Digital image correlation and fracture: an advanced technique for estimating stress intensity factors of 2D and 3D cracks (original) (raw)
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Advanced 2D and 3D Digital Image Correlation of the Full Field Displacements of Cracks and Defects
Full field mapping of displacements between successive images by digital image correlation is a powerful and well-established technique, used in fields as diverse as geo-tectonics, engineering mechanics and materials science. Analysis of three-dimensional images, such as computed X-ray tomographs, is also becoming routine. These techniques provide new ways to study and quantify deformation and failure processes: recently they have been applied to detect and study cracks and defects in engineering materials, for instance by coupling the displacement analysis with finite element codes to readily extract the crack propagation strain energy release rate (J Integral). Such analyses increase the richness of the data obtained, for example providing information on the mode of loading, and are suitable for the analysis of engineering components under complex states of stress. This work has highlighted areas where the development of image correlation methods that are optimised for analysis of discontinuities would be beneficial, for better detection of small cracks and the early development of damage against the background displacement field; improved precision in crack displacement field measurement by intelligent “masking’ or analysis algorithms and better integration with finite element software packages to make use of advanced tools for 2D and 3D deformation analysis. This paper reviews some of this recent work on the analysis of 2D and 3D damage in engineering materials, and describes developments in quantitative analysis of defects by image correlation. The examples covered include brittle crack propagation in nuclear graphite, fatigue loading in magnesium alloys and indentation damage in brittle and ductile materials.
Identification of a crack propagation law by digital image correlation
International Journal of Fatigue, 2012
It is proposed to use digital image correlation (DIC) to identify parameters governing crack propagation of commercially pure titanium. To achieve this goal, crack tip location, stress intensity factor, T -stress and plastic zone size are sought. Most of the DIC approaches are based upon local analyses of displacements, and their subsequent projection onto a set of mechanically relevant fields. It is proposed to perform these two sequential steps in a unique (and integrated) way, and to compare the results with a global approach to DIC with subsequent post-processing. A priori performances of two global approaches are compared, and a propagation law is identified form the series of raw images of a fatigue test on commercially pure titanium with the integrated approach that yields better results.
2013
Geomaterials exhibit better resistance to rupture in compression than in tension. The presence of cavities in these materials limits this property. In this work, we focus on the first stage of the rupture process: crack initiation, which is investigated in plaster, considered as a model material. The crack onset is studied in a rectangular specimen with a cylindrical cavity, loaded in compression. The study includes an experimental part and a theoretical part: the theory is based on the use of Leguillon’s mixed criterion, and the experimental part includes the determination of the mechanical properties of plaster and the compression tests. Due to the difficulty of the detection of the crack initiation by the naked eye, the technique of Digital Image Correlation is used to identify the crack onset with an objective criterion on the evolution of the strain field measured. The experimental results are in agreement with the theoretical results. Mots clefs: elasticity, fracture mechanics...
Identification of a crack propagation law by digital image correlation Florent Mathieua
It is proposed to use digital image correlation (DIC) to identify parameters governing crack propagation of commercially pure titanium. To achieve this goal, crack tip location, stress intensity factor, T -stress and plastic zone size are sought. Most of the DIC approaches are based upon local analyses of displacements, and their subsequent projection onto a set of mechanically relevant fields. It is proposed to perform these two sequential steps in a unique (and integrated) way, and to compare the results with a global approach to DIC with subsequent post-processing. A priori performances of two global approaches are compared, and a propagation law is identified form the series of raw images of a fatigue test on commercially pure titanium with the integrated approach that yields better results.
International Journal of Fracture, 2006
Digital image correlation is an appealing technique for studying crack propagation in brittle materials such as ceramics. A case study is discussed where the crack geometry, and the crack opening displacement are evaluated from image correlation by following two different measurement and identification routes. The displacement uncertainty can reach the nanometer range even though optical pictures are dealt with. The stress intensity factor is estimated with a 7% uncertainty in a complex loading set-up without having to resort to a numerical modelling of the experiment.
Multi-scale crack closure measurements with digital image correlation on Haynes.PDF
An experimental campaign was developed to study fatigue crack growth in Haynes 230, a Ni-based superalloy. The effects of crack closure were investigated with digital image correlation, by applying two different approaches. Initially, full field regression algorithms were applied to extract the effective stress intensity factor ranges from the singular displacement field measured at crack tips. Local closure measurements were then performed by considering crack flanks relative displacements. Two points virtual extensometers were applied in this phase. Experimental results were then compared to the reference da/dN -∆K eff curve: it was found that the correct estimation of crack opening levels shifts all the experimental points on the reference curve, showing that DIC can be successfully applied to measure crack closure effects.
Some experimental observations of crack-tip mechanics with displacement data.PDF
In the past two decades, crack-tip mechanics has been increasingly studied with full-field techniques. Within these techniques, Digital Image Correlation (DIC) has been most widely used due to its many advantages, to extract important crack-tip information, including Stress Intensity Factor (SIF), Crack Opening Displacement, J-integral, T-stress, closure level, plastic zone size, etc. However, little information is given in the literature about the experimental setup that provides best estimations for the different parameters. The current work aims at understanding how the experimental conditions used in DIC influence the crack-tip information extracted experimentally. The influence of parameters such as magnification factor, size of the images, position of the images with respect the crack-tip and size of the subset used in the correlation is studied. The influence is studied in terms of SIF and T-stress by using Williams' model. The concept of determination of the K-dominance zone from experimental data has also explored. In this regard, cyclic loading on a fatigue crack in a compact tension (CT) specimen, made of aluminium 2024-T351 alloy, has been applied and the surface deformation ahead of the crack tip has been examined. The comparison between theoretical and experimental values of K I showed that the effect of subset size on the measured K I is negligible compared to the effect of size of the image.
Investigation of fatigue crack closure using multiscale image correlation experiments
Engineering Fracture Mechanics, 2009
Two full-field macroscale methods are introduced for estimating fatigue crack opening levels based on digital image correlation (DIC) displacement measurements near the crack tip. Crack opening levels from these two full-field methods are compared to results from a third (microscale) method that directly measures opening of the crack flanks immediately behind the crack tip using two-point DIC displacement gages. Of the two full-field methods, the first one measures effective stress intensity factors through the displacement field (over a wide region behind and ahead of the crack tip). This method reveals crack opening levels comparable to the limiting values (crack opening levels far from the crack tip) from the third method (microscale). The second full-field method involves a compliance offset measurement based on displacements obtained near the crack tip. This method delivers results comparable to crack tip opening levels from the microscale two-point method. The results of these experiments point to a normalized crack tip opening level of 0.35 for R $ 0 loading in grade 2 titanium. This opening level was found at low and intermediate DK levels. It is shown that the second full-field macroscale method indicates crack opening levels comparable to surface crack tip opening levels (corresponding to unzipping of the entire crack). This indicates that effective stress intensity factors determined from full-field displacements could be used to predict crack opening levels.
Analysis of Fractured Samples with Digital Volume Correlation
Conference Proceedings of the Society for Experimental Mechanics Series, 2012
Synchrotron X-ray tomography was used to monitor in situ three dimensional (3D) fatigue crack propagation in a nodular graphite cast iron. Direct image analysis allows for the retrieval of the successive positions of the crack front, and the detection of local crack retardation, while volume correlation enables for the measurement of displacement fields in the bulk of the specimen. Stress Intensity Factors (SIF) are extracted from the measured displacement fields. It is possible to link the non propagation of a crack with crack closure in COD maps or with a local value of the measured SIF range.
Extended digital image correlation with crack shape optimization
International Journal for Numerical Methods in Engineering, 2008
The methodology of eXtended Finite Element Method is applied to the measurement of displacements through digital image correlation. An algorithm, initially based on a finite element decomposition of displacement fields, is extended to benefit from discontinuity and singular enrichments over a suited subset of elements. This allows one to measure irregular displacements encountered, say, in cracked solids, as demonstrated both on artificial examples and experimental case studies. Moreover, an optimization strategy for the support of the discontinuity enables one to adjust the crack path configuration to reduce the residual mismatch, and hence to be tailored automatically to a wavy or irregular crack path.