The Impact of Microstructure on Fracture in the Context of Micro-Scale Testing and Fragmentation of Brittle Composites

Visiting Scholar
Mija H. Hubler, Ph.D.
Assistant Professor
University of Colorado, Boulder

Tues., March 27, 11:00 am
Milner Exec. Boardroom (0560 MEK)

Abstract: Microstructure plays an important role in the fracture behavior of brittle and quasi-brittle materials. This talk will discuss two recent research topics which highlight the role of microstructure in fracture behavior:

Firstly, how microscale scratch tests can be used to assess fracture toughness of materials if the analysis considers the impact of the material microstructure. By analytically formulating the effect of the inclusions on the fracture toughness of a crack within a particle composite reduces the discrepancy of fracture toughness derived from different tests, and matches the trend of fracture toughness values as a function of particle volume fraction. Experiments are conducted with resin containing glass inclusions tested using two different experimental approaches: macroscale three-point single edge notch and micro-scratch. By exploring statistically representative sets of random inclusions using linear elastic fracture mechanics expressions, trends of the particle arrangement effect on the fracture toughness are observed. These trends can be quantified using spatial order metrics and reproduced using finite element modeling.

Secondly, how during dynamic fragmentation of concrete the microstructure governs the formation of nanoparticles. Fragments were generated by rapidly loading concrete test specimens under 1D axial compression at a constant strain rate and sampling the generated airborne dust during the impact event. The results show that the aerosol fragment size distributions generated by rapid fragmentation of concrete follow a bimodal distribution tied to microstructural features of the concrete mix. This experimental behavior is well described by introducing additional length scales in Gilvarry’s derivation of the Rosin-Rammler-Sperlin- Bennet (RRSB) particle distribution. On this basis, it is proposed that no new fragmentation theory is needed to describe aerosol particles if concrete fragmentation is regarded as a composite material explicitly considering microstructural features which create stress concentration on the microscale. By identifying a direct link between microstructural features and aerosol distributions, it will be possible to design concrete microstructures to limit aerosol exposure during dynamic failures such as structural collapse.

Bio: Mija H. Hubler began her study of structural engineering in 2002 at the University of Illinois at Urbana-Champaign. After graduating with a bachelor of science from the civil engineering department in 2006, she spent three years pursuing a master’s degree from Cornell University. She completed her doctoral work in 2013 at Northwestern University. She is currently an assistant professor at CU Boulder. She is a member of ACI committee 209, the International Association for Life-Cycle Civil Engineering, and the RILEM technical committee on material modeling and structural analysis. Her research interests include: structural engineering, construction materials, mechanics, numerical modeling, and experimental analysis.