Experimental Study On Deformation Measurement Under Step Load By Electronic Speckle Pattern Interferometry

In this paper,we hope to use the in-plane deformation measurement method of electronic speckle interferometry to introduce the shear amount in the image processing process to make the phase map generate "dislocation" to try to extract the strain information.The research content is mainly divided into three parts:(1)Establishment of strain measurement method.Firstly,the ESPI technique strain measurement formula is obtained by combining the in-plane displacement measurement method of electronic speckle interferometry,four-step phase shift technique,unwrapping technique and differential idea;Then,the in-plane displacement-sensitive electronic speckle interference optical path is used to carry out the step loading experiment for the three-point bending and the four-point bending aluminum beam.The phase-shift,image subtraction,unwrapping,and digital shearing are used to obtain the optical strain value,and corresponding The electrical strain value of the point to be measured is compared.It is proved by experiments that the strain error of the optical strain measured by the ESPI method is less than 3% with respect to the strain gauge,which shows the effectiveness of the method;Finally,the finite element model of the four-point bending beam of the Workbench is established.The numerical results are used to supplement the results of the optical strain test.The numerical method further proves that the ESPI technique strain measurement method is feasible.(2)Optimizing the accuracy of strain measurement.Firstly,the speckle contrast and speckle size are two important factors affecting the accuracy of phase extraction,and changing the aperture size and measuring distance can change the speckle size.Then,the size of the speckle is affected by two schemes: one is to ensure the measurement distance and the aperture size of the camera lens to change the focal length of the lens,and the other is to ensure that the focal length and the aperture of the camera lens are unchanged.distance.And based on these two schemes and ESPI technology in-plane deformation measurement method,the four-point bending step loading experiment of aluminum beam is carried out.By calculating the relative error of optical strain and electrical strain under different speckle particle size,the speckle size is proved.It can affect the measurement accuracy.(3)Application of strain measurement methods.Firstly,the basic theory of crack propagation of brittle materials is expounded,and the linear elastic criterion and elastoplastic criterion of fracture mechanics are explained.Then,the three-point bending step loading experiment of precast cracked cast iron is carried out by ESPI technology in-plane deformation measurement method.-Displacement curve analysis The crack propagation process is divided into three stages: the initiation stage,the stable expansion stage,and the unstable failure failure stage.It also shows that using the electronic speckle interferometry technology to calculate the phase cloud map and the displacement calculation phase cloud map is more intuitive to express the expansion process.At the same time,based on the ESPI technology in-plane deformation measurement theory,the crack opening displacement(CMOD)and the crack opening displacement(COD)are obtained.The load-CMOD curve and the load-COD curve are used to obtain the crack initiation load and the maximum bearing capacity of the specimen.Finally,the Abaqus three-point bending crack propagation model is established according to the extended finite element theory(XFEM),and the crack propagation process under the step load is numerically calculated.And the extended path,which proves that the numerical calculation results are consistent with the experimental conclusions.