Applied Mechanics and Materials Vols. 405-408 (2013) pp 2515-2519 Online available since 2013/Sep/03 at www.scientific.net © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMM.405-408.2515
Digital Image Correlation Test System with Automatic Transmission for Quasi-brittle Material Hongliang YUE1, a, Chunhui ZHANG1, b, Yan Wang2, c 1
School of Civil Engineering, Hebei University of Science and Technology, Shijiazhuang Hebei 050018, China
3
Department of Building Engineering, Shijiazhuang Vocational Technology institute, Shijiazhuang Hebei 050081, China a
b
c
[email protected],
[email protected],
[email protected]
Keywords: Digital image correlation, Automatic transmission, Fast Fourier transform
Abstract. To realize the automatic transmission for present digital image capture test system, a new system which consists of laptop computer, camera, fixing device and mechanical testing machine was presented and designed. The Monitor software, which was developed using MATLAB to realize automatic transmission function for the system, includes slow and high speed. The general stage and key stage for quasi-brittle materials can be effectively captured by the software. With MATLAB, the digital image correlation software (DIC) was developed. In DIC the first image is used as the base image. The images are sorted by photographing time (or file name), and the listing file is generated. The grid is drawn on the base image to divide the area into ‘facets’, the size of which can be adjusted between 1pixel and 100 pixels. Based on fast Fourier transform and convolution algorithm, the image comparison is realized and the deformation of the material is calculated. The designed system is applied to the compression test of a concrete specimen, and the results show that the system realizes automatic transmission image capture, and calculates accurate displacement. The displacement distribution is uniform in linear elastic stage, and varies in yielding stage. This is mostly due to localized failure and displacement discontinuity. Introduction Rock and concrete are as heterogeneous materials as other quasi-brittle materials [1-5], mechanical properties and fracture mechanism of which are particularly complex. Mechanical experiment [5-7] is an important method to study mechanical behavior of the quasi-brittle materials. Previous experimental [1,5-7] study shows that specimen deformation has dynamic (quick evolution in time, especially before destroy) and heterogeneous (the distribution of deformation and destroy are not uniform in space) characteristics. The former means that deformation and failure of materials is a dynamic process, deformation localization and catastrophic destroy is a rapid evolution process (generally within seconds), even though under quasi-static loading, therefore, quick observation is required. The latter means that the specimen appears microcrack localization before damage, accordingly, localized strain emerges on the surface of sample. The mechanical properties of materials in the localization band has great difference with out-band, similarly, stress and deformation in-band also are greatly different with out-band. Hence, a method for dynamically observing material deformation field in quasi-brittle material laboratory is urgently needed. At present, whether traditional or new measurement, for example, Scanning Electron Microscope [8], CT [9], infrared photography [10], is unable to dynamically observe material deformation field. Digital image technology developed in recent years is promising technology for solving this problem. High-speed camera is applied to capture image in experimental research [11-13], the method is available for dynamical capturing image in deformation or failure process of quasi-brittle materials. But the duration of continuous shooting is limited (a few seconds) for ordinary high-speed camera, so it is impossible to track the overall loading process. It is significant that the image capture system is developed into a system with automatic transmission function, with which all images in general stage (before yielding stage) and key stage (localized and catastrophic failure stage) can be captured.
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In this paper, the digital image correlation test system with automatic transmission function is presented and designed, the Monitor software with automatic transmission function, developed with MATLAB to capture images, includes slow and high-speed stage. Digital image correlation test system with automatic transmission The developed system consists of notebook computer, camera (1920000 pixels), the camera fixing device, mechanical testing machine, as shown in Fig 1. Automatic transmission is the key to the system. The Monitor software can be used to control the image capturing speed that fall into two stages: slow and high-speed stage. The former is mainly used for capturing image of material deformation in elastic stage, the rate is generally 1/(1~5s); The latter is capturing image of material deformation in localized and catastrophic failure stage, the rate is normally (1~8)/s. Interface of Monitor is as shown in Fig.2.
Fig.1 Digital image correlation test system with automatic transmission
Fig.2 Interface of Monitor O
test facet Ir(U)
Image i t=ti
X search facet
base image t=t1 mesh for search
L
Ir(U) It(U)
Y
Fig.3 Method of digital image correlation
Fig.4 Grid generation of the area
The location of each point in images obtained by digital image capture system, is represented by pixels U=U(u,v), gray level of each point in base image is represented by I=Ir(U), gray level of images captured after deformation is represented by I=It(U). In order to improve the analysis efficiency, base image is divided into a number of unique correlation areas, or ‘facets’, which typically contain a square subset of pixels. Position of facets vary during deformation, namely, facets move to a new location. Some areas are searched for the positions of facets after deformation; the new positions are determined by comparing gray level of facets, overall process as shown in Fig.3. The correlation between two facets could be described as
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∑∑ [( f − f )(g − g )] {∑∑ [( f − f ) ]∑∑ [(g − g ) ]} 2
2
(1)
0.5
Where f is facets in base image, g is gray level distribution in other images, is arithmetic mean operator, Fast Fourier transform (FFT) and convolution algorithm are used for correlative analysis. Examples
Y-Displacement /pixel
Y-Displacement /pixel
The system is applied to analyze concrete specimen under uniaxial compression. The concrete specimen is a cube, the size 150mm×150mm×150mm, In order to achieve better tracking effect, the surface of which is smeared with special material. Camera control parameters are as follows: the first stage takes 80s, the rate is 1/s; the second stage takes 84s, the rate is 4/s; loading equipment is electrohydraulic servo tester made in Changchun; the displacement-controlled mode is employed; loading speed is 0.1mm/s. The test is shown in Fig.1. The image taken just at loading (the first image) is defined as the base image, on which the grid is drawn to divide the area into ‘facets’, the size of the facet is 20pixel×20pixel, while a coordinate system is established in the image. The grid and the coordinate system are as shown in Fig 4.
x
y
y
Fig.6 Y displacement at the time of 50s (pixel)
Y-Displacement /pixel
Y-Displacement /pixel
Fig.5 Y displacement at the time of 10s (pixel)
x
y
x
Fig.7 Y displacement at the time of 80s (pixel)
y
x
Fig.8 Y displacement at the time of 100s (pixel)
In addition, the size of a pixel in images is calibrated, about 1pixel=0.454mm. The Monitor software captures the images of concrete specimen under loading, then, displacement is calculated by DIC software. There is Y displacement of concrete specimen at each time as shown in Fig.5 ~Fig.9. From Fig.5, it can be seen that Y displacement of specimen at the time of 10s is 2.3pixel×0.454mm/pixel=1.04mm, while displacement of electrohydraulic servo tester achieves 1.0mm, they inosculate with each other.
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Y-Displacement /pixel
local break
From Fig.5 to Fig.7, specimen deforms in the elastic stage, deformation on the surface of specimen is homogeneous. In Fig.7, local Y displacement changes greatly, it may be that local elements of specimen damage, owing to shear slip or others, and there is abnormal deformation in elements of specimen, thereby, local displacement of specimen is abnormal. There is the specimen as shown in Fig.10.
y
x
Fig.9 Y displacement at the time of 164s (pixel)
Fig.10 Localized failure at the time of 100s
350 300 250 200 150 100 50 0
Fig.11 Failure at the time of 164s
0
50
100
150
200
Fig.12 Curve of time VS force for sample
There is Y displacement of specimen at the time of 100s(Fig.8), while the specimen is yielding, and more local elements damage (Fig.10), thus, there is ‘fold’ about Y displacement in the area. There is Y displacement of specimen at the time of 164s (Fig.9), while the specimen has been completely destroyed, Y displacement of most points in the area is heterogeneous, and ‘fold’ group appears. The broken specimen is shown in Fig.11. There is the relation curve of time and force for sample (Fig.12). The curve is contrasted with specimen deformation in each stage from Fig.5 to Fig.9. Obviously, the analysis results above are reasonable. It can be found that Y displacement of the specimen distributes uniformly in linear elastic stage, the displacement distribution tends to heterogeneous when the sample destroy. This is mainly due to localized failure and corresponding ‘discontinuity’. Conclusions In this paper, the system is designed, which consists of notebook computer, camera, the camera fixing device, mechanical testing machine. Using MATLAB, the Monitor software is developed to realize the automatic transmission function. The digital image correlation software (DIC) is developed, in
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which the first image is used as the base image. The images are sorted by photographing time (or file name). Based on fast Fourier transform (FFT) and convolution algorithm, the displacement of sample is obtained by image correlation method. The system is applied to study on concrete specimen under compression, the results show that: (1) The images of quasi-brittle materials in general stage (before yielding stage) or key stage (localized and catastrophic failure stage) can be effectively captured by the system developed. (2) The system not only realizes automatic transmission image capture, but also accurately calculated the displacement of the specimen. (3) The displacement of the specimen distributes uniformly in linear elastic stage, the displacement distribution tends to non-uniform when the sample destroys. It is mainly due to localized failure and corresponding ‘discontinuity’. Acknowledgements This work was financially supported by the Chinese Natural Science Foundation (51274079, 51274110), the Hebei Natural Science Foundation (E2013208148), Guidance Project of Hebei Provincial Education Department (Z2012093) and Scientific Research Fund of Hebei University of Science and Technology (XL201235). References [1] L. Jing: Int J Rock Mech Min Sci Vol.40 (2003), p. 283 [2] Tang C.A, Tham LG, Lee PKK, Yang TH, Li LC: Int J Rock Mech Min Sci Vol.39 (2002), p. 477 [3] ZHANG Chunhui: Journal of China Coal Society Vol.34 (2009), p. 1460(In Chinese) [4] ZHANG Chunhui, YU Yongjiang, ZHAO Quansheng: Rock and Soil Mechanics Vol.30 (2009), p. 2837(In Chinese) [5] ZHANG Chunhui, ZHAO Quansheng, YU Yongjiang: Rock and Soil Mechanics Vol.32 (2011), p. 564(In Chinese) [6] Lei Dong, Wang Kefeng, Tang Xiaobo, Shao Guojian: Journal of Underground Space and Engineering Vol.7 (2011), p. 1158(In Chinese) [7] Xu X.H., Ma S.P., Xia M.F., et al: Theoretical and Applied Fracture Mechanics Vol.42 (2004), p. 131 [8] GE Xiu-run, REN Jian-xi, PU Yi-bin, et al: Macro and Meso Testing Study on Damage Mechanics of Geotechnical Media (Science Press, China 2004). [9] XIE Heping: Damage Mechanics of Rock and Concrete (University of Mining and Technology Press, China 1990). [10] ZHAO Yang-sheng, MENG Qiaorong, KANG Tianhe, ZHANG Ning, XI Baoping: Chinese Journal of Rock Mechanics and Engineering Vol.27 2008), p. 28(In Chinese) [11] Wu L. X., Liu S. J., Wu Y. H., et al: Int. J. Rock Mech. & Min. Sci. Vol.39(2002), p. 825 [12] D. J. W H I T E, W. A. TAKE, M. D. BOLTON: Geotechnique Vol.53(2003), p. 619 [13] MA Shaopeng: Chinese Journal of Rock Mechanics and Engineering Vol.23 (2004), p. 1410(In Chinese)
Progress in Industrial and Civil Engineering II 10.4028/www.scientific.net/AMM.405-408
Digital Image Correlation Test System with Automatic Transmission for Quasi-Brittle Material 10.4028/www.scientific.net/AMM.405-408.2515 DOI References [1] L. Jing: Int J Rock Mech Min Sci Vol. 40 (2003), p.283. http://dx.doi.org/10.1016/S1365-1609(03)00013-3 [2] Tang C. A, Tham LG, Lee PKK, Yang TH, Li LC: Int J Rock Mech Min Sci Vol. 39 (2002), p.477. http://dx.doi.org/10.1016/S1365-1609(02)00023-0 [7] Xu X.H., Ma S.P., Xia M.F., et al: Theoretical and Applied Fracture Mechanics Vol. 42 (2004), p.131. http://dx.doi.org/10.1016/j.tafmec.2004.08.002 [12] D. J. W H I T E, W. A. TAKE, M. D. BOLTON: Geotechnique Vol. 53(2003), p.619. http://dx.doi.org/10.1680/geot.2003.53.7.619
