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  • 1.
    Sadek, Mohamed
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013). Karlstad Universtity.
    COMPARISON OF DIFFERENT PROCEEDURES FOR COMPUTING THE STRESS INTENSITYFACTOR RANGE FOR FAT IGUE CRACK GROWTH TE ST ING AT 20 KHZManuscript (preprint) (Other academic)
  • 2.
    Sadek, Mohamed
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
    COMPUTATION AND MEASUREMENT OF MATERIAL DAMPING AT 20 KHZManuscript (preprint) (Other academic)
  • 3.
    Sadek, Mohamed
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
    Microstructure, mean stress and notch influence on fatigue strength and initiation of three bar steel grades in the very high cycle fatigue regimeManuscript (preprint) (Other academic)
  • 4.
    Sadek, Mohamed
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013). Karlstad Universtity.
    Very high cycle fatigue of automotive steels: Testing and computation at 20 kHz2020Doctoral 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.

  • 5.
    Sadek, Mohamed
    et al.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
    Bergström, Jens
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
    Hallbäck, Nils
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
    Burman, Christer
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
    20 kHz 3-point bending fatigue of automotive steels2018In: MATEC Web of Conferences, EDP Sciences, 2018, Vol. 165, p. 1-7, article id 22020Conference paper (Refereed)
    Abstract [en]

    The 20 kHz load frequency enables fatigue tests for very high cycle fatigue life, 109-1013 cycles, within conveniently short time. In automotive applications, many components are subjected to flexural loading and hence bending fatigue is an important test mode. Ultrasound fatigue test instruments have been used successfully in several assessments of fatigue strength and more commonly in uniaxial loading. Here, a 3-point bending fatigue test rig operating in resonance at 20 kHz load frequency has been designed to test plane specimens at R=0.1 loading. The test rig design and stress calculations are presented. Testing for fatigue strength was conducted using the staircase method with 15 specimens of each steel grade, specimens reaching 108 cycles were considered run-outs giving fatigue strength at 108 cycles. Additional 15 specimens of each grade were tested for S-N curves with the upper limit above 109 cycles. Two different common automotive steels, 38MnSiV5, a micro-alloyed ferritic-pearlitic steel, and 16MnCr5, a carburizing martensitic steel, were tested. The fatigue strengths achieved from the staircase testing are 340 and 419 MPa stress amplitudes for the 38MnSiV5 and 16MnCr5 steels, respectively. The S-N curves of the steels appear to be quite flat in the tested life range 107 - 109.

  • 6.
    Sadek, Mohamed
    et al.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
    Bergström, Jens
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
    Hallbäck, Nils
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
    Burman, Christer
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
    Computation of and testing crack growth at 20 kHz load frequency2016In: 21ST EUROPEAN CONFERENCE ON FRACTURE, (ECF21) / [ed] F. Iacoviello, L. Susmel, D. Firrao, G. Ferro,, Elsevier, 2016, p. 1164-1172Conference paper (Refereed)
    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.

  • 7.
    Sadek, Mohamed
    et al.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
    Bergström, Jens
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
    Hallbäck, Nils
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
    Burman, Christer
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
    Crack growth rates and mechanisms in 20 kHz FCP testing of automotive steels2019In: Proceedings of the 13th International Conference on the Mechanical Behaviour of Materials, 2019, Melbourne: ICM13 conference proceeding , 2019, , p. 10Conference 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.

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