Fatigue-Proof and Damage Tolerant Lightweight Design of Ti-6Al-4V Forgings

Bernd Oberwinkler

Research output: ThesisDoctoral Thesis

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Abstract

Conventional models for fatigue-proof design, which are predominantly developed based on ferrous and aluminum materials, are currently utilized for titanium components. However, the analyzed titanium alloy Ti-6Al-4V exhibits many characteristics and anomalies regarding fatigue, which cannot be represented by existing models. The thermomechanical processing, in this case forging and subsequent heat treatments, holds additional parameters with respect to the fatigue strength. The aim of this thesis was the development of appropriate models for fatigue-proof and damage tolerant lightweight design of forged Ti-6Al-4V components under consideration of microstructural variation during thermomechanical processing. For that purpose, specimens were taken from open- and closed-die forgings with different subsequent heat treatments and therefore varying microstructures. Furthermore, the fatigue and crack growth behavior was thoroughly analyzed with respect to microstructure. Based on these results, phenomenological models were developed to link fatigue and fracture behavior with microstructural parameters. Extensive fatigue tests were additionally performed for the development of fatigue models regarding mean stress and notch sensitivity, damage tolerance (influence of preexisting flaws), influence of multiaxial loading and surface state. The influence of operating temperature on the fatigue strength was discussed on existing test results and included in a phenomenological model. All developed models for the lifetime estimation of Ti-6Al-4V forgings were implemented by Böhler Schmiedetechnik GmbH & Co KG in Fortran in a postprocessor. Results of the forging simulation and finite element stress analyses are thereby used as input. Local variations of microstructure, owing to different local cooling rates or the like, can hence be considered. Postprocessing results are the distributions of damage and maximum allowable crack lengths in a component for a given load spectrum. The developed models for the lifetime estimation of Ti-6Al-4V components contribute in many respects to the approach of lightweight design. On the one hand, they enable an optimized utilization of the material capability of Ti-6Al-4V owing to improved dimensioning of a component (inter alia in combination with topology and shape optimization); on the other hand, they are the basis for a simulation-based optimization of the whole forging process. The result is an optimized component performance under service conditions.
Translated title of the contributionBetriebsfeste und schadenstolerante Auslegung von geschmiedeten Bauteilen aus Ti-6Al-4V
Original languageEnglish
QualificationDr.mont.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Christ, Hans-Jürgen, Assessor B (external), External person
  • Eichlseder, Wilfried, Assessor A (internal)
Publication statusPublished - 2010

Bibliographical note

embargoed until 12-04-2015

Keywords

  • Titanium alloy Ti-6Al-4V Fatigue Fracture Damage tolerance Forging

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