The Key Technology Of Strain Optical Measurement And Its Application In Special Equipment Monitoring

The rapid development of scientific research and engineering technology has higher and higher requirements for the measurement and monitoring of strain of materials and structural components under load.At present,the commonly used method of strain measurement and monitoring is the electrical measurement method represented by the resistance strain gage and the vibrating string extensometer,which has the characteristics of small error and large range of strain,but they have such problems as low survival rate of sensors,poor electromagnetic interference,and long creep state of steel wire string,so that it is difficult to achieve long-term equipment,continuous,on-line monitoring.According to the above problem,this paper study and solve the mechanical signals such as stress,strain and displacement of the structure surface measurement and long-term monitoring based on the optical methods,which uses digital speckle correlation method of whole-field strain optical measurement and fiber Bragg grating sensing technology of point strain optical monitoring.The strain of digital speckle correlation method is obtained through mathematical processing of displacement field,and the accuracy is greatly influenced by displacement field.Therefore,it is necessary to improve the sub-pixel search method of displacement field,and improve the accuracy and speed of measurement.When fiber Bragg grating strain sensing technology is used for long-term monitoring,the measured matrix and fiber are connected through the interlayer,the strain measured by fiber Bragg grating is different from that of matrix due to the shear deformation of the interlayer,which causes strain measurement errors.Therefore,it is necessary to study the function relationship between the strain measured of fiber Bragg grating and the strain of the matrix under the influence of the adhesive layer in order to improve the measurement accuracy.Therefore,the main work and innovation of this paper are as follows:1.The binary quadric surface fitting method of the sub-pixel displacement search algorithm is improved in this paper.Sub-pixel displacement of images before and after deformation is solved using the improved method and the binary quadric surface fitting method by the simulated speckle.It can be seen that the standard deviation of the improved quadratic fitting method is slightly greater than that of the binary quadric surface fitting method,while its mean error is much smaller than that of the binary quadric surface fitting method.2.A subpixel displacement measurement method based on the combination of particle swarm optimization and gradient algorithm was proposed for accuracy and speed optimization in gradient algorithm,which is a subpixel displacement measurement method better applied in engineering practice.An initial integer-pixel value was obtained according to the global searching ability of particle swarm optimization,and then gradient operators were adopted for a subpixel displacement search.A comparison was made between this method and gradient algorithm by simulated speckle images and rigid-body displacement in metal specimens.The results showed that the computational accuracy of the combination of particle swarm optimization and gradient algorithm method reached 0.01 pixels in the metal specimen.Also,computational efficiency and the anti-noise performance of the improved method were markedly enhanced.3.It has been a challenge for researchers to accurately measure high temperature creep strain online without damaging the mechanical properties of the pipe surface.To this end,a noncontact method for measuring high temperature strain of a main steam pipe based on digital image correlation was proposed,and a system for monitoring of high temperature strain was designed and developed.During the design of an optical window,finite element was used to analyze the air temperature field distribution of the pipe surface and its influence on an optical window,and the size of optical window was also provided.To verify the effectiveness of the proposed method and system,the experiment of spray painting and welding speckle field was carried out in the laboratory and compared with the resistance strain gauges.The results show that the three have good consistency,the error mainly lies in the angle deviation between the system and the specimen.The developed monitoring system of high temperature strain was tested on the real main steam pipe.The measured coefficient of thermal expansion shows good consistency with the data recorded in the literature.Therefore,the proposed method and system can be used to measure the creep strain of the main steam pipe of a boiler of an ultra-supercritical power plant in service,and to meet the actual measurement requirements for engineering.4.The grating ends bonding fiber Bragg grating sensor has the advantages of no multi-peak phenomenon of the gating bonding fiber Bragg grating,no direct force of the grating and all points under the same stress in the grating,etc.It is widely used in sensor packages such as substrate type and clamp type.However,due to the shear deformation of the adhesive layer,the strain measured by fiber Bragg grating is different from that of matrix,which causes strain measurement errors.Therefore,it is necessary to study the function relationship between the strain measured of fiber Bragg grating and the strain of the matrix under the influence of the adhesive layer in order to improve the measurement accuracy.In this investigation,to predict the strain transfer of the grating ends surface-bonded fiber Bragg grating,the strain transfer equation of the grating ends bonding fiber Bragg grating is derived and a theoretical model of the average strain transferred from the host material to the optical fiber is developed.The influence of the physical parameters on the average strain transfer rate of the adhesive layer and the matrix is given.The validity of the theoretical equation is verified by finite element method and experiment,which reveal the effectiveness of the proposed model and the superiority of the accuracy of measurement compared with the existing methods.