Study Of Rebar Video Extensometer

This project is supported by a grant from a large-scale iron and steel enterprise.In this paper the visual measurement method of the tensile process of rebar is discussed,where the standard sample is analyzed for acquiring dynamic deformation parameters through machine vision,then data acquired from samples is calculated and modeled for studying the elongation after fracture.This way can improve traditional measurements and realize the automatic method without manual marking.In the traditional mechanical property test of rebar,it is necessary to pretreat the rebar in advance,that is,the rebar was marked manually by ink or by punching.After the tensile test is completed,the marking position is manually measured,and then the parameters are analyzed.Since the manual marking is contingent and there are measurement errors,the accuracy of the testing process and the reliability of the data cannot be guaranteed.In addition,due to the irregular extension of the marks made by the tensile specimen in the axial direction of the specimen during the tensile process,the marks may tear and the system may fail.Therefore,aiming at the tensile visual stretching test method of rebar material mechanical properties,a non-contact rebar video extensometer for the whole process of rebar is developed.The traditional manual marking method is abandoned,and the feature points of thread edge required by the system are extracted to replace the traditional ink marking and hole marking.The method of obtaining feature points is used to realize the non-contact measurement of the whole process of stretching.The system can be used for non-contact measurement of the entire stretching process.Another important goal of this research is to achieve the planning goal of"Machine instead of human"in the cooperative iron and steel enterprise,thus reducing the reliance on the experience of testers and making the testing process more stable and reliable,with less labor intensity.The main research work of this paper is as follows:1.In order to obtain the feature points required for the measurement of displacement changes in the stretching process accurately,this project investigated the pre-processing algorithm for the stretching image of the rebar,using the segmented linear grayscale image enhancement method,the median filtering algorithm,the Ostu binarization algorithm,the Canny edge detection algorithm and the morphological gradient processing method to obtain a complete and coherent image of the edge of the rebar,laying the foundation for the subsequent operation of extracting feature points.2.Considering the randomness of the pull-off position,the Harris corner point detection algorithm was studied and improved to obtain accurate information on the pull-off position of the rebar and other feature points.3.The method of automatic marking of the edge features of the rebar was studied.By sequentially traversing the pixel coordinates of the image of the edge of the rebar to obtain the feature points required for the pull-off position,and using the information of the edge of the rebar at the spacing of the three horizontal ribs above and below the feature points instead of the traditional manual marking,a fully automatic non-marking test method was realized.This method ensures that the measured post-break elongation is valid regardless of the location of the break,as the distance between the break and the nearest thread feature required by the system is always greater than（1/2）₀,and the break location is always between the nearest thread feature required by the system.In this paper,the mechanical parameters of the material during the tensile process of the rebar were measured by the rebar video extensometer,and the post-break elongation of three groups of different diameters of rebar was measured by combining the above parameters.The error rate between the experimental results and the post-break elongation measured by the manual direct measurement method is less than 5%,which can meet the expected test performance requirements of the subject and comply with the relevant standards.