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!