Kwak, Tae young Master course Seoul National University

1. 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

2. Contents • Introduction • Digital Image Correlation • Evaluation of Accuracy for DIC • Summary

3. Introduction • Assessment of Shear Band Traditional methods measure overall soil behavior - Difficult to assessment of shear band Expansion of shear band by loading is related to failure mode of soil Evaluation of internal soil behavior near shear band - Importance of the digital image analysis

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)

5. Introduction • Methodology • Plane strain testfor 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. Introduction • Flow Chart of the Research • Specimen: EPK Kaolinite • Test Procedure Specimen preparation Plane strain test Transducers in test apparatus digital image acquisition Internal deformation using digital image analysis Overall stress-strain behavior Internal deformation and Shear band characteristics with shearing level

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

8. 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

9. Digital Image Correlation • Influence Factor for DIC Method Image resolution Size of pixel subset Deformation condition • Determined by pixel of digital camera • Higher pixel of digital camera, • higher image resolution Image pattern

10. Digital Image Correlation • Influence Factor for DIC Method Image resolution Size of pixel subset • 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 Deformation condition Image pattern

11. Digital Image Correlation • Influence Factor for DIC Method Image resolution Size of pixel subset • Rigid body movement / Deformation • Higher accuracy for rigid body • movement • Lower accuracy for high deformation • - Set-up DIC analysis interval shorter! Deformation condition Image pattern

12. Digital Image Correlation • Influence Factor for DIC Method Image resolution Size of pixel subset • 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.) Deformation condition Image pattern

13. 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

14. 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 ! • Size of pixel subset • Larger size of pixel subset, lower standard error ! • Image pattern • Distribution of soil particles with different color • 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

15. Evaluation of Accuracy for DIC • Evaluation of DIC method in cohesive soil for PS test OK OK 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

16. 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

17. 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)

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

19. Thank You!