The Influence of Thickness of Specimen on Fatigue Crack Propagation Properties in Spherical Graphite Cast Iron.

Usami, Hyoe; Sugiyama, Yoshihiro; Shiraki, Naoto; Umehara, Tsutomu

doi:10.2472/jsms.51.1386

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“…Figure 4a also shows that two samples with different thicknesses (12.7 and 6 mm) have almost the same da/dN curves and the same threshold behaviour. Past studies have reported that the threshold behaviour is affected by the specimen thickness due to differences in the oxide-induced closure 26,31 . However, in the present conditions without the presence of excess oxides, the difference in behaviour for the 12.7 and 6 mm low alloy steels clearly disappears.…”

Section: Resultsmentioning

confidence: 99%

Near-threshold fatigue crack propagation without oxide-induced crack closure

Tazoe¹,

Tanaka²,

Oka³

et al. 2020

Sci Rep

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An accurate value for the threshold stress intensity factor range, ΔK th , is a key parameter for studying crack-like defects. However, it is difficult to obtain accurate ΔK th values due to oxide-induced crack closure. In this study, we report conditions for minimizing the effects of oxide-induced crack closure near the threshold region, where a concave curve of the effect on the loading frequency on oxideinduced crack closure is achieved. the resulting conditions allow for an accurate determination of ΔK th , which is a key material parameter relating to the pertinent loading ratio. Almost all the mechanical structures are exposed to cyclic loading, which means that it is important to estimate the fatigue strength of mechanical structures to ensure safe operation. It is also well known that obtaining an accurate value for the threshold stress intensity factor range, ΔK th , is necessary for studying fatigue related problems. ΔK th is typically obtained via the ΔK-decreasing test based on ASTM E647 standard 1 , that the force is controlled to decrease step-by-step as the crack grows in the fatigue test, which is known to be affected by plasticity-induced 2-7 , roughness-induced 2,8-12 and oxide-induced crack closures 2,4,5,7,11,13-31. Among these types of crack closures, the roughness-induced closure is thought to be a material property because the roughness of the fracture surface is related to the microstructure of the material 10. In contrast, the plasticity-induced and the oxide-induced closures are considered to be dependent on the testing conditions 2-5,11,13-28. In order to obtain an accurate ΔK th value, these types of closures should be minimized. Many studies have been performed on the plasticity-induced closure 32-35 and have been summarized in the ASTM standard 1. The oxide-induced closure has also been studied in order to discuss the effects of various parameters 2,4,5,11,13-31. However, to the best of our knowledge, there have been no studies on the conditions that are necessary to minimize the oxide-induced closure at a low stress ratio when tested in air. According previous studies, it is considered that the large amount of oxides is generated on fracture surfaces by fretting oxidation 18 , causing the closure effect due to pushing up the crack closing point 2. Regarding the causes for the oxidation, the humidity 19,29 and the loading frequency 15,22,27,30 are important factors. Since high-moisture conditions accelerate oxidation, a low-humidity condition is recommended to minimize the oxide-induced closure. The effects of the loading frequency on the oxide-induced closure were studied by Bignonnet et al. 22 , who found that the magnitude of the closure in a structural steel at 7 Hz is smaller than that at 65 Hz. Todd et al 30. also reported a similar trend based on results from a MIL-S-24645 base metal. Conversely, Radon 36 reported that the ΔK th of an aluminium alloy tested at a high frequency tends to be smaller than that at a low frequency based on test results at 35 and 0.15 Hz. Skelton and...

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Section: Resultsmentioning

confidence: 99%