Multi-crack analysis of hydraulically pumped cone fracture in brittle solids under cyclic spherical contact |
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Authors: | Herzl Chai |
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Affiliation: | (1) Department of Solid Mechanics, Materials and Systems, Faculty of Engineering, Tel-Aviv University, Tel-Aviv, Israel |
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Abstract: | The evolution of surface damage in bilayers due to cyclic spherical indentation in the presence of incompressible lubricant
is studied using an all-transparent glass/polycarbonate system as a model for more practical applications such as dental crowns
and rolling contact fatigue. In situ observations and post-mortem material sectioning reveal that inner cone cracks evolve
sequentially from the contact edge inward by slow growth in a process controlled by stress shielding from preceding cracks.
The embryonic cracks are then accelerated by the action of fluid pressure into the flexural tensile stress at the lower part
of the coating, where crossover fracture leading to delamination between the coating and substrate may ensue.
A consistent FEM brittle fracture analysis incorporating multiple cracks, rate-dependent toughness and liquid pressure is
used to follow the damage evolution in the coating. Crack trajectories are determined incrementally under the dual constraint
K
I = K
II = 0, which maximize the tension at the crack tip upon the application of fluid pressure. The latter, evaluated at each increment
with the aid of a fluid entrapment model, helps drive the leading crack past the compression zone beneath the contact via
a hydraulic pump like action. In the early stages of fracture, the liquid pressure is reasonably well approximated by the
Hertzian radial surface stress at the crack mouth. Fluid trapped in secondary cracks accentuate the compression beneath the
contact. This helps squeeze more liquid into the tip of the leading crack in a zipping like action, which further enhance
the crack driving force in the far field. The analytic predictions generally collaborate well with the tests. |
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Keywords: | Cyclic indentation Lubricant Cone cracks Multiple cracks Hydraulic pumping |
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