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Helical crack-front instability in mixed-mode fracture.
1Department of Physics and Center for Interdisciplinary Research on Complex Systems, Northeastern University, Boston, Massachusetts 02115, USA.
Nature
|March 6, 2010
Summary
Mixed-mode I + III loading causes unstable crack segmentation, forming stepped fracture surfaces. Simulations reveal helical crack front deformations leading to finger-shaped daughter cracks and predictable surface roughness scales.
Area of Science:
- Materials Science
- Solid Mechanics
- Fracture Mechanics
Background:
- Planar crack propagation under pure tension (mode I) is typically stable.
- Superposing shear stress (mode III) leads to universally unstable crack growth.
- Mixed-mode (I + III) loading causes crack segmentation, creating stepped fracture surfaces with lance-shaped markings.
Purpose of the Study:
- Investigate the origin of mixed-mode I + III fracture instability.
- Develop a theory to predict the surface roughness scale of segmented cracks.
- Understand the 3D crack front evolution under mixed-mode loading.
Main Methods:
- Large-scale simulations of mixed-mode I + III brittle fracture.
- Utilized a continuum phase-field method for complete 3D crack-front evolution.
- Developed a new propagation law for curved cracks in three dimensions.
Main Results:
- Planar crack propagation is linearly unstable against helical deformations.
- Helical deformations evolve into segmented, finger-shaped daughter cracks.
- Facet coarsening observed, analogous to nonequilibrium growth phenomena.
- Derived a theoretical estimate for the unstable wavelength based on stress balance and cohesive forces.
Conclusions:
- Mixed-mode I + III loading induces instability through helical crack front deformations.
- The study provides a theoretical framework for predicting fracture surface roughness.
- Simulation results align with theoretical predictions and experimental data for facet rotation angles.
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