New perspective of fracture mechanics inspired by gap test with crack-parallel compression

Hoang Nguyen, Madura Pathirage, Masoud Rezaei, Mohsen Issa, Gianluca Cusatis, Zdenek P. Bazant*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

6 Scopus citations

Abstract

The line crack models, including linear elastic fracture mechanics (LEFM), cohesive crack model (CCM), and extended finite element method (XFEM), rest on the century-old hypothesis of constancy of materials' fracture energy. However, the type of fracture test presented here, named the gap test, reveals that, in concrete and probably all quasibrittle materials, including coarse-grained ceramics, rocks, stiff foams, fiber composites, wood, and sea ice, the effective mode I fracture energy depends strongly on the crack-parallel normal stress, in-plane or out-of-plane. This stress can double the fracture energy or reduce it to zero. Why hasn't this been detected earlier? Because the crack-parallel stress in all standard fracture specimens is negligible, and is, anyway, unaccountable by line crack models. To simulate this phenomenon by finite elements (FE), the fracture process zone must have a finite width, and must be characterized by a realistic tensorial softening damage model whose vectorial constitutive law captures oriented mesoscale frictional slip, microcrack opening, and splitting with microbuckling. This is best accomplished by the FE crack band model which, when coupled with microplane model M7, fits the test results satisfactorily. The lattice discrete particle model also works. However, the scalar stress-displacement softening law of CCM and tensorial models with a single-parameter damage law are inadequate. The experiment is proposed as a standard. It represents a simple modification of the three-point-bend test in which both the bending and crack-parallel compression are statically determinate. Finally, a perspective of various farreaching consequences and limitations of CCM, LEFM, and XFEM is discussed.

Original languageEnglish (US)
Pages (from-to)14015-14020
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Volume117
Issue number25
DOIs
StatePublished - Jun 23 2020

Keywords

  • Cohesive crack model
  • Finite element crack band model
  • Fracture energy
  • Quasibrittle materials
  • Softening damage

ASJC Scopus subject areas

  • General

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