Page 208 - Failure Analysis Case Studies II

Failure Analysis Case Studies II
D.R.H. Jones (Editor)
0 2001 Elsevier Science Ltd. All rights reserved 193

LOW-CYCLE FATIGUE OF TITANIUM 6A1-4V SURGICAL
TOOLS

H. VELASQUEZ, M. SMITH, J. FOYOS, F. FISHER and 0. S. ES-SAID*

Department of Mechanical Engineering. Loyola Marymount I Jniveraity. 7900 Loyola Blvd. Los Angeles.
CA 90045-8145, U.S.A.
and

G. SINES
Department of Materials Science and Engineering, University of California at Los Angeles, Los Angeles.
CA 90095-1595, U.S.A.
(Receit,ed 3 September 1997)

Abstract-A handle holder is used in heart valve replacement surgery to insert a mechanical flow valve into
the patient’s heart. In order to position the valve corrcctly, a 0.1” diameter titanium 6AI4V handle holder
shaft is bent until the proper angle is found. This bending of the tool makes it vulnerable to low-cycle fatigue
failure. Low cycle fatigue testing of titanium 6AMV specimens shows that surfacecracks appear approximately
20 cycles before failure occurs. It is recommended that the tool be carefully inspected before each use and
replaced when surface cracks appear. The data showed that the life of the tool may be increased by 30% if it
is bent in the same direction during its cntirc USC. 6 1998 Elsevier Science Ltd. All rights reserved.
Keywords: Fatigue data, handtool failures, safe life.

1. INTRODUCTION
During heart valve replacement surgery the patient receives a major incision that runs across the
chest. All access to the patient’s heart is through this opening in the chest. Selection of the optimal
replacement valve size follows. Once the valve has been selected it is placed in the patient’s heart.
A handle holder (Fig. 1) is used by the surgeon to place the valve during the surgical procedure.
The handle holder is a screwdriver-like tool made of titanium 6AI-4V equipped with a plastic tip
used to hold the prosthetic valve. The valve, held at the tool’s tip during positioning, is released
upon placement in the heart. To position the valve, it must be inserted through the incision,
maneuvered through the chest wall and finally put into place. The surgeon bends the thin shaft of
the handle holder until the proper angle is found to insert the valve. The reusable handle holder is
often bent several times during each surgical procedure and thus becomes vulnerable to low cycle
fatigue [l]. It is therefore essential that the surgeon knows when to replace the tool to avoid a
fatigue failure during surgery.
The objective of this study is to determine the fatigue life of the shaft. After conducting tests on
titanium 6A1-4V sample rods, use-limit recommendations were made.

2. INVESTIGATION METHOD

To determine whether a testing device to automate the cyclic loading would be required or if
manual testing was possible, the number of cycles (N) required for low-cycle fatigue failure of the
shaft was approximated by using the Coffin-Manson law [2] :

*Author to whom correspondence should be addressed.
Reprinted from Engineering Failure Analysis 5 (l), 7-1 1 (1998)