Research On Deformation Measurement Of Composite Materials Based On Digital Image Processing

Carbon fiber reinforced polymer(CFRP)is used in various fields because of its excellent mechanical properties.For the automotive field,it can realize the lightweight and reduce the fuel consumption.However,in order to apply CFRP to automobile,it must have excellent bearing performance,and the deformation in the bearing process should not be too large,which will lead to the failure of its connection with metal materials,so it is particularly important to accurately grasp the deformation of CFRP in various working conditions.At present,the deformation measurement of the structure is mainly divided into contact measurement and non-contact measurement.The contact measurement has a high requirement on the environment,and it needs to contact with the surface of the measured part,which to a certain extent causes errors to the measurement results,and can not measure the whole field deformation of the structure.As a non-contact measurement method,digital image processing has attracted more and more attention because of its high measurement accuracy,the ability to measure the displacement and strain of the whole field,and it does not need to arrange complex lines and sensors on the surface of the parts to be measured,thus it can improve the measurement efficiency.Digital image processing finds the position of each pixel point after the deformation by matching the speckle image before and after the deformation of the specimen collected by the camera,and then calculates the displacement field and strain field of the surface of the specimen to be measured,and obtains the deformation information of the specimen to be measured.The main work of this paper is as follows:(1)Based on the principle of digital image processing method,this paper compares the correlation functions representing the correlation degree of image sub areas before and after deformation,determines to choose the normalized covariance correlation coefficient with better unimodal property to measure the matching degree of image sub areas,and introduces the image pre-processing method.It is found that the smaller the filtering window,the better the processing effect.(2)On the basis of MATLAB,the correlative algorithm is compiled to make the simulated speckle map,and the whole pixel correlative search method(global search method and three-step search method)is verified in calculation accuracy and running speed.Through processing the simulated speckle map with known deformation parameters,it is found that the three-step search method is better than the global search method,and the three-step search method is improved to improve displacement measurement After the whole pixel displacement is obtained,the sub-pixel interpolation is carried out,and the calculation results of quadric surface fitting method and gradient method are compared.It is found that the calculation results of quadric surface fitting method are more accurate and reliable.(3)After the displacement field is obtained,the local least square fitting method is used to transform it into strain field,so that the deformation information of the sample can be obtained,and the DIC method used in this paper is compared with the calculation results of commercial software GOM,and the calculation errors of the two methods under different deformation conditions are calculated respectively.