Characterisation of fatigue crack tip field in the presence of significant plasticity

Characterisation of a fatigue crack tip in the presence of significant plasticity has been challenging due to the lack of suitable tools and lack of knowledge of material constitutive information under cyclic loading. In this paper, Digital Image Correlation (DIC) and integrated finite element (FE) analyses have been used to characterise the crack-tip field beyond the small-scale yielding (SSY) regime in a stainless steel 316L of a compact-tension (CT) specimen under mode I loading conditions. The non-linear characteristics of the near-tip deformation field were verified by the poor fit to the William’s regression and the overestimation of the stress intensity factor K. The extent of the crack tip plasticity was estimated using a detailed constitutive material model and compared with the estimated by Irwin. The displacement fields local to a stationary fatigue crack were mapped using DIC, and inputted into the FE model as boundary conditions so that an integrated FE analysis was carried out. Fatigue pre-cracking was simulated in the FE analysis prior to the full-field analysis of the fatigue crack tip, including stress/strain distributions ahead of the crack tip and the crack opening displacement (COD) under selected loading conditions. Although a distinct “knee” was captured as an indication of crack opening from the compliance curves in both the DIC measurements and the FE analyses, consistent with the existing knowledge on the phenomenon of crack closure, it does not appear to correlate with the crack driving force measured by the J-integral

J-integral analysis: an EDXD and DIC comparative study for a fatigue crack

Synchrotron Energy Dispersive X-ray Diffraction (EDXD) and Digital Image Correlation (DIC) have been applied to map simultaneously the 2D elastic strain and displacement fields of a propagating fatigue crack in the HAZ of a welded Cr2Ni4MoV bainitic steel. The position of the crack tip was tracked via a phase congruency analysis of the displacement field, and also by detection of its cyclic plastic zone. Both types of full field data provided independent inputs to finite element/J-integral analyses that directly quantified the elastic cyclic stress intensity factor range applied to the crack. No knowledge was required of the specimen geometry, crack length or applied loads. The agreement between the two analyses in this controlled study shows that strain mapping by synchrotron EDXD can provide a reliable method to study the crack fields in more complex problems, such as interactions between crack closure, residual stresses and applied loading

A 3D full-field study of cracks in a nuclear graphite under mode I and mode II cyclic dwell loading conditions

Three‐dimensional (3D) full‐field deformation around crack tips in a nuclear graphite has been studied under mode I and mode II cyclic dwell loading conditions using digital volume correlation (DVC) and integrated finite element (FE) analysis. A cracked Brazilian disk specimen of Gilsocarbon graphite was tested at selected loading angles to achieve mode I and mode II cyclic dwell loading conditions. Integrated FE analysis was carried out with the 3D displacement fields measured by DVC injected into the FE model, from which the crack driving force J‐integral was obtained using a damaged plasticity material model. The evolution of near‐tip strains and the J‐integral during the cyclic dwell loading was examined. Under cyclic dwell, residual strain accumulation was observed for the first time. The results shed some light on the effect of dwell time on the 3D crack deformation and crack driving force in Gilsocarbon under cyclic mode I and II loading conditions

J-integral analysis: An EDXD and DIC comparative study for a fatigue crack

Synchrotron Energy Dispersive X-ray Diffraction (EDXD) and Digital Image Correlation (DIC) have been applied to map simultaneously the 2D elastic strain and displacement fields of a propagating fatigue crack in the HAZ of a welded Cr2Ni4MoV bainitic steel. The position of the crack tip was tracked via a phase congruency analysis of the displacement field, and also by detection of its cyclic plastic zone. Both types of full field data provided independent inputs to finite element/J-integral analyses that directly quantified the elastic cyclic stress intensity factor range applied to the crack. No knowledge was required of the specimen geometry, crack length or applied loads. The agreement between the two analyses in this controlled study shows that strain mapping by synchrotron EDXD can provide a reliable method to study the crack fields in more complex problems, such as interactions between crack closure, residual stresses and applied loading

Impact of phonons on dephasing of individual excitons in deterministic quantum dot microlenses

Optimized light-matter coupling in semiconductor nanostructures is a key to understand their optical properties and can be enabled by advanced fabrication techniques. Using in-situ electron beam lithography combined with a low-temperature cathodoluminescence imaging, we deterministically fabricate microlenses above selected InAs quantum dots (QDs) achieving their efficient coupling to the external light field. This enables to perform four-wave mixing micro-spectroscopy of single QD excitons, revealing the exciton population and coherence dynamics. We infer the temperature dependence of the dephasing in order to address the impact of phonons on the decoherence of confined excitons. The loss of the coherence over the first picoseconds is associated with the emission of a phonon wave packet, also governing the phonon background in photoluminescence (PL) spectra. Using theory based on the independent boson model, we consistently explain the initial coherence decay, the zero-phonon line fraction, and the lineshape of the phonon-assisted PL using realistic quantum dot geometries

Systematic study of the influence of coherent phonon wave packets on the lasing properties of a quantum dot ensemble

Kohärente Phononen können die Licht-Materie-Wechselwirkung in Halbleiter Nanostrukturen stark ändern. Bei einem Ensemble von Quantenpunkten (QP) als aktivem Lasermedium sind Phononen im Stande, die Laserintensität deutlich zu verstärken oder abzuschwächen. Die Physik des gekoppelten Phonon-Exziton-Licht-Systems wird von verschiedenen Mechanismen dominiert, die im Experiment nicht eindeutig unterschieden werden können, da die komplizierte Probenstruktur zu einem komplexen Verspannungspuls führt, der auf das QP-Ensemble trifft. Hier zeigen wir durch eine umfassende theoretische Studie, wie die Laseremission durch Phononpulse verschiedener Form und QP-Ensembles verschiedener spektraler Verteilung beeinflusst wird. Dies erlaubt einen Einblick in die grundlegenden Wechselspiele des gekoppelten Gesamtsystems. Dadurch können wir zwischen zwei Mechanismen unterschieden: der adiabatischen Verschiebung des Ensembles und dem Schüttel-Effekt. Dies ebnet den Weg zu einer gezielten Kontrolle der Laser Emission durch kohärente Phononen.Coherent phonons can greatly vary light–matter interaction in semiconductor nanostructures placed inside an optical resonator on a picosecond time scale. For an ensemble of quantum dots (QDs) as active laser medium, phonons are able to induce a large enhancement or attenuation of the emission intensity, as has been recently demonstrated. The physics of this coupled phonon–exciton–light system consists of various effects, which in the experiment typically cannot be clearly separated, in particular, due to the complicated sample structure a rather complex strain pulse impinges on the QD ensemble. Here we present a comprehensive theoretical study how the laser emission is affected by phonon pulses of various shapes as well as by ensembles with different spectral distributions of the QDs. This gives insight into the fundamental interaction dynamics of the coupled phonon–exciton–light system, while it allows us to clearly discriminate between two prominent effects: the adiabatic shifting of the ensemble and the shaking effect. This paves the way to a tailored laser emission controlled by phonons.</p

Editor's Choice: Contemporary treatment of popliteal artery aneurysm in eight countries: A Report from the Vascunet collaboration of registries.

To access publisher's full text version of this article, please click on the hyperlink in Additional Links field or click on the hyperlink at the top of the page marked Files. This article is open access.To study contemporary popliteal artery aneurysm (PA) repair.Vascunet is a collaboration of population-based registries in 10 countries: eight had data on PA repair (Australia, Finland, Hungary, Iceland, New Zealand, Norway, Sweden, and Switzerland).From January 2009 until June 2012, 1,471 PA repairs were registered. There were 9.59 operations per million person years, varying from 3.4 in Hungary to 17.6 in Sweden. Median age was 70 years, ranging from 66 years in Switzerland and Iceland to 74 years in Australia and New Zealand; 95.6% were men and 44% were active smokers. Elective surgery dominated, comprising 72% of all cases, but only 26.2% in Hungary and 39.7% in Finland, (p < .0001). The proportion of endovascular PA repair was 22.2%, varying from 34.7% in Australia, to zero in Switzerland, Finland, and Iceland (p < .0001). Endovascular repair was performed in 12.2% of patients with acute thrombosis and 24.1% of elective cases (p < .0001). A vein graft was used in 87.2% of open repairs, a synthetic or composite graft in 12.7%. Follow-up was until discharge or 30 days. Amputation rate was 2.0% overall: 6.5% after acute thrombosis, 1.0% after endovascular, 1.8% after open repair, and 26.3% after hybrid repair (p < .0001). Mortality was 0.7% overall: 0.1% after elective repair, 1.6% after acute thrombosis, and 11.1% after rupture.Great variability between countries in incidence of operations, indications for surgery, and choice of surgical technique was found, possibly a result of surgical tradition rather than differences in case mix. Comparative studies with longer follow-up data are warranted

In situ synchrotron investigation of degenerate graphite nodule evolution in ductile cast iron

Ductile cast irons (DCIs) are of increasing importance in the renewable energy and transportation sectors. The distribution and morphology of the graphite nodules, in particular the formation of degenerate features during solidification, dictate the mechanical performance of DCIs. In situ high-speed synchrotron X-ray tomography was used to capture the evolution of graphite nodules during solidification of DCI, including degenerate features and the effect of the carbon concentration field. The degeneration of nodules is observed to increase with re-melting cycles, which is attributed to Mg-loss. The dendritic primary austenite and carbon concentration gradients in the surrounding liquid phase were found to control nodule morphology by locally restricting and promoting growth. A coupled diffusion-mechanical model was developed, confirming the experimentally informed hypothesis that protrusions form through liquation cracking of the austenite shell and subsequent localised growth. These results provide valuable insights into the solidification kinetics of cast irons, supporting the design of advanced alloys