| Two approaches to fully automatic speckle interferometry, CASI(I) and CASI(II), have been developed. Rather than using a photographic camera as in conventional optical speckle interferometry, CASI utilizes a video camera and an image grabber for its data acquisition. To maximize the capability of the resolution-limited digital camera, an optimal sampling rate has been analyzed and established for both laser and white-light speckle patterns.; The data processing of each CASI involves a two-step fast Fourier transform. Typically two speckle patterns of a specimen, one before and one after deformation, are captured by a video camera and registered into the memory of a frame grabber or a computer. The two speckle patterns are then segmented into two series of small subimages. Corresponding subimage pairs selected from both series are analyzed pointwise by a two-step FFT. The resulting spectra in CASI(I) and CASI(II) give rise to, respectively, one and two signal peaks. Interrogations of these signal peaks reveal the local displacement vector between the two exposures. By analyzing all subimage pairs of the two speckle patterns, 2-D displacement field of an object can be determined completely.; This technique retains all nondestructive features of optical speckle method, and provides an extended range of measurement. Although direct outputs of CASI are displacements, 2-D strain fields {dollar}(epsilonsb{lcub}xx{rcub}, epsilonsb{lcub}yy{rcub}, epsilonsb{lcub}xy{rcub}){dollar} of the object can be differentiated from the displacement fields. Very large plastic deformation can be analyzed by registering many speckle patterns at different deformation stages, and resolving the incremental strain between each two consecutive speckle patterns. Dynamic incremental deformations can also be inspected by capturing many speckle patterns using a high-speed CCD camera. The system has been applied to material testings, as well as experimental fracture mechanics.; Although their approaches are quite different, it has been proved that both schemes of CASI have provided equivalent operations to direct cross-correlation method. However due to the high efficiency of FFT, CASIs have achieved dramatically higher processing speed, 117.0 times faster in CASI(I) and 68.3 times faster in CASI(II). In fact, it is the magic power of CASI that makes it possible to resolve a 2-D deformation field at nearly one thousand points in less than 4 minutes using a personal computer-486. |
