Study On Improved Extended Digital Image Correlation Method And Its Application

Digital image correlation (DIC) method is one of the most commonly used testmethods in experimental mechanics. It proves to be a very promising test method forengineering application. Standard DIC is not applicable to measure the discontinuousdeformation field directly. However, the displacement mode of extended finiteelement method (FEM), which is chosen as the shape function of extended digitalimage correlation (X-DIC) method, could break the application limitations of standardDIC in the discontinuous deformation field measurement. The main content of thispaper is to improve the algorithm of X-DIC, with the aim to improve themeasurement accuracy of X-DIC and enlarge its engineering application scales.This paper improved and optimized X-DIC method in three aspects. Firstly,single-element correlation scheme and core area method for post-processing ofmeasured results are proposed. Compared to the integral correlation scheme, theproposed method could reduce the error transfer between the subset effectively andimprove the measurement accuracy to some extent. Secondly, ring template isproposed for estimating the integer pixels displacement. And the scheme of the initialvalue selection for the Newton-Raphson iterative method of improved X-DIC is alsoproposed. The above-mentioned work could reduce the non-convergence phenomenonin the iterative procedure. What’s more, the ring template possesses the rotationinvariance and the bi-linear displacement hypothesis could describe the rigid bodyrotation completely, which make improved X-DIC could measure the deformationincluding rigid body rotation with arbitrary angle. Thirdly, the selection scheme of theoptimum subset is proposed as well. Correlation coefficient is used to determine thesize and the location of the optimum subset, which makes the measurement of theelastic constants and the deformation of the crack tip and other mechanical parameterswith higher accuracy.Simulated images and experiments are used to analysis the measurementaccuracy of improved X-DIC. For single-element correlation scheme, themeasurement accuracy of single subset could represent the measurement accuracy ofimproved X-DIC. Some conclusions are reached through the accuracy analysis: if thedisplacement hypothesis of the shape function could describe the actual deformation of the subset completely, the subset with large size could reduce the random error andimprove the measurement accuracy, and subset with fewer nodes would get highermeasurement accuracy; when the displacement mode of the cracked element is chosenfor the shape function to measure the discontinuous deformation, the measurementaccuracy is lower than that of using conventional element to measure the deformationfield on each side of the crack respectively; when improved X-DIC based on thefour-node quadrilateral element displacement hypothesis is used to measure thedeformation which is in accordance with this hypothesis, the maximum absolute errorwill be located on the node of the subset; and as the given displacement approachesthe integer pixels or half pixels, the measurement error gets minor.At last, improved X-DIC is used to solve the practical engineering problems: theelastic constants of the light guide plate of mobile phone are measured; theelastic-plastic deformation of the molded pulp pallet in the dropping test isinvestigated; large deflection deformation of the polyurethane specimen is measured;the deformation in the region near the crack of the metal specimen is measured; theplane strain fracture toughness of polycarbonate is measured; and the bendingdestruction behavior of the liquid crystal faceplate used for mobile phone is studied.