Damage from low-temperature irradiation and the subsequent degradation of materials performance pose sig-nificant challenges for the storage of radioactive materials and for peripheral components in some nuclear reactor designs. Fully understanding the mechanical behavior of such materials requires test data for strain rates in both the quasi-static (< 10/s) and dynamic (>> 10/s) regimes. While dynamic testing has generally been avoided in the past for neutron irradiated (contamination concerns) and ion irradiated (insufficient volume) materials, surface-sensitive Richtmyer-Meshkov instability (RMI) tests were used in the present work to overcome these limitations. Here, nanopillar compression, nanoindentation, and RMI testing data from a helium implanted surface layer (similar to 10 mu m thick) were compiled to explore the effects of helium bubbles on the materials strength of high-purity copper at strain rates of 0.001/s - 10(8)/s. While nano-mechanical testing revealed increases in yield strength and hardness with increasing helium dose from 1000 to 4000 appm He, RMI indicated no significant changes in strength as compared to unimplanted copper. This discrepancy in behavior was rationalized through a combination of recent literature and follow-on molecular dynamics (MD) simulations, leading to the conclusion that the nanoscale helium bubbles acting as dispersed barriers to dislocation motion at quasi-static strain rates collapse under shock loading and cease to be effective barriers at high strain rates.

The California Maternal Quality Care Collaborative (CMQCC) proposed an algorithm to reduce maternal mortality by screening for cardiac disease in pregnancy, including use of maternal heart rates. The effect of this guideline was assessed at an outpatient obstetrics office by studying rates of heart rate screening before and after implementation. This study reviewed 100 return in-person prenatal visits monthly from January 2019 to November 2021. Maternal heart rate, demographics, and presence of chronic hypertension or cardiac disease were noted. Rates of heart rate collection were compared pre- and post-guideline implementation. Per the CMQCC guideline, a resting heart rate of ≥ 110 bpm was deemed high enough to warrant further evaluation. This retrospective cohort included 3,478 visits (100 visits per month, fewer visits in April and May 2020 due to COVID19 pandemic). Several visits (n=382, 11%) were to patients with chronic hypertension and 159 (4.6%) had underlying cardiac disease. Overall, 1798 (51.7%) visits recorded a maternal heart rate, while 1680 (48.3%) did not. More visits had heart rates recorded post- than pre-guideline (67.6% versus 21.4% of visits, respectively; p < 0.001). Maternal heart rates ≥ 110 bpm were recorded at 130 visits (3.7%). Pre-guideline, 29.5% of patients with heart disease and/or chronic hypertension had heart rate recorded compared to 20.1% of patients without these conditions (p=0.005). Post-guideline, patients with heart disease and/or chronic hypertension were more likely to have heart rate recorded compared to those without these conditions (78.8% versus 65.8%, p < 0.001). Elevated heart rates ≥ 110 were recorded at 34 of 308 (11.0%) visits to patients with known hypertension and/or cardiac disease, compared to 96 out of 1490 (6.4%) to patients without these diagnoses (p=0.005). Maternal heart rate screening significantly increased after guideline implementation. Patients with underlying cardiac disease and/or chronic hypertension were nearly twice as likely to have a heart rate ≥ 110 bpm when heart rate was assessed.View Large Image Figure ViewerDownload Hi-res image Download (PPT)

The mechanisms responsible for the collapse of helium-filled bubbles during the passage of shock waves in monocrystalline copper are revealed. Both internal pressure (caused by pre-existing helium atoms) and bubble size are varied in molecular dynamics simulations to understand the atomistic scale deformation as they are subjected to shock compression at pressures of 48, 123, and 170 GPa, corresponding to particle velocities of 1.0, 2.0, and 2.5 km/s. Both empty and helium filled bubbles serve as dislocation sources, generating intense, localized plastic regions. There are distinct differences in the collapse of empty voids compared to He-filled bubbles, the former requiring less stress and generating a greater density of dislocations for a given shock strength. A generalized model for dislocation emission is proposed, where the inclusion of shear stress generated by the helium bubble increases the critical stress to generate dislocations at the defect surface, demonstrating the change in plastic deformation.

There has been a challenge for many decades to understand how heterogeneities influence the behavior of materials under shock loading, eventually leading to spall formation and failure. Experimental, analytical, and computational techniques have matured to the point where systematic studies of materials with complex microstructures under shock loading and the associated failure mechanisms are feasible. This is enabled by more accurate diagnostics as well as characterization methods. As interest in complex materials grows, understanding and predicting the role of heterogeneities in determining the dynamic behavior becomes crucial. Early computational studies, hydrocodes, in particular, historically preclude any irregularities in the form of defects and impurities in the material microstructure for the sake of simplification and to retain the hydrodynamic conservation equations. Contemporary computational methods, notably molecular dynamics simulations, can overcome this limitation by incorporating inhomogeneities albeit at a much lower length and time scale. This review discusses literature that has focused on investigating the role of various imperfections in the shock and spall behavior, emphasizing mainly heterogeneities such as second-phase particles, inclusions, and voids under both shock compression and release. Pre-existing defects are found in most engineering materials, ranging from thermodynamically necessary vacancies, to interstitial and dislocation, to microstructural features such as inclusions, second phase particles, voids, grain boundaries, and triple junctions. This literature review explores the interaction of these heterogeneities under shock loading during compression and release. Systematic characterization of material heterogeneities before and after shock loading, along with direct measurements of Hugoniot elastic limit and spall strength, allows for more generalized theories to be formulated. Continuous improvement toward time-resolved, in situ experimental data strengthens the ability to elucidate upon results gathered from simulations and analytical models, thus improving the overall ability to understand and predict how materials behave under dynamic loading.

The interaction of shock waves with non-planar free surfaces can cause atoms to eject from the surface, leading to the formation of ejecta. These non-planarities in the free surface can occur due to machining of the free surface or can be induced in the shock wave itself due to the presence of heterogeneities in the material. Both cases lead to the formation of ejecta. While the effect of machining on ejecta has been well studied, the latter has not been a topic of significant investigations. In this work, molecular dynamics simulations are used to systematically investigate the effect of size and concentration of He bubbles in Cu with planar free surfaces on ejecta production. It is shown that the presence of defects leads to the formation of non-planarity in the shock wave itself producing ejecta as the front reaches the flat free surface. The cluster size and velocity of ejected particles greatly exceeds that of pure Cu; the radius, density, and nature of the helium-filled voids alter the mass, velocity, and size distribution of the ejected matter.

Background: Internationally, the COVID-19 pandemic severely curtailed access to hospital facilities for those awaiting elective/semi elective procedures. For allergic children in Ireland, already waiting up to 4yr for an elective oral food challenge (OFC), the restrictions signified indefinite delay. At the time of the initiative there were approx 900 children on the Chidren’s Health Ireland(CHI) waiting list. In July 2020, a project was facilitated by short term(6wk) access to an empty COVID stepdown facility built, in a hotel conference centre, commandeered by the Health Service Executive Ireland(HSE). The aim was to the achieve rapid rollout of an off-site OFC service, delivering high throughput of long waiting patients, while aligning with hospital existing policies and quality standards, international allergy guidelines and national social distancing standards. Methods: The working group engaged key stakeholders to rapidly develop an offsite OFC facility. Consultant Paediatric Allergists, Consultant Paediatricians, trainees and Allergy Clinical Nurse Specialists were seconded from other duties. The facility was already equipped with hospital beds, bedside monitors(BP, Pulse, Oxygen saturation) bedside oxygen. All medication and supplies had to be brought from the base hospital. Daily onsite consultant anaesthetic cover was resourced and a resuscitation room equipped. Standardised food challenge protocols were created. Access to onsite hotel chef facilitated food preparation. A risk register was established. Results: After 6wks planning, the remote centre became operational on 7/9/20, with the capacity of 27 OFC/day. 474 challenges were commenced, 465 (98%) were completed, 9(2%) were inconclusive. 135(29.03%) OFC were positive, 25(5%) causing anaphylaxis. No child required advanced airway intervention. 8 children were transferred to the base hospital. The CHI allergy waiting list was reduced by almost 60% in only 24 days. Conclusions: OFCs remain a vital tool in the care of allergic children, with their cost saving and quality of life benefits negatively affected by delay in their delivery. This project has shown it is possible to have huge impacts on a waiting list efficiently, effectively and safely with good planning and staff buy in – even in a pandemic. Adoption of new, flexible and efficient models of service delivery will be important for healthcare delivery in the post-COVID-19 era.

The effect of He concentration and morphology on ejecta production is investigated via molecular dynamics simulations. Identical He concentrations are inserted into Cu single crystals as interstitial atoms or bubbles near a flat free surface. The resulting ejecta is quantified through total mass, cluster size, and velocity of ejected particles. The presence of He increases total ejected mass as compared to pure Cu; He bubbles produce 56% more mass than atomic He. This increase is attributed to non-planarities in the shock front and reflected pulse due to He bubbles, akin to ejecta resulting from traditional Richtmeyer–Meshkov instabilities.

We examine the effect of grain size on the dynamic failure of tantalum during laser-shock compression and release and identify a significant effect of grain size on spall strength, which is opposite to the prediction of the Hall-Petch relationship because spall is primarily intergranular in both poly and nanocrystalline samples; thus, monocrystals have a higher spall strength than polycrystals, which, in turn, are stronger in tension than ultra-fine grain sized specimens. Post-shock characterization reveals ductile failure which evolves by void nucleation, growth, and coalescence. Whereas in the monocrystal the voids grow in the interior, nucleation is both intra- and intergranular in the poly and ultra-fine-grained crystals. The fact that spall is primarily intergranular in both poly and nanocrystalline samples is a strong evidence for higher growth rates of intergranular voids, which have a distinctly oblate spheroid shape in contrast with intragranular voids, which are more spherical. The length of geometrically-necessary dislocations required to form a grain-boundary (intergranular) void is lower than that of grain-interior (intragranular) void with the same maximum diameter; thus, the energy required is lower. Consistent with prior literature and theory we also identify an increase with spall strength with strain rate from 6 x 10(6) to 5 x 10(7) s(-1). Molecular dynamics calculations agree with the experimental results and also predict grain-boundary separation in the spalling of polycrystals as well as an increase in spall strength with strain rate. An analytical model based on the kinetics of nucleation and growth of intra- and intergranular voids and extending the Curran-Seaman-Shockey theory is applied which shows the competition between the two processes for polycrystals. (C) 2018 Published by Elsevier Ltd on behalf of Acta Materialia Inc.

While silicon carbide (SiC) has been predicted to undergo pressure-induced amorphization, the micro structural evidence of such a drastic phase change is absent as its brittleness usually prevents its successful recovery from high-pressure experiments. Here we report on the observation of amorphous SiC recovered from laser-ablation-driven shock compression with a peak stress of approximately 50 GPa. Transmission electron microscopy reveals that the amorphous regions are extremely localized, forming bands as narrow as a few nanometers. In addition to these amorphous bands, planar stacking faults are observed. Large-scale non-equilibrium molecular dynamic simulations elucidate the process and suggest that the planar stacking faults serve as the precursors to amorphization. Our results suggest that the amorphous phase produced is a high-density form, which enhances its thermodynamical stability under the high pressures combined with the shear stresses generated by the uniaxial strain state in shock compression. (C) 2018 Published by Elsevier Ltd on behalf of Acta Materialia Inc.