Laser Speckle Interferometry Research Articles

Determination of double-Determination of double- K criterion for crack propagation in quasi-brittle fracture Part I: experimental investigation of crack propagation

The results of experimental investigations using laser speckle interferometry on small size three-point bending notched beams and using photoelastic coating and the strain gauges on very large size compact tension specimens of concrete are presented in detail. The investigations showed that there exists a stage of stable crack propagation before unstable fracture occurs. The results are in agreement with other researchers' investigations using moire interferometry, holographic interferometry, dye-impregnation method and microscope. Further detailed study shows that the three different states, i.e., crack initiation, stable crack propagation and unstable fracture can be distinguished in the fracture process in concrete structures. In order to predict the crack propagation during the fracture process in quasi-brittle materials a double-K criterion is proposed. The double-K criterion consists of two size-independent parameters. Both of them are expressed in terms of the stress intensity factors. One of them reflects the initial cracking toughness, denoted with Kini, which can be directly evaluated by the initial cracking load, Pini, and the precast crack length, a0, using a formula of LEFM. The other one refers to the unstable fracture toughness, denoted with Kun, which can be obtained inserting the maximum load, Pmax, and the effective crack length, a, into the same formula of LEFM. The values of the two parameters, KIcini and KIcun, obtained from the small size three-point bending notched beams and the large size compact tension specimens show that KIcini and KIcun are size-independent. Evaluating with the K-resistance curves obtained from the same test data, it is found that the proposed double-K criterion is equivalent to it in basic principle, but, the double-K criterion can be applied more easily than the K-resistance curve. Finally, as a practical example, the application of the double-K criterion to the prediction of the crack propagation in a concrete dam is discussed.

Speckle-Interferometrie

With the electronic laser speckle interferometry (ESPI) it is possible to make deformation and strain fields visible. An easy to use sensor is presented. For demonstration the inhomogeneous deformation behaviour of laser-welded aluminium sheet is shown. At lower load the deformation is homogeneous, at higher load there is a deformation concentration in the welding zone.

Three‐component recording of ultrasonic transient vibration by optical heterodyne interferometry

The generation and detection of elastic waves by photoelastic methods, called ‘‘laser ultrasonics,’’ is now a rather commonly used method in research. The aim of this paper is to adapt an existing method, based on laser speckle interferometry, in order to record the three components of transient vibration induced by an ultrasonic wave at a point on the surface of a solid model. A detailed description of the method is given. The reliability of the method is experimentally demonstrated. As an illustration, maps of the local three‐dimensional transient vibration and of the particle trajectories of different points at the surface of a solid are reported.

A procedure for calculating through laser speckle interferometry the elastic constants of isotropic materials

The displacement suffered by the points located on one face of a parallelepipedic specimen made of isotropic elastic material is studied, while subject to simple compression, which generates not only strain but also both a translational and a rotation effect. The displacement is measured by double-exposure speckle photography. The elastic constants of a material are computed by measuring the in-plane displacement of three points. The effect of the in-plane and out-of-plane displacements due to translation and rotation are analyzed. Extreme values for the tilt angle are estimated, as well as the maximum diameter of the read-out beam. The systematic uncertainty of the method is also studied. Both Young's modulus and Poisson's ratio are experimentally measured in aluminum.

Quantization of surface rust by using laser imaging techniques

Laser speckle interferometry 1,2 and image processing 3,4 have been used to detect and quantize the rust build-up on metal surfaces under water. Speckle information from the sample metal surface was captured by a CCD camera and a frame grabber card. Software techniques were used to convert the image data files into ASCII files in an appropriate format. Three-dimensional surface plots were generated to define the numerical values for the amout of rust build-up.

A digital speckle correlation interferometer using an image fibre

The performance of a digital speckle correlation interferometer using an image fibre is investigated theoretically and experimentally. An expression is derived for the visibility of correlation fringes in terms of spatial filtering properties of the image fibre. The visibility of fringes depends mainly on the resolution of the image fibre as well as the in-plane displacement of diffuse objects. The results show that a computerized video system combined with the image fibre is suitable for laser speckle interferometry.

Reconstruction of sequential cardiac in-plane displacement patterns on the chest wall by laser speckle interferometry

Time-average speckle interferometry has been applied to obtain displacement patterns on the chest wall produced by cardiac action, in the absence of breathing, during various phases of the cardiac cycle. This has been achieved by an electronic shutter, controlled by the electrocardiogram of the subject. The recorded holographic plates processed under identical conditions are scanned by the pointwise method to obtain the absolute displacements at various locations corresponding to the activities of the various cardiac chambers and values. These data are transformed to a 40 x 30 matrix by an interpolation method and, from this, three-dimensional displacement plots are reconstructed by an IBM PC/AT computer. These patterns shown the displacements over the entire cardiac area corresponding to the activities of various regions during the cardiac cycle. The apex and aortic valve areas show the maximum displacements during the systolic phase. During the diastolic phase the activities over the low pressure atrial regions are also observed. The results obtained outline the functional details of the normal heart and the activities over various areas are in agreement with that of other noncontact techniques.

Measuring the curvatures of machined surfaces using laser speckle interferometry

Dimensional accuracy of components is of paramount importance to the manufacturing industry. Very often curvatures are used in specifying the profile of components which, after being machined, can be difficult to measure with profilometers or coordinate measuring devices. This paper presents a non-contacting and whole-field method of measuring curvatures using a moving laser source and an image-shearing camera. The resulting interference fringe pattern, which is related to the slope of the profile, enables easy computation of local curvatures with only one differentiation. To enhance the viability of this technique in the industrial environment, optical fibres were used so that the desired movement of the laser source could be achieved readily. In this paper, the technique described was applied to measuring the profile of wire-cut cylinders having circular and elliptical cross-sections.

Study of fracture process zone in rock laser speckle interferometry. Technical note

Study of fracture process zone in rock laser speckle interferometry. Technical note

Study of the fracture process zone in rock by laser speckle interferometry

Observation of the growth and formation of microcrack process zone by laser speckle interferometry for three kinds of rock. The effect of crack width and grain size on the process zone feature are also studied. The relation between the load F and crack tip opening displacement is examined by the same method

Laser speckle interferometry applied to studying transient vibrations of a cantilever beam

In this paper, the applicability of laser speckle interferometry for studying the transient vibrations of mechanical structures is demonstrated. The displacement of the free end due to the transverse vibrations following a transverse impact at the tip of a cantilever beam is computed theoretically and compared with that obtained by the experimental method. The two results are found to be in close agreement.

Three-dimensional reconstruction of cardiac displacement patterns on the chest wall during the P, QRS and T-segments of the ECG by laser speckle interferometry.

A noninvasive and noncontact technique based on the principle of laser speckle interferometry has been developed to record the cardiac displacements observed on the chest wall. These displacements are then reconstructed in the form of three-dimensional plots, during the P, QRS and T-waves of the ECG. A comparison of these patterns shows that the mechanical activity of each region varies significantly during these phases of cardiac cycle. As these displacements depend on the clinical status of the heart, its use with a cardiac patient shows the functional changes of the affected regions in the form of alteration of these patterns.

The use of optical techniques for monitoring fluid-structure interactions

The use of coherent light from laser sources has revolutionized the capabilities of measurement techniques in experimental fluid mechanics. In particular, laser Doppler velocimetry has become a standard tool for measuring nonintrusively the small-scale structure of turbulence in fluids. Similarly, laser vibrometers are becoming common instruments for detecting vibrations on solid surfaces. Interferometric measurements (homodyne or heterodyne) can provide very accurate surface vibration measurements, with resolution of a fraction of an Angstrom; subsequently, they are a powerful tool for monitoring fluid-structure interactions, especially for small scale phenomena. At larger scales, another powerful tool is the laser speckle and holographic interferometry which, when combined with optical Fourier transform techniques, can yield a direct visualization of wave propagation of the ω-k space (wave vector filtering). A recent effort has also been made to design a simple and rugged fiber optic laser probe that would be used to measure noninvasively the vector displacement of a vibrating structure. Such a probe allows the identification of both in-plane (short wavelength, acoustically inefficient) and out-of-plane (long wavelength, acoustically efficient) modes of vibration, and the conversion between these modes at discontinuities such as ribs on the surface. The relation between such vector measurements with acoustical holography and structural intensity will be discussed together with the use of scanning techniques. [Work supported by ONR.]

Mechanism of Microcrack Formation in Concrete

Formation and extent of microcracks in the fracture process zone of concrete is studied by laser speckle interferometry. Main emphasis is given to detection of microcracks, measurement of the width of microcracking zone, and assessment of process zone length. Crack opening displacements measured within the process zone were substantially higher than the values conventionally measured at the crack mouth. Measured critical process zone lengths (length of the micro-cracking region at peak load) were found to be specimen-size independent.

Cardiac functional analysis by laser speckle interferometry

The laser speckle interference pattern during movement of a rough surface is employed to measure the respective displacements. The purpose of this work is to apply this technique in the form of laser speckle displacement cardiography to analyse the displacement patterns during the I and II heart sounds. The recording is performed by illuminating the chest over the cardiac region by collimated laser beam controlled by an ECG operated electric shutter. By analysis the 3-D displacement patterns are obtained. A comparison shows that the displacement at the apex, right ventricle, aortic and mitral valvular regions are significantly higher during I sound than that of II sound.

A new speckle carrier method in speckle interferometry

A new speckle carrier technique is presented, which can solve the problem in one-beam laser speckle interferometry, that the direction of the displacement vector is ambiguously determined when the point-wise method is used. The method is very easy and convenient and the results are good. Some discussion about the relationship between the carrier frequency and the displacements is given.

Experimental determination of the center of resistance of the upper first molar in a macerated, dry human skull submitted to horizontal headgear traction

Laser speckle interferometry, recently developed for noninvasive measurement of small displacement (microns), was used to study the movement of maxillary molars submitted to extraoral traction. Using an antenna-headgear at different horizontal levels, loadings of 700 gm were applied perpendicular to the long axes of these teeth. Bodily movement was observed when the traction line passed slightly occlusal to the trifurcation of the roots. Therefore, the center of resistance of the upper molar was experimentally localized at this level. Laser speckle interferometry proved to be a promising technique for dental research.

Statistical analysis of one-beam subjective laser-speckle interferometry

The use of laser speckle for displacement and deformation measurement has found wide applications in mechanics and metrology. Traditionally, a deterministic approach has been adopted to analyze the process. In this paper a statistical approach is utilized, resulting in a better understanding of the limitations and potentials of one-beam subjective laser-speckle interferometry (or speckle photography). We find that the amplitude modulation of the information-carrying cosine fringes is controlled by three factors: a decorrelation factor γ, a displacement factor Ω, and a displacement-gradient factor ψ. We introduce the concept of an ambiguity factor that sets a critical value for the product γΩψ. We then propose a criterion for fringe discernibility from which the upper limit of measurement is established.

Reconstruction of Cardiac Displacement Patterns on the Chest Wall by Laser Speckle Interferometry

Laser speckle interferometry has been used to determine the displacement pattern on the chest wall produced by the heart action during the systolic phase of the cardiac cycle. The time averaged specklegram, recorded in the image plane, is scanned by the pointwise method. From the values of the spacings of Young's fringes, measured at various points, the corresponding displacement pattern in the form of a 40 x 30 matrix, equal to that of the cardiac region, is reconstructed and color-coded by a PDP 11/23 image processor. This pattern shows significant variations in displacements at various locations. Depending on the clinical status of the subjects, further changes in the pattern are observed. The reconstructed cardiac displacement patterns, obtained by this new noncontact and noninvasive technique, provide information on the functioning of various parts of the heart in cardiac disorders which are in qualitative agreement with that of two-dimensional echocardiography.

Contouring by laser speckle

A technique is presented to determine surface contours of 3-D objects by laser speckle interferometry. A double exposure specklegram is recorded by giving the object a small tilt between exposures. Surface contours are obtained by Fourier filtering of the specklegram.