SOME REMARKS ON JKR EXPERIMENTS

Two important questions on the validity of the mechanical analysis com monly used to deduce adhesion strength from a Johnson, Kendall and Rob erts. (JKR) test are investigated in detail, based on systematic exper iments conducted on a JKR machine which allows the relations between t he contact area and the load, and between the deformation in the centr e of the contact area and the load to be simultaneously characterized. First, the existence of finite size effects associated with the use o f small elastomer lenses is clearly demonstrated, and a way to elimina te them is proposed by intercalating an elastomer ribbon with an adjus ted thickness between the lens and the rigid plate which holds it. Sec ond, kinetic effects associated with the fact that the unloading seque nce is never performed at a vanishing velocity are also analysed. It i s 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 ad hesion strength is velocity-dependent, care has to be taken to ensure a correct analysis of the data. It is shown that monitoring the full r elaxation trajectory, at fixed deformation, allows such an adhesion st rength to be characterized as a function of the velocity.