Damage tolerance and strength increase of drivetrain components

Jürgen Maierhofer

Research output: ThesisDoctoral Thesis

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Abstract

For the damage tolerant design of components, the external load and the component’s geometry as well as residual stresses and defects such as cracks or non-metallic inclusions are of importance. The load carrying capacity is given for long-term fatigue endurance by the crack growth threshold, for finite lifetime by the fatigue crack growth rate; both depend on the load ratio, but also on the size of the defect. In this work the fatigue crack growth behaviour of the quenched and tempered steel 25CrMo4 is investigated in detail by means of single edge notched bending (SENB) specimens at various load ratios. The influence of flaw size is studied by different notch depths. At the notch root, short fatigue cracks are introduced by cyclic compression. The growth behaviour of these cracks in the notch stress field is monitored in detail, whereby also information about the build-up of crack closure and the transition from short to long crack behaviour is gained. In order to investigate the influence of compressive residual stresses – as introduced by various mechanical surface treatment processes such as shot peening or deep rolling – straight beams with convex fillets are subjected to controlled flat rolling, thereby introducing residual stresses varying along the ligament of the specimen. In these specimens, again fatigue cracks are introduced and monitored, showing the combined influence of residual stresses and crack length on the fatigue crack growth behaviour. In all cases, special attention is paid to the evolution of the fatigue crack growth threshold as a function of crack length, stress ratio and residual stresses. Based on the evolution of the fatigue crack growth threshold as a function of crack length (crack resistance curve), an analytical model for describing the fatigue crack growth rate, as well as an extended Kitagawa-Takahashi diagram are developed. The model is based on the NASGRO equation, which is modified to describe the build-up of crack closure with increasing crack length and thereby the short crack behaviour. With this extended NASGRO model it is possible, due to a combined view of load stresses and residual stresses, to describe the crack growth also in the presence of residual stresses. In summary, the results of this work provide a more accurate way to estimate the lifetime or service intervals of cyclically loaded components in the presence of flaws and residual stress fields.
Translated title of the contributionSchadenstoleranz und Festigkeitssteigerung von Antriebskomponenten
Original languageEnglish
QualificationDr.mont.
Supervisors/Advisors
  • Pippan, Reinhard, Assessor A (internal)
  • Gänser, Hans-Peter, Assessor B (internal)
Publication statusPublished - 2014

Bibliographical note

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Keywords

  • damage tolerance
  • deep rolling
  • crack growth model
  • Kitagawa-Takahashi diagram
  • residual stresses
  • short cracks
  • crack closure

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