Effect of Crack-Parallel Compression or Tension on Structural Scaling strength of Materials - as applied to Concrete, Composite and Metal

ABSTRACT

In all widely used fracture test specimens, the compressive or tensile stress parallel to the plane of growing crack is negligible, and thus its effect cannot be revealed. The classical fracture models, including the cohesive crack model, cannot capture any effect of such crack-parallel normal stress and strain, except parametrically, because they do not figure as the basic thermodynamic variables. To capture these effect, the fracture process zone must be modeled tensorially, by a strain-softening damage constitutive law. Here it is shown experimentally, and documented by crack band finite element simulations with microplane model M7, that the crack-parallel normal stresses have a major effect in quasibrittle materials such as concrete. They are shown to cause a major decrease or increase of the Mode I (opening) fracture energy Gf (or fracture toughness KIc. The experiments introduce a modification of the standard three-point bend test, whose idea is the use plastic pads with a near-perfect yield plateau to first generate compression, and a gap at end supports to close later and generate bending. The experiments show and the microplane model confirms that a small crack-parallel compression greatly increases Gf (even doubling it or more), but a higher compression reduces Gf greatly, which represents the case of compression splitting. The microplane model predictions fit the test results closely. Thus validated, it shows that crack-parallel tension reduces Gf, and further that a compressive or tensile stress normal to the specimen plate has a similar major effect on Gf. High crack-parallel stresses arise, e.g., in shear failure of reinforced concrete beams or slabs, and they are typical for prestressed concrete, as well as in hydraulic fracturing of shale.

BIOGRAPHY

Hoang received his Bachelor's degree in Civil Engineering from Bach Khoa University (old name Ho Chi Minh city University of Technology, Vietnam) in 2014. He then earned his Master's degree in Theoretical and Applied Mechanics (TAM) from Northwestern University (USA) in 2017. Since then, he has been jointly advised by Professor Horacio Espinosa and Prof. Zdeněk Bažant to pursue a Ph. D. degree also in TAM. Hoang is interested in multiphysics and multiscale phenomena in rock-like materials (primarily concrete), and how to apply his understanding of those phenomena into sustaining structural serviceability and improving other human-structure interaction activities. Aside from research, he enjoys reading natural and health science books and jamming mash-up from random songs.