High Precision Deformation Measurement Based On Radial Basis Function

Electric speckle pattern interferomery (ESPI) and digital image correlation (DIC) are popular modern optical methods for surface deformation measurement. In ESPI, the essence of the measurement is the monitoring of brightness changes in each and every speckle and calculating the changes in phase relationships within individual speckles cause changes in the speckle irradiances. As for DIC, its basic principle is to match the same physical points between the two images recorded before and after deformation. The two methods both offer the advantages of non-contact and full-field deformation measurement. However, there still exist quite a few deficiencies in terms of displacement and strain measurement accuracy. In this research, ESPI and DIC have been studied, and high accuracy deformation measurement methods based on radial basis function (RBF) were proposed. Main achievements of the research are listed as following:1. RBF was combined with ESPI, and a novel pre-filtering RBF method and a local data based moving-RBF method were proposed for speckle denoising. The selection of important parameters was also discussed. Furthermore, a method of non-destructive strain field determination based on the combination of the temporal phase measurement interferometry (TPMI) with RBF was presented. Optimal sampling intervals of images with different sizes were given based on numerical experiments.2. Two methods based on DIC and RBF were proposed to obtain the accurate strain field. One is a combined method. RBF was applied to remove the noisy discrete displacement data first. After that, the strain was computed by a local least-squares algorithm. The other is a partial derivative of RBF (PD-RBF) based strain estimation method which integrated denoising with differential process. Furthermore, a shape parameter selection method based on rate of convergence was suggested. The new method simplifies the choice of the good shape parameter. Moreover, a gradient-entropy based RBF method was developed to obtain good result.3. Experimental investigation and analysis on different materials and objects were performed based on the proposed methods. A) Amplitude-fluctuation electronic speckle pattern interferometry (AF-ESPI) was incorporated with RBF to measure vibration characteristics of sandwich panels with honeycomb core. The influence of presence of a pre-notch at different location was also investigated. B) Strain distribution of a three-point bending beam with pre-notch was also determined determination based on the combination of the TPMI with RBF interpolation. C) DIC was used to measure the fracture process of the cast iron. RBF was applied to denoise the displacement data. The initial crack load was determined by analyzing results of the strain-rate of the area around the tip of the pre-notch after strain calculation. The predicted fracture initiation and crack propagation patterns are consistent with the observed cracking behavior. The load capacity was evaluated by the method combined DIC with the double-K fracture criterion and the simplified double-K fracture model.