Page 212 - Failure Analysis Case Studies II

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,,- T
SHA
f

Fig. 6. Surface cracks.

6). In fact from the time that cracks become apparent to the time when failure occurs, the data
shows that at least 10 more cycles may be safely performed. Clearly, the onset of cracking can be
easily used to warn of impending failure. Since the low cycle fatigue testing was conducted on 3/16
diameter specimens, 1/10 diameter rods were also tested to ensure that the surface cracks would be
visible under normal visual inspection. This testing showed that the cracks are visible on the 1/10”
diameter handle holder shafts.
The data also showed that the life of the specimens in group 1 which were bent in one direction
only was about 30% longer than that of the specimens in group 2 which were tested in reverse
bending. It is recommended that the tool be bent in the same direction throughout its life whenever
possible.

5. CONCLUSION

Cyclic loading test on titanium 6A14V specimens showed that appearance of surface cracks can
act as a low cycle fatigue warning system in the 1/10 diameter tool shaft. It is recommended that
the tool receive a careful visual inspection before each surgery and be replaced whenever surface
cracks become visible. To extend tool life it is also rccommcnded that the tool be bent in one
direction only whenever possible. The Coffin-Manson equation was useful in deciding testing
procedure, but should not be used for service life prediction in this critical application.
Acknowkdgments-The authors wish to thank Ms Rachel Adams of the Mechanical Engineering Department of Loyola
Marymount University for typing the paper. The study was partially funded by a grant from the Medical Carbon Research
Institute, Austin, Texas.

REFERENCES

1, http :i,/www.heartlab.rri.uwo.ca/valve.surgery.html.
2. Dieter, G. E., Mechanical Metallurgy, McGraw-Hill. New York, 1976, pp. 412413