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Computation of and testing crack growth at 20 kHz load frequency
Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013). (Materials Engineering)ORCID-id: 0000-0001-6029-2613
Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013).
2016 (Engelska)Ingår i: 21ST EUROPEAN CONFERENCE ON FRACTURE, (ECF21) / [ed] F. Iacoviello, L. Susmel, D. Firrao, G. Ferro,, Elsevier, 2016, s. 1164-1172Konferensbidrag, Publicerat paper (Refereegranskat)
Abstract [en]

Fatigue properties are evaluated in a large span of fatigue lives ranging from a few load cycles to more than 1013 load cycles. If the interest is focused on fatigue lives above 10(7) load cycles, we speak of the very high cycle fatigue (VHCF) range. For evaluation of properties in the VHCF range one often needs to use higher load frequencies to be able to perform testing within a reasonable time. Therefore, the influence of load frequency on fatigue strength and fatigue crack growth is an important issue, both from testing and design perspectives. Within an EU-RFCS research project on the frequency influence on high strength steel fatigue properties the present study has been conducted on fatigue crack growth testing to determine threshold values and crack growth material parameters. The testing was analyzed by FE-computation to determine geometry factors for AK-determination. The testing was performed in a 20 kHz ultrasound resonance instrument. In such a system the whole load train needs to be designed to run at a resonance frequency of 20 kHz, and it implies that the specimen needs to be designed and computations performed by dynamic computational methods. As the crack grows the dynamic response of the specimen will change, and hence calculation to obtain the geometry factor is made with a progressing crack length. A uniaxial tensile load at 20 kHz frequency is applied to a single edged notched side-grooved flat specimen. The specimen dimensions are calculated in order to have a resonance frequency of 20 kHz, which is the frequency used for the experiments. Dynamic FEM computation, with a 3D-model and a quarter symmetry was used with one of the symmetry planes parallel to and in the crack growth line. To avoid crack surface interpenetration during the simulations a rigid thin sheet was introduced and used as a counter-face to the crack surface. The solution obtained was then combined with the breathing crack model proposed by Chati et. al. (1997) in order to solve for the irregularities observed when crack surface interpenetration occurs. Finally, the whole load train was considered. Thus, also the computed frequencies were very close to frequencies observed in experiments. The computation of stress intensities was made for varying crack lengths in a series of simulations. The geometry factor relation was determined and used in 20 kHz crack growth testing to control the actual stress intensity at the advancing crack tip. Comparison of computations and experimental results were made.

Ort, förlag, år, upplaga, sidor
Elsevier, 2016. s. 1164-1172
Serie
Procedia Structural Integrity, ISSN 2452-3216 ; 2
Nyckelord [en]
Crack growth, High frequency, Dynamic analysis, steel, threshold testing
Nationell ämneskategori
Materialteknik
Forskningsämne
Materialvetenskap
Identifikatorer
URN: urn:nbn:se:kau:diva-62609DOI: 10.1016/j.prostr.2016.06.149ISI: 000387976801029OAI: oai:DiVA.org:kau-62609DiVA, id: diva2:1130577
Konferens
21st European Conference on Fracture (ECF), JUN 20-24, 2016, Catania, ITALY
Tillgänglig från: 2017-08-10 Skapad: 2017-08-10 Senast uppdaterad: 2019-12-19Bibliografiskt granskad
Ingår i avhandling
1. Very high cycle fatigue of automotive steels: Testing and computation at 20 kHz
Öppna denna publikation i ny flik eller fönster >>Very high cycle fatigue of automotive steels: Testing and computation at 20 kHz
2020 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Karlstad: Karlstads universitet, 2020. s. 41
Serie
Karlstad University Studies, ISSN 1403-8099 ; 2020:7
Nyckelord
Very high cycle fatigue, ultrasonic fatigue testing, automotive steels, fatigue strength, stress intensity computation, crack growth
Nationell ämneskategori
Maskinteknik
Forskningsämne
Materialteknik
Identifikatorer
urn:nbn:se:kau:diva-76028 (URN)978-91-7867-090-1 (ISBN)978-91-7867-100-7 (ISBN)
Disputation
2020-02-14, 21A342, 10:15 (Engelska)
Opponent
Handledare
Projekt
FREQTIGUE
Tillgänglig från: 2020-01-27 Skapad: 2019-12-19 Senast uppdaterad: 2020-01-27Bibliografiskt granskad

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