1. 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

2. Page  1 Contents 1.Introduction 2.Digital Image Correlation 3.Evaluation of Accuracy for DIC 4.Summary

3. Page  2 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

4. Page  3 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)

5. Page  4 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

6. Page  5 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

7. Page  6  Concept of Digital Image Correlation –Evaluation of displacement between two digital images Determine displacement vector for selected subset Digital Image Correlation After Before ?

8. Page  7  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 Digital Image Correlation

9. Page  8  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

10. Page  9  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

11. Page  10  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!

12. Page  11  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.)

13. Page  12  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

14. Page  13  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 resolution (mm/pixel) Volume changeDistortion 0.5%1.5%5.0%0.5%1.5%5.0% 150×1500.08520.14770.23350.09780.15310.1212 300×3000.02830.05210.11710.02580.05570.0941 600×6000.01470.04080.10670.01160.01010.0585 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 Evaluation for cohesive soil in plane strain test is needed With alternative methodology instead of DEM program

15. Page  14  Evaluation of DIC method in cohesive soil for PS test Evaluation of Accuracy for DIC Image resolution Size of pixel subset Deformation condition 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 size Uniqueness ↔ Spatial distribution Small deformation interval Accuracy & Precision for Analysis ↔ Analysis efficiency OK

16. Page  15 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 ↔ △ ε v ) Exclude upper & near shear band section - Consideration of soil disturbence Exclude upper & near shear band section - Consideration of soil disturbence

17. Page  16 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( △ ε LVDT ↔ △ ε DIA )

18. Page  17 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

19. Thank You!