The strength of an adhesive contact in the presence of interfacial defects

Adhesive contacts which possess a dominant stress concentration, such as at the contact edge in
spherical junctions or at the detachment front in a peeling film, are well studied. More complex
adhesive junction geometries, such as mushroom-shaped fibrils in bioinspired micropatterned
dry adhesives, have exhibited a complex dependence of adhesive strength on the presence of
interfacial defects within the contact. This has led to the emergence of statistical variation
of the local behavior among micropatterned sub-contacts. In order to examine the interplay
between geometry and interfacial defect character in control of the adhesive strength, the model
system of a stiff cylindrical probe on an elastic layer is examined. Both experiments (glass on
PDMS) and cohesive zone finite element simulations are performed, with analytical asymptotic
limits also considered. The thickness of the elastic layer is varied to alter the interfacial stress
distribution, with thinner layers having a reduced edge stress concentration at the expense
of increased stress at the contact center. The size and position of manufactured interfacial
defects is varied. It is observed that for the thickest substrates the edge stress concentration
is dominant, with detachment propagating from this region regardless of the presence of an
interfacial defect within the contact. Only very large center defects, with radius greater than
half of that of the contact influence the adhesive strength. This transition is in agreement
with analytical asymptotic limits. As the substrate is made thinner and the stress distribution
changes, a strong decay in adhesive strength with increasing center defect radius emerges. For
the thinnest substrate the flaw-insensitive upper bound is approached, suggesting that this
decay is dominated by a reduction in the contact area. For penny-shaped defects at increasing
radial positions, the adhesive strength for the thinnest substrates becomes non-monotonic. This
confirms an intricate interplay between the geometry-controlled interfacial stress distribution
and the size and position of interfacial defects in adhesive contacts, which will lead to statistical
variation in strength when defects form due to surface roughness, fabrication imperfections, or
contaminant particles.