An Experimental Study of Ratchetting During Indentation of 316L Stainless Steel

Abstract

This paper deals with fatigue problems of 316L stainless-steel surgical structures used in the correction of pelvic obliquity. The structures consist of implants, milled rods, and screws. SEM observations show that fatigue degradation of the elements in complete contact under shearing forces is governed by blunting of the rod pyramid. This involves the occurrence of a micro-clearance at joints leading excessive damage of the junctions. An experimental procedure based on cyclic indentation tests is developed to predict the life of the prosthesis. Results indicate that components exhibit ratchetting. The progressive deformation associated with this phenomenon increases linearly with the logarithm of the number of cycles and leads to the degradation of the surgical assemblies by an excessive accumulated blunting of the rod pyramid. In addition, it appears that viscous effects and tensile, as well as compressive residual stresses of less than 200 MPa, do not play an important role on the ratchetting rate. However, load amplitude and maximum load are important. Phenomenological relationship describing this evolution is established.