A novel technique for the dynamic characterization of metals from a single Taylor impact test is proposed. This computational characterization procedure is based on the formulation and solution of a ...first class inverse problem, in which the silhouette of the Taylor specimen’s final shape is expressed as a vector of its geometrical moments and used as input parameter. The inverse characterization problem is reduced to an optimization problem where the optimum material parameters for the Cowper–Symonds material model are determined. The optimization process is performed by a range adaptation real-coded genetic algorithm. Numerical example for the characterisation of 1018 steel is implemented and presented to validate the methodology presented in this paper. The effectiveness and simplicity of the proposed characterization procedure makes it an appropriate tool for the characterization of metals at high strain rates.
Large variability in dissolved organic carbon (DOC) uptake rates has been reported for headwater streams, but the causes of this variability are still not well understood. Here we assessed acetate ...uptake rates across 11 European streams comprising different ecoregions by using whole‐reach pulse acetate additions. We evaluated the main climatic and biogeochemical drivers of acetate uptake during two seasonal periods. Our results show a minor influence of sampling periods but a strong effect of climate and dissolved organic matter (DOM) composition on acetate uptake. In particular, mean annual precipitation explained half of the variability of the acetate uptake velocities (VfAcetate) across streams. Temperate streams presented the lowest VfAcetate, together with humic‐like DOM and the highest stream respiration rates. In contrast, higher VfAcetate were found in semiarid streams, with protein‐like DOM, indicating a dominance of reactive, labile compounds. This, together with lower stream respiration rates and molar ratios of DOC to nitrate, suggests a strong C limitation in semiarid streams, likely due to reduced inputs from the catchment. Overall, this study highlights the interplay of climate and DOM composition and its relevance to understand the biogeochemical mechanisms controlling DOC uptake in streams.
Plain Language Summary
Headwater streams receive and degrade organic carbon and nutrients from the surrounding catchments. That degradation can be assessed by measuring the uptake of simple compounds of carbon or nitrogen such as acetate or nitrate. Here we determine the variability in acetate and nitrate uptake rates across headwater streams and elucidate the mechanisms behind that variability. The balance between nutrients, the composition of the organic materials present in the streams, and the climatic background is at interplay.
Key Points
Across ecoregions, mean annual precipitation predicts 50% of the variability in dissolved organic carbon uptake
The interplay between dissolved organic matter composition and nutrients availability underlies the climatic pattern and drives uptake
Dissolved organic carbon uptake (as acetate) is negatively related to ecosystem respiration across regions
The Colombian aeronautical industry has stimulated research into the mechanical behaviours of materials under the different loading conditions that aircrafts are generally exposed during their ...operation. Calima T-90, the first military aircraft that was built in Colombia, is used for the primary flights training of Colombian Air Force pilots, and is thus often exposed to adverse operating situations, such as hard landings, which cause impact loads and can subsequently produce impact fatigue. The Calima T-90's structure is mainly comprised of composites materials, generating assemblies and subassemblies of different components of it. The main method of bonding these components is by using adhesive joints with different thicknesses. This study aimed to characterise these adhesive joints as they were placed under typical loads. For this purpose, an evaluation of the effects of adhesive thickness on the mechanical performances of such joints was accomplished and conducted using quasi-static loading conditions and impact fatigue, employing single lap-joint (SLJ) specimens. Cyclic impacts were induced using a drop weight impact testing machine, in order to obtain the characteristic impact-fatigue life diagrams. The evolution of dynamic parameters was then analysed such as residual strength, impact contact time and stiffness degradation; this was done for all the adhesive thicknesses tested. A modified phenomenological prediction model for strength degradation and a cycle to failure prediction model in terms of the adhesive thickness of the joint, mass and the impact strain rate were proposed. It was found that the impact fatigue life of the joints decreased as adhesive thicknesses increased under the same linear momentum, however, this did not occur when the specimens were subjected to similar impact energy values. Subsequently, an analysis of the fracture surfaces of the specimens was conducted that, considered the mechanical interactions between the substrate and the adhesive, fiber tearing and debonding from the composite matrix was found as well as delamination, resulting in a beneficial adhesion mechanism.
Numerical simulations of high strain-rate events require robust characterization of constitutive material parameters in order to predict accurately the deformation process. Historically, this ...characterization has been performed using non-standardized techniques such as the Taylor impact test, where specimen's final deformed geometry is used to determine the material constitutive parameters by inverse analysis. However, validation of material parameters found in this way, in terms of internal plastic strains, is an open research problem. In this work, material constitutive parameters of steel AISI 1010 found by inverse analysis from Taylor tests at ~5×103s−1 were validated. This was done by comparing the Vickers hardness profile along the axis of a deformed specimen against the numerically calculated hardness from a finite element model of the Taylor impact, which used the constitutive parameters found by inverse analysis. To map the simulated effective plastic strains found in the model into Vickers hardness, an experimentally-derived analytical relation between compressive quasi-static plastic strains and Vickers hardness on cylinders of the same material was used. It was found that the material constitutive parameters of steel AISI 1010 characterized by inverse analysis, from silhouettes of deformed Taylor tests, are capable to predict the plastic-strain distribution inside the deformed material.
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•Constitutive parameters of steel AISI 1010 found by inverse analysis from Taylor impact tests.•Constitutive parameters determined from silhouette of deformed Taylor specimens.•Validation of constitutive parameters by mapping the effective plastic strains into Vickers hardness.•Comparison of Vickers hardness of deformed specimens with numerically calculated hardness from FE model of Taylor impacts.•Experimentally-derived analytical relation between compressive quasi-static plastic strains and Vickers hardness.
The main aim of this paper is to investigate the behaviour of adhesively bonded CFRP joints subjected to cyclic low-velocity impacts and to compare this with fracture in specimens tested in standard ...fatigue (i.e. non-impacting, constant amplitude, sinusoidal fatigue). It is seen that the accumulated energy associated with damage in impact-fatigue is significantly lower than that associated with similar damage in standard fatigue and that the mechanisms of failure are very different for the two loading regimes. For both types of loading, fracture initiates in the adhesive layer and then propagates into the 0° ply of the composite adjacent to the adhesive layer. However, the fracture surfaces after impact-fatigue are generally less uniform and exhibit more signs of high rate/brittle fracture than seen in the fracture surfaces after standard fatigue testing. Various parameters are proposed to characterise damage in standard and impact-fatigue and it is shown that crack velocity, accumulated absorbed energy and normalised maximum force are all useful parameters for characterising damage evolution.
The response of adhesive joints to three fatigue regimes, namely; constant amplitude sinusoidal loading (standard fatigue, SF), cyclic in-plane impacts (impact-fatigue, IF) and a combination of the ...two (CSIF), has been investigated. The samples used in this study were carbon fibre-reinforced polymer (CFRP) lap–strap joints (LSJs) bonded with a rubber modified epoxy adhesive. It was observed that fatigue fracture at very low load amplitudes occurred in IF. Two main patterns of failure were observed in SF; cohesive failure in the adhesive, which was linked to slow fatigue crack growth behaviour, and a mixed-mode failure, involving failure in both the adhesive and the CFRP. In addition, it was observed that the transition from cohesive to mixed-mode failure was accompanied by crack growth acceleration. In IF it was seen that all failure was of a mixed-mechanism nature. In the combined standard and impact-fatigue it was seen that the introduction of a relatively small number of impacts between SF blocks drastically changed the dynamics of fatigue crack propagation, increasing the crack rate. A further observation was that cavitation of rubber particles in the adhesive, which is seen as evidence of active toughening, was affected by the addition of impact loading.
There is increasing interest in the effects of low-velocity impacts produced in components and structures by vibrating loads. This type of loading is known as impact-fatigue. The main aim of this ...paper is to investigate the behaviour of adhesive joints exposed to low-velocity impacting, to study the impact-fatigue life and to compare this loading regime with standard fatigue (i.e. non-impacting, constant amplitude, sinusoidal fatigue). To this effect, bonded aluminium single lap joints have been subjected to multiple impacting tensile loads and it has been shown that this is an extremely damaging load regime compared to standard fatigue. Two modifications of the accumulated time-stress model have been proposed to characterise the impact-fatigue results presented in this paper. The first model has been termed the
modified load-time model and relates the total cumulative loading time of the primary tensile load wave to the mean maximum force. The second model attempts to characterise sample damage under impact-fatigue by relating the maximum force normalised with respect to initial maximum force to the accumulated loading time normalised with respect to the total accumulated loading time. This model has been termed the
normalised load-time model. It is shown that both models provide a suitable characterisation of impact-fatigue in bonded joints.
The use of osteosynthesis materials when complex fractures are presented is well known. However, the use of these materials has not achieved a correct fixation and reduction of all bone fragments. ...Therefore, an adhesive for bones would provide a simple and quick method to fix this kind of fractures. The aim of this work is to propose and to evaluate an adhesive based on chitosan hydrogels that could have a potential use as a bone adhesive underwater and will not develop cytotoxic effects. Ionically and covalently crosslinked hydrogels based on chitosan were used in this study. Butt joint test with bovine cancellous bone specimens were used in order to measure the tensile bond strength (TBS) in ideal (completely dry) and physiological (immersed in water at 37 °C) conditions. Additionally, TBS was estimated as a function of time of bone specimens immersed in water at 37 °C. Cell viability was studied using MTT assay and cell morphology on the adhesive surface was examined by scanning electron microscope. Mechanical studies revealed that only covalently crosslinked hydrogels maintain their TBS at physiological condition with respect to the dry environment. In addition, it was observed that the TBS, using only covalently crosslinked hydrogels adhesives, dramatically changes as a function of time and its behavior increases as calcium carbonate and hydroxyapatite is added. Finally, in vitro cell testing of covalently crosslinked hydrogel with calcium carbonate and hydroxyapatite exhibited excellent biocompatibility. Therefore, this formulation is proposed as a potential candidate for clinical use in orthopedic surgery.
To assess the effectiveness and safety of an intravitreal injection of 1.25 mg bevacizumab (IVB) as a preoperative adjunct to small-gauge pars plana vitrectomy (PPV) compared with PPV alone in eyes ...with tractional retinal detachment secondary to proliferative diabetic retinopathy.
This prospective, double-masked, randomized, multicenter, active-controlled clinical trial enrolled 224 eyes of 224 patients between November 2013 and July 2015. All eyes underwent a baseline examination including best-corrected visual acuity, color photos, optical coherence tomography, and fluorescein angiography. Data were collected on intraoperative bleeding, total surgical time, early (<1 month) postoperative vitreous hemorrhage, and mean change in best-corrected visual acuity at 12 months. P < .05 was considered statistically significant.
A total of 214 patients (214 eyes) were randomized in a 1:1 ratio to PPV plus IVB (study group 102 eyes) or PPV plus sham (control 112 eyes). Iatrogenic retinal breaks were noted intraoperatively in 35 eyes (34.3%) in the study group, and 66 eyes (58.9%) in the control group (P = .001). Grade 2 intraoperative bleeding was noted in 32 (31.3%) eyes in the study group and 58 (51.7 %) eyes in the control group (P = .001). Endodiathermy was necessary in 28 (27.4 %) eyes in the study group, compared with 75 (66.9%) eyes in the control group (P = .0001). Mean surgical time was 71.3 ± 32.1 minutes in the study group and 83.6 ± 38.7 minutes in the control group (P = .061).
Preoperative IVB seems to reduce intraoperative bleeding, improving surgical field visualization, and reducing intraoperative and postoperative complications. NOTE: Publication of this article is sponsored by the American Ophthalmological Society.
The present work focuses on the discussion of the λ Bootis nature of the multiperiodic δ Scuti star HD 192640 (29 Cyg), through a comprehensive asteroseismic modeling. Some of the most recent ...asteroseismic tools are used to check whether the observed low metallicity is internal, i.e., intrinsic, present throughout the star, or due to superficial processes as accretion, diffusive settling, radiative levitation, mass loss, etc. The modeling method uses some of the most recent tools, including: (1) effects of rotation on equilibrium models, on the adiabatic oscillation spectrum, and its influence in multicolor observables, (2) nonadiabatic stability of radial and nonradial modes, (3) inclusion of the atmosphere-pulsation interaction for a more accurate multicolor mode identification, and (4) ratio between radial modes n = 4 and n = 5 in the framework of Petersen diagrams. The analysis performed reveals that the models fulfilling all the constraints are those in the middle of the main sequence (MS), with subsolar metallicity, except some other unlikely possibilities. Therefore, this study does not support the idea of the λ Bootis stars being zero-age MS or pre-MS stars interacting with their primordial cloud of gas and dust, but suggest the explanation of their nature as submetallic MS objects. Nevertheless, more accurate multicolor photometric observations are required for a more conclusive study using the procedure presented here, since the observational errors are too large for a definitive rejection of any of the possible explanations.
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