The most accurate method to determine the fatigue limits is to conduct the conventional fatigue tests actually, but it is very laborious, taking many hours and requiring many specimens. Then, various methods are proposed for estimating fatigue limits rapidly, one of which is temperature measurement of fatigue specimens under gradually increasing loads.
Recently one of the present authors has devised a new accurate temperature measuring method, using the ceramic condenser, the capacitance of which changes with temperature according to the Curie-Weiss law.
By this method, it is easy to measure the temperature of rotating specimen accurately without any contact part like slip ring, by searching for the resonant frequency of L-C circuit consisting of coil and ceramic condenser attached to the specimen. This method is readily applicable to the rotating bending fatigue testing machines of various types widely used in Japan.
In this investigation, the behaviors of rising temperature in the rotating bending specimens of Ono type fatigue testing machine under gradually increasing loads were measured and compared with the fatigue limits which were determined by conventional fatigue tests. The most specimens were machined from S20C, S45C, SNC-2 and 14S-T6 as received bars except some that were of S20C which were fully annealed in vacuum after machining.
The behavior of rising temperature due to increasing stress amplitude were expressed typically in three parts, the gradually increasing part, the linearly increasing part and the rapidly increasing part. And for all materials, the fatigue limits were very close to the stress amplitude, at which the temperature began to deviate from the linear part to the rapid part, with maximum error 2.8 percent (non-conservative side).