The Improved Sampling Moiré Method For Deformation Measurements And Its Application In Geotechnical Engineering

With the rapid development of image acquisition and image processing technology,the vision based non-contact deformation measurement method has great advantages compared to the traditional contact deformation measurement method,especially in the whole field strain measurement.At present,digital image correlation,including particle image testing,template matching,as the most used vision-based deformation measurement method,has been widely used in indoor test and field deformation monitoring.However,it requires complex calculation,is time-consuming and cannot be used for periodic patterns.Therefore,with the support of the national key research and development plan(2016YFC080200)and natural science foundation of China(51978254),a series of original research including sampling moiré method for measuring deformation and its application in geotechnical engineering have been carried out in this paper,which will provide theoretical guidance for applying sampling moiré method to actual engineering displacement monitoring and indoor test displacement measurement.Firstly,Sampling moiré method(SMM)provides accurate measurement only when the displacement is smaller than half of grating pitch due to defects on phase difference calculation.To overcome the shortage of SMM,we improve the performance of SMM by adapting the integer-pixel digital image correlation(DIC)which is used to the estimate the displacement and phase difference of moiré fringe.The improved SMM(called by DIC-SMM)is capable of measuring displacements larger than half of the grating pitch with high accuracy.To validate the accuracy of the improved SMM(DIC-SMM),the displacement measurement accuracy of the improved SMM under various grating pitches and sampling pitches with multiple noise levels is analysed by computer simulation.Then the proper range of sampling pitch is obtained.Two independent experiments with short and long–range sight distances are conducted to validate the accuracy of the improved SMM(DIC-SMM).In conclusion,with a mean measurement accuracy of up to 1/350 of a pitch,the improved SMM(DIC-SMM)can be used for measuring high-accuracy in-plane displacement whether the displacement is greater than a half of grating pitch or not.In order to explore the influence of environmental factors on the measurement accuracy of sampling moiré method,the effect of the sampling pitch,Gaussian noise level,interpolation method,light illumination,uneven width of grating and rotation of photography axis on the displacement measurement accuracy are studied through theoretical analysis and computer simulation.As a result,the law of influence of environmental factors on the measurement accuracy of sampling moiré method are obtained.Secondly,since the displacement filed proposed by SMM contains a lot of noise,we proposed a sub-pixel SMM(SSMM)by applying sub-pixel interpolation operation before the down-sampling in traditional SMM,and the sampling pitch was selected as a non-integer nearest to the grating pitch in the sub-pixel resolution image.Meanwhile,the filter method was used to eliminate the symmetric error caused by interpolation.As a result,the period of moiré fringe was enlarged greatly and the measurement accuracy was also increased.baaed on the principle of calculating the phase distribution of phase-shift moire fringe,the phase calculation process in SSMM was simplified to speed up the calculation.To investigate the efficiency of sub-pixel SMM(SSMM),the computer simulation is applied to analyze the impacts of the interpolation method,the number of sub-pixel interpolation and fringe number on the accuracy of sub-pixel SMM(SSMM).The most suitable value of interpolation point between two integer pixel position was 9.Then a simple tensile experiment was conducted to validate the efficiency of this method,and maxmum diference between SSMM and Fiber Bragg Grating(FBG)was lower than 50 με.Thirdly,a whole field distortion correction method based on sampling moirémethod is proposed to correct the distorted image.After the grating picture is taken with the distorted camera,the distortion displacement field is obtained by sampling moiré method.The difference between the real distortion displacement field and the distortion model is taken as the objective function,and the improved spotted hyena algorithm is proposed to increase the convergency speed and local search ability.Then the improved SHO is used to search the distortion model parameters by minimize the objective function.the image can be coreretcted with the distortion model parameters,and the result experiment shows that the proposed method can correct more than 80%distorted displacement.Finally,the experiment setup based improved sampling moire method was establised,then the pull-out test of anchor soil interface and the bearing test of stiffened deep cement mixing pile were carried out baed on the established system.The shear characteristics of anchor soil interface,the development law of shear band and the stress distribution in the sample under different anchor diameters and anchor bond lengths were obtained based on the interfacial pull-out tests of anchor soil sample under different sizes.The deformation distribution of soil and bearing mechanism were obtained based on the bearing test of different types of stiffened deep mixed piles.By the way,a new pile top force sensor is developed to monitor the load-sharing ratio of pile top.In two geotechnical tests,sampling moiré method is introduced to measure the deformation distribution of anchor and pile during loading,and the method for making grating pattern on the surface of structure with grating mask is proposed for geotechnical objects.