An investigation of the S/N fatigue life gage

Abstract

The S/N fatigue life gage is a small sensor which is similar in appearance to a foil strain gage. The S/N fatigue life gage is bonded to the surface of a mechanical structure using standard strain-gage techniques. The S/N gage changes resistance permanently as a continuous function of fatigue experience. This gage was developed by Mr. Darrell R. Harting of the Boeing Company, Seattle, Washington. The gage is produced commercially by Micro-Measurements, Inc. and distributed by W.T. Bean, Inc., Detroit, Michigan. This investigation describes the results of a series of reverse bending tests on S/N fatigue life gages which were mounted on Ti-6A1-4V titanium specimens. Each S/N gage was subjected to various constant strain loadings for varying numbers of cycles. The results of this investigation show that the performance of the S/N fatigue gage under random cyclic loading is predictable. Another result of the tests indicates that the S/N gage experiences an above normal increase in resistance well in advance of actual gage failure. Finally it was observed that a decrease in S/N gage resistance will occur: immediately after the mean cyclic strain level is lowered; and whenever the gage is subjected to any substantial rest period.