Stress intensity in viscoelastic contacts

Dynamic mechanical contacts with nanometer-to-micrometer dimensions are important in scanned probe microscopy, ultra-low load indentation, microelectromechanical systems, compact discs, etc. The response of these contacts is poorly understood if they involve adhesive viscoelastic materials, such as polymers and self-assembled monolayers. We have studied dynamic contacts to styrene–butadiene latex films. Plots of load vs. displacement show substantial adhesion hysteresis between the loading and unloading portions. The hysteresis is at least partially due to creep, as indicated by the continued increase in penetration after the start of unloading. Thermodynamic works of adhesion were estimated from fits to the loading–unloading data obtained at small loading and unloading rates. Theoretical models that include adhesion but neglect long-range creep effects could not fit the data at all loading rates. Creep tests were carried out under constant load. A model due to Hui, Baney, and Kramer (HBK), which predicts the response of an adhesive viscoelastic contact under increasing load, was used to extract a mode I stress-intensity functional from the data. When this functional is normalized by the square root of the displacement rate, it is shown empirically, to have a simple, nearly universal time-dependence. Variations of this universal form due to load, range of interaction potential, glass transition temperature, and probe shape are weak. This result supports the suggestion of HBK that the stress-intensity functional provides a useful way to characterize adhesive contacts to viscoelastic materials.