Signal Processing for Laser-Speckle Strain-Measurement Techniques

Contactless and nondestructive material testing is of increasing interest in modern material sciences, where the measurement of the material properties of fibers and foils has become important in the development of new materials like composites, fiber bundles, or fiber-reinforced ceramics. However, strain measurement methods making use of laser speckle shifts induced by translation and deformation of the specimen turned out to be very useful when measuring stress-strain relations or thermal expansions of specimens to which strain gauges are not applicable, either due to the geometric dimensions of the specimen or for environmental conditions, e.g., high temperatures. Using laser-optical methods, one is confronted with the problem of calculating the speckle-pattern-shift values from a time series of images in the presence of the speckle decorrelation effects. In this paper, we give an overview of the most common methods and present a novel algorithm based on the maximum-likelihood principle, which yields sufficient accuracy for the common measurement tasks. Moreover, we show the application of two different optical strain-measurement setups used to measure mechanical and thermal strain.