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Polymers are finding increasing use in biomedical engineering applications. One of the major problems in designing with polymers is predicting when the material will fail and what stress values should be used in the design of components in order that the material will not fail prematurely. Recent work on modeling the creep rupture behavior of more than 15 polymers has produced a breakthrough in predicting the failure times as well as the upper stress and lower stress limits of the polymers [1,2]. The upper stress relates to the maximum stress that can be applied to the material before instantaneous fracture. The lower stress limit relates to the minimum stress that can be sustained by the material without failure and, in some cases, this also corresponds to the fatigue endurance limit of the polymer. The creep rupture times of some medical plastics such as high density polyethylene (HOPE), polycarbonate (PC), polypropylene (PP), polyoxymethylene (POM), polyvinylchloride (PVC), polysulphone (PSU), and polymethyl-methacrylate (PMMA) were analyzed at 20°C in air. In order to optimize the parameters for creep rupture prediction, a nonlinear regression analysis program was used . Special attention was made for Delrin (POM from Du Pont) which is of interest in cardiovascular applications related to wear of mechanical heart valves  This matenal was tested in air at 37, 60, and 80°C. The effect of saline solution was also studied Here, the lower stress limit was reduced from 20 MPa to 5 MPa. All the specimens brittle fractured This may account why Delrin used in the artificial hip joint prosthesis  where the contact stresses exceed this limit was known to wear severely. However, in the disc of the mechanical heart valve  no failure occurs because the stresses were probably below the lower stress limit.
failure prediction, design stresses, medical plastics, creep, upper stress limit, lower stress limit, creep rupture modeling
Senior lecturer, National University of Singapore,