Development Of Visual Measurement System For Material Deformation Based On CCD

Strain is one of the most important parameters in material measurement. Thematerial constitutive relation made of stain and stress can be used to characterize themechanical behavior of materials. So a rapid and accurate measurement of the strainis an important research direction in material mechanics. In this thesis, theextensometer’s development status and the image processing algorithms in videoextensometer have been discussed. And a visual measurement system for materialdeformation based on CCD has been designed using the technologies of visualmeasurement. The designed system can realize non-contact and high-precisionmaterial deformation test.In the thesis the measurement principle of the visual measurement system hasbeen described. Firstly, the optical structure has been analyzed and optimized. Allcomponent brackets of the measurement system have been designed usingSolidWorks. The relationships between each other have been determined and thesystem assembly has been simulated. And then, an equivalent pixel algorithm hasbeen put forward to replace the camera calibration. It can easily convert materialdeformation from the pixel space to the actual space. In order to adapt theillumination change, a digital image pretreated algorithm based on the combination ofbrightness adjustment and defogging enhancements has been proposed, and a clearimage including laser mark and specimen mark is obtained. The image characteristicsof colors and brightness have been analyzed, a thresholding segmentation based on Crchrominance has been applied to recognize the laser mark, and the other thresholdingsegmentation has been used to obtain the specimen mark based on the brightnessgradient in the region of interest. And at last, the platform of the visual measurementsystem has been developed. The error sources which will affect the visual measurement system’s precision have been analyzed, and some correction methodshave been proposed to improve the system’s accuracy and stability effectively.The experiments have been done to verify feasibility of the visual measurementsystem:(1) Two marks can respectively recognized in different light and whether ornot there is the chuck of the test machine, and the mark recognition error is less than0.5pixels.(2) A repeated tensile tests are done to verify the system feasibility withdifferent materials. And a compared test has been tested with the testing machine. Andthe ideal and stable stress-strain curves have been obtained.