<p>Authors has reported the effect of the loading sequence on thermal fatigue in a mixing tee in the previous study. Strain-controlled fatigue tests were conducted using specimens and the strain around a hot spot, which was estimated from wall temperatures measured in a mock-up test, in order to investigate the effect of the loading sequence. It showed that the character of strain with periodic overload caused the reduction of the fatigue life. In this study, the effect of the single overload on the fatigue crack growth rate was investigated to clarify the causes. Plate specimens with a notched crack were used to measure the change in fatigue crack growth rate after a single overload. Fatigue crack growth tests were conducted by controlling the strain which was measured by a strain gauge attached 14 mm away from a pre-crack. The specimen was made of type 316 stainless steel which was same as used in the previous study. Tensile and compressive overloads were applied during constant amplitude cycling. The overload ratio, which was defined as the ratio of overload size to baseline constant amplitude, was changed. The constant amplitude tests were conducted under the strain ratio of −1.0 which was defined as the ratio of the minimum strain to the maximum strain. The crack opening point was also obtained by the measured load-displacement curve using unloading elastic compliance method. The crack growth rate increased after the single compressive overload. The accelerating rate increased with the overload ratio. In contrast, not only the acceleration but also the retardation of the crack growth rate was observed for some tensile overload cases. The crack growth rate increased for relatively-small tensile overload cases and decreased for relatively-large tensile overload cases. The change in the effective strain range<i> </i>after the single overload, which was defined as the difference between the maximum strain and the crack opening strain, was also examined. The effective strain range after the compressive single overload increased and was nearly equal to the strain range. It means that the crack opened fully during loading cycle after the single overload. The increased effective strain range decreased gradually with the crack propagation. The effective strain range after the tensile single overload increased for the cases where the crack growth rate accelerated. The crack became fully open right after the tensile overload. The increased effective strain range decreased immediately after the overload. The effective strain range decreased for the cases of the retardation of the fatigue crack growth rate. The decreased effective strain range became less than the value before the overload. The crack growth rates after the tensile and compressive single overloads were correlated with the effective strain intensity factor range rather than the strain intensity factor range.</p>

Effects of a Single Overload on Fatigue Crack Growth of Type 316 Stainless Steel

Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2021/volume/85314

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