The laser speckle method used to measure strain and displacement of object surface is possessed of advantage in high precision and sensitivity. A series of practical laser speckle technique proposed by us and its application is presented in this paper.
The laser speckle method which is used to measure strain and displacement of rock samples or hydrostructural models has such advantages as simplicity in setup, non-contacting and higher accuracy and sensitivity. The information of displacement and deformation of a sample's surface can be recorded in a photoplate and measured by Point-by-point with the specklegram or Shown as the whole field state by displacement contour. Owing to the substance of calculated information of specklegram is to measure spatial spectura of displacements instead of themselves, so their sen- sitivity and accuracy have been greatly Improved. since 1980, the laser speckle method with its obvious advantages was lntroduced in the field of rock test and structural model test of hydropower pro- jects. In recent years, a series of practical laser speckle techniques improved and developed by us as follows:
the method of superimposition translation when measuring minor or larger displacement;
the method of determining direction of speckle displacement vector;
sandwich speckle method without accurate reduction (SSM);
combined speckle method (CSM);
automatic date-processing of speckle patterns;
hybrid analysis of laser speckle and numerical calculation for 2D and 3D problem.
These techniqes have been applied to rock tests and structural model tests of five practical large hydropower projects in China. The tests results show that the relative error of displacement is less than 4% between the speckle method and the dial indicator and also less than 10 % between the speckle mot hod and finite element method. Thus the laser speckle techniques with high accuracy, effective and non-contacting as a set of measuring means are providing for rock in-situ tests and structural model tests of hydropower projects, and especially adapted to geomechanical model tests which have higher gravity, lower strength and lower moduls
When diffusely reflecting object is illuminated by laser light and imaged by an optical system, radom speckles will modulate the image field. If the object surface is well focused, the image speckles will appear to move as if attached to the object surface. Because laser speckle can serve as markers for points on the object surface, so it becomes a new no-touched method to measure displacement and deformation of the object surface.
The experimental arrangement of laser speckle method is schematically shown in Fig. 1. Where the camera is focused on the object surface and the photoplate is put on the back focal plane. The laser speckle information can be recorded by a photoplate with isochronous double exposure before and after the deformation of the object. The information of the object's surface displacement can be acquired from specklegram which has been treated as follows:
First method is called the point-by-point method. When the specklegram is put on the optical arrangement as shown in Fig. 2, the displacement for a point can be calculated.