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ASTM has recommended practices for fatigue testing to obtain representative data and to allow comparison of data from different sources. This paper describes testing techniques that follow the guidelines of ASTM but that are modified to study the response of specific materials and to allow an extrapolation of the data to design service lives. The particular material illustrated is a cold-worked type 316 stainless steel (Unified Numbering System [UNS] S31600) in room temperature air. This and other austenitic alloys are characterized by having a relatively stable inelastic strain for a high percentage of life even up to lives beyond 108 cycles.
The testing program included (a) low-cycle tests under fully reversed total-strain control and (b) high-cycle tests under fully reversed stress control. The latter tests were performed in a stress sequence of the staircase method. The test specimen and testing frequency for each type of test were designed to maximize the ability to measure strain and stress and to minimize any instabilities of strain or temperature.
For the high-cycle fatigue tests in room temperature air, the specimen temperature rises as a function of the cyclic work rate. The temperature history was measured and the inelastic strains were estimated from a finite-element heat conduction analysis. Failure was characterized by a continuous increase in temperature but run-out (nonfailure) exhibited an initial rise and a decrease to a stable temperature which was a function of the stress range.
The data for inelastic and elastic strains and their histories were analyzed and relationships were formulated for extrapolation. From the staircase method of testing, the fatigue strength was statistically determined at 106 cycles and 3.2 × 108 cycles, an interval of 2.5 decades. An extrapolation was made for an equal interval to 1011 cycles. It is proposed that the techniques developed in this paper can be used to generate fatigue data for design of structures using austenitic stainless steels.
ConsultantMember of ASTM, Westinghouse Research and Development Center, Pittsburgh, PA
Advisory engineerMember of ASTM, Westinghouse Nuclear Energy Systems, East Pittsburgh, PA
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