Filtern
Erscheinungsjahr
Dokumenttyp
- Zeitschriftenartikel (57)
- Vortrag (40)
- Beitrag zu einem Tagungsband (32)
- Monografie (3)
- Buchkapitel (2)
- Beitrag zu einem Sammelband (2)
- Forschungsbericht (2)
Schlagworte
- Fracture mechanics (25)
- Fatigue strength (19)
- Bruchmechanik (16)
- Schwingfestigkeit (14)
- Weldments (10)
- Fatigue crack propagation (9)
- Fatigue crack propagation threshold (7)
- Short crack propagation (7)
- Short cracks (7)
- FITNET (6)
Organisationseinheit der BAM
- 9 Komponentensicherheit (52)
- 9.4 Integrität von Schweißverbindungen (49)
- 9.0 Abteilungsleitung und andere (7)
- 5 Werkstofftechnik (6)
- 5.1 Mikrostruktur Design und Degradation (5)
- 9.6 Additive Fertigung metallischer Komponenten (5)
- 8 Zerstörungsfreie Prüfung (3)
- 8.5 Röntgenbildgebung (3)
- 4 Material und Umwelt (2)
- 9.3 Schweißtechnische Fertigungsverfahren (2)
Paper des Monats
- ja (1)
Eingeladener Vortrag
- nein (40)
In the approval procedure of transport packages for radioactive materials, the mechanical and thermal safety assessment is carried out in Germany by competent authority BAM. In recent years BAM was involved in several licensing procedures of new spent fuel and HLW package designs, where the cask body was made of Ductile Cast Iron (DCI). According to IAEA regulations package designs have to fulfill requirements for specific conditions of transport. Type B(U) packages must withstand the defined accident conditions of transport. The temperature range from -40°C up to the operational temperature has to be considered. For the cask material DCI, it is necessary to determine the brittle fracture behavior. The German guideline BAM-GGR 007 defines requirements for the fracture mechanics of DCI. Due to complex structure of the cask body and the dynamic loading a fracture mechanical assessment in an analytical kind is not always possible. Experience of recent design approval procedures show that the application of numerical calculations are applicable to determine the fracture mechanical load in the cask body. At the first step a numerical analysis has to be done to identify the loading state at the whole cask body. Secondly an analysis of a detail of the cask body is made considering the displacement boundary conditions of the global model. An artificial flaw is considered in this detailed model to calculate the fracture mechanical loading state. The finite element mesh was strongly refined in the area of flaw. The size of the artificial flaw is characterized by the ultrasonic inspection used for the quality assurance of the package. The applicant developed additional analysis tools for calculation of stress intensity factor and/or J-Integral. The assessment approach by BAM led to the decision to develop own tools to the possibility for independent check of the results.
The paper describes the authority assessment approach for the DCI fracture mechanics analysis. The validation procedure incl. the development of own tools is explained. BAM developed a post-processor called JINFEM to determine the fracture mechanical loads. The regulatory 1 m puncture bar drop test is used to give an example of the assessment procedure.
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 which became part of the widely used
NASGRO approach, the R6 procedure, a method for improving the ligament yielding correction f(Lr ) of
R6 proposed by the authors of the present paper 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.
The paper gives an overview on safe life and damage tolerance methods applied to railway axles. It describes failure scenarios due to fatigue crack initiation and propagation. Besides common aspects of design, specific features such as corrosion and impact damage from flying ballast are discussed which may reduce the fatigue strength of axles during service. Potential effects of non-metallic inclusions from the steel manufacturing process are addressed in the context of the very high number of loading cycles railway axles are designed for. With respect to damage tolerance general lines of fracture mechanics residual lifetime analyses are introduced. More specific discussion is provided on aspects such as the threshold value of fatigue crack propagation and reliability aspects of non-destructive inspection.
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.
On July 9, 2008 a high speed train derailed in Cologne main Station, Germany at a low speed because an axle was broken. Fortunately, the derailment happened at a low speed so that nobody was injured. The reason for the broken axle was investigated and it turned out that most likely large inclusions located shortly undemeath the surface in a T-transition were the origin of the final crack. Basing on that result, a systematic investigation on existing safety assessments of railway axles was performed. This results in an analysis of the production process of axles and in a critical review of existing of existing assessments. Improvements and future developments are outlined.
The net section limit load FY is a key input parameter for the accuracy of any
elastic-plastic flaw assessment procedure of the R6 type. Unfortunately available limit load
solutions are of variable quality since they have been obtained over decades by different
methods. As a consequence the results of the fracture analyses such as the critical load or
crack size are limited in their accuracy and are often significantly conservative. A further
problem is that common limit load solutions based on ligament yielding are inadequate in a
number of cases even for through crack configurations and should be replaced by some
kind of local yielding solutions. In the present paper a simple and straightforward reference
load definition is proposed instead of the limit load which strictly corresponds to a ligament
yielding parameter Lr = 1 in the R6 Routine and similar approaches such as SINTAP and
FITNET. This can be determined by finite element simulation for any geometry. In addition to
a previous study on thin wall notched plates the method is applied to plates containing
shallow semi-elliptical surface cracks. The results demonstrate that the approach provides a
suitable extension and improvement of the existing methods.
The net section limit load FY necessary for assessment at static loading is a key input for the accuracy of any elasticplastic flaw assessment procedure of the R6 type. Unfortunately available limit load solutions are of variable quality since they have been obtained over decades by different methods. As a consequence the results of the fracture analyses such as the critical load or crack size are limited in their accuracy and are often significantly conservative. Further, common limit load solutions based on ligament yielding are inadequate in a number of cases even for through crack configurations and should be replaced by some kind of local yielding solutions. In the present paper a simple and straightforward reference load definition is proposed instead of the limit load which strictly corresponds to a ligament yielding parameter Lr = 1 in the R6 Routine and similar approaches such as the European SINTAP and FITNET methods. This can be determined by finite element simulation for any geometry. In addition to a previous study on thin wall notched plates the method is applied to plates containing shallow semi-elliptical surface cracks. The results demonstrate that the approach provides a suitable extension and improvement of the existing methods.
The safety assessment of railway axles is based on a two-stage approach: fatigue strength design and regular inspections which, in terms of a general safety philosophy refer to safe-life and damage tolerance concepts. Starting with a recent failure case, a broken axle of a German high speed train, a discussion is presented on issues of both safety levels. These include ideas for finite life design, the treatment of in-service effects on the fatigue strength due to flying ballast damage and corrosion pits, the effect of corrosion on fatigue crack initiation and propagation, potential effects of non-metallic inclusions in steels, the way to detect them by quality control measures and reliability aspects of non-destructive testing with respect to the detection of fatigue cracks. Proposals are made how the safety level could be further improved.
In the design approval of transport packages for radioactive materials, the mechanical and thermal safety assessment is carried out in Germany by competent authority BAM. In recent years BAM was involved in several licensing procedures of new spent fuel and HLW package designs, where the cask body is of Ductile Cast Iron (DCI). According to IAEA regulations package designs have to fulfill requirements for specific conditions of transport. Type B(U) packages must withstand the defined accident conditions of transport. The temperature range from -40°C up to the operational temperature has to be considered. For the cask material DCI, it is necessary to determine safety against brittle fracture. The German guideline BAM-GGR 007 defines requirements for fracture mechanics of packagings made of DCI. Due to complex cask body structure and the dynamic loading a fracture mechanical assessment by analytical approaches is not always possible. Experience of recent design approval procedures show that the application of numerical calculations are applicable to determine the stresses and stress intensity factors in the cask body. At the first step a numerical analysis has to be done to identify
the loading state at the whole cask body. Secondly an analysis of a detail of the cask body is made considering the displacement boundary conditions of the global model. An artificial flaw is considered in this detailed model to calculate the fracture mechanical loading state. The finite element mesh was strongly refined in the area of the flaw. The size of the artificial flaw is based on the ultrasonic inspection acceptance criteria applied for cask body manufacture. The
applicant (GNS) developed additional analysis tools for calculation of stress intensity factor and/or J-Integral. The assessment approach by BAM led to the decision to develop own tools to the possibility for independent proof of the results.
The paper describes the authority assessment approach for DCI fracture mechanics analysis. The validation procedure incl. the development of own tools is explained. BAM developed a postprocessor to determine the fracture mechanical loads. A horizontal 1 m puncture bar drop test is used to give a detailed description of the assessment procedure.
Die Sicherheit von Radsatzwellen beruht gegenwärtig auf zwei Säulen: der Schwingfestigkeitsauslegung und periodischen Inspektionen. Ausgehend von einem Schadensfall, der gebrochenen ICE 3-Welle von Köln, 2008, sollen ausgewählte Aspekte beider Säulen diskutiert und Vorschläge zur weiteren Verbesserung gemacht werden. Diese betreffen u.a. Fragen einer Betriebszeitbegrenzung, Veränderungen der Schwingfestigkeit während des Betriebs infolge von Korrosion und Steinschlag, den Einfluss von Korrosion auf Schwingrissinitiierung und -fortschritt, den potentiellen Einfluss nichtmetallischer Einschlüsse auf die Schwingfestigkeit, die Möglichkeiten sie im Rahmen der Qualitätssicherung zu detektieren sowie Fragen der Bruchmechanik und der Zuverlässigkeit zerstörungsfreier Schwingriss-Prüfverfahren im Rahmen eines Konzepts regelmäßiger Inspektionen.
Design methodology of vessel produced by L PBF stainless steel using representative specimens
(2022)
This work presents the preliminary results of an ongoing project with a double objective: on the one hand, the characterisation of the mechanical properties against fatigue damage of an additively manufactured 316 stainless steel produced by laser powder bed-based (L-PBF) technology; on the other hand, the implementation of numerical simulation techniques able to predict the mechanical behaviour of the material in order to optimise and reduce the design costs of vessels used in the chemical sector. The current state of the work developed in this research framework allows showing the first batch of experimental results of crack propagation rate (FCGR) and high cycle fatigue (HCF) tests. The geometry of the vessels studied presents three clearly differentiated regions, either in terms of thickness (11-15 mm) or concerning the inclination of the walls to the direction of manufacturing (0º - 45º). The experimental campaign carried out so far allows identifying the differences in behaviour when comparing different extraction locations around the vessel. This is due to the variations in thermal cycles that the deposited material undergoes during the manufacturing process. Therefore, this causes variations in the microstructure which lead to changes in the response of the material. In this work, these differences are analysed qualitatively and quantitatively from the results of FCGR and HCF, thus allowing to locate the regions with the highest risk in terms of structural integrity against fatigue. This preliminary phase together with the numerical simulation of the additive manufacturing process are key to achieving a reliable description and modelling of the material. The latter will make it possible to address the priority aim of this project, involving the manufacture of independent samples whose properties are representative of the original material extracted from the reference vessels. It is, therefore, a comprehensive methodology for the design of additively manufactured components based on the localised fatigue mechanical properties of representative specimens.
The robust determination of the threshold against fatigue crack propagation DKth is of paramount importance in fracture mechanics based fatigue assessment procedures. The standards ASTM E647 and ISO 12108 introduce operational definitions of DKth based on the crack propagation rate da/dN and suggest linear fits of logarithmic DK– da/dN test data to calculate DKth. Since these fits typically suffer from a poor representation of the actual curvature of the crack propagation curve, a method for evaluating DKth using a nonlinear function is proposed. It is shown that the proposed method reduces the artificial conservativeness induced by the evaluation method as well as the susceptibility to scatter in test data and the influence of test data density.
Bei der Auslegung von Achsschenkeln für Nutzfahrzeuge wird für den Überlastfall ein fail-safe-Kriterium (kontrolliertes Versagen) als Basisphilosophie angesetzt. Gefordert wird, dass sich der Achsschenkel auch unter Akzeptanz begrenzten Risswachstums hinreichend elastisch-plastisch verformen kann, ohne dass es zum Bruch kommt. Der Fahrer soll anschließend anhand der bleibenden Verformungen bemerken können, dass mit seinem Fahrzeug etwas nicht in Ordnung ist. Eine Voraussetzung für dieses Bauteilverhalten ist eine hinreichende Bruchzähigkeit des Werkstoffs. Diese, wie auch die Rissspitzenbeanspruchung, müssen ermittelt werden, um den bruchmechanischen Sicherheitsnachweis führen zu können. Zur Ermittlung der mechanisch-technologischen und bruchmechanischen Werkstoffeigenschaften wurde ein Prüfkonzept entwickelt, bei dem die Proben an der potenziell kritischen Stelle direkt aus den geschmiedeten Achsschenkeln entnommen werden. Die experimentelle Werkstoffcharakterisierung bildet dann die Beanspruchungsbedingungen beim Überlastereignis ab. Im Rahmen der Produktion stehen jedoch nur Kerbschlagarbeitswerte zur Verfügung und aufwändige bruchmechanische Versuche sollen vermieden werden. Deshalb wurde eine Korrelation zur Abschätzung der vorhandenen Bruchzähigkeit aus der Kerbschlagarbeit herangezogen. Es ergibt sich eine Ergebnismatrix, die die Einflussgrößen Versagenswahrscheinlichkeit, Temperatur und Bruchzähigkeit verknüpft. Abschließend wird ein erster Vergleich der abgeschätzten mit den experimentell ermittelten Bruchzähigkeitswerten gezogen. Dieser weist aus, dass die abgeschätzten Werte geringfügig konservativ sind.
In the framework of the fatigue assessment of welded components, several methods are available in design standards such as the nominal stress, hot-spot stress, notch stress and linear elastic fracture mechanics approaches. The present paper aims at comparing two advanced local approaches for the fatigue strength assessment of different welded joints made of steel. The first one is IBESS which is based on short crack fracture mechanics. The second one is the PSM which instead involves the strain energy density approach. Both methods will be briefly presented, and the fatigue life prediction results discussed. The results obtained for the joint geometries considered in this work show advantages and drawbacks of the approaches, which are thoroughly analysed as well.
The effect of the environmental conditions on the threshold against fatigue crack propagation
(2022)
The threshold against fatigue crack propagation (ΔKth) is a crucial parameter for the damage tolerance assessment of engineering components subjected to cyclic loading and it is composed by two distinct components, one intrinsic, dependent on the elastic material properties and the lattice type, and one extrinsic, related to the occurrence of crack closure effects. An important issue is that several factors can influence ΔKth and, in general, the fatigue crack propagation behavior. In this work, the influence of the experimental procedure, air humidity, stress ratio and test frequency on da/dN-ΔK data has been investigated. Results are discussed with their potential causes and consequences on the calculations of the residual lifetime.
The Kitagawa-Takahashi (KT) diagram is a proven concept for describing the fatigue limit in presence of a defect or crack. It can be determined empirically with great experimental effort. It can also be estimated by means of the El Haddad relationship if the endurance limit and the long fatigue crack propagation threshold are available in reasonable accuracy. A third option is the determination using the cyclic R-curve, which describes the dependency of the fatigue crack propagation threshold on the crack growth at the short crack propagation stage. This can be experimentally determined using a closure-free initial pre-crack. It can then be applied to the determination of crack arrest for a given applied load and a given defect or crack size. Compared to the other two methods mentioned above, this option has considerable advantages: It can be applied to any component and any stress ratio. It allows the treatment of multiple cracks and provides estimations of the S-N curve in the finite life regime as well as at the endurance limit. Compared to the empirical determination of the KT diagram, the experimental effort is significantly lower and compared to the El Haddad approach it avoids problems such as the use of non-conservative long fatigue crack propagation thresholds (when the conventional load reduction method is applied to materials prone to corrosion) and the mathematical predetermination of the curve shape. The work introduces the method and provides a critical discussion as well as quantitative comparison between the different methods.
Additive manufacturing (AM) is becoming increasingly important in engineering applications due to the possibility of producing components with a high geometrical complexity allowing for optimized forms with respect to the in-service functionality. Despite the promising potential, AM components are still far from being used in safety-relevant applications, mainly due to a lack of understanding of the feedstock-process-properties-performance relationship. This work aims at providing a full characterization of the fatigue behavior of the additively manufactured AISI 316L austenitic stainless steel and a direct comparison with the fatigue performance of the wrought steel. To this purpose, a set of specimens has been produced by laser powder bed fusion (L-PBF) and subsequently heat treated at 900 °C for 1 hour for complete stress relief, whereas a second set of specimens has been machined out of hot-rolled plates. Low cycle fatigue (LCF) and high cycle fatigue (HCF) tests have been conducted for characterizing the fatigue behavior. The L-PBF material had a higher fatigue limit and better finite life performance compared to wrought material. Both, LCF and HCF-testing revealed an extensive cyclic softening.
Turbinenscheiben sind thermisch und mechanisch hochbeanspruchte, sicherheitsrelevante Komponenten in Gasturbinen. Ihre Integrität wird in Bauteiltests unter Überdrehzahlbedingungen nachgewiesen. Kontext des Aufsatzes ist die Erarbeitung einer zusätzlichen Bewertungsebene auf der Grundlage von Versuchen unter monotoner Beanspruchung, die die Beanspruchung im Bauteil realistisch wiedergeben soll. Betrachtet wird ein halbelliptischer Riss, der bei der weiteren Ausbreitung als Umfangsriss wächst (rim peeling). Die Simulation dieser Stelle im Bauteil schließt die mechanische und thermische Beanspruchung sowie sie Dehnungsbehinderung ein. Letztere wird anhand zweier Parameter, der Spannungsmehrachsigkeit ℎ und des (allerdings modifizierten) Constraint-Parameters nach Newman, der die Spannungen über den plastisch verformten Bereich mittelt, bestimmt. Erwartungsgemäß zeigt sich, dass die höchste Dehnungsbehinderung weder an den Oberflächenpunkten, noch an den tiefsten Punkten, sondern an einer Stelle zwischen beiden auf der Rissfront auftritt. Als Versuchsgeometrie für die monotonen Tests soll eine Kreuzprobe mit wanddurchdringenden Rissen ausgehend von einer Innenbohrung dienen. Durch geeignete Wahl der Risstiefe sowie der in beiden Richtungen angreifenden Kräfte gelingt es, bauteiladäquate Zustände zu erzeugen.
Am Beispiel eines abgesetzten zylindrischen Bauteils (Achsschenkel) werden Probleme der analytischen bruchmechanischen Bauteilbewertung diskutiert. Als Basis-philosophie wird ein Fail Safe-Kriterium (kontrolliertes Versagen) angesetzt. Die Forderung ist, dass sich der Achsschenkel, bevor er bricht, in einem Ausmaß plastisch verformen kann, bei dem der Fahrer bemerkt, dass mit seinem Fahrzeug etwas nicht in Ordnung ist. Um das zu gewährleisten, muss der Werkstoff einen bestimmten Riss-widerstand aufweisen. Dazu werden im hier vorgestellten ersten Teil des Projektes mittels FEM Rissspitzenbeanspruchungen in Abhängigkeit von der Auslenkung des Zylinders und angenommener Rissgrößen ermittelt. Diese finden als Zielgrößen in der weiteren Analyse Anwendung. Da eine einfach umzusetzende Lösung angestrebt wird, werden zusätzlich analytische Rechnungen an Ersatzgeometrien (halbunendliche Platte und Vollzylinder), aber mit den realen Spannungen am Bauteil durchgeführt. Es zeigt sich, dass die Ergebnisse wegen des Steifigkeitsunterschieds zwischen Bauteil und Ersatzgeometrie unvertretbar konservativ sind. Alternativ wird ein hybrides Konzept vorgestellt, für das die Modellparameter mittels FEM bestimmt werden. Es liefert sehr genaue Ergebnisse. Im Rahmen der analytischen Rechenvorschrift können die Lösungen auf andere Werkstoffe übertragen werden, ohne dass zusätzliche FEM-Analysen notwendig sind.
The fracture mechanics-based IBESS approach is applied to the determination of FAT classes of butt welds with crack Initiation along the weld toe. The aim is an investigation of the effect of the geometrical parameters toe radius, flank angle, reinforcement and secondary notches such as roughness or undercuts. The influence of these parameters is discussed both individually and in combination; however, excluding statistical distributions of them and the material data. The results, when compared with conventional FAT classes for butt welds, are encouraging with respect to a potential contribution of IBESS to the discussion of more advanced quality criteria for welds. To that purpose, demands for further research are proposed.
This paper presents the application of short crack propagation models based on the cyclic R-curve for assessing the fatigue strength of additively manufactured (AM) materials containing fabrication defects. Chapetti’s and IBESS models were implemented in combination with Murakami’s √area parameter, considering recently published data on laser powder bed fusion processed AISI 316L stainless steels. Estimated S-N curves and Kitagawa-Takahashi diagrams predict fairly well the experimental data, especially the origin of the failure from internal or surface defects. These results provide an indication that the methods based on the cyclic R-curve constitute suitable tools for fatigue behavior assessment of AM materials.
Determination of fatigue crack propagation thresholds for steel in presence of environmental effects
(2021)
The experimental determination of the resistance to fatigue crack propagation in steel, particularly in the socalled near-threshold regime, is a crucial issue for safety-relevant components which are designed to operate for a large number of loading cycles before undergoing periodic inspections. This work summarizes the most relevant results of extensive experimental campaigns conducted at BAM and IPM over the last years, which have been devoted to the determination of fatigue crack propagation characteristics on medium and high strength steels. The influence of environment, oxide-induced crack closure and test methodologies in the near-threshold regime are discussed.
Für die Bestimmung der Restlebensdauer zyklisch beanspruchter Bauteile kommt dem Schwellenwert für Ermüdungsrissausbreitung, Δ𝐾𝑡ℎ, und dem sich anschließenden Schwellenwertbereich der 𝑑𝑎𝑑𝑁⁄−Δ𝐾-Kurve große Bedeutung zu. Dem stehen Unsicherheiten bei der Bestimmung und Analyse gegenüber. Der Aufsatz befasst sich mit der Ermittlung des sogenannten intrinsischen Schwellenwerts, der als robuster Eingangsparameter für eine „rissschließfreie“ Analyse der Restlebensdauer von entscheidender Bedeutung ist.
Für die Bestimmung der Restlebensdauer zyklisch beanspruchter Bauteile kommt dem Schwellenwert für Ermüdungsrissausbreitung ∆Kth und dem sich anschließenden Schwellenwertbereich der da/dN-∆K-Kurve große Bedeutung zu. Dem stehen Unsicherheiten bei der Bestimmung und Analyse gegenüber. Der Aufsatz befasst sich mit der Ermittlung des sogenannten intrinsischen Schwellenwerts, der als robuster Eingangsparameter für eine „rissschließfreie“ Analyse der Restlebensdauer von entscheidender Bedeutung ist.
If a component is cyclically loaded, its load carrying capacity is considerably lower than in the monotonic loading case. This general observation applies in particular to L-PBF parts. The causes of this are mainly material defects such as pores and unwelded regions (Chapter 8) and a pronounced surface roughness in the as-built condition (Chapter 9). In addition, effects due to the anisotropy of the microstructure (Chapter 6) and a complex residual stress pattern (Chapter 7) play an important role. A consequence is that common strategies of fatigue assessment cannot be transferred to L-PBF applications without modifications. Due to the inhomogeneity of the material, the determination of representative material properties and the transfer to the component is a problem, and this is also the case with regard to the consideration of defects, surface roughness and residual stresses. The chapter gives a brief introduction to these problem areas.
The safe fatigue design of metallic components fabricated by additive manufacturing (AM) is still a largely unsolved problem. This is primarily due to (a) a significant inhomogeneity of the material properties across the component; (b) defects such as porosity and lack of fusion as well as pronounced surface roughness of the asuilt components; and (c) residual stresses, which are very often present in the as‐built parts and need to be removed by post‐fabrication treatments. Such morphological and microstructural features are very different than in conventionally manufactured parts and play a much bigger role in determining the fatigue life. The above problems require specific solutions with respect to the identification of the critical (failure) sites in AM fabricated components. Moreover, the generation of representative test specimens characterized by similar temperature cycles needs to be guaranteed if one wants to reproducibly identify the critical sites and establish fatigue assessment methods taking into account the effect of defects on crack initiation and early propagation. The latter requires fracture mechanics‐based approaches which, unlike common methodologies, cover the specific characteristics of so‐called short fatigue cracks. This paper provides a discussion of all these aspects with special focus on components manufactured by laser powder bed fusion (L‐PBF). It shows how to adapt existing solutions, identifies fields where there are still gaps, and discusses proposals for potential improvement of the damage tolerance design of L‐PBF components
Aero-engine turbine disks are safety-relevant components which are operated under high thermal and mechanical stress conditions. The actual part qualification and certification procedures make use of spin-tests conducted on productionsimilar disks. While these tests provide, on the one hand, a reliable definition of the critical conditions for real components, on the other hand they represent a relevant cost item for engine manufacturers. The aim of this work is to present part of a fracture mechanics-based procedure under development which aims at replacing the tests on production-similar disks with lab tests on fracture mechanics specimens. In particular, the rimpeeling failure mode is considered as case study. A semi-circular surface crack is modelled at the most stressed region at the diaphragm of a turbine disk, with the crack plane perpendicular to the radial direction. The crack is therefore subjected to a biaxial stress state and grows under increasing rotational speed until it triggers the rim-peeling failure. The finite element simulation of the cracked disk considers the real thermal and mechanical loading conditions. In order to design a lab representative specimen, beside the crack driving force, expressed in terms of -integral, also the constraint to plastic deformation e.g., stress triaxiality, at the crack-tip must be similar for the same crack in the specimen and in the disk. This has been achieved and as expected, both the highest -integral and constraint factor are calculated at the same location along the crack front for both disk and specimen. The results of the structural integrity assessment in the form of a Failure Assessment Diagram (FAD) show good agreement between designed specimen and disk both in terms of expected failure mode and value of the critical speed. Probabilistic aspects are also considered in the calculations.
Using the case study of a fail-safe design criterion for a steering knuckle of a commercial vehicle, the effect of stiffness/constraint on the crack driving force in the component is discussed. The problem arises when assessment procedures such as R6, BS 7910 or SINTAP/FITNET are applied in conjunction with substitute geometries for determining the model parameters K-factor and limit load (or a substitute for the latter), as it is common practice. In the example, the conservatism was so pronounced that this procedure was in fact unusable. A way out could be the finite element-based determination of the model parameters and their use in the analytical framework. This procedure is useful and effective if the finite element-based calculations are used in parameter sensitivity analyses.
Die sichere Auslegung von Bauteilen kommt an der Anwendung bruchmechanischer Methoden heute nicht mehr vorbei. Das vorliegende Buch bietet eine Einführung in diese Methoden mit besonderen Schwerpunkten auf dem aktuellen Stand der bruchmechanischen Kennwertermittlung und den Methoden der analytischen bruchmechanischen Bauteilbewertung. Es beschreibt die Mechanismen von Risswachstum und Bruch, die grundlegenden Konzepte der Bruchmechanik, die analytische Ermittlung der Rissspitzenbelastung in Bauteilen sowie die Anwendung der Bruchmechanik in unterschiedlichen Kontexten. Der Schwerpunkt liegt dabei auf der praktischen Anwendung u. a. anhand von Demonstrationsbeispielen, die im Detail nachvollzogen werden können.
The impact of crack closure and environmental effects on the experimental determination of the fatigue crack propagation threshold is a major problem for the assessment of cyclically loaded components, especially at low stress ratios 𝑅. In this work, the influence of the experimental procedure and air humidity on d𝑎∕d𝑁 − 𝛥𝐾 data at different 𝑅 is discussed. Unlike the results at 𝑅 = −1, the threshold values obtained at 𝑅 ≈ 0.8, i.e. under negligible crack closure levels, show a very small scatter band regardless of the variation of the test parameters and environmental conditions.
A fracture mechanics model which shall be applied to the fatigue strength determination of weldments has to focus on various aspects such as: (a) the description of mechanical and physical short fatigue crack extension which is characterised by yielding conditions which do not permit the application of the common ΔK concept and by the gradual build-up of the crack closure effect, (b) a consistent methodology for determining the initial crack size, (c) based on this, the determination of a fatigue limit, (d) the treatment of multiple crack propagation at load levels above this limit, (e) the variation of the local geometry along the weld toe, and (f) statistical effects.The paper gives alimited overview of the work the authors did in this field during the last years within the German project cluster IBESS. A model is presented and briefly discussed which covers the questions above.
Ermüdungsrisse gehen in technischen Materialien in der Regel von Werkstoffdefekten auf oder nahe der Oberfläche aus. Bei Schweißverbindungen können Schweißnahtdefekte, z. B. oberflächennahe Schlackeneinschlüsse die Risseinleitungsphase erheblich verkürzen, so dass der Hauptteil der Lebensdauer auf die Phase der Kurzriss- und zu geringerem Anteil auf die Langrissausbreitung entfällt. Bei nichtgeschweißten Bauteilen aus technischen Werkstoffen starten Ermüdungsrisse häufig an Einschlüssen, oberflächennahen Poren oder anderen Ungänzen. Einerseits ermöglicht dies prinzipiell die Anwendung bruchmechanischer Methoden zur Ermittlung der Schwingfestigkeit, andererseits müssen die Methoden in der Lage sein, spezielle Aspekte des Kurzrisswachstums wie den graduellen Aufbau des Rissschließeffekts und die lokale Rissspitzenplastizität zu erfassen. Im Beitrag werden ein geeignetes analytisches Modell sowie erste Validierungen an Proben aus verschiedenen Aluminiumlegierungen vorgestellt. Der abschließende Teil des Beitrags geht auf Besonderheiten von Schweißverbindungen (Schweißnahtdefekte, Eigenspannungen etc.) im Kontext des vorgestellten Modells ein. Eine Erweiterung des Modells für Schweißverbindungen aus Stahl wird im Rahmen eines kürzlich gestarteten DFG-AiF-Clusterprojekts erfolgen.
The idea to determine the fatigue strength of weldments by means fracture mechanics was originally based on the empirical observation that the major portion of the fatigue life in weldments is frequently spent in crack propagation. The present paper gives a brief overview on the basic principles of fracture mechanics-based determination of fatigue strength and on special features which have to be considered for weldments. Based on this information, the ongoing research project IBESS is introduced and briefly reviewed.
Die Sicherheit von Radsatzwellen beruht gegenwärtig auf zwei Säulen: der Schwingfestigkeitsauslegung und periodischen Inspektionen. Ausgehend von einem Schadensfall, der gebrochenen ICE 3-Welle von Köln, 2008, werden ausgewählte Aspekte dieser beiden Säulen diskutiert und Vorschläge zur weiteren Verbesserung gemacht. Diese betreffen u. a. Fragen einer Betriebszeitbegrenzung, Veränderungen der Schwingfestigkeit während des Betriebs infolge von Korrosion und Steinschlag, den Einfluss von Korrosion auf Schwingrissinitiierung und -fortschritt, den potenziellen Einfluss nichtmetallischer Einschlüsse auf die Schwingfestigkeit, die Möglichkeiten, sie im Rahmen der Qualitätssicherung zu detektieren, sowie Fragen der Bruchmechanik und der Zuverlässigkeit zerstörungsfreier Schwingriss-Prüfverfahren im Rahmen eines Konzepts regelmäßiger Inspektionen.
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 yield or limit load is a key parameter with respect to the accuracy of flaw assessment based on R6 type procedures such as the R6 routine, the SINTAP and FITNET method, the standard BS 7910 and others. In a number of previous papers two of the present authors proposed the use of a reference load instead of the common limit load, which not only provided more exact fracture mechanics predictions, but showed also a wider and more general application range than the conventional parameter. Here the method has been extended to biaxial tensile loading and it has been successfully validated by a thorough comparison with finite element results and alternative solutions available in the literature.
The Fatigue Crack Propagation threshold as a design criterion - actual developments and open issues
(2014)
The paper provides a discussion on damage tolerance options applied to railway axles and factors influencing the residual lifetime as well as the required inspection interval. These comprise material properties such as the scatter of the da/dNΔK curve, the fatigue crack propagation threshold ΔKth and the toughness of the material. Parameters affecting axle loading such as the press fit, rotating bending, load history and mixed crack opening modes are discussed. Finally the influence of the initial crack geometry on residual lifetime is simulated.
The aim of this paper is a collection of stress intensity factor solutions for cracks in railway axle geometries which the authors of the present special issue developed and/or used for damage tolerance analyses. These solutions comprise closed form analytical as well as tabled geometry functions and they refer to solid as well as hollow axles and various crack sites such as the T- and V-notch and the axle body.
Railway structures such as rails and axles are safety relevant components since any failure includes the risk of catastrophic derailment. This paper gives a brief overview on the most important potential failure scenarios driven by fatigue crack initiation and propagation and discusses the effects of important influencing factors such as vehicle weight and speed, track quality and environmental conditions. The present paper summarizes earlier work of the authors in this field.
Based on a large set of 2D finite element analyses, parametric equations for weld toe stress concentration factors and through-thickness stress profiles are provided for butt-welded plates. These cover Single-Vand Double-V joints subject to tensile and bending loading. Compared to the most widespread solutions in the literature, which have also been tested against the finite element data in this work, the application range is extended particularly with respect to the weld toe radius(0.1≤ρ≤4 mm)andangle(10 °≤α≤60 °). Furthermore, two values of excess weld metal are considered (h=0.75;2.5mm). The analyses are performed for a plate thickness of 10 mm. Besides the extended validity range, the proposed equations show also an improved accuracy compared to the existing solutions.
Semi-probabilistic method for residual lifetime of aluminothermic welded rails with foot cracks
(2016)
One of the most frequent and dangerous failure modes in continuous welded rails is fatigue crack Propagation terminated by brittle fracture. Due to the brittleness of the weld material and HAZ and the scatter in its mechanical properties, a statistical approach is necessary. The paper deals with surface cracks at the foot base of aluminothermic welded rails, developing a probabilistic methodology for determining the day by day prospective failure probability. The investigations presented here comprise weld material characterization, simulation of fatigue crack propagation and finally the determination of the failure probability using the Monte Carlo method. The effect of various parameters, such as axle weight, Initial crack size, residual stresses, fatigue crack propagation threshold and date of inspection were analyzed.
The results show that, independent of the date of the last inspection, almost any failure event happens in wintertime. This is in accordance with practical experience. However, from the proposed analysis it is evident that the main parameter controlling rail fracture is not only the minimum local temperature, but the temperature range over the whole year. Finally, the results are compared to the standard rail classification method.
Starting from an introduction into important Basic questions of failure analysis and fracture mechanics, the author specifies what kind of questions in failure analysis can be effectively solved by fracture mechanics (and which can't). He illustrates his discussion with a number of case studies. Much more pronounced than in the design stage the benefit of fracture mechanics in failure analysis depends on ist accuracy. This is limited by both, intrinsic factors of the method and the availability and quality of the input information. The author discusses the various aspects and provides the participants with background information helpful for better understanding the prospects and limitations of fracture mechanics in failure analysis and the conditions of its application.
Unterschiedliche Werkstoffeigenschaften reagieren auf jeweils spezifische Weise auf Gittertyp, Gefüge und Defekte. Für die schadenstolerante Betrachtung von Werkstoff und Bauteil ist das Verständnis dieser Zusammenhange essentiell. Der Beitrag gibt mit Hinblick auf additiv gefertigte metallische Bauteile (Selective Laser Melting) einen kurzen, keineswegs vollständigen Überblick über Faktoren, die die Steifigkeit, Festigkeit, Duktilität, Zähigkeit, Ermüdungsrissausbreitung und Schwingfestigkeit beeinflussen. Es wird aufgezeigt, wie die bruchmechanische Betrachtung zur Quantifizierung der zusammenhänge beitragen kann.
A discussion is provided on the relation between non-destructive testing (NDT) and fracture mechanics. The basic tasks behind this are to guarantee the safety of a component at a potential hazard loading event, to specify inspection intervals or, alternatively, of demands on NDT for a fixed inspection regime, to plan accompanying measures for cases of temporary continued operation of structures in which cracks have been detected, and, finally, fatigue strength considerations which take into account initial defects.
The burst of a disc in rotating machinery can cause catastrophic damage of the equipment and, more importantly, it can represent a mortal threat to anyone in the sphere of influence of the event. In order to minimize the danger associated to a rotating component failure, burst testing is required by the authorities in order to set safety margins to the normal operating speeds. Moreover more accurate predicting tools are required for designing the components. This paper presents the results of a numerical and analytical study on the assessment of the crack driving force for discs containing surface flaws. The aim is to provide a simple, though reliable, tool in order to be able to calculate the possible dangerous in-service speed for a rotating component and, therefore, to set burst margins above the normal operating conditions of the equipment. An R6-like flaw assessment procedure is adopted considering different assumptions in the definition of the critical speed and the analytical predictions are compared with the results of elastic-plastic finite element analysis of disc. In particular, the validity and potentiality of the method is proved for surface flaws in different positions in the disc.
Kernstück der angestrebten IBESS-Prozedur ist eine analytische Methode zur bruchmechanischen Beschreibung der Rissausbreitung unter Schwingbelastung. Da man es im Bereich der Schwingfestigkeit mit kurzen Rissen zu tun hat, auf die die etablierten Methoden der Langrissbruchmechanik nicht anwendbar sind, werden Lösungen entwickelt, die es erlauben, sowohl die rissnahen Plastizitätsverhältnisse als auch die mit wachsendem Riss graduell zunehmenden Rissschließeffekte quantitativ zu beschreiben. Unter Einbeziehung der sogenannten "zyklischen R-Kurve", deren experimentelle Methodik ebenfalls im Rahmen des Projekts weiterentwickelt wird, ist es zudem möglich, Ausgangsrissgrößen vorherzusagen, die auf Dauerfestigkeitsniveau gerade so noch arretieren würden. Dies erlaubt umgekehrt eine Abschätzung der Dauerfestigkeit an realen Bauteilen wie eben Schweißnähten sowie die statistisch basierte, analytische Vorhersage des Zeitfestigkeitsastes der Wöhlerkurve. Die im Modell berücksichtigten Parameter bzw. deren statistische Verteilung schließen Werkstoffdaten und die lokale Nahtgeometrie (Nachtübergangsradius, Nahtanstiegswinkel, Einbrandkerben oder Oberflächenrauhigkeit) ein. Im Vortrag wird das analytische Modell vorgestellt und vor dem Hintergrund der im IBESS-Cluster bereitgestellten experimentellen Information diskutiert.
Bruchmechanischen Bewertungsmethoden liegt stets eine tatsachlich vorhandene oder fiktive Anfangsrisslange zu Grunde. Schweißverbindungen sind mit strukturellen Imperfektionen wie beispielsweise lnhomogenitaten in der Mikrostruktur und den Werkstoffeigenschaften behaftet, welche auf der Werkstoffseite berücksichtigt werden können. Imperfektionen wie der Kantenversatz und der Nahtübergangsradius, die als außere geometrische Imperfektionen aufgefasst werden können, haben vor allem Auswirkungen auf den Spannungszustand und werden daher auf der Lastseite erfasst. Innere geometrische Imperfektionen wie Einschlüsse oder Poren, von welchen Risse initiieren und sich ausbreiten, können als Ausgangsrissgröße aufgefasst werden. Im Teilprojekt A1 des Forschungsclusters IBESS wurden außere geometrische Imperfektionen an Schweißverbindungen ermittelt und statistisch ausgewertet. Des Weiteren wurden Schweißverbindungen fraktografisch im Rasterelektronenmikroskop (REM) bezüglich innerer geometrischer Imperfektionen untersucht, jedoch konnten aufgrund der hohen Schweißgütekeine schadigungsrelevanten inneren Imperfektionen (Ausgangsdefekte) detektiert werden.
Um dennoch eine Ausgangsrisskonfiguration zu bestimmen, wurden "Heat-Tinting" Versuche eingesetzt, mit denen auf der Bruchflache verschiedene Anrissstadien markiert werden können. Mit dem .Heat-Tinting" Verfahren können sehr kleine Anrisse detektiert und in Abhangigkeit von der Beanspruchung frühe Anrisskonfigurationen ermittelt werden.
Des Weiteren haben Zerbst und Madia im Teilprojekt A3 ein Modell entwickelt, mit dem für den Grundwerkstoff mittels zyklischer R-Kurven eine Ausgangsrissgröße rechnerisch ermittelt werden kann. Unter Verwendung der Ergebnisse aus den Versuchen zur Ermittlung der Anrissstadien soll dieses Modell validiert werden, um auch für Schweißverbindungen Anwendung zu finden.
Die Projektbezeichnung IBESS steht für .Integrale Bruchmechanische Ermittlung der Schwingfestigkeit von Schweißverbindungen". Ziel des von acht Partnern vorangetriebenen AiF-DFG-Forschungsclusters IBESS ist die Entwicklung eines Ansatzes zur bruchmechanischen Ermittlung der Schwingfestigkeit von Schweißverbindungen. Die Arbeiten umfassen grundlegende Untersuchungen zum Wachstum und Arrest kurzer Risse, zur Ausbildung und Beeinflussung von Schweißeigenspannungen unter zyklischer Last, zum Einfluss der lokalen Nahtgeometrie auf die Rissausbreitung, zum Mehrfachriss-Phänomen oberhalb des Dauerfestigkeitsniveaus sowie zum Koaleszenzverhalten benachbarter Risse. Die gewonnenen Erkenntnisse fließen in ein analytisches Modell zur Ermittlung der Schwingfestigkeit ein, das physikalisch begründete und mithin sehr viel individuellere Vorhersagen des Ermüdungsverhaltens von Schweißnähten ermöglichen soll, als das auf konventioneller Basis bislang möglich ist. Ein nicht geringer Teil der Aktivitäten des Clusters ist mit der Bereitstellung von Daten zur Validierung befasst. Diese wird auf verschiedenen Ebenen vorgenommen, von der Beschreibung des Wachstuns von Einzelrissen über ihr Zusammenwachsen bis hin zur kompletten Wöhlerkurve. Ziel dieses Vortrags ist ein kurzer Überblick über das IBESS-Gesamtprojekt, das dem Zuhörer eine Einordnung der im IBESS-Vortragsblock vorgestellten Einzelaktivitäten ermöglichen soll.
Special aspects of welded components such as geometrical imperfections, inhomogeneity, strength mismatch and welding residual stresses have to be taken into account in a fracture mechanics analysis since they affect the crack driving force in the component as weil as the fracture resistance of the material. The treatment of components subjected to static loading is based on methods such as the European SINT AP procedure, which has been validated in a nurober of case studies. In the present paper the influence of strength mismatch and residual stresses on the fracture resistance and the assessment of a component are discussed.
Bruchmechanik: Konzepte, Kennwertermittlung, Bauteilbewertung. Kurzeinführung Betriebsfestigkeit
(2015)
Bei der Bestimmung der Restlebensdauer zyklisch beanspruchter Bauteile kommt dem Schwellenwert für Ermüdungsrissausbreitung, K111, oft entscheidende Bedeutung zu. Dabei ist seine Anwendung weniger gesichert als bislang angenommen. Der Vortrag diskutiert Fragen der experimentellen Ermittlung des Schwellenwertes sowie seiner Anwendung auf Bauteile. Auf beiden Gebieten haben sich in den letzten Jahren Probleme ergeben, die bestehendes Wissen teilweise wieder in Frage stellen oder modifizieren. Im Beitrag diskutiert werden der Einfluss unterschiedlicher Rissschließmechanismen auf K111, Konsequenzen, die sich daraus hinsichtlich der Ermittlung und Übertragung des Kennwertes ergeben, sowie die Effekte von Einflussgrößen wie Spannungsverhältnis und Rissgröße, aber auch des Werkstoffgefüges und der damit korrelierten mechanischen Eigenschaften.
Welding is one of the most common methods in industrial practice for joining components. Its main advantages are high speed in manufacturing combined with low costs and, usually, a high degree of flexibility, integrity and reliability. Nevertheless, welding is a highly complex metallurgical process and, therefore, weldments are susceptible to material discontinuities, flaws and residual stresses which may lead to structural failure and life time reduction. As a consequence weldments are an important field of fracture mechanics methods although its application is more complex than for homogeneous or non-welded structures. The aim of the paper is to provide an overview on the current state of fracture mechanics application to weldments. It starts by discussing the specific features which any fracture mechanics analysis of weldments has to take into account. Then, the experimental determination of fracture toughness, fatigue crack propagation and tensile properties of weldments is addressed. Finally, the analytical determination of the crack driving force in components and structural integrity assessment approaches for weldments are presented.
Recently three of the authors of this paper presented analytical solutions for reference loads of plate geometries with semi-elliptical surface cracks subjected to tension, bending, combined tension-bending and biaxial tension. These solutions were shown to provide more accurate crack driving force estimates than the conventional limit load solutions available in the literature, and the method behind them allowed for a wider application range. Within the present paper a methodology for the fracture analysis of thick-wall pressurized pipes using the R6 assessment method and considering both, biaxial and combined tension-bending loading is developed and validated. The analyses are carried out analytically, and the comparison between the predicted critical loads and experimental burst test failure loads shows satisfying agreement, this way demonstrating the potential of the proposed method.
The idea to determine the fatigue strength of weldments by means fractuie mechanics was originally based on the empirical observation that the major portion of the fatigue life in weldments is frequently spent in crack propagation. The present paper gives a brief OverView on the basic principles of fracture mechanics-based determination of fatigue strength and on special features which have to be considered for weldments. Based on this Information, the ongoing research project IBESS is introduced and briefly reviewed.
ln den letzten Jahren hat sich gezeigt, dass die Ermittlung des Schwellenwertes DeltaKth nach den bestehenden Standardvorschriften für verschiedene Materialien zu nichtkonservativen Werten führt. Das ergibt sich im Vergleich mit Schwellenwerten, die man mit Anrissen erhält, die hinsichtlich der Ober- und der Unterlast komplett im Druckbereich erzeugt wurden. Dort, wo die Diskrepanz auftritt, verstärkt sie sich mit abnehmendem Spannungsverhältnis. ln der vorliegenden Studie wurden an einem mittel- und einem hochfesten Stahl bei unterschiedlichen Spannungsverhältnissen Schwellenwerte bestimmt. Anwendung fanden die konventionelle Lastabsenkungsmethode nach ASTM-Standard und ein Verfahren, bei dem der Langrissscbwellenwert als Endpunkt der sogenannten Zyklischen R-Kurve erhalten wird. Dieses folgt im Wesentlichen der von Pippan et al. entwickelten Methode, ergänzt diese jedoch durch eine eigene Verfahrensvariante.
In a number of previous papers, the authors have proposed a model for fracture mechanics based prediction of the S-N characteristics of metallic components with large microstructural defects. Here, an extension to materials that do not show large defects onto the fracture surfaces is provided. In such cases, an approach based on a so-called cyclic R-curve analysis is proposed for the determination of the initial flaw size, which has to be used in the calculation of fatigue crack propagation. The principle is explained and demonstrated by a first application to a welded joint.
Unterschiedliche Werkstoffeigenschaften reagieren auf jeweils spezifische Weise auf Gittertyp, Gefüge und Defekte. Für die schadenstolerante Betrachtung von Werkstoff und Bauteil ist das Verständnis dieser Zusammenhänge essentiell. Der Beitrag gibt mit Hinblick auf additiv gefertigte metallische Bauteile(Selective Laser Melting) einen kurzen, keineswegs vollständigen Überblick über Faktoren, die die Steifigkeit, Festigkeit, Duktilität, Zähigkeit, Ermüdungsrissausbreitung und Schwingfestigkeit beeinflussen. Es wird aufgezeigt, wie die bruchmechanische Betrachtung zur Quantifizierung der Zusammenhänge beitragen kann
Analytical flaw assessment
(2018)
The paper provides a review on analytical flaw assessment methods with the focus on fracture under monotonic loading and fatigue crack propagation. The first topic comprises linear elastic as well as elastic-plastic fracture mechanics approaches. It essentially follows their historical development. Topics which are separately discussed are reference/Limit loads, the treatment of secondary stresses, strength mismatch, constraint, unstable crack propagation (monotonic R-curve analyses) and statistical aspects. With respect to fatigue crack propagation the analytical treatment of crack closure and constraint and the Determination of the cyclic elastic-plastic crack driving force is discussed. Finally, cyclic Rcurve analyses are briefly addressed.
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 additive manufacturing technology of Selective Laser Melting (SLM) experiences a rapid development within an increasing marked of quite different application fields. The properties of SLM materials and structures are influenced by a number of tech-nological parameters such as the metal powder (particle size, homogeneity, cleanliness), the laser tool (power, beam diameter, pulse lengths), the scanning operation (speed, sequence and orientation of melting paths), parameters of the over-all equipment (design and preheating of the base plate, currents and turbulence in the protective gas atmosphere) and, last not least, the hatching strategy including the build-up direction of the structure with respect to the loading direction of the component.
For the perspective use of SLM structures as load carrying, safety-relevant components the knowledge of their mechanical properties is necessary. It is essential to understand these in the context of the manufacturing-related features and at the back-ground of the basic characteristics of metallic materials: crystal lattice, microstructure and material defects. The paper provides an overview on factors which affect the mechanical parameters stiffness, strength, ductility, toughness, fatigue crack propagation and fatigue strength in the context of selective laser melting.
Damage development and damage tolerance of structures manufactured by selective laser melting
(2017)
The additive manufacturing technology of Selective Laser Melting (SLM) experiences a rapid development within an increasing marked of quite different application fields. The properties of SLM materials and structures are influenced by a number of tech-nological parameters such as the metal powder (particle size, homogeneity, cleanliness), the laser tool (power, beam diameter, pulse lengths), the scanning operation (speed, sequence and orientation of melting paths), parameters of the over-all equipment (design and preheating of the base plate, currents and turbulence in the protective gas atmosphere) and, last not least, the hatching strategy including the build-up direction of the structure with respect to the loading direction of the component.
For the perspective use of SLM structures as load carrying, safety-relevant components the knowledge of their mechanical properties is necessary. It is essential to understand these in the context of the manufacturing-related features and at the back-ground of the basic characteristics of metallic materials: crystal lattice, microstructure and material defects. The paper provides an overview on factors which affect the mechanical parameters stiffness, strength, ductility, toughness, fatigue crack propagation and fatigue strength in the context of selective laser melting.
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.
Der Vortrag behandelt in grundsätzlicher weise die Anwendung bruchmechanischer Methoden auf die rechnerische Ermittlung der Wöhlerkurve. Dabei werden die Besonderheiten des Kurzrisswachstums (Notwendigkeit der elastisch-plastischen Beschreibung der zyklischen Rissspitzenbelastung, gradueller Aufbau des Rissschlißphänomens) ebenso diskutiert wie das Problem des Mehrfachrissewachstums und Rissarrest. Die Ausführungen werden an einem Beispiel illustriert.
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.
Konventionell wird in der Bauteilauslegung zwischen Konzepten der Schwing- und Betriebsfestigkeit auf der einen und der Bruchmechanik auf der anderen Seite unterschieden. Werden in der Schwingfestigkeitsbewertung Gesamtlebensdauern ermittelt, so sind es in der Bruchmechanik Restlebensdauern bei Vorhandensein eines größeren, d.h., bereits zerstörungsfrei auffindbaren Risses. Daneben gibt es seit längerem Bestrebungen, bruchmechanische Ansätze auch für die Ermittlung der Gesamtlebensdauer fruchtbar zu machen. Der besondere Wert einer solchen Kombination besteht darin, dass die Bruchmechanik die explizite theoretische Behandlung von Eingangsparametern (Kerben, Oberflächenrauhigkeit, Mittelspannung, etc.) erlaubt, die in den konventionellen Ansätzen mit vergleichsweise hoher Ungenauigkeit halbempirisch einbezogen werden. Der Beitrag stellt einen von den Autoren entwickelten bruchmechanischen Ansatz zur Ermittlung von Wöhlerkurven vor, der eine Reihe innovativer Merkmale wie die Anwendung der elastischplastischen Bruchmechanik auf kurze Risse, die Ermittlung der Ausgangsrissgröße aus Rissarrestbetrachtungen, die Beschreibung der transienten Entwicklung des Rissschließeffekts mit Hilfe zyklischer R-Kurven (Risslängenabhängigkeit des Schwellenwertes von der Risstiefe), die theoretische Erfassung von Mehrfachrisswachstum bei Lastniveaus oberhalb der Dauerfestigkeit und statistische Aspekte umfasst. Das diskutierte Beispiel ist eine Schweißverbindung (Stumpfnaht) und wurde im Rahmen des laufenden Cluster-Projektes IBESS (Integrale bruchmechanische Ermittlung der Schwingfestigkeit von Schweißverbindungen) generiert.
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.
Analytische bruchmechanische Ermittlung der Schwingfestigkeit von Schweißverbindungen (IBESS-A3)
(2016)
Ziel des DFG-AiF-Forschungsclusters IBESS war die Entwicklung einer Methodik zur bruchmechanischen Ermittlung von Wöhlerkurven in Schweißverbindungen. Dem vorliegenden Teilprojekt A3 kamen dabei zwei Aufgaben zu: die Entwicklung einer analytischen bruchmechanischen Methodik und die Koordinierung des insgesamt acht Partner umfassenden Clusters. Dieser Bericht fasst die Ergebnisse der erstgenannten Aufgabe zusammen. Die bruchmechanische Methode ist durch folgende Aspekte charakterisiert. (a) Sie beschreibt sowohl Kurz- als auch Langrisswachstum. Ersteres umfasst mechanisch und physikalisch kurze Risse. Mechanisch kurze Risse weisen Abmessungen in der Größenordnung der plastischen Zone auf, weshalb sie zwar bruchmechanisch, nicht jedoch auf der Basis des linear-elastischen K-Konzepts charakterisiert werden können. Im Projekt wurde entsprechend eine Methode zur Bestimmung eines „plastizitätskorrigierten“ zyklischen K-Faktors entwickelt, die auf dem zyklischen J-Integral beruht. Physikalisch kurz bedeutet, dass die Rissschließeffekte im Ausgangsstadium der Rissentwicklung zunächst noch nicht vorhanden sind, dann jedoch graduell aufgebaut werden, bis sie mit Erreichen des Langrissstadiums eine risstiefenunabhängig konstante Größe erreichen. Beschrieben wird dieser Effekt durch die Anwendung der sogenannten zyklischen R-Kurve, der Funktion des Schwellenwerts gegen Rissausbreitung von der Risstiefe. Mittels Rissarrestbetrachtungen des kurzen Risses werden (b) die Ausgangsrissgröße für die weitere Bruchmechanikanalyse und (c) die für N = 107 definierte Dauerfestigkeit der Bauteile bestimmt. Die Analyse erfolgt (d) statistisch, Schwankungen der lokalen Nahtgeometrie entlang des Nahtübergangs über ein Modell äquidistanter Abschnitte einbezogen werden. (e) Eigenspannungen werden sowohl im unbehandelten Schweißzustand als auch nach zyklischer Belastung berücksichtigt. Das Modell wird an insgesamt 33 Wöhlerkurven validiert, die an drei Schweißverbindungsformen (Stumpfstoß, Kreuzstoß, Längssteife) mit je zwei unterschiedlichen Ausführungen, zwei Werkstoffen (S355NL und S960QL), sowie im spannungsarmgeglühten und ungeglühten Zustand experimentell ermittelt wurden.
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.