Modified Shape of Dynamic Master Curves due to Adiabatic Effects

AbstractWithin a joint project of IWM/Freiburg and MPA/Stuttgart the fracture toughness of a 22 NiMoCr 3 7 steel (A 508 Cl.2) was characterized at IWM with SE(B)10/10- und SE(B)40/20-specimens at -20 °C and high crack loading rates in the range of 103 to 106 MPa√m s-1, see Böhme et al. (2012 and 2013). The single temperature Master Curve evaluation according to ASTM E1921 and Wallin (2011) resulted in part in 5%-lower-bound fracture toughness versus temperature curves below the deterministic ASME lower bound KIR-reference-curve. At a first glance, this seems to violate the ASME KIR-concept, however, possibly this just indicates, that the conventional MC-evaluation has to be modified for elevated loading rates. Adiabatic heating in the vicinity of the crack tip could be one reason for that, as already argued in Schindler (2013 and 2015).Therefore, additional SE(B)-tests at temperatures of -20 °C, 0 °C and +20 °C were performed at IWM within the current follow-up joint IWM-MPA project. The new IWM-results show in agreement with previous investigations by Viehrig et al. (2010) and Schindler et al. (2013 and 2015) that the Master Curves at elevated loading rates are steeper than at quasistatic loading, probably due to local adiabatic heating in the vicinity of the crack tip. Therefore, the temperature field around the crack tip has been measured with a high speed infrared camera and has been compared to results of a numerical simulation. Up to crack initiation, a local adiabatic rise in temperature of the order of magnitude of about 60 K was measured and calculated in the vicinity of the crack tip at a crack loading rate of about 106 MPa√m s-1. In order to take into account this adiabatic effect, the dynamic master curves were evaluated by applying an adjusted MC shape parameter. This finally leads to more plausible results for the dynamic Master Curves. Thus, the choice of a rate dependent shape parameter p should be considered for future modifications of the elevated loading rate appendix of ASTM E1921

Investigation of cleavage fracture under dynamic loading conditions: Part II numerical analysis

Part I of this study was an extensive fractographic investigation that covered the topics local crack arrest, and cleavage fracture-inducing mechanisms under dynamic loading situations. Also, it produced data regarding the origin of cleavage fracture. Now, this data is used for the numerical part of this study. First, the development of temperature and strain rate increase at the origin of cleavage fracture is conducted, and linked to discrepancies regarding experiments and the Master Curve concept in a phenomenological way. Then, cleavage fracture controlling mechanical field variables at the origin of fracture are analyzed, whereas very similar conditions regarding crack initiation and propagation are found when compared to quasi-static data. The influence of wave phenomena is examined as well. Finally, micromechanical simulations showed that a local temperature increase at the particle–matrix interface does not influence fracture behavior either, and that conclusively, the actual physical mechanism of cleavage fracture initiation (crack initiation and instability) takes place under the same conditions at elevated loading rates as under quasi-static conditions. Ultimately, the mechanisms responsible for the shortcomings of the Master Curve concept under dynamic loading conditions are identified, and current local approach concepts need to be adjusted to consider local crack arrest to be reliable

Untersuchung und Modellierung von sprödem Versagen und lokalem Rissarrest bei ferritischen Stählen unter dynamischer Beanspruchung

The main objective of this study is to obtain a better understanding of the micro mechanisms that lead to cleavage fracture of ferritic-bainitic steels under dynamic loading conditions. Also, the new-found knowledge was used to improve local cleavage fracture models. Initially, an extensive data base of dynamic fracture toughness experiments was provided. A combined fractographic and mostly numerical approach with the finite element method was chosen to identify and quantify the relevant micro mechanisms, whereas the finite element method was also used to calculate the probability of failure, and perform the model modification. It was shown that adiabatic heating is a highly complex phenomenon which controls cleavage fracture initiation collectively with the increase in local strain rate. The macroscopic fracture behavior, as well as the origin of fracture, is significantly impacted by this interaction. Also, local crack arrest is highly relevant under dynamic conditions, and detectable in form of so-called cleavage fracture islands on the fracture surface. Thereupon, macroscopic fracture behavior is fundamentally changed. The mechanical field variables at the origin of cleavage fracture initiation are identical with those witnessed under quasi-static conditions. The initiation mechanism can therefore solely be described by the use of adequate temperature- and strain rate dependent material properties in combination with the consideration of heat generation and heat conduction. However, the application of established local cleavage fracture concepts under dynamic conditions showed that it is not possible to predict fracture behavior correctly. The discrepancy strongly correlates with the observed amount of local crack arrest incidences. Finally, a micro-mechanistically motivated model modification in the shape of a local arrest condition is proposed. It considerably improves the analysis accuracy and applicability of local cleavage fracture models subjected to dynamic loading conditions

Investigation of cleavage fracture under dynamic loading conditions: Part I fractographic analysis

The fracture toughness of ferritic steels under dynamic loading conditions on the one hand shows decreasing values with elevated loading rates but on the other hand the shape of the temperature dependent fracture toughness curve has turned out to be different from the static Master Curve according to ASTM E 1921. This difference is often explained by adiabatic heating in the crack tip region, yet it is not clear if there are other additional mechanisms under dynamic loading conditions that contribute to these changes. This work is dedicated to systematically identifying and quantifying additional mechanisms regarding cleavage fracture under dynamic loading conditions. In part I of this study an extensive fractographic analysis of the fracture surfaces was conducted for various crack tip loading rates and testing temperatures. The primary fracture-inducing mechanism was found to be identical to the dominant one under quasi-static conditions (carbide cracking). Yet, the dynamic loading conditions appear to change the origin of fracture, promote local crack arrest, and cause multiple fracture initiation sites that lead to global failure. These results also question the reliability of current local approach concepts if used to assess fracture probability at elevated loading rates. The fractographic results are used in, and complemented by, part II of this study which deals with the numerical analysis of other additional mechanisms such as inertia

Application of the Criterion of Disposability as a Form of Indirect Discrimination in the Establishment of an Employment Relationship

Pracodawcy często w ogłoszeniach o pracy jako jeden z wymogów rekrutacyjnych wymieniają dyspozycyjność. Nie jest jednak jednoznaczne, czego w istocie pracodawcy, formułując takie kryterium, oczekują od kandydatów. Skutkuje to ryzykiem, że pracodawcy, wymagając od kandydatów dyspozycyjności, narażą się na zarzut dyskryminacji pośredniej ze względu np. na płeć, wiek, sytuację rodzinną, niepełnosprawność lub stan zdrowiaEmployers often list disposability as one of the recruitment requirements in job advertisements. However, it is not clear what employers actually expect from candidates when formulating such a criterion. This results in the risk that employers, by requiring applicants to be disposable, will expose themselves to charges of indirect discrimination on the basis ofe.g. sex, age, family situation, disability or health

Dopuszczalność strajku personelu medycznego z perspektywy interesów pracowników, pracodawców oraz interesu społecznego

The aim of the article is to assess the admissibility of a medical staff strike from the perspective of the employees’, employers’, and social interest. The applicable labor law regulations do not give an unambiguous answer to the question whether medical workers have the right to strike, since striking is a special instrumentality for employees to protect their rights. On the one hand, the labor law does not literally deprive medical staff of the right to strike. On the other hand, there are numerous arguments supporting the notion that a strike of medical staff causes a threat to the health and life of patients who are deprived of medical care during a strike. Regulating the issue is therefore of significant importance, not only social, but also to the employment relationship parties.Celem artykułu jest dokonanie oceny dopuszczalności strajku personelu medycznego z perspektywy interesu pracowników, pracodawców oraz interesu społecznego. Obecnie obowiązujące regulacje prawa pracy nie dają jednoznaczniej odpowiedzi na pytanie, czy osobom należącym do personelu medycznego przysługuje prawo do strajku. Strajk jest bowiem szczególnym środkiem przysługującym pracownikom w celu ochrony ich praw. Z jednej strony przepisy prawa pracy literalnie nie pozbawiają personelu medycznego prawa do strajku. Z drugiej jednak strony istnieją liczne argumenty przemawiające za tym, iż strajk personelu medycznego powoduje zagrożenie dla zdrowia i życia pacjentów, którzy pozbawiani zostają w ten sposób opieki medycznej. Uregulowanie kwestii prawa personelu medycznego do strajku ma więc istotne znaczenie nie tylko społeczne, a także dla samych stron stosunku pracy

Neue Untersuchungen zu Wachstum und Struktur von Fluorapatit-Gelatine-Nanokompositen

Die vorliegende Dissertation beschäftigt sich mit Wachstum und Aufbau von Fluorapatit-Gelatine-Nanokompositaggregaten. Diese Aggregate werden im sogenannten Doppeldiffusionsversuch biomimetisch erzeugt und ihre äußere Form bzw. Formentwicklung lässt sich anhand eines fraktalen Modells bis ins Detail nachvollziehen. Sie zeigen einen komplexen inneren Aufbau, in dem die Makromoleküle der organischen Komponente einerseits im Zentrum jeder Nanoeinheit und andererseits zu Strängen, den sogenannten Fibrillen, zusammengelagert am Aufbau der Kompositaggregate beteiligt sind. Im Fall des Kompositkeims ist die innere Architektur in hoher Detailstufe verstanden, auch wenn -- insbesondere bezüglich der späteren Wachstumsphasen -- eine Reihe ungeklärter Fragestellungen verbleibt. Ein zentrales Ergebnis der vorliegenden Arbeit bildet die Entdeckung eines weiteren Wachstumstypen, der im Vergleich zu den bekannten, fraktalen Kompositaggregaten grundsätzliche Unterschiede bezüglich des inneren und äußeren Aufbaus zeigt. Der Grund für die andersartige Formentwicklung liegt in der Versteifung der organischen Komponente durch eine vorangegangene Einlagerung von Calciumionen, wie sowohl experimentell als auch mit atomistischen Computersimulationen gezeigt werden konnte. Aufgrund der hohen Komplexität des Systems ist es bislang allerdings nicht möglich, lokale Ionen-Konzentrationen und pH-Werte vor bzw. während Nukleation und Wachstum der Kompositaggregate im Doppeldiffusionsversuch zu bestimmen. Deshalb wurde ein Ersatzversuch -- der sehr ähnlich strukturierte Aggregate erzeugt, sich aber mit rechnerischen Methoden analysieren lässt -- entworfen und untersucht. Anhand dieser Ergebnisse konnte erstmals die "Geschichte" von Fluorapatit-Gelatine-Nanokompositaggregaten detailliert nachvollzogen werden. Da über die Rolle der Gelatine beim Wachstum der Kompositaggregate nur wenig bekannt ist, wurde eine Reihe von Versuchen durchgeführt, in denen Gelatinen mit verschiedenen Molekülmassenverteilungen eingesetzt wurden. Es stellte sich heraus, dass für selbstorganisiertes und insbesondere fraktales Wachstum der Kompositaggregate lange, möglichst wenig gestörte Makromoleküle von zentraler Wichtigkeit sind. Um die Funktion der organischen Komponente für das Kompositwachstum näher zu untersuchen, wurden Oberflächen von Kompositkeimen mit rasterkraftmikroskopischen Methoden studiert. Durch Säuberung der Oberflächen konnten Austrittsstellen der organischen Komponente durch die Oberfläche der Kompositkeime identifiziert werden. Damit konnte gezeigt werden, dass die organische Komponente aus dem Inneren des Festkörpers teilweise durch die Oberfläche dringt und somit während des Wachstums weit in das Gel hineinreichen sollte. Für die mesoskopische Strukturbildung der Kompositaggregate spielen intrinsische elektrische Felder eine essenzielle Rolle. Deshalb wurde bislang eine Wirkung externer elektrischer Felder auf das Wachstum der Kompositaggregate vermutet. Im Rahmen der vorliegenden Arbeit wurde herausgearbeitet, dass es zwar zu keiner direkten Beeinflussung kommen kann, jedoch in den elektrodennahen Bereichen des Gels eine Ordnung der organischen Moleküle durch externe elektrische Felder zu erwarten ist. Dies könnte eine Wirkung auf wachsende Kompositaggregate zeigen. Da diese Effekte auch aufgrund der elektrischen Felder um die dipolaren Kompositaggregate zu erwarten sind, könnte eine ähnliche Strukturierung der Gelatine in der Nähe der wachsenden Kompositaggregate stattfinden. Insgesamt wurden in dieser Arbeit eine Reihe grundlegender Beiträge zur Erforschung der biomimetisch erzeugten Fluorapatit-Gelatine-Nanokompositaggregate geleistet. Es konnten neue Erkenntnisse zur inneren und äußeren Architektur der Kompositaggregate, zu Mechanismen der Morphogenese und deren wichtigsten Einflussgrößen sowie zum Verständnis der chemisch-physikalischen Vorgänge auf atomarer Größenskala gewonnen werden. Als besonders fruchtbar erwies sich die Verbindung von Experimenten mit theoretischen Untersuchungen, so dass dieser Weg auch in Zukunft grundlegende Erkenntnisse bei der Erforschung der Biomineralisation verspricht und weiterhin verfolgt werden sollte

Investigation and modeling of local crack arrest in ferritic-bainitic steels under dynamic loading

Local crack arrest is usually irrelevant under quasi-static loading conditions in the ductile to brittle transition region. Elevated loading rates, however, allow cleavage fracture due to the dynamic embrittlement also at higher testing temperatures compared to static loading. This behavior is generally accompanied by local crack arrest events. In addition, adiabatic heating processes in the crack tip region increase local temperature as well, which further promotes crack arrest. This complex interaction between crack initiation and crack arrest at elevated loading rates substantially changes macroscopic fracture behavior, whereas its investigation is the core of this work. An experimental database of dynamic fracture mechanics experiments for the reactor pressure vessel steel 22NiMoCr3-7 is examined in this work that was previously tested at crack tip loading rates of about 10(3) to 10(5) MPa√m /s. Recent fractographic examinations and statistics regarding the occurrence and characteristics of local crack arrest incidences are shown for different loading rates and testing temperatures. Furthermore, an existing local probabilistic cleavage fracture model is used to describe macroscopic fracture behavior for the provided experimental database, and also compared to other assessment methods (i.e. Master Curve). The shortcomings in the numerical assessment methods can be linked to the amount of observed local crack arrest incidences, and a micromechanically motivated model modification is proposed to consider the mechanism of local crack arrest. The agreement between experimental results and numerical cleavage fracture assessment can be significantly increased by using the modified model