Effect of Hardness and Stress Ratio on Threshold Stress Intensity Factor Ranges for Small Cracks and Long Cracks in Spheroidal Cast Irons

Abstract

Threshold stress intensity factor ranges, ∆K th s, of spheroidal cast iron, which are composed of ferritic, ferrite-pearlitic, or pearlitic structures, have been estimated by ∆K-increasing tests for small cracks and by ∆K-increasing and ∆K-decreasing tests for long cracks at several stress ratios. With respect to the effects of the hardness and stress ratio, ∆K th estimated by the ∆K-decreasing tests for long cracks is evidently different from that for small cracks, while ∆K th estimated by the ∆Kincreasing tests for long cracks is similar to that for small cracks. Moreover, in the case of long cracks, the values of ∆K th estimated by the ∆K-decreasing tests were larger than those estimated by the ∆K-increasing tests, irrespective of the type of matrix structures. These tendencies are attributed to the fact that the crack closure stress of the long cracks in the ∆K-decreasing tests is relatively higher than that of the long cracks in the ∆K-increasing tests. On the fracture surface of the long cracks in the ∆K-decreasing tests, oxide layers produced by fretting are observed at sites around the crack tip, although such layers are not observed on the fracture surface in the ∆K-increasing tests.

Influence of small defects and nonmetallic inclusions on the high and very high cycle fatigue strength of an ultrahigh‐strength steel

“…However, this value is significantly larger than the long-crack threshold of ΔK th,lc = 2.4 MPa ffiffiffiffi ffi m p determined by fatigue crack growth measurements in ambient air (Figure 8). Deviations from ΔK th,lc for large defects have been reported for martensitic stainless steels 4 and spheroidal cast iron, 33 but they are mainly found at load ratios different from R = À1 and could be explained by the influence of mean stress on the crack tip constraints. 4 The extremely slow crack growth rates measured in the near-threshold regime as shown in Figure 8 rather suggest environmental influences that enable crack growth at extremely low stress intensity factors.…”

A practical method for fatigue limit prediction in ductile cast irons

“…The most significant benefit of applying the parameter model is that the fatigue limit of metallic materials containing defects can be predicted within an error margin of about ±10%, without the need for performing a fatigue test. Accordingly, a quantitative assessment of the fatigue limit of DCIs has been conducted by utilizing the parameter model in many studies …”

“…Accordingly, a quantitative assessment of the fatigue limit of DCIs has been conducted by utilizing the ffiffiffiffiffiffiffiffiffi area p parameter model in many studies. [6][7][8][9][10][11][12][13][14][15][16][17] It should be noted that Equation ( 1) is applicable only to small defects in the small crack region for which the threshold SIF range depends on the crack size, ie, ΔK th ∝ ffiffiffiffiffiffiffiffiffi area p ð Þ 1=3 : 20 The fatigue limit, σ w , for larger defects beyond the small crack region can be predicted by the conventional method based on linear elastic fracture mechanics by using ΔK th that is a material constant for long cracks. [20][21][22][23] The approximate upper limit of ffiffiffiffiffiffiffiffiffi area p to which Equation ( 1) is applicable is reported to be 2000 μm for ferritic DCI 9 and 1400 to 1500 μm for ferritic-pearlitic and pearlitic DCIs.…”

“…Many investigations have been conducted into the effects of shape, size, and distributions of graphite and casting defects, as well as microstructures, on the fatigue strength of DCIs. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Considering the issues tackled in those studies, in order to realize a reliable design for the safe life (ie, infinite life) of DCI products, the following two approaches first need to be established:…”

“…Many investigations have been conducted into the effects of shape, size, and distributions of graphite and casting defects, as well as microstructures, on the fatigue strength of DCIs . Considering the issues tackled in those studies, in order to realize a reliable design for the safe life (ie, infinite life) of DCI products, the following two approaches first need to be established: A method to predict the fatigue limit for a given matrix structure and discontinuity. A method to quantify the statistical influences. The principal aim of this study is to propose a possible technique (1) by which a reasonably accurate prediction result is derived more simply than ever before.…”