Fatigue strength of small‐notched specimens under variable amplitude loading within the fatigue limit diagram

Abstract

A B S T R A C T Although the fatigue limit diagram is defined in principle for constant stress amplitude, it is often considered that fatigue failure would not occur, even in varying loading, if applied stresses were kept within the fatigue limit diagram. However, it was shown in the case of small-notched specimens that fatigue failure occurred in some special cases of variable amplitude loading, even when all stress amplitudes were kept within the fatigue limit diagram. The cause of this phenomenon was examined using two-step stress and repeated two-step stress patterns in which the first step stress was chosen to be equal to the fatigue limit with zero mean stress and a mean stress was superposed on the second step stress. A non-propagating crack was formed by the first step stress. This crack functioned as a pre-crack for the second step stress with high mean stress. Consequently, fatigue failure occurred even when all stress amplitudes were kept within the fatigue limit diagram. It was an unexpected fracture caused by the interference effect of a non-propagating crack and a mean stress change.Keywords fatigue limit diagram; non-propagating crack; notch; repeated two-step loading; short crack; variable amplitude loading. N O M E N C L A T U R En 1 = number of cycles of 1st step stress in one block of repeated two-step stress n 2 = number of cycles of 2nd step stress in one block of repeated two-step stress R = stress ratio t = notch depth K = stress intensity factor range ( K eff ) th = effective threshold stress intensity factor range σ 0.2 = 0.2% offset stress σ 1 = 1st step stress amplitude of repeated two-step stress σ 2 = 2nd step stress amplitude of repeated two-step stress σ a = stress amplitude σ B = tensile strength σ m = mean stress σ m 1 = mean stress of 1st step stress of repeated two-step stress σ m 2 = mean stress of 2nd step stress of repeated two-step stress σ y = yield stress I N T R O D U C T I O NThe effect of mean stress on fatigue limit is usually evaluated using the fatigue limit diagram 1 as shown in Fig. 1.

Effect of small notch and absorbed hydrogen on the fatigue fracture in two‐step stress test within fatigue limit diagram

“…If the stress amplitude and mean stress were kept constant within fatigue limit diagram, fatigue fracture would not occur. In the previous study, however, it was shown that fatigue failure occurred in some special cases even when all stress amplitudes were kept within fatigue limit diagram 7–9 . In the previous study, 9 repeated two‐step loading pattern was examined.…”

“…The major reason why two-step stress sequence was used is to compare with the repeated two-step stress sequence used in the previous report. 9 From the viewpoint of practical application, this kind of mean stress change may occur due to the change in operating condition of machine. The first step stress (σ a1 ) was chosen just equal to the fatigue limit for zero mean stress in this test.…”

“…In the previous study, however, it was shown that fatigue failure occurred in some special cases even when all stress amplitudes were kept within fatigue limit diagram. [7][8][9] In the previous study, 9 repeated two-step loading pattern was examined. In the repeated two-step loading, multiple repetitions of mean stress change by repeated block loadings occur.…”