Filtern
Dokumenttyp
- Zeitschriftenartikel (2)
- Beitrag zu einem Tagungsband (2)
- Vortrag (1)
Sprache
- Englisch (5) (entfernen)
Schlagworte
- Crack propagation (5) (entfernen)
Organisationseinheit der BAM
Eingeladener Vortrag
- nein (1)
An analytical fracture mechanics model for predicting the finite life fatigue strength of components is presented which Combines a number of well established and newly developed approaches such as Murakami’s and McEvily's approach for describing the transient behaviour of crack closure of short cracks, the analytical (long) crack closure function of Newman, the R6 procedure modified by a method for improving the ligament yielding correction proposed by the authors and other elements. Basic assumption is the preexistence of initial flaws such that the crack initiation or nucleation stage is small and can be neglected. The application of the model is demonstrated for small tension plates of aluminium Al 5380 H321 with artificial initial defects generated by FIB technology, the size of which was fixed on the basis of fractographic investigations on broken, smooth specimens.
Recently two of the authors of the present paper proposed a model for a fracture mechanics
based prediction of the S–N characteristics of metallic components with large microstructural
defects and supported this by a validation exercise on tensile plates made of an aluminium
alloy AL5380 H321. Here the authors extend the study using a number of further
data sets from the literature for which data were available at different R ratios. These data
include two aluminium alloys, Al 2024-T3 and Al 7075-T6, and a ductile cast iron, EN-GJS-
400-18-LT. Despite of necessary assumptions for the compensation of partially missing
input information the results were fairly reasonable with the exception of one data set.
The authors identify high applied stress levels in combination with potential multiple crack
initiation as the probable root of the problem and propose a scheme how the model can be
extended for taking into account crack initiation.
The determination of the fatigue life in technical alloys containing large and small defects must rely on a propagation model which accounts for short and long crack growth. Recently an analytical model which incorporates propagation in the short crack regime and plastic correction for the crack driving force has been presented by two of the authors.
This work is intended to show further validation of the model, taking into account data sets for different materials with different testing conditions.
Despite the assumptions about missing parameters, the value of which had to be taken from the literature, the predictions showed a fairly good approximation of the fatigue lives. A possible interpretation of the results in terms of multiple crack initiation and propagation at higher loads is proposed.
The Kitagawa-Takahashi (KT) diagram is a well-established tool in the fatigue community for the prediction of the fatigue limit of metallic materials in presence of defects. Nevertheless, its determination is associated with a large number of fatigue tests carried out on specimens with artificial defects (notches) to describe the fatigue limit-defect size relationship in the short crack regime. The preparation of the specimens and the execution of the tests is time expensive. Therefore, few phenomenological models have been introduced in the past to provide a first approximation of the KT diagram based solely on the fatigue limit for smooth specimen and the fatigue crack propagation threshold for long cracks. Despite the use of such models is widely spread, these suffer from problems related to the uncertainties of the material parameters. Furthermore, the mechanics of short cracks, which is the physical mechanism behind the concept of the fatigue limit, is not considered. This work discusses the advantages and drawbacks of using short fatigue crack propagation models for predicting the fatigue limit of flawed metallic materials. The KT diagrams for a EA4T railway axle steel in as-received and full-hardened condition are approximated by means of the cyclic R-curve method.