Abstract
Fatigue tests of two-level multiple-step wave stress were carried out on austenitic stainless steel SUS 316 by use of a varying loading apparatus combined with a rotating fatigue testing machine. Discussions of the test results and the analysis of equivalent alternating stress were presented. The following results were obtained.
(1) In comparably small strain hardening materials such as carbon steel or Mg-alloy, the equivalent alternating stress could be obtained from the fatigue data under alternating stress σa combined with the mean stress σm by using the rectificating coefficient η of 0.50. On the other hand, remarkable strain hardening was observed in the test specimens of SUS 316 fractured under the condition of ∞>A≥1.0 (A=σa/σm), and little effect of σm on the fracture life was detected. However, from the test specimens fractured under the condition of two-level multiple-step wave or two-level two-step wave stress, little strain hardening was observed because both stress ranges of σa1 and σa2 (σa1>σa2) were alternating stresses. Therefore, η of 0.50 was found to be usable, if the stress shape of two-level multiple-step wave was assumed to be changed partially to alternating stress of pulsating stress.
(2) The stress shape of two-level multiple-step wave could be translated to the shape of partial alternating stress. However, the equation of Σn1/Neq=Σn2/Neq=1/2 was hardly applicable in two-level multiple-step wave stress except for the case of partial alternating stress. Then, by using the linear damage rule and the medium of partial alternating stress, the equivalent alternating stress σ'aoe and the equivalent number of cycles to fracture N'eq were obtained analytically from the basic data of alternating fatigue tests.
