Experimental Evaluation of the Size Effect on Flat and Curved Sliding Motions

The experimental behaviour of Concave Surface Slider devices has been deeply investigated in the recent past. Several static and dynamic tests have shown non-negligible dependencies of the friction coefficient on important response parameters, such as sliding velocity and contact pressure at the sliding interfaces. Moreover, an additional decay trend can be noticed, as the isolator is subjected to multi-cycle excitations: precisely, sliding motions produce heating phenomena, which lead to exponentially decreasing values of friction coefficient. Such a dependency can be expressed as a function of the dissipated energy, developed during motion, in terms of integral of the vectorial force-displacement relationship. It has been proved that the decay curve parameters change, when different couples of sliding velocity and contact pressure values are applied to the tested device; thus, the friction coefficient can be generally considered as a function of sliding velocity and contact pressure, whereas the degradation due to sliding interfaces heating can be modelled through an exponential decay function, with parameters which also depend on sliding velocity and contact pressure. Nonetheless, experimental results may be affected by sizes of the tested sliding pads: precisely, if low-to-medium vertical loads are applied to the isolator, a non-constant contact pressure distribution at the sliding interfaces is expected for large sliding pads, differently to what is assumed in designing the devices. This aspect may result into changes in the friction coefficient value and, consequently, can lead to unexpected responses. In this work results of a wide experimental campaign carried out on both Flat and Double Concave Surface Slider devices are discussed. Particularly, three different pad diameters have been implemented for both the Flat and the Curved sliders, and the same testing protocol has been applied, in order to evaluate the effects of the pad size on the frictional response. Bidirectional motions have been considered, with cloverleaf orbit, as ruled for bidirectional dynamic tests by standard code UNI:EN15129:2009. Results have been analyzed by investigating the common dependencies of the friction coefficient with respect to sliding velocity, contact pressure and cyclic decay, by varying device typology and pad sizes.