Microscopic Damage Evolution During Very High Cycle Fatigue (VHCF) of Tempered Martensitic Steel

AbstractDimensioning of high-strength steels relies on the knowledge of Wöhler-type S/N data and the assumption of a fatigue limit for applications where the number of load cycles exceeds 107. Very high cycle fatigue (VHCF) experiments applied to a 0.5C-1.25Cr-Mo tempered steel (German designation: 50CrMo4) revealed surface crack initiation at prior austenite grain boundaries in medium strength condition (37HRC) and internal crack initiation at non-metallic inclusions at high strength condition (48HRC). Despite the formation of small cracks during cycling up to 109 cycles, it seems that the medium strength condition exhibits a real fatigue limit. Application of automated electron back-scattered diffraction (EBSD) within the shallow-notched area of electro-polished fatigue specimens had shown that prior austenite grain boundaries act as effective obstacles to crack propagation. High resolution thermography during cycling of the specimens allowed the identification of local plasticity, which led to crack initiation at a later stage of the fatigue life. It was found that Cr segregation rows play a decisive role in the crack initiation process. By means of high-resolution electron microscopy in combination with focused ion beam milling (FIB), evolution of cyclic plasticity and crack initiation was correlated with the material's microstructure. The results are discussed in terms of the completely different crack initiation mechanisms of medium and high strength variants of the same steel. EBSD and crack propagation data are used to adapt numerical modeling tools to predict crack initiation and short crack propagation

Phason-phonon coupling in decagonal quasicrystals

Quasikristalle haben neben den von Kristallen bekannten phononischen Freiheitsgraden weitere sogenannte phasonische Freiheitsgrade. Diese basieren auf alternativen Positionen bestimmter Atome im Quasikristall. Ein Sprung eines Atoms auf eine solche alternative Position wird Flip genannt. Beschreibt man den Quasikristall eingebettet in einem höherdimensionalen Hyperraum, so können die phasonischen Freiheitsgrade wie die phononischen als Verzerrungen beschrieben werden. Eine solche phasonische Verzerrung bewirkt im ganzen Quasikristall Flips. Die räumliche Verteilung dieser Flips in Abhängigkeit von der phasonischen Verzerrung wurde konstruiert. Die phasonischen Verzerrungen liefern in einer erweiterten Elastizitätstheorie auch einen Beitrag zur elastischen Energie. Dadurch hat ein Quasikristall neben den phononischen elastischen Konstanten auch sogenannte phasonische und insbesondere auch eine Phason-Phonon-Kopplungskonstante, die eine Wechselwirkung zwischen einer phononischenSchermode und einer phasonischen Verzerrung beschreibt. Es wurde eine Simulationsreihe entwickelt, mit der sowohl alle phononischen wie die phasonischen elastischen Konstanten und die Kopplungskonstante für einen dekagonalen Quasikristall in Molekulardynamiksimulationen bestimmt werden können

Phason-elastic energy in a model quasicrystal

The standard two-dimensional decagonal binary tiling quasicrystal with Lennard-Jones potentials is metastable at zero temperature with respect to one phason strain mode. By calculating the frequencies of local environments as a function of phason strain, a correction for the potentials is predicted, which stabilizes the quasicrystal

Growing Cities, Growing Food: Urban Agriculture on the Policy Agenda

The first part of these proceedings contains papers addressing such themes as the impact of urban agriculture on households, policy options for urban agriculture, and integrating agriculture in urban policies. Part 2 contains city case studies from Africa, Latin America, Asia and EuropeThe first part of these proceedings contains papers addressing such themes as the impact of urban agriculture on households, policy options ... Part 2 contains city case studies from Africa, Latin America, Asia and Europe

Investigation of changing failure mechanisms in the VHCF regime caused by different strengths of a martensitic steel

The present study gives an overview of recent investigations dealing with the fatigue behaviour of the tempered martensitic steel 50CrMo4 (Fe-0.5wt%C-1wt%Cr) in the HCF and VHCF regime by taking into account a variation in material strength, by modifying the heat treatment parameters. The parameters for the tempering treatment were adapted to receive two material conditions with 37HRC and 57HRC, respectively. Subsequently, fatigue specimens were machined from the heat-treated bars for fatigue tests in an ultrasonic (f=20000Hz) and a resonance (f=95Hz) fatigue testing machine under fully reversed loading (R=-1) at laboratory air atmosphere. It was found that the dominant fatigue and fracture mechanisms change with increasing material strength. For 37HRC moderate-strength specimens crack initiation was shown to occur on the specimen surface within Cr depleted bands (segregation bands) as the dominant fatigue damage mechanism. Contrary to that, only internal crack initiation at non-metallic inclusions was observed for the high strength 57HRC condition. Furthermore, the completely different crack initiation mechanisms of the two heat treatment conditions were assessed by applying the Murakami approach relating the fatigue limit with the size of non-metallic inclusions

Investigation of changing failure mechanisms in the VHCF regime caused by different strengths of a martensitic steel

The present study gives an overview of recent investigations dealing with the fatigue behaviour of the tempered martensitic steel 50CrMo4 (Fe-0.5wt%C-1wt%Cr) in the HCF and VHCF regime by taking into account a variation in material strength, by modifying the heat treatment parameters. The parameters for the tempering treatment were adapted to receive two material conditions with 37HRC and 57HRC, respectively. Subsequently, fatigue specimens were machined from the heat-treated bars for fatigue tests in an ultrasonic (f=20000Hz) and a resonance (f=95Hz) fatigue testing machine under fully reversed loading (R=-1) at laboratory air atmosphere. It was found that the dominant fatigue and fracture mechanisms change with increasing material strength. For 37HRC moderate-strength specimens crack initiation was shown to occur on the specimen surface within Cr depleted bands (segregation bands) as the dominant fatigue damage mechanism. Contrary to that, only internal crack initiation at non-metallic inclusions was observed for the high strength 57HRC condition. Furthermore, the completely different crack initiation mechanisms of the two heat treatment conditions were assessed by applying the Murakami approach relating the fatigue limit with the size of non-metallic inclusions