The Damage Operator Approach: Fatigue, Creep and Viscoplasticity Modeling in Thermo-Mechanical Fatigue 2011-01-0485
In the last decades the development time of vehicles has been drastically reduced due to the application of advanced numerical and experimental methods. Specifications concerning durability and other functional attributes for every new model improve for every vehicle.
In particular, for machines and components under variable multiaxial loading, fatigue evaluation is one of the most important steps in the design process. Appropriate material testing and simulation is the key to efficient life prediction.
However, the life of automotive components, power plants and other high-temperature facilities depends mostly on thermo-mechanical fatigue (TMF). This is due to the normally variable service conditions, which contain the phases of startup, full load, partial load and shut-down.
Within this context, there are several requirements for making accurate life predictions: a proper material model with material parameters derived from testing, modeling of stress-strain response during cyclic loading, a multiaxial fatigue criterion, a proper damage accumulation model and component testing to evaluate the correlation between life prediction models and experiments.
For high temperatures the influence of creep, creep fatigue and viscoplastic stress relaxation have to be considered both in an accurate way and efficiently.
The present paper presents the damage operator approach enabling online continuous damage calculation for isothermal and non-isothermal loading with mean stress correction. The cycle closure point, cycle equivalent temperature, threshold temperature and separate rainflow counting (mandatory for the equivalent temperature approach) are not necessary any more for the damage operator approach. In fact, for constant temperature, both approaches are equivalent for the second run of block loading. It also highlights an extension of the strain-life approach to take into account viscoplastic effects and creep.