Measurement of mixed-mode stress intensity factors using digital image correlation method Rui Zhang n , Lingfeng He School of engineering and transporting, South China University of Technology, Guangzhou 510640, China article info Article history: Received 28 April 2011 Received in revised form 11 January 2012 Accepted 11 January 2012 Available online 6 March 2012 Keywords: Digital image correlation Stress intensity factors Mixed mode Least squares Crack tip location abstract Applications of the digital image correlation method (DIC) for the determination of the mixed-mode stress intensity factors (SIF) is investigated in this paper. Experiments were performed on an edge fatigue cracked aluminum specimen using a special loading device, which is an appropriate apparatus for experimental mixed-mode fracture analysis. The full-field displacements around the crack-tip region of the test sample were calculated using DIC. And then the SIF associated with unavoidable rigid- body displacement motion were calculated simultaneously from the experimental data. The effect of the rigid body motion on the measured displacements was then eliminated using the computed rigid body translation and rotation. A coarse-fine searching method was developed to determine the crack- tip location. For validation, the SIF thus determined is compared with theoretical results, confirming the effectiveness and accuracy of the proposed technique. Therefore it reveals that the DIC is a practical and effective tool for full-field deformation and SIF measurement. Crown Copyright & 2012 Published by Elsevier Ltd. All rights reserved. 1. Introduction Cracks are inclined to initiate and grow at stress concentration zones such as notches, holes, or other mechanical defects that are inherent in structural components. Knowledge of the severity of cracks is necessary in order to predict the fatigue crack growth rate, critical length and fatigue life. According to linear elastic fracture mechanics, the stress intensity factor is the key para- meter which quantifies the severity of cracks as it reflects the effect of loading, crack size and crack shape. Also, the displace- ment field, the stress field and strain field near the crack tip can be characterized by SIF. An accurate knowledge of the stress intensity factor is rather important to prevent brittle fractures arising from cracks. Conventional experimental techniques, such as strain gauge, photo elasticity and moire interferometry, have been used to determine SIF or as a way to validate theoretical and numerical results. Strain gage [1] was used to determine strain near the tip of a crack, and then SIF was determined from measured strains. This method has made little progress, which is limited to finite points. Optical methods such as photo elasticity [2] and moire interferometry[3] based on full field information has been used to determine the SIF. Although this optical technique greatly improves the accuracy of SIF measurement, it also has some limitations. For example, the experimental equipment is complicated and the determination of opaque materials is difficult. Compared with the above mentioned methods for the determination of SIF, DIC has follow attractive characterizes: (1) full-field measurement compared with strain gauge, thus sufficient information could be used; (2)doesn’t have such limit when combined with high speed camera; (3) various materials both opaque and transparent materials could be tested and simple experimental set-up compared with photo elasticity; Moreover, unlike other optical methods, DIC is suitable for measurements at many scales for its lack of an inherent length scale, thus, it has been performed on macroscale[4,5], microscale [6,7] and nanoscale [8] measurements. Due to its simplicity and effectiveness, recently, DIC as one kind of optical methods has been used to study fracture parameters for a cracked body subjected to mode I loading [5,7,9–11], from which, John Lambros investigate crack growth in functionally graded materials from microscale view[5] and macro scale view[7] respectively, proving their effectiveness equally. How- ever, cracks in real structures are often subjected to the combined loading of mode I and mode II, and some work have been done on mixed mode study [12–14], and all of them measured SIF at micro scale, from which 1 pixel correspond to several um. And in this paper we study mixed fracture at macroscale, from which 1 pixel correspond to several tens um. In this paper, DIC is used to determine SIF for cracks in aluminum plate. First, the edge-notched aluminum alloy speci- men was subjected to fatigue loading in order to simulating the crack exists in the actual components. Then DIC method is applied to extract full-field crack-tip displacement fields for an Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/optlaseng Optics and Lasers in Engineering 0143-8166/$ - see front matter Crown Copyright & 2012 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2012.01.009 n Corresponding author. E-mail addresses: zrui0310@163.com (R. Zhang), lfhe@scut.edu.cn (L.F. He). Optics and Lasers in Engineering 50 (2012) 1001–1007