Integral method
The basic assumption of critical plane criteria that a load state on only one specific plane is responsible for total damage induced in the point examined does not seem to be wholly legitimate in the case where changes of principal directions during load history is very frequent (as e.g. is the case of a random multiaxial loading). On the other hand, the integral criteria expect an integration of the damage parameter related to individual planes over all of them. This in turn means that an average equivalent load over all planes is the resulting value of integral methods. The criteria are thus much closer to IDS methods that also process complete load effect in the point than to critical plane criteria that are related to some plane found in this point.
A relatively significant drawback of integral methods is their time consuming computation. In contrast to critical plane criteria, where some optimization routines were programmed, the integration process expects relatively small step in rotation of examined planes so that the final damage was computed acceptably accurately. This seems to be the main reason, why the integral criteria are not implemented in any commercial fatigue solver today and also why the methods of this type are designed only for use in the solution setting the exceedance of the local fatigue limit (where the assumption of an constant amplitude loading is hidden). The discrete integration in PragTic utilizes a concept of globe analogy.
The integral criteria programmed in PragTic are these:
More:
criteria using Ilyushin deviatoric space (IDS)
© PragTic, 2007
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