W. F. Ranson

Determination of displacements using an improved digital correlation method

Abstract An improved digital correlation method is presented for obtaining the full-field in-plane deformations of an object. The deformations are determined by numerically correlating a selected subset from the digitized intensity pattern of the undeformed object. The improved numerical correlation scheme is discussed in detail. The displacements of a simple object, as computed by the correlation routine, are shown to agree with theoretical calculations.

Application Of Digital Correlation Methods To Rigid Body Mechanics

The random nature of white light speckle is combined with newly developed video digital data acquisition procedures to experimentally determine parameters of interest for a rigid body dynamics problem. Digital correlation of subsequent images during a dynamic event is shown to yield displacements and velocities of discrete points in the rigid body and, therefore, the angular velocity and linear velocity of the rigid body.

Determination of plastic strains at notches by image-proceessing methods

An image-processing method was developed to quantitatively extract the level of plastic deformation in metal specimen made of 304 stainless steel under remote tensile loading. The effective strain distribution around the notch tip was obtained and compared with the finite-element results. An exponential decay of the plastic strain concentration with distance from the notch tip was observed.

Acoustical-speckle interferometry

When a continuous or pulsed wave of ultrasound is scanned over an object, an echo emanating from the object may be detected. If the echo has a random variation in amplitude vs. position of the beam, the resulting variation is referred to as acoustical speckle. An analysis is presented for utilizing acoustical speckle to determine displacements on the surface or interior of an object. Two experimental configurations, pulse-echo and continuous wave scanning, are treated in the verification of the analysis.

Designing with scattered-light photoelasticity

Scattered-light photoelasticity can be utilized in locating ‘critical’ stress points in a structural component and can be further utilized in experimentally reshaping the structural components surface to minimize stress in critical regions. The theory and techniques to accomplish this experimental design are noted in the paper.

Digital Image Correlation Of White Light Speckle Including The Effects Of Image Distortion

An image correlation theory of white light speckle is presented which describes the light intensity as a continuous function for both the reference and deformed images. The reference image is divided into small regions corresponding to small subimages of the object surface. The subimage forms the basic element of the process of image correlation. The subimage of the undeformed intensity pattern is mapped to the deformed surface in order to compare its location to a corresponding location on the deformed intensity surface. This mapping function mathematically translates, rotates and deforms small subimages until the best fit is obtained with the recorded data. The following is thus obtained: (1) translation, (2) rotation, and (3) deformation.

Extending the laser-specklegram technique to strain analysis of rotating components

A technique involving sandwich-speckle interferometry has been investigated for application in making strain measurements on rotating structures. The technique has proven to be effective in relaxing stringent timing requirements for recording laser photographs and provides extended ranges of displacement measurement. Application of the technique to an experimental rotating specimen has demonstrated the potential of the method for making accurate strain measurements.