Spannungsmehrachsigkeit

Fracture mechanical strength verification for machine components taking into account the stress multiaxiality
FKM 2017
Issue number 328
Project no. 304
181 pages

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Final report

Abstract:
The influence of the stress multiaxiality (constraint) on
the fracture behavior and the cyclic crack propagation in metallic materials was investigated as part of the project.
In detail, the following aspects of fracture mechanics component evaluation
were considered under consideration of the constraint:

- static verification according to the failure assessment diagram (FAD) concept;
- cyclic crack propagation behavior in samples of different geometry and with small initial defects;
- static verification in the presence of high thermal stresses.

Two
analytical approaches were compared and validated to quantify the influence of the constraint within the framework of the FAD concept. For this purpose, a large number of static
fracture mechanics tests from the literature were evaluated based on the two analytical methods
. It was shown that the consideration of the constraint according to the
R6/FITNET procedure enables a reliable and less conservative component evaluation
in principle. However, limitations with regard to the determination of relevant
material parameters for the estimation of the constraint correction function were shown,
which should be taken into account in assessment practice.
In experimental investigations of cyclic crack growth, the dependence
of the crack propagation rates on the standard specimen geometry used was confirmed. This
resulted in a tendency towards lower crack propagation rates with increasing constraint level,
which effect can be explained by lower plasticization at the crack tip
. This assumption was confirmed by tests on a high-strength quenched and tempered steel,
where a significantly lower constraint influence was observed. In further experimental
investigations on samples with small surface cracks, which mainly took place in the
Paris area and at high nominal stresses, no significant difference
to the long crack behavior was found.
A significantly improved fracture mechanics evaluation for components with high thermal
stresses was achieved with a so-called Vg method. This and two alternative
methods from the current regulations were compared and validated using numerical simulations under
variation of several relevant influencing variables.
The objective of the research project was achieved.