PowerPoint Template
Download
Report
Transcript PowerPoint Template
Wednesday seminar
Assessment of Shear Band Characteristics in
Cohesive Soils using Digital Image Analysis
Technique for Plane Strain Tests
- Concept & Evaluation of accuracy for DIC-
Kwak, Tae young
Master course
Seoul National University
Contents
1. Introduction
2. Digital Image Correlation
3. Evaluation of Accuracy for DIC
4. Summary
Page 1
Introduction
Assessment of Shear Band
Expansion of shear band by loading is related to
failure mode of soil
Traditional methods measure overall soil behavior
- Difficult to assessment of shear band
Evaluation of internal soil behavior near shear
band - Importance of the digital image analysis
Page 2
Introduction
Object of This Research
– Internal deformation of specimen during compression shearing
• Internal behavior with stress-strain response
• Spatial distribution for deformation
– Assessing the shear band characteristics for cohesive soil
• Generation of shear band, corresponding to the stress state and strain level
• Characteristics of shear band for soil type (NC vs. OC)
Page 3
Introduction
Methodology
– Plane strain test for the observation of failure surface
• Internal behavior of the soil follows the surface behavior at plane strain
condition.
• Occurrence of shear band characteristics is clear at plane strain test.
– Digital image analysis for the internal deformation of specimen
• Quantitative measurement for internal deformation value of specimen
• Available for high accuracy and continuous evaluation during shearing
Page 4
Introduction
Flow Chart of the Research
– Specimen: EPK Kaolinite
– Test Procedure
Specimen preparation
Plane strain test
Page 5
Transducers in test
apparatus
digital image acquisition
Overall stress-strain
behavior
Internal deformation using
digital image analysis
Internal deformation and Shear band
characteristics with shearing level
Digital Image Correlation
Concept of Digital Image Correlation
– Evaluation of displacement between two digital images
• Determine displacement vector for selected subset
Before
After
?
Page 6
Digital Image Correlation
Digital Image Correlation Procedure
– Select the subset pixel from the first picture. And then consider
expected deformation of the selected subset pixel
– Perform cross correlation for each pixel by moving 1pixel, so correlation
between two subset pixels can be found out
– Determine the location of highest correlation peak
– Interpolate the surrounding of highest correlation peak
Page 7
Digital Image Correlation
Influence Factor for DIC Method
Image resolution
Size of pixel subset
Deformation condition
Determined by pixel of digital camera
Image pattern
Page 8
Higher pixel of digital camera,
higher image resolution
Digital Image Correlation
Influence Factor for DIC Method
Image resolution
Size of pixel subset
Determined by size of pixel subset
Deformation condition
Image pattern
Page 9
Larger size of pixel subset,
higher uniqueness
Difficulty in observing the behavior for
each location of the soil at too large
size of pixel subset
Digital Image Correlation
Influence Factor for DIC Method
Image resolution
Size of pixel subset
Rigid body movement / Deformation
Deformation condition
Image pattern
Page 10
Higher accuracy for rigid body
movement
Lower accuracy for high deformation
- Set-up DIC analysis interval shorter!
Digital Image Correlation
Influence Factor for DIC Method
Image resolution
Size of pixel subset
Image pattern classification
Deformation condition
Grid pattern (Alshibli et al.)
- Same pattern makes difficult to
classify
Image pattern
Color of soil particle (Jang et al.)
Arbitrary pattern by spray (Kim et al.)
Page 11
Evaluation of Accuracy for DIC
Evaluation of Accuracy for DIC Method (Jang et al, 2009)
– Comparison between criterion displacement and analyzed displacement
• Criterion displacement : Exact particle movement
› Make a particle movement by DEM program
• Analyzed displacement : Measured movement by DIC method
› Use initial digital image and deformed digial image by DEM program
N
[( u
Std _ E rr
u D E M ) ( v D IA v D E M ) ]
2
D IA
i
N
(u D E M ,v D E M ):D isp la ce m e n t b y D E M p ro g ra m
(u D IA ,v D IA ):D isp la ce m e n t b y D IC m e th o d
Page 12
2
Evaluation of Accuracy for DIC
Evaluation of Accuracy for DIC Method (Jang et al. 2009)
– Image resolution
• Higher image resolution, lower standard error !
– Deformation condition
• Larger deformation, higher standard error !
– SizeImage
of pixel subset Volume change
Distortion
resolution
•Limitation
Larger sizefor
of pixel
subset,
lower
standard error
! with DEM program
DIC
method
for granular
soils
0.5%
1.5%
5.0%
0.5%
1.5%
5.0%
(mm/pixel)
– Image
pattern DEM program is not available for cohesive soil
However,
150×150
0.1477
0.2335
• Distribution
of 0.0852
soil particles
with different
color0.0978
300×300
0.0283
0.0521
0.1171
0.0258
0.1531
0.1212
0.0557
0.0941
Evaluation for cohesive soil in plane strain test is needed
With
alternative
methodology
instead
of DEM
program
600×600
0.0147
0.0408
0.1067
0.0116
0.0101
0.0585
Page 13
Evaluation of Accuracy for DIC
Evaluation of DIC method in cohesive soil for PS test
Image OK
resolution
Size of pixel subset
Deformation
condition
Image
pattern
OK
Digital camera
: 12-million pixels (4288 x 2848 Pixels) digital camera Nikkon D90
Arbitrary pattern for cohesive soil by spray
(Kim et al.,2011)
Adequate pixel subset size
Uniqueness ↔ Spatial distribution
Small deformation interval
Page 14
Accuracy & Precision for Analysis ↔ Analysis efficiency
Evaluation of Accuracy for DIC
Evaluation of DIC method in cohesive soil for PS test
1) Evaluation by void-ratio
Almost rigid body movement
Comparison between void ratio change
and digital image analysis(△e ↔△εv)
Exclude upper & near shear band section
- Consideration of soil disturbence
Page 15
Evaluation of Accuracy for DIC
Evaluation of DIC method in cohesive soil for PS test
2) Evaluation by LVDT
Volume change during consolidation
Comparison between LVDT and digital
image analysis(△εLVDT ↔△εDIA)
Page 16
Summary
Assessment of shear band localization
– Importance of digital image analysis
– Investigate overall stress-strain behavior and observe deformation of specimen
during shearing
Introduction of digital image correlation
– Concept of the digital image correlation
– Influence factors for accuracy of the DIC method
Evalation of accuracy for DIC
– Literature review about Evaluation of accuracy in granular soil
– Suggestion about method for evaluation of accuracy in cohesive soil
Page 17
Thank You!