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Crack tip deformation measurement using single-element extended ...
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利用单单元扩展数字图像相关方法测试裂纹尖端变形场,陈金龙,张晓川,由于裂纹尖端位移场会对疲劳断裂实验中裂纹扩展路径产生较大影响,因此,发展一种测试裂纹尖端位移场的实验方法就显得尤为重要。
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中国科技论文在线
Crack tip deformation measurement using single-element
extended digital image correlation
#
CHEN Jinlong
1,2
, ZHANG Xiaochuan
1
, ZHAN Nan
1*
5
(1. Department of Mechanics, Tianjin University, Tianjin 300072;
2. Tianjin Key Laboratory of Modern Engineering Mechanics, Tianjin 300072)
Foundations: Specialized Research Fund for the Doctoral Program of Higher Education (No. 20090032110006);
National Natural Science Foundation of China (No. 10772133 and 11072172)
Brief author introduction:Chen Jinlong(1966-), male, professor, Main research: Experimental solid mechanics,
Biomechanics, and non-destructive detection technique. E-mail: jlchen66@tju.edu.cn
Abstract: Since the crack tip displacement field influences the crack paths under both fatigue and
fracture conditions strongly, it is very important to develop a test method with high accuracy to
measure this deformation parameter. Single-element extended digital image correlation (X-DIC) using 10
the enrichment function to describe the crack tip singularity, which could measure the discontinuous
deformation directly. Simulated images are used to verify the performance of single-element X-DIC,
the measured crack tip displacement field is in accordance with the given value. Displacement fields of
the compact tension specimen under pure mode I loading condition was measured. The influence to the
measurement accuracy caused by the crack width,subset size and subset location was discussed. All 15
results in this paper show that single-element X-DIC could be used to measure the crack tip
displacement field with high accuracy.
Key words: Experimental solid mechanics; crack tip deformation; extended digital image correlation
(X-DIC); subset
20
0 Introduction
The path that a crack follows is strongly influenced by the elastic and plastic displacement
field at the crack tip
[1]
. Although electrometric method is used widely for engineering application,
strain gauge must be adhered to the specimen, it is a contact and non-full-field test method.
Photoelastic stress analysis, Moire interferometry and other traditional optical measurement 25
methods could successfully characterize the elastic stress fields around cracks. However, these
methods require the strict test environment and the operation of fringe is too trivial.
Digital image correlation (DIC) method possesses many advantages such as its simple
experimental steps, high accuracy and high computational efficiency, so it has been applied to
measure the deformation of the materials on the multi-scales extensively
[2–5]
. In recent years, 30
many works to improve the measurement accuracy and extend the application scope of DIC have
been done
[6-8]
. However, standard DIC has two drawbacks, which are the difficulty to measure the
displacement field near the discontinuous region and the measured results need smoothing
[9]
.
Although change of the correlation steps could make standard DIC suitable to measure the
discontinuous deformation
[10, 11]
, it usually results in a higher workload and some artificial errors. 35
Extended digital image correlation (X-DIC) takes the displacement mode of extended finite
element method (X-FEM) as its shape function, which extends its application scope to measure the
discontinuous deformation field directly and without the smoothing process
[12, 13]
. Chen and Zhang
have investigated the error map of X-DIC, and developed single-element X-DIC with higher
accuracy
[14, 15]
. When X-DIC is used to measure the discontinuous deformation, initial crack is 40
usually considered with no width. However, the gap of crack face in the image usually possesses
several pixels. In this paper, the principle of single-element X-DIC is introduced firstly. Then,
simulation verification shows this technique is suitable to measure the crack tip deformation field.
At last, displacement field of the specimen under pure mode I loading condition is measured and
the influence to the measurement accuracy caused by the crack width and subset size is discussed. 45
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