PSI - Issue 47

Jan Patrick Sippel et al. / Procedia Structural Integrity 47 (2023) 608–616

609

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Author name / Structural Integrity Procedia 00 (2019) 000 – 000

© 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 IGF27 chairpersons Keywords: Ultrasonic testing, natural frequency, defect size, crack propagation, high-strength steels. 1. Introduction Many parts and components such as automotive engines, turbines in the energy sector or rotors of helicopters as well as axles of high-speed trains are subjected to cyclic loadings with cycles exceeding the traditional fatigue limit of 10 7 cycles by far as described in Christ (2018), Shanyavskiy (2013) or Bathias (2001). This failure at cycles above the traditional fatigue limit is called very high cycle fatigue (VHCF) and is constantly gaining attention due to the lightweight construction efforts, coupled with increasing speeds and the demand for higher lifetimes. Most of the VHCF related research is focused on the crack initiation stage with the aim to describe and explain the characteristic mechanism of this lifetime regime. However, the underlying mechanism of fine granular area (FGA) formation during this stage is still not fully understood as discussed in the review by Sippel and Kerscher (2020). To further analyze the underlying mechanisms as well as achieve the data base for safe design, several tests must be performed due to the relatively large scatter observed in this regime. To reach the VHCF regime in the laboratory in an economical way, ultrasonically based test benches are common and have established themselves as the standard. Many different effects during the ultrasonic testing, like the size effect described for example in Furuya (2010) as well as the frequency effect described for example in Mayer et al. (2001) can be found in literature. However, since more than 90% of the lifetime is spent for the specific FGA formation during the early crack initiation as described by Murakami et al. (2000), Bayraktar et al. (2006) or Hong et al. (2014), the late crack propagation outside the fisheye is often neglected. In contrast to conventional testing using servo-hydraulic test setups, force control cannot be realized while testing with ultrasonic test setups. The loading of the specimen is realized by means of excitation with natural frequency of the oscillating system resulting in a standing longitudinal wave with the maximum stress amplitude forming in the center of the specimen. Therefore, due to the ultrasonic testing, however, special features can arise during late crack propagation with respect to natural frequency and the resulting stress amplitude, which can be reflected as features in the fracture surface. The objective of this paper is to describe the different fracture surfaces observed after the ultrasonic based testing of two high-strength steels, namely AISI 52100 and AISI 4140. Furthermore, these differences are quantitatively compared by means of roughness measurements using confocal microscopy. Subsequently, an approach for the differences in fracture surfaces is established by evaluating the fracture surface morphologies using a scanning electron microscope (SEM). Additionally, first comparative investigations regarding generator power, frequency, and amplitude characteristics during the ultrasonic based testing of both materials is given. © 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 IGF27 chairpersons

Nomenclature area FiE

area of the fisheye fine granular area

[mm²]

FGA

FiE

fisheye

K C,AISI 52100 K C,AISI 4140

fracture toughness of AISI 52100 fracture toughness of AISI 4140

[MPa m [MPa m [MPa m

1/2 ] 1/2 ] 1/2 ]

K max,FiE

maximum stress intensity factor at the border of the fisheye

N f

number of cycles to failure

R z,c R z,l SE

average surface roughness measured circular around the crack initiation site average surface roughness measured longituginal along the crack path

[µm] [µm]

secondary electron

SEM

scanning electron microscope

SIF

stress intensity factor

[MPa m 1/2 ]

∆ T max VHCF

maximum allowed temperature increase of the specimen during fatigue

[K]

very high cycle fatigue