Evaluation of Cyclic Service Load by Micro-Fractography

Abstract

For prevention of fatigue fracture troubles in structural parts, the establishment of a procedure to evaluate service load by micro fractography is desirable. Fatigue tests under block loading with four or five load steps and random loading with a constant minimum load were carried out using CT test specimens of stainless steel (SUS 304). Micro-fractographic analysis was performed on the test specimens and the relationship between the applied stress and the striation spacing observed was investigated. The results obtained are summarized as follows:
(1) The striation spacing corresponding to the maximum stress amplitude in the block loading tests was equal to that in the constant load test.
(2) Retardation of fatigue crack propagation was recognized in the striation spacing corresponding to the stress amplitude lower than the maximum amplitude in the block loading test.
(3) The data of fatigue crack propagation rate obtained in the block and random loading tests fitted well to a straight line when plotted against m factorial mean ΔK value.
(5) The random test loads could be evaluated almost correctly from the striation spacings measured, except for small load spectra which gave unresolvably small striation spacings.