Correlação de imagens digitais em ensaios de compressão diametral em rochas

Abstract

Rock compression tests are performed to determine the mechanical characteristics of the material, such as fracture orientation and limits of rupture. These tests facilitate understanding the behavior of rocks under stress. They are characterized as destructive tests, as the specimen is normally unusable after the test. In substitution or comparison to the strain gauges used to measure the specimen deformations, digital image correlation (DIC) techniques are used. These techniques stand out by the measurement without the contact with the specimen, as the extensometers can be damaged in the current tests. The present work involves the application of DIC 2D using a freeware and a low-cost digital camera in indirect tensile tests by a diametric compression, commonly known as Brazilian test. In this study, tests seeking the best way to collect images and measure their pixels were performed, images were treated, and algorithms were used to process the images. The objectives were: studying crack propagation, analyzing displacement and strain, comparing DIC values with values obtained by extensometers, and making tests with low-cost camera feasible. The results demonstrated that the use of the USB microscope provides displacement values which are very close to the values obtained using DSLR camera (12% difference) and also very close to the values measured by an extensometer (0,10mm difference). DIC reduces the amount of time necessary for the test, as the test using extensometer requires discharging the stress to remove the extensometer at 70% of the pre-peak in order to avoid damaging the extensometer. The trajectory analysis tool provides a good estimate of the direction of deformation and the break of the specimen.