An Experimental Study of Friction During Planar Elastic Impact

Low-speed oblique elastic impacts frequently occur in heat exchangers and other equipment having loosely supported tubes or pipes. In order to experimentally study friction during such impacts, a pendulum-type impact apparatus was developed. A hardened steel sphere at the end of the pendulum collides with a flat steel surface for a range of approach velocities and angles. The present investigation examined eight velocities from 8 to 93 mm/s and impact angles (measured from the common normal) of 0 to 75 deg for each velocity. The normal and tangential contact force waveforms were measured using a triaxial piezoelectric force transducer which was dynamically calibrated. As expected, the results show that the tangential force is less than the limiting Amontons-Coulomb friction predictions at low impact angles. Two regimes of stick-slip and gross-slip friction are clearly distinguished by a new friction parameter called the specific traction ratio. Tangential force reversal was observed at low impact angles, indicating local tangential oscillations. The stick-slip results are consistent with a partial-slip model where the contact zone has a central sticking region surrounded by a ring area undergoing slip.