IMPACT OF DELRIN DISC - LUBRICATED AND DRY CONTACTS

A Delrin disc is used in the Bjork-Shiley tilting disc heart valve (H. L. Milligan, B.M. Johnson and P.A. Engel, ASTM J. Test. Eval., 22 (5) (1994) 474-484; C.G. Montero et al,, Ann, Thorac. Surg., 37 (4) (1984) 328-336), where dynamic contact occurs between metallic cylindrical s truts and the disc, during valve action. To investigate the wear behav ior of the Delrin disc, impact wear tests were performed by using a pi votal hammer device with a spherical striking face. For the biomedical wear study (Milligan et al.), raised temperature (36.6 degrees C) and lubrication (simulating blood flow) was always provided, but in the p resent work both dry and lubricated contacts were investigated in orde r to compare wear mechanisms. For a small (sharp) indenter radius, pla stic deformations of the Delrin occurred under the initial impact cycl es. These incremental plastic deformations diminished under subsequent impacts, and were later followed by completely elastic contact, contr ibuting 'zero wear'. Beyond this region, a measurable wear regime woul d start. At severe contact stress (sharp indenter),an accelerated wear ing pattern was seen to start at some stage during the measurable wear range. Taking into account plastic deformations existing at the begin ning of the wear process, the present work discusses an impact wear mo del. The combined effects of wear, shear traction and crack propagatio n are discussed.