Filtern
Erscheinungsjahr
Dokumenttyp
- Zeitschriftenartikel (54)
- Beitrag zu einem Tagungsband (15)
- Vortrag (8)
- Monografie (2)
- Beitrag zu einem Sammelband (2)
- Buchkapitel (1)
Sprache
- Englisch (82) (entfernen)
Schlagworte
- Fracture mechanics (25)
- Fatigue strength (18)
- Weldments (10)
- Fatigue crack propagation (8)
- Fatigue crack propagation threshold (7)
- Short crack propagation (7)
- Short cracks (7)
- Initial crack size (6)
- Multiple crack propagation (6)
- Crack propagation (5)
Organisationseinheit der BAM
- 9 Komponentensicherheit (40)
- 9.4 Integrität von Schweißverbindungen (39)
- 5 Werkstofftechnik (6)
- 5.1 Mikrostruktur Design und Degradation (5)
- 9.0 Abteilungsleitung und andere (5)
- 9.6 Additive Fertigung metallischer Komponenten (4)
- 8 Zerstörungsfreie Prüfung (3)
- 8.5 Röntgenbildgebung (3)
- 4 Material und Umwelt (2)
- 4.0 Abteilungsleitung und andere (1)
- 4.2 Material-Mikrobiom Wechselwirkungen (1)
- 5.2 Metallische Hochtemperaturwerkstoffe (1)
- 5.4 Multimateriale Fertigungsprozesse (1)
- 5.5 Materialmodellierung (1)
- 8.0 Abteilungsleitung und andere (1)
- 9.3 Schweißtechnische Fertigungsverfahren (1)
- S Qualitätsinfrastruktur (1)
- S.0 Abteilungsleitung und andere (1)
Paper des Monats
- ja (1)
Eingeladener Vortrag
- nein (8)
The paper provides an overview on the results of a German cluster project on the use of fracture mechanics to the determination of the fatigue strength of weldments with fatigue cracks originating at the weld toes. The approach includes (a) a concept for short crack propagation for which the common ΔK concept is not applicable and the crack closure effects are still being gradually build-up, (b) a method for determining fatigue life relevant initial crack sizes as they are needed in any fracture mechanics analysis and (c) multiple cracking and crack coalescence at load levels higher than the endurance limit. The analyses are stochastically performed. Both, the endurance limit and the finite life branch of the S-N curve are determined.
Besides a brief introduction into the approach, validation examples are presented. These comprise different weldment types (butt welds, cross joints and longitudinal stiffened plates), two steels (S355NL and S960QL) of quite different strengths, different weld geometries due to different welding techniques (WIG, MAG), as-welded and stress relieved welds and different stress ratios varying from R = -1 to R = 0.5.
The paper provides an overview on material defects which may serve as fatigue crack initiation sites and can cause final fatigue failure of a component. These comprise nonmetallic inclusions in Steel and aluminum alloys, cavities such as pores, micro-shrinkages and un-welded regions in cast, sinter and additively manufactured alloys, graphite nodules, shrinkages and other items in modular cast iron, regions of defective microstructure, microcracks and secondary notches such as undercuts and surface roughness. Besides their origin, the effect and mechanisms on fatigue crack initiation and propagation are discussed.
The considerations are proceeded and accompanied by a Brief discussion of some Basic aspects such as the stages of crack propagation along their length scale, the overcoming of crack arrest and the question when a secondary notch can be treated as a crack.
The application of fracture mechanics to the determination of the fatigue behavior of weldments is discussed with the focus on classic fatigue, i.e., the overall fatigue life and the fatigue strength in terms of an S-N curve and the endurance limit. The following issues are addressed: specific features of short fatigue crack propagation, an adequate initial crack size, multiple crack propagation and its statistical treatment as well as welding residual stresses. As an example, an approach of the authors is applied to the determination of FAT classes for a butt weld with varying weld toe geometry.
The fatigue crack propagation threshold ΔKth is of paramount importance for any kind of damage tolerant design, but in contrast to this importance, the determination and application of the parameter is not on the firm ground as previously assumed. The paper discusses questions of its experimental determination as well as of its application to components. In both fields, new questions have been raised with the potential to challenge or modify long-standing knowledge. Against this background, the paper is an attempt to systematize the established knowledge as well as questions and open issues.
The paper deals with the prediction of the fatigue Limit of notched specimens. Some authors in the literature have shown that this issue can be solved by employing an approach based on critical distances, others used elastic fracture mechanics. In this paper the fatigue limit of notched specimens is given by the non-propagating condition of mechanically small surface cracks. According to the so-called cyclic R-curve method, the crack driving force of a growing small crack is compared to its resistance force, which incorporates the gradual build-up of crack closure. The fatigue limit is determined by that applied nominal stress, for which the tangency condition of crack driving and resistance force is satisfied. The approach has been modified to incorporate plasticity effects in the mechanically short crack Regime and it has been applied to a mild and a high-strength steel, in case of an infinite plate with circular hole under remote tensile stress. The method has been successfully applied to the evaluation of fatigue notch-sensitivity in case of surface roughness.
Although fatigue crack propagation and fracture cause a large part of failure events in industrial practice, fracture mechanics in failure analysis seems to be still a side issue. Starting from an introduction into important basic questions of failure analysis and fracture mechanics, the authors specify what kind of questions in failure analysis can be effectively solved by fracture mechanics (and which can't). They illustrate their discussion with a number of 13 case studies from the literature. Much more pronounced than in the design stage the benefit of fracture mechanics in failure analysis depends on its accuracy. This is limited by both, intrinsic factors of the method and the availability and quality of the input information. The authors discuss the various aspects and provide the reader with some background information which, as they believe, will be helpful for better understanding the prospects and limitations of fracture mechanics in failure analysis and the conditions of its application.
Two of the authors of the present paper proposed a definition of a reference load F0 which can be used as an alternative option to the common limit load for cases where the definition of the latter might be problematic in the frame of flaw assessment procedures such as R6, BS 7910, SINTAP or FITNET. The reference load is defined as that load at which the ligament yielding parameter Lr approaches one. F0, in general, varies along the crack front. It has to be obtained by finite element analyses but the results can be approximated by analytical expressions. In a previous study of the authors such a solution was presented for tension loaded plates with semi-elliptical surface cracks. In the present paper it is extended to pure bending and to combined bending and tension.
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 two-part paper series provides an overview on the state-of-the-art in the application of engineering fracture mechanics to weldments limited to butt and fillet welds with crack initiation at weld toes. In the present second part, one focus is on welding residual stresses, their characteristics and stability under cyclic loading and their effect on structural integrity. Subsequently, features will be addressed which are essential for applying fracture mechanics to overall fatigue life and fatigue strength considerations of weldments. These comprise fatigue life relevant initial crack sizes and multiple crack initiation and Propagation due to various stress peaks along the weld toe. A concept is briefly introduced which covers all these aspects.
The two-part paper provides an overview on the state-of-the-art in the application of engineering fracture mechanics to weldments. This, of course, cannot be exhaustive but is limited to butt and fillet welds with crack initiation at weld toes. In the present first pari, the authors briefly focus on the susceptibility of welds to cracks and other defects. Following this, they discuss in more detail the consequences of material inhomogeneity across the weld for fracture mechanics. Inhomogeneity causes scatter in fracture toughness and strength mis-match effects which both have to be considered in fracture toughness testing, crack driving force determination and fracture assessment of welded components. Part 2 of the paper series will add a discussion of welding residual stresses and questions of applying fracture mechanics to residual as well as total lifetime estimation of welds under cyclic loading.