PSI - Issue 66

A.R. Pelton et al. / Procedia Structural Integrity 66 (2024) 265–281 Pelton/ Structural Integrity Procedia 00 (2025) 000–000

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Table 1: Measured and calculated large crack and small crack stress intensity threshold, ∆ K th , values in superelastic Nitinol

∆ K th (MPa √ m)

R

Large Crack

Small Crack

Small Crack Methodology

Reference

0.1

2.48

1.33

Calculated

(Robertson and Ritchie 2008)

0.5

2.02

0.81

Calculated

(Robertson and Ritchie 2008)

0.7

1.15

0.71

Calculated

(Robertson and Ritchie 2008)

0.75

-

0.34

Experimental

(Malito, Haghgouyan et al. 2024)

Fig. 11: (left) ∆ K th, small crack extrapolation from (Robertson and Ritchie 2008, Robertson, Pelton et al. 2012). The calculated extrapolated value for the small crack threshold is approximately 40-60% less than the corresponding large crack threshold. (right) a Kitagawa-Takahasi diagram based on the work from (Malito, Haghgouyan et al. 2024). For this investigation, the authors created focused ion beam starter cracks on superelastic Nitinol wire and testing was conducted under mode I conditions with tension-tension fatigue. This diagram shows a region of “likely no fatigue fracture” (green) and “likely fatigue fracture” (gold) under conditions of a prestrain of 6%, mean strain of 3% and R = 0.75. This diagram indicates that the critical small crack length is on the order of 10µm, whereby defect sizes greater than this value can tolerate lower stress amplitudes.

Fig. 12: Gumbel probability plot of the defect size with 95% confidence bands for defect size versus a function of the probability, F. Experimental data were obtained from SEM analysis of twenty extracted wires that were fatigued to fracture under various mean strain – strain amplitude conditions. After (Brambilla, Berti et al. 2024).