A Fracture Mechanics Model for Cavitation Damage in Mechanical Heart Valve Prostheses

Abstract

In the 1994 Replacement Heart Valve Guidance of the Food and Drug Administration (FDA), both cavitation and damage tolerance analyses are required for mechanical heart valves (MHV). Cavitation results from a sequence of events. First, vaporous bubbles are generated in the blood flow. They then collapse to form high-speed micro liquid jets, striking against the solid valve boundary, and subsequently causing pits on the surface. Micro cracks may initiate around the pitted areas under repeated jet impacts superimposed with cyclic loading. These events are, of course, closely related to the shape and size of an MHV. The factors specified in the current FDA guidance and considered by many authors for calculating working stresses, including static stresses, dynamic stresses, residual stresses, and stress concentrations, appear to be inadequate. The local high pressure caused by cavitation jets is another important factor which may aggravate crack propagation. The present paper is aimed at quantitatively assessing the influence of cavitation jets on the safe service life of an MHV using a damage tolerance approach. A new fracture mechanics model for estimating service lives of MHV prostheses is proposed, in which bubble dynamics and cavitation phenomenon are incorported. Numerical results show that the local high pressure is dominant, and is large enough to cause a crack to propagate at a greater rate, and resulting in much shorter fatigue life for an MHV.