Research On Structural Deformation Measurement Method Based On DIC Technology

As a new non-contact optical measurement method,digital image correlation method has been developed rapidly since it was put forward in 1980s.Because it has many advantages such as high precision,non-contact and full field measurement,so it has been applied into many fields.In the field of civil engineering,the advantages of digital image correlation method not only make up the shortage of existing deformation measurement methods but also solve many problems of civil engineering experiment.At present,there are many methods of structural displacement and deformation measurement,this paper summarizes some common existing displacement and deformation measurement methods,and make a comparison with digital image correlation method.This paper summarizes the research status of digital image correlation method at home and abroad,and introduces the application of digital image correlation method in civil engineering field.This paper introduces the typical digital image correlation method system composition,and describes some principles,including the shape function,correlation function and other theoretical principles,and three common sub-pixel displacement algorithm:surface fitting method,gradient method,N-R method,and make a comparison between their calculation accuracy and computational efficiency through the simulation of speckle patterns.This paper introduces the pixel-real distance conversion system and the production technology of simulated speckle pattern.The displacement field is calculated by three sub-pixel displacement measurement algorithms that are the surface fitting method,gradient method and N-R method.The displacement field is filtered by median filter,and the corresponding strain field is obtained by the difference method and the local displacement field least square method.By comparison,it is concluded that:(1)The calculation accuracy of the displacement field is favorable to the calculation of the strain field;(2)The calculation accuracy of the strain field can be improved by smoothing the displacement field;(3)Based on the displacement field local least square method,the strain is smoothed to smooth the noise in the displacement field.This paper puts forward a method based on the least square method of long-distance under the condition that the small deformation of the least square method is not good for calculation.That means the displacement field will become smooth through increasing the calculation step of the displacement field.The larger the calculation step is,the corresponding decrease of the calculation point.In order to increase the number of calculation points,the displacement field is proposed in multi-step calculation,the displacement field is fitted with the least square fitting,and multiple strain fields are obtained.Finally,the multiple strain matrix is recombined to get a new strain matrix.The calculation accuracy is higher than the direct least square fitting method.In this paper,the least squares method is proposed to solve the strain based on cubic spline function smoothing technique.Through the median filter processing of the displacement field,it found that smoothing the noise to the displacement field is a simple and effective method to improve the strain precision.Cubic spline function has many excellent properties,so the cubic spline function is considered to fit the displacement matrix,and its smoothing effect is very effective.In order to verify the feasibility of the least squares fitting based on the step length method and the least squares method based on cubic spline function smoothing technique,the paper uses the pure bending beam experiment and the tensile experiment with perforated plate to verify the two methods.The displacement field is calculated by n-r method,and then the strain is calculated by two improved strain calculation methods.The calculation results of DIC method are compared and verified by using the finite element model.At the same time,in order to expand the application of digital image correlation method,a large-scale measurement technology based on DIC-edge detection is proposed.