Strain energy release rates of a pressure sensitive adhesive measured by the shaft-loaded blister test

Abstract

The elastic solution to the shaft-loaded blister test (SLBT) was adopted to measure the applied strain energy release rate ( G ) of Kapton® pressure sensitive adhesive (PSA) tape bonded to a rigid substrate. The substrates used were either aluminum or Teflon®, a high-energy surface and low-energy surface, respectively. The values of G were calculated from three different equations: (1) load-based, (2) hybrid, and (3) displacement-based. An experimental compliance calibration was utilized to measure the film's effective tensile rigidity, ( Eh ) eff , the results of which are presented in an appendix. Plastic deformation at the contact area of the shaft tip and adhesive results in an overestimated displacement ( w 0 ) (relative to the elastic model), leading to disagreement among the values of G calculated. Estimation of the effective membrane stress in the film, ( N eff ), as well as the reasonable agreement between the value of ( Eh ) determined from a stress-strain experiment and the compliance calibration, suggest that, in spite of the plastic deformation, the assumption of linear elasticity in the crack growth region and hence the validity of the model, is reasonable. The compliance calibration has been shown to improvethe agreement among the values of G calculated from the three equations. Using the load-based equation, the assumed "correct" value of G may be obtained for a thin adhesive coating independent of the film's stiffness even in the presence of plastic deformation at the shaft tip. Comparing the value of G obtained by a pull-off test and the 90° peel test for a single ply indicates that the value of G obtained by the SLBT is of reasonable magnitude, being less than that obtained by the more firmly established pull-off test, and also that undesirable plastic deformation is reduced relative to the 90° peel test. An experimental configuration for studying the effects of liquids on the fracture energy has been demonstrated for the SLBT. This study indicates that the SLBT is an attractive and convenient test method for measuring the strain energy release rate of adhesive films, because of the insensitivity of the load-based equation to the coating stiffness, plus the independence of the value of G on the plastic deformation at the shaft tip, and the reduced plastic deformation at the crack front relative to the 90° peel.