Full Program »

Nanoindentation investigation of fib-milled microstructures to assess nanoscale strength and fracture toughness of cement

Fracture and strength measurements made at the typical scales of concrete laboratory testing are often explained and modeled by introducing the idea of microcracks, small material flaws within the cement paste matrix. Current experiments, however, are unable to reveal how fractures are generated within the microstructure. The small scale of these cracks and the volumes around them pose many challenges for measurements and modeling. As a result, modeling the initiation, propagation, interdependence, and coalescence of microcracks for macroscopic material failure is a formidable challenge because basic input data is missing. New, small scale experimental techniques are required to quantitatively assess the complex material behaviors that lead to microcracking, and eventually failure, in cement paste.
Focused Ion Beam (FIB) milled sample structures, including micropillars and wedge splitting geometries, are designed to be tested using nanoindentation to apply loads and measure displacements. The micropillar is compressed by the nanoindenter tip in an unconfined compression test, giving plastic strength properties of the tested material. The wedge-splitting test is a novel method for inducing controlled fracture by driving a wedge through a notch to introduce well-controlled tension across a crack. A sharp nanoindenter tip serves as an effective wedge. After application to cement pastes with various water/cement ratios, the experimental results are organized into statistical models of failure properties within the individual material phases in cement paste. Improved fundamental understanding of failure in concrete materials will lead to rational optimization of material design at the nanoscale and more efficient use of natural resources and more sustainable and resilient structures.

Author(s):

Rahnuma Shahrin    
North Carolina State University
United States

Christopher Bobko    
North Carolina State University
United States