Digital Image Correlation technique: Application to early fatigue damage detection in stainless steel (original) (raw)
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Fatigue damage analysis in a duplex stainless steel by digital image correlation technique
Fatigue & Fracture of Engineering Materials & Structures, 2008
analysing the mechanical behaviour of materials in situ during mechanical tests. The originality of the present study is to use this technique on the micro-structural scale, in order to understand and to obtain quantitative values of the fatigue surface damage in a two-phased alloy. In this paper, low-cycle fatigue damage micromechanisms in an austenitic-ferritic stainless steel are studied. Surface damage is observed in real time, with an in situ microscopic device, during a low-cycle fatigue test performed at room temperature. Surface displacement and strain fields are calculated using digital image correlation from images taken during cycling. A detailed analysis of optical images and strain fields measured enables us to follow precisely the evolution of surface strain fields and the damage micromechanisms. Firstly, strain heterogeneities are observed in austenitic grains. Initially, the austenitic phase accommodates the cyclic plastic strain and is then followed by the ferritic phase. Microcrack initiation takes place at the ferrite/ferrite grain boundaries. Microcracks propagate to the neighbouring austenitic grains following the slip markings. Displacement and strain gradients indicate probable microcrack initiation sites.
A digital image correlation method for fatigue test experiments
Optics and Lasers in Engineering, 2009
In this paper, a method based on the digital image correlation (DIC) technique is proposed to monitor the crack growth process during a cyclic fatigue test. Stroboscopic illumination is used to acquire DIC speckle pattern images while the test sample is dynamically loaded. The proposed DIC algorithm uses the fact that the load is periodic to increase the accuracy of the displacement field estimates (a sinusoidal fitting method is introduced for this purpose). Using the appropriate post-processing both the crack lengths and the stress intensity factors can be estimated in function of the number of fatigue cycles. A validation test case on an aluminum U-profile will be presented in the paper.
Stress Intensity Factor Gauging by Digital Image Correlation: Application in Cyclic Fatigue
Strain, 2007
A fatigue crack in steel (CCT geometry) is studied via digital image correlation. The measurement of the stress intensity factor change during one cycle is performed using a decomposition of the displacement field onto a tailored set of elastic fields. The same analysis is performed using two different routes, namely, the first one consists in computing the displacement field using a general correlation technique providing the displacement field projected onto finite element shape functions, and then analysing this displacement field in terms of the selected mechanically relevant fields. The second strategy, called integrated approach, directly estimates the amplitude of these elastic fields from the correlation of successive images.
Usage of Digital Image Correlation in Analysis of Cracking Processes
Image Processing & Communications
In this paper, the analysis of the possibilities of using Digital Image Correlation (DIC) based on Graphics Processing Unit (GPU) for strain analysis in fatigue cracking processes is presented. The basic assumption for the discussed displacement and strain measurement method under time variable loads was obtaining high measurement sensitivity by simultaneously minimizing the measurement time consumption. For this purpose special computing procedures based on multiprocessor graphic cards were developed, which significantly reduced the total time of displacement and strain analysis. The developed digital procedure for correlation of images has been used for an example of displacement analysis in the method of fatigue crack propagation testing in airplane riveted joints. In this paper are presented the results of the researches of the team run by professor Antoni Zabłudowski
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.
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.
Surface Deformation or displacement of any material and structure at a macro, micro or nano level because of the external effect (both mechanical and thermal) is the point where failure of the material is initiated. To know why and how materials are failed many researchers and engineers have been conducted different investigation on the field of mechanics of materials. Specially, knowing the value and behaviour of displacement, strain, and stresses induced in any loaded members and structure is the crucial way to reduce the failure of material. For this reason, many researchers have been conducted numerous investigation which modifies, and improves an accuracy and computational time of the DIC. Nowadays, digital image correlation (DIC) is a technique which gained immense attention on the field of experimental mechanics due to its versatility measurement accuracy including strain measurement. DIC is a full field, non-contact, optical, strain and displacement measurement technique; it is simple, cost effective and high accuracy as compared with the other strain measurement technique.
Digital Image Correlation Technique in Selected Mechanical Tests
2017
The paper shows how modern contactless Digital Image Correlation (DIC) method can be implemented for examination of material behaviour under various types of loading. DIC method was used to evaluate material straining under monotonic tension conducted by the use of flat specimens having artificial defects in the form of U and V notches. This technique was also examined during capturing of strain distribution in dynamic tests on Split Hopkinson Pressure Bar. On the basis of DIC results the strain maps at various stages of material deformation were elaborated in order to indicate characteristic features of a material behaviour. It enabled an analysis of damage zone evolution up to specimen fracture.