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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. Introduction Digital Image Correlation - PowerPoint PPT Presentation
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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
Wednesday seminar
Page 2
Contents
1. Introduction
2. Digital Image Correlation
3. Evaluation of Accuracy for DIC
4. Summary
Page 3
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 4
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 5
Introduction
Methodology– Plane strain test for the observation of failure surface
• Internal behavior of the soil follows the surface behavior at plane strain condi-tion.
• 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 6
Introduction
Flow Chart of the Research– Specimen: EPK Kaolinite– Test Procedure Specimen preparation
Internal deformation and Shear band characteristics with shearing level
Transducers in test apparatus
digital image acquisition
Plane strain test
Internal deformation using digital image analysis
Overall stress-strain behavior
Page 7
Concept of Digital Image Correlation– Evaluation of displacement between two digital images
• Determine displacement vector for selected subset
Digital Image Correlation
AfterBefore
?
Page 8
Digital Image Correlation Procedure– Select the subset pixel from the first picture. And then consider ex-
pected 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
Digital Image Correlation
Page 9
Influence Factor for DIC Method
Digital Image Correlation
Image resolution
Size of pixel subset
Deformation condition
Image pattern
Determined by pixel of digital camera
Higher pixel of digital camera, higher image resolution
Page 10
Influence Factor for DIC Method
Digital Image Correlation
Image resolution
Size of pixel subset
Deformation condition
Image pattern
Determined by size of pixel subset
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
Page 11
Influence Factor for DIC Method
Digital Image Correlation
Image resolution
Size of pixel subset
Deformation condition
Image pattern
Rigid body movement / Deformation
Higher accuracy for rigid body movement
Lower accuracy for high deformation - Set-up DIC analysis interval shorter!
Page 12
Influence Factor for DIC Method
Digital Image Correlation
Image resolution
Size of pixel subset
Deformation condition
Image pattern
Image pattern classification
Grid pattern (Alshibli et al.) - Same pattern makes difficult to classify
Color of soil particle (Jang et al.) Arbitrary pattern by spray (Kim et al.)
Page 13
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
Evaluation of Accuracy for DIC
DEM DEM
DIA DIA
(u ,v ):Displacement by DEM program(u ,v ):Displacement by DIC method
2 2[( ) ( ) ]_
N
DIA DEM DIA DEMi
u u v vStd Err
N
Page 14
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 !– Size of pixel subset
• Larger size of pixel subset, lower standard error !– Image pattern
• Distribution of soil particles with different color
Evaluation of Accuracy for DIC
Image resolu-tion
(mm/pixel)
Volume change Distortion
0.5% 1.5% 5.0% 0.5% 1.5% 5.0%
150×150 0.0852 0.1477 0.2335 0.0978 0.1531 0.1212
300×300 0.0283 0.0521 0.1171 0.0258 0.0557 0.0941
600×600 0.0147 0.0408 0.1067 0.0116 0.0101 0.0585
Limitation for DIC method for granular soils with DEM program
However, DEM program is not available for cohesive soil
Evaluation for cohesive soil in plane strain test is needed With alternative methodology instead of DEM program
Page 15
Evaluation of DIC method in cohesive soil for PS test
Evaluation of Accuracy for DIC
Image resolution Size of pixel subset Deformation condi-tion Image pattern
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 sizeUniqueness ↔ Spatial distribution
Small deformation intervalAccuracy & Precision for Analysis ↔ Analysis efficiency
OK OK
Page 16
Evaluation of Accuracy for DIC
Evaluation of DIC method in cohesive soil for PS test1) 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 17
Evaluation of Accuracy for DIC
Evaluation of DIC method in cohesive soil for PS test2) Evaluation by LVDT
Volume change during consolidation
Comparison between LVDT and digital image analysis(△εLVDT ↔△εDIA)
Page 18
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
Thank You!