Ausgewählte Einflüsse auf das Rissfortschrittsverhalten am Beispiel zweier AlSi-Gusslegierungen

Translated title of the contribution: Influence study on the crack growth behaviour of two cast AlSi-alloys

Research output: ThesisDiploma Thesis

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Abstract

Cast aluminium parts are often found in vehicle engine components such as cylinder head or gearbox housing. With increasing demands in lightweight design, reduced component weight is needed which leads to higher local stresses. Hence, technical crack initiation and subsequent component failure due to cyclic crack growth occurs in cyclic loaded parts. Aluminium castings possess heterogenous microstructure and defects such as gas pores due to the manufacturing process. These imperfections act as preferred nucleii for crack initiation and subsequent crack propagation until complete burst failure. The obtained microstructure determines both the static strength but also crack initiation and crack propagation resistance. Within this work, the fracture mechanical threshhold as well as crack propagation is examined for two aluminium cast materials AlSi7Mg and AlSi10Mg. The following influences are studied: 1) Testing parameters: the effect of increasing or decreasing stress intensity over number of load cycles on the threshhold is evaluated using polished specimens. They possess an initial crack which is introduced by compressive fatigue testing at low stress intensities. This minimizes the size of the plastic zone during initial crack growth. The stress intensity threshhold increases if testing with decreasing stress crack tip intensity is used for this ductile aluminium material. Hence, it is recommended to use only fracture mechanical threshhold tests with increasing stress intensity. 2) Effect of grain size: tests on specimen made of AlSi7Mg showed that an increase in grain size also heightens the threshhold. Crack closure effects are the main reason for this behaviour due to the enlarged fracture surface roughness. 3) Influence of stress ratio: tests on specimen made of AlSi7Mg showed that a decrease in stress ratio also heightens the threshhold. the reason for this are reduction of the effective stress intensity and crack closure effects. 4) Influence of microstructure represented by dendritic arm spacing DAS and effect of chemical composition: Investigations on the aluminium casting AlSi7Mg showed only minor changes in the DAS-parameter. The threshhold value tends to increase with the dendritic arm spacing. Beside that, the microstructure changes in dependency of the chemical composition. The investigated aluminium alloys exhibited an eutectoid fraction of fifty up to eight-five percent. Tests revealed that the grain size is the uppermost dominant factor on stress intensity threshhold. Local changes in eutectoid microstructure are neglegible. Finally, analytical investigations on the measured crack propagation curves are applied using different single- and multiparametric equations to parametrize the threshhold and long-crack growth region. A modell which correlates the threshhold value to the characteristic parameters such as grain size and stress ratio is given.
Translated title of the contributionInfluence study on the crack growth behaviour of two cast AlSi-alloys
Original languageGerman
QualificationDipl.-Ing.
Supervisors/Advisors
  • Redik, Sabine, Co-Supervisor (internal)
  • Grün, Florian, Supervisor (internal)
Award date19 Dec 2014
Publication statusPublished - 2014

Bibliographical note

embargoed until null

Keywords

  • crack propagation
  • threshhold stress intensity
  • stress intensity
  • AlSi-cast alloys

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