Ingenieurwissenschaften und zugeordnete Tätigkeiten
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- Fatigue strength (2)
- Fracture mechanics (2)
- Multiple crack propagation (2)
- Bruchmechanik (1)
- Crack driving force (1)
- IBESS-Projekt (1)
- Initial crack size (1)
- Notches (1)
- Schweißverbindungen (1)
- Schwingfestigkeit (1)
Eingeladener Vortrag
- nein (4)
The work aims at addressing the modelling and implementation of criteria for multiple crack propagation, including interaction and coalescence, for a more reliable fracture mechanics-based prediction of stress-life curves for weldments.
A large experimental work is presented in which micro-cracks have been made visible by heat-tinting at successive stages of fatigue life of the welded specimens. Here the correlation between the number of initiation sites and the applied stress level has been also investigated.
The criteria have been implemented in in-house software, which allows multiple fatigue crack propagation, and validated against selected experimental tests. The results have shown that the modelling of multiple crack propagation and interaction is crucial for the prediction of the fatigue strength of weldments, both in finite and infinite life regime.
Approximation of the crack driving force for cracks at notches under static and cyclic loading
(2017)
The work deals with the efficient calculation of the elastic-plastic crack driving force (J-integral for monotonic loading andΔJ-integral under cyclic loading) for short cracks at notches as essential parameter for the reliable static and fatigue assessment of notched structures. The J- or ΔJ-integral is calculated based on analytical solutions for stress intensity factors, estimated by means of well-known weight function solutions in the case of cracks under power-law stress distributions. A plasticity-correction function is applied to the stress intensity factors to obtain the final expression of the crack driving force. The comparison between analytical solutions and finite element calculations in case of cracks at the weld toe in welded joints shows good agreement.
If fracture mechanics shall be applied to the total lifetime respectively the fatigue limit of components (within the meaning of the S-N curve approach) it has to address four challenges:
(a) It has to adequately describe so-called short crack propagation, which cannot be based on the common long crack concepts for principle reasons. Since the crack size is in the order of the plastic zone size, the modelling of short crack propagation cannot be based on the common linear elastic Delta K concept. Instead, an elastic-plastic parameter such as the cyclic J integral has to be applied. A second point is that the crack closure concept has to be modified in that the crack opening stress is not a constant, crack size- independent parameter but shows a transient behaviour with increasing short crack size.
(b) It has to provide a meaningful definition of the initial crack dimensions as the starting point for an S-N curve relevant (residual) lifetime analysis. This can be based either on the (statistical) size of material defects which can be treated as cracks or by the size of the crack which would arrest subsequent to early crack propagation, whatever is larger.
(c) It has to cope with the problem of multiple cracks for load levels higher than the fatigue limit such as it occurs in many applications in the absence of very large initial defects.
(d) This requires consequent statistical treatment taking into account variations in the local geometry of the area where crack initiation has to be expected as well as the scatter in the initial crack size and in the material data used for the analyses.
Ermittlung der Schwingfestigkeit von technischen Legierungen mittels bruchmechanischer Methoden
(2017)
Die Arbeit beschreibt die Arbeitsphasen zur Entwicklung einer Prozedur zur bruchmechanischen Ermittlung der Schwingfestigkeit von technischen Legierungen.
In dieser Hinsicht sind die experimentelle Ermittlung von Werkstoffkennwerten, die analytische Modellierung des Ermüdungsrisswachstums und eine ausreichende Validierung von Bedeutung.
Um die Schwingfestigkeit von Komponenten adäquat ermitteln zu können, müssen die folgenden Aspekte in der Modellierung betrachtet werden: Die statistische Eingabe von Modellparametern, die Berücksichtigung von Kurzrisswachstum und ggf. die Mehrfachrisswachstum, bzw. die Rissinteraktion.