Research On Digital Speckle Correlation Method And Its Applications In Mechanical Engineering Measurement

Two-dimensional digital speckle correlation mothod is an effective optical measurement technique for full-field displacement and strain estimation, which has been widely used in the fields of experimental mechanics and engineering measure and test. However, deficiency existed in terms of the calculation accuracy, calculation efficiency and the versatility. An improved algorithm here aims at making this method a widely applicatory measuring method. It means that ideally this method can be applied in various conditions. The key results of this dissertation are:In view of the inefficiency of point-by-point search algorithm, an adjacent domain search algorithm is proposed. This algorithm can greatly improve the efficiency of the integer pixel search. A grayscal linear change model λ used in the gray gradient-based method for subpixel-accuracy displacement measurement is proposed. This proposed method could deal with the gray change or illumination variation on the specimen surface during the experiment. A formula of displacement measurement errors by the gradient-based DSCM method was derived. The errors were found to explicitly relate to the image grayscale errors consisting of sub-pixel interpolation algorithm errors, image noise, and subset deformation mismatch at each point of the subset.A new algorithm of simulated random speckle pattern is proposed. In this way, the distribution of spots with respect to speckle diameter and location can be controlled and optimized. Toner transfer technique is exploited to provide an efficient method to realize speckle patterns on the specimen surface for DSCM. This technique provides us with a novel approach for artificial speckle pattern realization. Lens distortion practically presents in optical imaging system using in DSCM system, and gives rise to additional errors in the displacement and strain measurement. Camera calibration procedure based on Harris method is performed to obtain the coefficients of radial distortion and tangential distortion. The three-point bending test results show that the camera lens calibration method can effectively eliminate the effect of lens distortion and improve displacement and strain measuremen accuracy.This dissertation proposes a digital speckle correlation strain measurement method based on a finite element (FE) algorithm. Both simulation and experiment processing, including tensile strain deformation, show that the proposed method can achieve nearly the same accuracy as the cubic spline interpolation method in most cases and higher accuracy in some cases, such as the simulations of uniaxial tension with and without noise. The results show that it also has a good noise-robustness. Finally, this method is used in the uniaxial tensile testing for Dahurian Larch wood specimens with or without a hole, and the obtained strain values are close to the results which were obtained from the strain gauge and the cubic spline interpolation method.The other important part of my work is the application of DSCM in engineering measure and test. Some key issues are discussed in the prosess of testing. The key results of this part of work includes:(1) The thermal deformation fields and coefficient of thermal expansion of45steel at different temperatures are measured and compared with the existing literature value. The repeated experimental results confirm the effectiveness and accuracy of the noncontact varying temperature deformation measuring system.(2) The mixed mode stress intensity factors are calculated using displacements from digital speckle correlation method. With the crack angle increasing, the SIF of mode I in unilateral crack increase all along, but the SIF of mode II in unilateral crack shows an initial increase, followed by a decrease. Crack-tip plastic zones were determined using an anisotropic yield criterion.(3) A mechanical creep testing machine is developed by author and used in combination with DSCM. Creep strain-time curves of45steel at room temperature were measured using digital speckle correlation method.Actual measurement results demonstrate that the proposed DSCM algorithm is a high accurate measurement method and possesses preferable practicability and adaptability.