Research On The Vision-based Non-contact Planar Structure Stress Distribution Measurement Method

In the automotive industry, many important components usually require analysis of the productstrength, stiffness analysis, durability analysis, impact shock test after trial. For the stress andstrain measurement, limited local data can be accessed using traditional testing tools, such asstrain gages, which is not enough for a comprehensive assessment of product performance andimproving product quality. Digital image correlation is a new non-contact optical measurementmethod by matching points between front and rear deformation deformation image using regionalmatching technology.Currently there are some deficiencies in the measurement accuracy of this method. Purpose ofthis paper is to study how to improve the application range and the measurement accuracy of planedigital image correlation method. The main research achievements are as follows:(1) Based on themethod which automatically extracts and matches the features of speckle pattern, a kind of methodis presented to estimate the initial value of the deformation. This method utilizes SIFT (ScaleInvariant Feature Transform) algorithm, which has rotation, light and perspective invariance, toextract the before-and-after deformation coordinates of the same points. According to thecoordinates, the initial value of the deformation is calculated. This estimation method can also beapplied in the condition of large deformation and acquire more accurate initial value;(2)Considering the error caused by the problem that the optical axis is not perpendicular to theobject surface, a block is designed to measure the angle. Parameters of the block are analyzed.With the captured block image handled, the angle between the optical axis and the object surfaceis calculated to remove the error;(3)In order to compare the performance of the sub-pixelalgorithms of plane digital image correlation method, a speckle pattern with non-uniform strain isapplied. The results show that Newton-Raphson method has highest accuracy of the threealgorithms;(4) A set of plane digital image correlation calculation program is written in Matlablanguage. This program includes a kind of subpixel displacement calculation method namedNewton-Raphson algorithm, and it can complete various computing tasks of digital imagecorrelation.At last, DIC method is applied in tensile tests of plates with holes, whose strain can not bemeasured by strain gauge test. The results are consistent with the finite element analysis method.Results of this study improve the calculation range and degree of automation of plane digitalimage correlation on one hand and on the other hand provide a guarantee for the accuracy ofcamera mounting in the actual experiment.