Relation between work of adhesion and work of fracture for simple interfaces

A study was conducted of the relation between work of adhesion and work of fracture for adhesive-substrate systems exhibiting fracture at the interface. Materials and test conditions were selected to eliminate contributions from irreversible, energy-consuming processes in the bulk of the adhesive or substrate. Values for W _a were determined from contact angle measurements made at room temperature with the adhesive in the liquid state; values for F were determined from inverted blister tests conducted at temperatures low enough for the adhesive to be in the solid state. The independent variable, W _a, ranged from about 40 mJ/m² to 144 mJ/m². The dependent variable, F, was found to range from 0.12 J/m² to 20 J/m², with most under 5 J/m². The excess of F over W _a was found to increase exponentially with W _a, and was proof of the occurrence of irreversible processes in specimens as they were loaded and fractured. The exponential behavior of (F – W _a) with W _a suggested that the irreversible process was orientation hardening. The absence of detectable permanent deformation of any kind in the bulk substrates or at the fracture surfaces, plus the incapability of the adhesives to sustain significant irreversible processes, led to the conclusion that the orientation hardening must have taken place in the substrate, within a thin layer (and small volume) adjacent to the interfacial plane.