PSI - Issue 24

Guido Violano et al. / Procedia Structural Integrity 24 (2019) 251–258

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G. Violano and L. A ff errante / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 3: Contact radius vs force relation obtained during the loading phase. The markers refer to experimental data, the black line to the JKR fit.

Fig. 4: The strain energy release rate G as a function of the contact line velocity v p = − da / dt . Data refer to experiments performed at di ff erent driving velocities of the indenter ( V = 0 . 0002 , 0 . 002 , 0 . 02 mm / s). The linear fit returns n = 0 . 2.

The second parameter β can be estimated from the load vs contact radius data measured during the unloading. In this regard, Fig. 5a shows the unloading data obtained by changing the displacement velocity V over three orders of magnitude ( V = 0 . 0002 , 0 . 002 , 0 . 02 mm / s ). Coloured curves are instead obtained by solving eq. (1) with the values of β that gives the best fit of the experimental data. Notice n is independent of V as it is a characteristic constant of the elastomer. On the contrary, β is proportional to V . The e ff ect of the maximum force ¯ P max reached at the end of the loading phase is investigated in Fig. 5b, for fixed displacement velocity of the indenter V = 0 . 002 mm / s. For smooth surfaces, where complete contact occurs in the region r ≤ a , we expect the pull-o ff force to be independent of ¯ P max (Dorogin et al. (2017)). Both numerical and experimental data confirm this prediction. In fact, we found a very slight increase in the pull-o ff force with ¯ P max as a result of the stretching of polymer chains occurring before breaking the adhesive bond. This could be the reason of a slight increase in the work of adhesion during pull-o ff . Finally, Figs. 6a-b show the pull-o ff force in terms of the displacement velocity V (Fig. 6a) and maximum load ¯ P max (Fig. 6b). As already above predicted, the pull-o ff force is strongly influenced by V , while negligible e ff ects are observed by changing ¯ P max . Moreover, the figures show a quite good agreement between experimental values and numerical fit.