Issue 30

P. Lorenzino et alii, Frattura ed Integrità Strutturale, 30 (2014) 369-374; DOI: 10.3221/IGF-ESIS.30.44 369 Focussed on: Fracture and Structural Integrity related Issues Application of Digital Image Correlation (DIC) in resonance machines for measuring fatigue crack growth P. Lorenzino Dpto. de Ingeniería de los Materiales. INSA de Lyon, Francia. pablo.lorenzino@insa-lyon.fr G. Beretta, A. Navarro Escuela Técnica Superior de Ingeniería, Universidad de Sevilla. navarro@us.es A BSTRACT . This paper presents a simple experimental procedure that greatly facilitates the use of digital image correlation (DIC) techniques in fatigue test conducted in resonant testing machines, without the need of test interruptions. This is possible due to the implementation of USB interface optical microscopes of very small dimensions, so that they can be mounted on the specimen as a contact extensometer. Thus, the microscope- sample assembly oscillates at the resonance frequency of the test. This is how, although the resonant testing machine is in motion, and although the specimen is subjected to fatigue, changing its dimensions according to the applied load, the acquired image is completely static. This allows the evaluation of the plastic deformations generated by the crack growth, avoiding the elastic ones. Preliminary results on monitoring cracks with this technique on flat specimens with cylindrical notches of 1050 aluminium alloy are also presented. K EYWORDS . Digital Image Correlation; Fatigue test; Resonance Machines; Crack growth. I NTRODUCTION here is a growing interest in using digital image correlation (DIC) to characterize materials for mechanical properties. DIC techniques allow the strain fields arising from application of a stress to a component to be defined. The procedure involves producing a spot pattern on the surface of the target component and assessing relative motion in the spots between the initial and final states in order to characterize the resulting strain fields. This requires taking photographs of the target element before and after loading (see Fig. 1), and using an appropriate algorithm to assess relative motion between spots to obtain detailed information about the resulting strain fields [1-3]. Successful implementation of DIC entails minimizing any relative motion between the image acquisition system and the testing machine in order to avoid errors in calculating strain fields. This hinders use of the technique with resonance machines because they are mounted on springs and hence in continuous motion. Notwithstanding this difficulty, DIC has aroused much interest for fatigue mechanics studies as it provide the means for analyzing plastic deformations arising during crack propagation. T