experimental conditions.     

Through-the-thickness displacements measurement in laminated composites using electronic speckle photography

This paper describes an application of electronic speckle photography to measurement of through-the-thickness displacements in composites. Purpose is to assess the accuracy of this technique in presence of the strong gradients and rotations at the layer interfaces of these materials. The software for data acquisition and processing was in-home developed. Experimental results quantifying the thickness distribution of in-plane and transverse displacements in laminated beams are reported for some kinds of beams with metallic, polymeric and carbon fibre layers, to assess the influence of different constituent materials and optical properties. The experimental results are compared with the predictions of a theoretical model previously developed by the author, substantiated by the exact elasticity solution. In all the examined cases, electronic speckle photography appeared suited to accurately measure displacements with a magnitude of technical interest.     

Digital speckle photography: visualization of mesoflow through clustered fiber networks

Digital speckle photography (DSP) is used for velocity field measurements inside a fiber network. The width of the channels in which the flow is measured is typically less than 1 mm. Therefore a microscope is used to image the fiber network. When we sample 30 images/s and separate the moving parts of the images from the stationary parts, the velocity field can be deduced with DSP.