Rißfortschritt an Bauteilen bei Drucküberlasten

FKM 1995 Project No. 159
Abstract:

Many components in mechanical and plant engineering are subject to stress sequences that are characterized by a succession of small high-frequency amplitudes and relatively few large amplitudes. The few large amplitudes can be regarded as overloads. In addition, incorrect operation, misuse or even malfunctions can cause overloads that are superimposed on the normal operating stresses. For overloads that cause high tensile stress, it is known that these have a positive influence on the crack propagation life. For compressive overloads, on the other hand, a reduction in service life, i.e. an acceleration in crack propagation, is expected. Precise knowledge of the influence of compressive overloads is of decisive importance for a meaningful estimation of the service life reserves of cracked components exposed to compressive overloads. The subject of this work is the experimental and theoretical/numerical determination of the effects of compressive overloads on the crack propagation life, which are essentially due to sequence effects described by a changed crack opening process. For this purpose, experimental investigations were carried out on standard fracture mechanics specimens and on specimens similar to components using the heat-treatable steel 42 CrMoV 4 7, which is representative of the steels used in mechanical engineering. Crack opening load measurements and numerical FE calculations provide information on the influence of compressive overloads on crack opening behavior. A Newman model based on the Ougdale model and linear crack propagation models were used for computational estimation. The main results of the tests and calculations are -Compressive overloads do not lead to an acceleration of cyclic crack propagation. Nominal stresses and local stresses must be seen in the context of the overall system in which the cracked component is integrated. -A reduction in crack propagation life was only observed at high compressive overloads (in the range of 50% of the tensile yield strength); however, no significant acceleration as a result of a sequence effect was observed. Compressive overloads have no influence on the threshold valuel lKth. The numerical simulation of the crack opening stresses confirms the test results and shows the influence of the constraint on the crack oiling behavior and the crack propagation velocity. The comparison of calculations using different models with the test lifetimes for the component-like specimens shows the good applicability of linear prediction models, e.g. according to Paris on the basis of effective stress intensity factors llKeff, for an estimation of the influence of compressive overloads on crack propagation lifetime. A calculation concept for the evaluation of compressive overloads in the estimation of the service life of cracked components is presented.

Scope of report:
136 pages incl. images and tables, 62 references

Start of work:
0106.1991

End of work:
3106.1993

Funding body:
BMWi I AIF-Nr 8636

Research institutes:
Institute for Mechanical Plant Engineering and Structural Durability (lfB) Clauslhal University of Technology (TUC), in charge

Management:
Prof. Dr.- lng. H. Zenner Industrieanlagen Betriebsgesellschaft (IABG), Abteilung TAB Head: Dr. W. Bergmann Fraunhofer Institute for Mechanics of Materials (IWM), Freiburg

Head:
Prof. Dr. E. Sommer

Author:
Dipl.-lng. H Buschermöhle (lfB) Dr. D. Memhard (IWM) Dr.-lng.lng. M Vorm wald (IABG)

Chairwoman of the working group:
Dr.-lng. Ch Berger

Chairman of the advisory board:
Prof. Dr.-lng. H. Kipphan Heidelberger Druckmaschinen AG, Heidelberg