Bruchmechanik und schadenstolerante Konstruktion im Automobilbau

Translated title of the contribution: Fracture Mechanics and Damage Tolerant Design in Automotive Engineering

Andreas Leitgeb

Research output: ThesisDoctoral Thesis

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Abstract

Goals of this study Over the last years, lightweight design has become highly important in automotive engineer-ing, which has led to a continuously increasing use of aluminium alloys. For cylinder heads of internal combustion engines, where elevated temperatures of locally above 200 °C can occur, also the accelerated ageing behaviour of precipitation hardened aluminium alloys has to be considered. Therefore lifetime estimation has to include, aside from other influences, flaws, like pores, cracks or oxide inclusions, which can occur during the die casting process, as well as the ageing condition. As service intervals generally cannot be enforced in the automotive industry, the safe-life concept, based on S/N curves, which are measured with standardized smooth, defect-free specimens is used for lifetime estimation. To describe the influence of flaws in the component, these can be considered either during the stress calculation or by adjusting the S/N curve by equivalence coefficients. Consideration of flaws in the lifetime estimation To enable S/N curves to account for cracks or flaws in the component, a combination of frac-ture mechanics and stress-based approaches is needed. In lifetime estimation concepts based on fracture mechanics, a crack is assumed in the material, whose size has to be estimated from the detection limit in non-destructive testing. With few additional data from tensile and crack growth tests it is possible to define the behaviour of the S/N curve for different influences such as flaw size, temperature and duration of ageing, or stress ratio. To combine damage mechanics and fracture mechanics, a correlation between the fatigue limit and the fatigue crack growth threshold stress intensity factor has to be found that is able to account for different ageing conditions. Based on the El Haddad equation, an engineering estimate is proposed which allows to calculate the fatigue limit of a die cast aluminium alloy depending on flaw size, temperature, and duration of ageing. By utilising the parallels between the two approaches it is also possible to minimize the testing effort for assessing, e.g., the mean stress sensitivity or other fatigue parameters. To define the length and direction of crack extension in a component the concept of stress intensity factors is widely used in linear elastic fracture mechanics. For simple structures like beams, discs or the spacing between rivet holes analytical approaches for the geometry factor of the singularity can be found. If the components become more complex, these approaches based on simple geometries cannot be used any longer. Under these circumstances the Finite Element Method provides a convenient solution for stress intensity factor calculations. For this purpose various element types are provided, everyone with its advantages and disadvan-tages, which can have a large influence on the simulation result. Some of the problems and limitations of the implementation of these calculation methods in ABAQUS 6.9-1 are dis-cussed and some easy engineering workarounds are presented. Special focus is laid on crack modelling during pre-processing, mesh dependence, and assessment of closure effects by ac-counting for the contact behaviour of the crack flanks. The combination of damage mechanics and fracture mechanics material models has high po-tential for realistic lifetime estimation by considering important parameters, e.g., ageing and inhomogeneities. If the Extended Finite Element Method is consequently developed further, it shows promising possibilities to describe flaws during the simulation process.
Translated title of the contributionFracture Mechanics and Damage Tolerant Design in Automotive Engineering
Original languageGerman
QualificationDr.mont.
Supervisors/Advisors
  • Eichlseder, Wilfried, Assessor A (internal)
  • Pippan, Reinhard, Co-Supervisor (internal)
Publication statusPublished - 2010

Bibliographical note

no embargo

Keywords

  • Lifetime estimation
  • fatigue
  • fracture mechanics
  • finite element simulation

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