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 formore 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.
AconsistentFEMbrittle 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
constraintKI =KII = 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 theleading 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.
