PSI - Issue 54

Naveen Kumar Kanna et al. / Procedia Structural Integrity 54 (2024) 196–203 © 2023 The Authors, Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of the scientific committee of the ICSI 2023 organizers

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Keywords: Fatigue; Crack length measurement; DC Potential Drop

1. Introduction Failure of components subjected to cyclic loading is governed by crack initiation followed by the propagation of short and long fatigue cracks (Schijve (2003)). For a detailed analysis of the fatigue behavior, it is essential to determine the crack initiation and investigate the propagation behavior of short and long fatigue cracks. Especially the investigation of the propagation of short cracks is a challenge. Direct Current Potential Drop (DCPD) measurements have been widely used to measure the fatigue crack initiation, crack arrest or propagation. In the DCPD technique, a direct current is passed through a specimen. When a crack is initiated and propagates, the potential lines in front of the crack tip get densified, so the potential drop measured by the potential probes increases. This method offers advantages like flexibility, especially during elevated temperature measurements, reliability and reproducibility (Lambourg (2020)). The DCPD method can be easily automated for performing crack propagation experiments (Bär (2001)). However, the method offers low sensitivity to detect crack initiation and measure the propagation of short cracks, especially when the Johnson equation is used to determine the crack length from the measured potential drop. Alternatively, the alternating current potential drop (ACPD) method is more precise in detecting small surface cracks. But this method requires complicated evaluation and expensive equipment (Moreno (2003), Raja (2010)). In this article, a new method suggested by Bär (2020), to experimentally characterize the initiation and propagation of semi-elliptical fatigue cracks with a multiple probe potential drop measurement for an improved resolution is applied and a simple approach suggested by Tiedemann (2016) is employed to calculate the length of semi-elliptical fatigue cracks from the measured potential data. For this purpose, a low alloyed, quenched and tempered C45E steel has been investigated.

Nomenclature a

crack depth

t

specimen thickness crack width specimen width

c

w A y 0

crack area

half distance between the welding points of the potential probes slope of the fitting curve (Tiedemann equation) degree of curvature of the fitting curve (Tiedemann equation)

q

r

a notch c notch Δσ 0 ΔK th

depth of the notch width of the notch

fatigue limit

threshold against fatigue crack propagation

P i U i

relative potential actual potential

U i,0

mean potential of the first cycles without a crack