In the shaft-loaded blister test (SLBT), plastic deformation often occurs at the contact area between the shaft tip and adhesive layer, leading to a larger displacement (blister height) than if the film was loaded elastically. As a consequence, incorporating the displacement variable into the analysis can result in misleading values of the applied strain energy-release rate, G. In this work, the influence of plastic yielding at the contact area on G of a thin film was investigated as a function of some common SLBT experimental variables, namely, substrate hole diameter, film thickness, and shaft-tip diameter. Test specimens consisted of plies of pressure-sensitive adhesive tape adhered to a rigid glass substrate. G was calculated from the following equations: (1) load-based, (2) hybrid, (3) displacement-based, and (4) combination. Decreasing the film thickness, increasing the hole diameter, or decreasing the shaft-tip diameter lead to more plastic yielding at the contact area as well as to an increase in blister height. The increased blister height resulting from plastic deformation leads to disagreement among the values of G calculated from the different equations when the displacement variable was included in the calculation. However, the load-based equation, which does not include the displacement, was determined to be independent of plastic yielding and the "correct" equation for calculating G. In addition, the film tensile rigidity (Eh) was calculated using an experimental compliance calibration. The effects of film thickness on the mechanical behavior of the film ( bending plate vs. stretching membrane) as well as methods to determine the displacement resulting from plastic deformation are also discussed.