Research On Robust Digital Image Correlation Method And Its Application In Strain Measuring

Vehicle developing involves choosing materials, analyzing parts' strength, testing vehicle performance and so on. Aiming at the mechanical property related testing during the process,traditional testing methods, like strain gauge and extensometer, can not measure subsize specimen or large deformation, and these methods can only acquire limited local average data. Digital image correlation(DIC) technology is a novel non-contact testing method. It transforms the deformation testing of the specimen surface to intensity feature matching. DIC takes advantages of being able to testing wide range objects, high testing accuracy, measuring whole deformation of the specimen surface and so on, therefore this method is widely applied in material testing nowadays.Study of DIC method now mainly aims at improving algorithm's accuracy and extending the application of this technology. Research contents of this paper are listed as follows:(1) Simulated speckle images, with deformation parameters controlled, are effective bases for algorithm analysis. A reverse mapping method is proposed for generating deformation speckle images.This method is easily understood and is liable to realizing by programming.(2) Correntropy based robust digital image correlation method(RDIC) is proposed. When the shape function can not accurately describe the deformation of the subset, or there is noise in the imaging system, the intensity of the images is affected, and the large intensity bias has a bad effect on the calculation. However, kernel size of the correntropy criterion can automatically find the intensity bias of the speckle image and gives a lower weight to the bias to improve the accuracy of the digital image correlation method.(3) As for the strain calculation of the material surface, displacement field obtained via RDIC is firstly smoothed by the regularized method to punish the displacement noise. Then the smoothed displacement field is differenced to acquire smoother strain distribution.(4) Robust digital image correlation method is applied to the experiments. For continuous deformation, results gotten by RDIC and extensometer are compared to verify the effectiveness of the RDIC method. For discontinuous deformation, RDIC method is applied to test stress concentration around the hole of the carbon fiber specimen, the deformation distribution around the crack of the carbon fiber and the deformation around the dense holes to further extend the application of RDIC.