Composite Connection Structure Detection Based On Digital Image Correlation

Carbon fiber reinforced plastic(CFRP)is widely used in mechanical materials and high temperature and ablative resistant materials due to its characteristics of light weight,high strength,high temperature resistance,corrosion resistance and excellent thermodynamic properties,and are the most widely used composites at present.At the same time,the connection structure of composite material is an indispensable and weak part of the whole structure,and the damage of the connection structure will lead to the catastrophic damage of the whole structure.Therefore,it is necessary to monitor the composite connection structure.As a non-contact measurement method,digital image correlation method has attracted more and more attention because of its high measurement accuracy,and it can measure the displacement and strain of the whole field.Moreover,it does not need to lay out complex lines and sensors on the surface of the measured parts,so as to improve the measurement efficiency.This paper focuses on the bolt connection structure of CFRP material and aluminum alloy material,and takes the bolt vibration state of the connection structure as the research object.Based on the digital image correlation method,the deformation information of the connection bolt in the composite connection structure is obtained by analyzing the collected speckle images before and after the deformation of the connection bolt,and calculating its displacement field and strain field.The main work of this paper is as follows :(1)The basic content of digital image correlation method used in composite connection structure is analyzed,including basic measurement function,integral pixel displacement search method,sub-pixel displacement search method,Strain field measurement,etc.The influencing factors that may cause the measurement error of DIC algorithm are discussed.(2)In The digital image correlation measurement system platform based on composite material connection structure is built,and the checkerboard method is used to calibrate the camera.(3)In order to make the features of the image more prominent,the image is pre processed by graying and filtering.At the same time,an algorithm is written to make the simulated speckle pattern,and the calculation accuracy and running speed of the sub-pixel displacement search method are verified.The DIC core algorithm used in this paper to measure the deformation of the joint bolt in the composite joint structure is discussed,and the spatial resolution of the camera is improved by image Mosaic technology.(4)Based on the vibration state of the connecting bolt of composite plate structure,according to the scope of the bolt to be measured and the application of graphic stitching technology,this paper takes experimental photos in three different situations.DIC algorithm was used to analyze the images obtained in different experiments to obtain strain and displacement cloud images,strain and displacement curves,as well as strain and displacement data information.The obtained data are analyzed to verify the effectiveness of the algorithm and graphic stitching technology in this paper,and the loosening of bolts can be inferred from the obtained data.