Synchrotron X-Ray Topography of Dislocation Arrays

1986 ◽  
Vol 82 ◽  
Author(s):  
J. C. Bilello
Keyword(s):  
Thin Films ◽  
Plastic Deformation ◽  
Crack Initiation ◽  
Large Scale ◽  
Crack Initiation And Propagation ◽  
X Ray ◽  
Gaas Substrates ◽  
Initiation And Propagation ◽  
Dislocation Arrays

ABSTRACTThe application of relatively low resolution x-ray topography methods, typically ∿ 1 micrometer, is limited in studies which involve large scale dislocation networks. However, the ability to non-destructively image wide areas for “thick” specimens at high intensity with a tunable x-ray source makes the synchrotron an ideal probe for a range of problems previously inaccessible by other methods. Some examples will be discussed such as: (a) crack initiation and propagation in fatigued bicrystals, (b) real-time in situ plastic deformation studies in strain-annealed Mo crystals, and (c) strain distributions in vapor deposited and LPE thin films on Si and GaAs substrates.

scholarly journals Hydrogen enhanced cracking studies on Fe–3wt%Si single and bi-crystal microcantilevers

2017 ◽  
Vol 375 (2098) ◽  
pp. 20160410 ◽  
Author(s):  
T. Hajilou ◽  
Y. Deng ◽  
N. Kheradmand ◽  
A. Barnoush
Keyword(s):  
Stress Concentration ◽  
Crack Initiation ◽  
The Other ◽  
Bending Test ◽  
Propagation Path ◽  
Transgranular Fracture ◽  
Post Mortem ◽  
Crack Initiation And Propagation ◽  
Initiation And Propagation

Hydrogen (H) enhanced cracking was studied in Fe–3wt%Si by means of in situ electrochemical microcantilever bending test. It was clearly shown that the presence of H causes hydrogen embrittlement (HE) by triggering crack initiation and propagation at the notch where stress concentration is existing. Additionally, the effect of carbon content and the presence of a grain boundary (GB) in the cantilever were studied. It was shown that in the presence of H the effect of carbon atom on pinning the dislocations is reduced. On the other hand, the presence of a GB, while the chemical composition of material kept constant, will promote the HE. Crack initiation and propagation occur in the presence of H, while the notch blunting was observed for both single and bi-crystalline beams bent in air. Post-mortem analysis of the crack propagation path showed that a transition from transgranular fracture to intragranular fracture mechanism is highly dependent on the position of the stress concentration relative to the GB. This article is part of the themed issue ‘The challenges of hydrogen and metals’.

Towards a Better Understanding of Crack Growth in Nickel-Cast Alloys Under Creep-Fatigue and Thermo-Mechanical Fatigue Conditions

2019 ◽  
Author(s):  
Karl Michael Kraemer ◽  
Falk Mueller ◽  
Christian Kontermann ◽  
Matthias Oechsner
Keyword(s):  
Crack Initiation ◽  
Crack Growth ◽  
Load Cycle ◽  
Crack Initiation And Propagation ◽  
Mechanical Fatigue ◽  
Initiation And Propagation ◽  
Creep Fatigue ◽  
Cast Alloys ◽  
Crack Growth Rates

Abstract To ensure the feasibility of gas turbines, despite rising commodity prices and emission restrictions, an enhancement of both their efficiency and flexibility is necessary. The consequential higher loading of components at high temperature conditions calls for an increased use of damage tolerant design approaches. To still guarantee a safe operation, a sound understanding and reliable estimations for crack growth under service conditions is indispensable. In this paper, the results from several projects in this field conducted at the TU Darmstadt and involved partners are summarized to identify and describe the various influences on crack growth under creep-fatigue and thermo-mechanical fatigue (TMF) loading. The activation of damage mechanisms under TMF loading and interactions between them are dependent of the temperature cycle and the respective load phasing. Depending on the type of loading (force- vs. strain-control), contrary influences of the phase shift on the TMF crack growth rates are found. This can partially be attributed to the differences in mean stress evolution. Crack initiation and propagation under creep-fatigue and TMF conditions are also often connected with significant scattering of initiation sites and crack growth rates. One reason for this non-uniform behaviour is the interaction of geometric discontinuities with the microstructure. To investigate the role of the local grain structure for crack initiation and propagation, in-situ observation techniques for crack tip movement and local strain fields were applied. Harsh gradients in the local deformation behaviour were identified as origins of secondary crack initiation. To describe crack growth under creep-fatigue and TMF conditions, the linear accumulation model “O.C.F.” was developed. It is based on the contributions of fatigue, creep and oxidation to crack growth per load cycle. This model is capable to reproduce the effects of time-dependent damage, different load ratios and TMF phase shifts, as well as component geometry. Substantial advantages of this method are its independence from empiric correction factors to assess changing load cycle forms and the possibility to give analytic estimations without the need of extensive data processing. The model is currently validated for three nickel cast alloys, also including single crystalline (SX) and directionally solidified (DS) cast variants, different creep-fatigue and TMF loading scenarios and crack geometries. The model’s linear formulation allows assessing the dominant driver of crack growth at each stage of an experiment. These predictions are compared with fractographic investigations and in-situ observations of crack paths to identify the mechanisms of crack growth under different TMF load cycle forms.

scholarly journals High-speed X-ray visualization of dynamic crack initiation and propagation in bone

2019 ◽  
Vol 90 ◽  
pp. 278-286 ◽  
Author(s):  
Xuedong Zhai ◽  
Zherui Guo ◽  
Jinling Gao ◽  
Nesredin Kedir ◽  
Yizhou Nie ◽  
...  
Keyword(s):  
Crack Initiation ◽  
High Speed ◽  
Dynamic Crack ◽  
Crack Initiation And Propagation ◽  
X Ray ◽  
Initiation And Propagation
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