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Experimental Micromechanics Group at North Carolina State University

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North Carolina State University
Nanomechanical Testing and Multiscale Poromechanics
Christopher Bobko

December 2015

Problems

Materials commonly used in civil infrastructure, including cement, concrete and asphalt, as well as shales and mudstones frequently encountered in petroleum exploration and extraction, are multiscale heterogeneous porous composites. Multiple length scales, complicated chemical compositions, and varying microstructures in these materials present formidable challenges to researchers who seek to understand and model fundamental material behaviors. The complexity also hinders efforts by engineers to rationally optimize material properties and processes. Our research aims to provide insights into fundamental material behaviors by finding common links within classes of materials, and by understanding the mechanical microstructures that result in the wide variation of material properties. Experimental results and multiscale models are linked together to provide new insights at both small and large scales.

Approach

Our research probes the nanostructure and small-scale mechanical behavior of complex heterogenous materials. The primary research tools include nanoindentation testers and a nanoscratch tester, which provide continuous records of the load and displacement of a diamond tip with known geometry as it penetrates into a test material. For heterogeneous materials, we employ a grid nanoindentation technique (top figure) with hundreds of indents that probe composite behavior at smaller scales (bottom figure) using carefully chosen test parameters. This provides sufficient data to apply statistical deconvolution analysis that identifies properties and volume fractions of individual phases. Our group currently works to expand the application of these techniques to new materials and more difficult material behaviors, including viscoelastic and strength properties.

Findings

Recent research has focused on the small scale behavior of cements and concretes with enhanced sustainability, including the use of high volume fractions of fly ash, blast furnace slag, porous lightweight aggregates, and natural fibers. To help assess the long-term performance of these materials, their performance after fire damage was assessed. We found that hydration products of blended cements had the same nanoindentation responses as those from ordinary Portland cement, both before and after damage, but relative volume fractions differed. We have also found that the presence of porous aggregates changes the microstructure of cement hydration products. We hypothesize that these changes are related to internal curing, where secondary water stored in the porous aggregates is released some time later in the hydration process. 

Impact

The production, transportation, and use of concrete accounts for between 5-9% of total CO2 emissions worldwide. Material modifications to reduce these emissions can only be rationally studied and designed with improved fundamental understanding of the origin of material behaviors. Similarly, improving the efficiency and safety of petroleum exploration and production in shales depends on fundamental understanding of mechanical behavior of the material.

Selected Publications

Zanjani Zadeh, V. and Bobko, C. (2014) "Nano-mechanical properties of internally cured kenaf fiber
reinforced concrete using nanoindentation." Cement and Concrete Composites 52: 9-17.
Zanjani Zadeh, V. and Bobko, C. (2014) "Nanomechanical characteristics of lightweight aggregate concrete
containing supplementary cementitious materials exposed to elevated temperature." Construction and
Building Materials 51: 198-206.
Zanjani Zadeh, V. and Bobko, C. (2013) "Nanoscale mechanical properties of concrete containing blast
furnace slag and fly ash before and after thermal damage." Cement and Concrete Composites 37: 215-221.
Bobko, C., Gathier, B., Ortega, J.A., Borges, L., Ulm, F.-J, and Abousleiman, Y. (2011) "The nanogranular
origin of friction and cohesion in shale - a strength homogenization approach to interpretation of
nanoindentation results." International Journal for Numerical and Analytical Methods in Geomechanics 35
(17): 1854-1876.


Core Competencies

  • Grid nanoindentation experiments on heterogeneous porous materials
  • Assessing nanomechanical morphologies and mechanical properties
  • Multiscale poromechanics modeling for inverse problems, upscaling prediction, and optimization

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Top: grid nanoindentation on cement paste Middle: indentation schematic Bottom: AFM image of C-S-H nanoparticles in cement paste (Nonat, 2004)

Current Research Team Members:
 Yuriy Veytskin (Ph.D. candidate)
 Rahnuma Shahrin (Ph.D. candidate)
 Ye Zhang (M.S. candidate)

Recent PhD Graduates:
 Vahid Zanjani Zadeh (2013), now at Bechtel

Current Collaborations:
 Cassie Hintz and Y. Richard Kim, NSCU,
Nanoindentation investigation of
viscoelasticity and cohesion of asphalt
binder and mastic
 Korukonda Murty, NCSU, Assessing
zirconium hydride nanostructures
precipitating in zirconium alloys