A qualitative picture of the relationships between adhesion experiments conducted with flat and spherical punches has been developed. By using a spherical punch with a relatively low value of a/h (ratio of the contact radius to the adhesive layer thickness) one can maintain a well-defined geometry during the debonding process, allowing one to quantify the relationship between the crack driving force, G, and the resultant crack velocity. This G(v) relationship governs the debonding process in any test or practical situation. By using a properly aligned flat punch, one can access much larger values of a/h, where cavitation throughout the contact zone precedes crack growth originating from the outer periphery of the contact zone. In this the balance between cavity growth and fibril extension can be very sensitive to the details of the G(v) relationship. For this reason, differences between systems which are difficult to observe when using a spherical punch can sometimes be readily observed when using a flat punch.
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