Some remarks on JKR experiments

Two important questions on the validity of the mechanical analysis commonly used to deduce adhesion strength from a Johnson, Kendall and Roberts (JKR) test are investigated in detail, based on systematic experiments conducted on a JKR machine which allows the relations between the contact area and the load, and between the deformation in the centre of the contact area and the load to be simultaneously characterized. First, the existence of finite size effects associated with the use of small elastomer lenses is clearly demonstrated, and a way to eliminate them is proposed by intercalating an elastomer ribbon with an adjusted thickness between the lens and the rigid plate which holds it. Second, kinetic effects associated with the fact that the unloading sequence is never performed at a vanishing velocity are also analysed. It is shown that a constant and very low unloading rate is not a guarantee that the fracture will propagate at a constant speed. Thus, if the adhesion strength is velocity-dependent, care has to be taken to ensure a correct analysis of the data. It is shown that monitoring the full relaxation trajectory, at fixed deformation, allows such an adhesion strength to be characterized as a function of the velocity.