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Crack growth rates and mechanisms in 20 kHz FCP testing of automotive steels
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).ORCID iD: 0000-0001-6849-2409
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).ORCID iD: 0000-0001-6029-2613
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
2019 (English)In: Proceedings of the 13th International Conference on the Mechanical Behaviour of Materials, 2019, Melbourne: ICM13 conference proceeding , 2019, , p. 10Conference paper, Published paper (Refereed)
Abstract [en]

During the past decades, Very High Cycle Fatigue (VHCF) research has developed into an active and prioritized research area. An increased interest in testing up to 107-1010 load cycles, realized within a reasonably short amount of time, has been enabled by the development of 20-30 kHz ultrasonic fatigue testing equipment. Here, a study is presented on fatigue crack propagation at 20 kHz of three different automotive steels tested at R=-1 and R=-0.24 load ratios. However, the high load rates provokes new challenges, as theoretically finding the best practice method to compute the stress intensity factor considering 20 kHz dynamic effects (inertia forces and damping). Calibrating the fatigue load system, monitoring, controlling and performing precise measurements of the growing crack during the tests are some examples of experimental challenges encountered. Here, a best practice method for computing the stress intensity factor is presented together with a complete 20 kHz fatigue crack growth testing procedure.Three different bar steel grades have been tested; a ferritic-pearlitic, a quenched and tempered martensitic and a carburizing steel grade. Crack propagation test results differentiated between the three steel grades and were depending on load ratio R. The obtained test results at 20 kHz were found to agree with results of the same steel grades tested at conventionally used load frequencies. The SEM fractography analysis revealed ductile transgranular crack propagation mechanisms, also this in agreement with the same steel grades tested at lower frequencies.

Place, publisher, year, edition, pages
Melbourne: ICM13 conference proceeding , 2019. , p. 10
Keywords [en]
VHCF; steels, crack propagation rate; stress intensity factor; stress intensity threshold
National Category
Mechanical Engineering
Research subject
Materials Science
Identifiers
URN: urn:nbn:se:kau:diva-76024ISBN: 978-1-922016-65-2 (print)OAI: oai:DiVA.org:kau-76024DiVA, id: diva2:1380854
Conference
13th International Conference on the Mechanical Behaviour of Materials (ICM13), 11-14 June 2019, Melbourne, Australia
Projects
FREQTIGUEAvailable from: 2019-12-19 Created: 2019-12-19 Last updated: 2020-01-14Bibliographically approved
In thesis
1. Very high cycle fatigue of automotive steels: Testing and computation at 20 kHz
Open this publication in new window or tab >>Very high cycle fatigue of automotive steels: Testing and computation at 20 kHz
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Mechanical fatigue failure occurs in components subjected to cyclic loading. A crack initiates at critical regions in the component and propagates during repeated loading. The expected fatigue life depends on the level, type and frequency of the loading. Generally, as implied by the Wohler’s SN curve, higher applied cyclic load leads to lower fatigue lifes and vice versa. When designing mechanical components carrying cyclic loading, engineers take into account the fatigue limit, i.e. the material specific maximum load allowed for the desired fatigue life. In automotive machinery, components are often required to withstand a very high amount of load cycles before failing. Hence, fatigue data for the very high cycle fatigue (VHCF) regime becomes significant.

In this thesis, an ultrasonic fatigue testing system, with 20 kHz loading frequency, was used to determine the fatigue properties of three high strength low alloyed automotive steels (a ferritic-pearlitic, a martensitic and a carburizing martensitic steel) in the VHCF regime. Theoretical modelling taking into account the influence of the high load frequency was developed and utilized to control the experimental testing. Fatigue strength, crack initiation mechanisms and crack propagation behaviour in the VHCF regime were studied. More specifically, fatigue strength (σN) at 108 cycles, in both uniaxial and bending loading, crack growth rate (da/dN) and threshold behaviour (ΔKth) in the low stress intensity factor regime were determined using specially designed specimens and test rigs.

The fatigue failure of the automotive steels in the VHCF regime, revealed fracture surfaces with fine granular area (FGA) formation close to the initiation point, with a transition to flat transgranular crack growth inside the fish-eye area. The stress intensity thresholds of FGA and fish-eye transitions are described and related to crack tip plastic zone sizes and steel strength.

The effect of damping at 20 kHz, on automotive steels, was thoroughly studied by experimental measurements and theoretical investigations. The effect of introducing damping in dynamic analysis of stress and stress intensity computations for fatigue strength and crack growth testing at 20 kHz was clarified. Best practice for theoretical modelling and computation of stress intensities in crack growth testing at 20 kHz, including the effect of damping, along with guidelines for best practice testing procedure are provided.

Abstract [en]

In automotive machinery, components are often required to withstand a very high amount of loading cycles before failing. Hence, fatigue data for the very high cycle fatigue (VHCF) regime are of importance for the engineers designing such components. In this thesis, the VHCF properties of three different high strength microalloyed automotive steel grades are investigated. Through theoretical computation and modelling, as well as experimental testing, knowledge of fatigue strength as well as crack initiation and propagation behaviour is gained.

The influence of high load frequency on both theoretical computation and experimental testing of fatigue properties was analyzed and conclusions were drawn. An ultrasonic fatigue testing system at 20 kHz load frequency was used to perform uniaxial and bending fatigue, and crack growth testing.

Place, publisher, year, edition, pages
Karlstad: Karlstads universitet, 2020. p. 41
Series
Karlstad University Studies, ISSN 1403-8099 ; 2020:7
Keywords
Very high cycle fatigue, ultrasonic fatigue testing, automotive steels, fatigue strength, stress intensity computation, crack growth
National Category
Mechanical Engineering
Research subject
Materials Engineering
Identifiers
urn:nbn:se:kau:diva-76028 (URN)978-91-7867-090-1 (ISBN)978-91-7867-100-7 (ISBN)
Public defence
2020-02-14, 21A342, 10:15 (English)
Opponent
Supervisors
Projects
FREQTIGUE
Available from: 2020-01-27 Created: 2019-12-19 Last updated: 2020-01-27Bibliographically approved

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Sadek, MohamedBergström, JensHallbäck, NilsBurman, Christer

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