Critical Loading Rate Research Articles

Optimizing the inclined plate settler for a high-rate microaerobic activated sludge process for domestic wastewater treatment: A theoretical model and experimental validation

This study used a theoretical model and an experimental system to investigate the feasibility of using inclined plate settlers (IPSs) to upgrade the secondary clarifier for bulking sludge recovery in the high-rate microaerobic activated sludge (MAS) process that achieved enormous reduction in aeration energy consumption during domestic wastewater treatment. Compared to bulking sludge concentration (1.2–4.2 g/L) (and thus varied sludge zone settling velocities), both the experimental results and the theoretical predictions demonstrated that the plate length-to-spacing (L/D) ratio (3.75–15) of the IPSs likely plays a decisive role in determining its treatment capacity of the IPSs for bulking sludge recovery. The theoretical model was finally simplified to predict the critical surface loading rate of the IPSs as a function of the L/D ratio. The maximum biomass concentration in the micro-aeration tank of the high-rate MAS process was set at 4.5 g/L to assure effective sludge/water separation and bulking sludge recovery. Lastly, the continuous operation of the high-rate MAS-IPS process over a lengthy period achieved stable effluent turbidity of <4 NTU and COD and NH4+-N removals of 97 ± 2% and 93 ± 6%, respectively, when the treatment capacity of secondary clarifier was enhanced by >28%.

Insights into Anaerobic Co-Digestion of Lignocellulosic Biomass (Sugar Beet By-Products) and Animal Manure in Long-Term Semi-Continuous Assays

Biogas production through anaerobic digestion has proven to be one of the most important pillars of the transition into the circular economy concept, a sustainable approach for biorefinery. This work aims to extend and improve knowledge in the anaerobic co-digestion of complementary substrates, given insights into wastes biodegradability and the influence of manure composition on the anaerobic process stability. Anaerobic co-digestion of sugar beet by-products with two kinds of animal manure (pig and cow) was investigated in semi-continuous assays, analyzing both common and non-classical parameters. Co-digestion with manure clearly mitigated the inhibitory effect of volatile fatty acids at high organic loading rates, leading to increases in methane production by 70% and 31% in comparison with individual digestion of sugar beet by-products, for co-digestion with pig and cow manure, respectively. Non-classical parameters could give more insight into the coupling/uncoupling of the anaerobic digestion phases and the involved microorganisms. Indirect parameters indicated that the process failure at the critical organic loading rates was mainly due to methanogenesis inhibition in the co-digestion with pig manure, while in co-digestion with cow manure or in individual digestion of sugar beet by-products, both hydrolysis–acidogenesis and methanogenesis phases were affected. Biomethanation degree refers to the maximum methane potential of organic wastes. Sugar beet by-products required a long digestion-time to reach high biodegradability. However, short digestion-times for co-digestion assays led to a high biomethanation degree.

Effect of 17-4PH stainless steel powders interaction on feedstocks

The objective of this work is to investigate the effects of 17-4PH stainless steel powders interaction on properties of feedstocks. γ-aminopropyltriethoxy silane (KH550), γ-(2,3-epoxypropoxy)propyltrimethoxy silane (KH560), and propyltrimethoxy silane (N313) were used to modify 17-4PH stainless steel powders, and described as KH550-m, KH560-m and N313-m with amino, epoxy and propyl group, respectively. The properties of feedstocks prepared from the pristine, KH550-m, KH560-m and N313-m were compared with that of equal amount of KH550-m and KH560-m (KH550-m/KH560-m). The experimental results revealed that the critical solids loading, density and melt flow rate of KH550-m/KH560-m feedstocks were 62.8 vol%, 5.24 g/cm3 and 51.42 g/10 min, respectively, which were higher than that of others. Besides, the debinded parts shape retention of KH550-m/KH560-m was also better than others. The good properties of KH550-m/KH560-m feedstocks originate from the strong interaction between KH550-m and KH560-m, which comes from the reaction and hydrogen bond between amino group and epoxy group.

Experimental study on ductile-to-brittle behavior and crystal orientation in micro-crack induced severing of 4H-SiC and sapphire

For the scribing-to-severing method, it is difficult to sever single-crystal SiC and sapphire without edge fracture and near-zero material removal due to material hardness, brittleness and crystal orientation. According to the micron-scale brittle behavior, a crack-free microgroove-apex is proposed on single-crystal SiC and sapphire surfaces to induce the micro-crack propagation. The objective is to understand how the ductile-to-brittle behavior and crystal orientation to influence the severing performance. In order to induce the micro-crack propagation in severing, the microgroove-apex was machined on the workpiece surface by the micro-grinding. First, the micro-indentation experiments were performed to explore the ductile-to-brittle behavior; then, the severing force, efficiency and accuracy were investigated in severing experiments, respectively. It is shown that the micro-crack propagation ratio derived from the micro-indentation may display the ductile-to-brittle behavior in relation to the severing performance. With larger harness and less micro-crack propagation ratio, the 4H-SiC leads to larger severing force, longer severing time and less form error than the sapphire does, respectively. Moreover, the severing force and the severing time are sensitive to the crystal orientation. Because the micro-crack propagation behavior is changed by the loading rate, the static severing is transferred to the dynamic severing at the critical loading rate, thus leading to different severing behavior. Along an accurate micro-crack propagation, the precision severing depends on a sharpness and smoothness microgroove-apex.

Loading rate dependency of Mode I interlaminar fracture toughness for unidirectional composite laminates

This study was conducted to investigate the effect of the loading rate on the Mode I interlaminar fracture toughness of unidirectional carbon/epoxy laminates. Double cantilever beam (DCB) test geometry was employed for both quasi-static and dynamic fracture tests. A novel dual electromagnetic Hopkinson bar was employed to load the DCB specimens dynamically and symmetrically with velocities in the range of 10–30 m/s. A hybrid experimental-numerical method was used to determine the interlaminar fracture toughness for both quasi-static and dynamic loading conditions using the virtual crack closure technique (VCCT). The results indicate the presence of a critical loading rate for interlaminar fracture, below which the fracture toughness remains constant and beyond which the fracture toughness increases rapidly. Fractography results suggest that the failure mechanism transitions from a fiber/matrix interface failure under quasi-static loading to a brittle cleavage fracture of the matrix material with microbranching under dynamic loading.

Combined effect of loading rate and percentage by weight of MWCNTs on inter laminar shear strength (ILSS) and flexural strength of CFRP

The mechanical performances of fiber reinforced polymer (FRP) composites depend upon the factors like types of reinforcement added and loading rate to which it is subjected. The present paper investigates the influence of multi-walled carbon nanotubes (MWCNTs) doping and strain rate on the properties like inter laminar shear strength (ILSS) and flexural strength of composites. Two samples i.e., neat carbon/epoxy composites and 0.25 wt. % of MWCNTs reinforced CFRP were prepared and tested. The specimens were tested on Hounsfield H50KS, universal testing machine (UTM). The SBS and three-point bend tests were done as per ASTMD 2344 and ASTMD 7264 standards. The hand lay-up technique assisted by vacuum bagging was used to fabricate the symmetrically designed CFRP laminates with eight layers of 0.5 mm thickness each. The prepared samples were tested at the three different crosshead speeds of 1, 100 and 250 mm/min. The results obtained in this paper indicate that the carbon/epoxy composites doped with MWCNTs showed higher strength as compared to neat CFRP at the corresponding loading rates. Also, it was detected that there was the existence of a critical loading rate at which the mechanical properties, ILSS and flexural strength was enhanced up to its maximum limit. Moreover, the FESEM images of fractured samples were taken to observe the inter laminar shear and bending failure.

Avalanche Fatalities during Atmospheric River Events in the Western United States

Abstract The occurrence of atmospheric rivers (ARs) in association with avalanche fatalities is evaluated in the conterminous western United States between 1998 and 2014 using archived avalanche reports, atmospheric reanalysis products, an existing AR catalog, and weather station observations. AR conditions were present during or preceding 105 unique avalanche incidents resulting in 123 fatalities, thus comprising 31% of western U.S. avalanche fatalities. Coastal snow avalanche climates had the highest percentage of avalanche fatalities coinciding with AR conditions (31%–65%), followed by intermountain (25%–46%) and continental snow avalanche climates (&amp;lt;25%). Ratios of avalanche deaths during AR conditions to total AR days increased with distance from the coast. Frequent heavy to extreme precipitation (85th–99th percentile) during ARs favored critical snowpack loading rates with mean snow water equivalent increases of 46 mm. Results demonstrate that there exists regional consistency between snow avalanche climates, derived AR contributions to cool season precipitation, and percentages of avalanche fatalities during ARs. The intensity of water vapor transport and topographic corridors favoring inland water vapor transport may be used to help identify periods of increased avalanche hazard in intermountain and continental snow avalanche climates prior to AR landfall. Several recently developed AR forecast tools applicable to avalanche forecasting are highlighted.

On the micromechanics of void dynamics at extreme rates

In an earlier work, we derived evolution equations governing the dynamic growth of voids considering the combined retarding effects of micro-inertia and dislocation kinetics. Here we derive closed-form approximate solutions to these governing equations that accurately capture the transient evolution of void size history. In addition to being invaluable to the development of computationally-efficient dynamic ductile failure frameworks, these closed-form models help reveal deeper insights into the underlying physics governing complex dynamic void growth. In particular, we show that the transient effects of inertia are important for families of voids that nucleate from coarse, micron-sized second phase particles. For nano-sized voids the finite kinetics of dislocation substructure evolution towards a saturated state play a vital role. In spite of the significant complexity associated with capturing an evolving substructure, an elegant closed-form approximation presents itself. These closed-form models are further bolstered by good agreement with molecular dynamics calculations. Lastly, we derive closed-form models of void size distribution evolution to confirm an experimentally observed transition in fracture surface characteristics above a critical loading rate.

Correlation between strain rate sensitivity and α relaxation of metallic glasses

An inherent correlation between the strain rate sensitivity and α relaxation of metallic glasses (MGs) is observed. This correlation can be attributed to the secondary term which incorporates the nonaffine displacements of atoms in the analytical expression of the elastic modulus of amorphous solids. The observed correlation supports the proposition that stress and temperature play equivalent role in the glass transition of MGs. Besides, an ideal liquid state of MGs is observed in the supercooled liquid region when they are deformed below a critical loading rate. This observation would benefit the application of MGs in the fabrication of micro parts for MEMS (Micro Electro-Mechanical Systems).

An Evaluation Method for the Bursting Characteristics of Coal Under the Effect of Loading Rate

Changes in loading rate can affect the strength performance and bursting characteristics of coal. Coal samples from the same source were subject to mechanical tests for multi-loading rates. These tests were carried out to determine changes in evaluation indicators for the law of bursting liability due to increases in the loading rate. The results show that indicators, including the uniaxial compressive strength and the bursting energy index, first increased and then decreased with the increase in loading rate. The value of the loading rate, which corresponds to the strength turning point, is named the “critical loading rate”. The results of evaluating the bursting liability of coal (from the same source) show all the possibilities. The critical loading rate corresponds to the highest bursting liability. The “loading rate sensitivity” is proposed to describe the comprehensive response of the bursting characteristics to the loading rate. This index, combined with the scale conversion method, makes the field application of laboratory experimentation possible. This method for evaluating the bursting liability of coal under the effect of loading rate was systematically established and shows good applicability.

A fast numerical procedure for the simulation of inter-laminar damage growth in stiffened composite panels

A novel numerical methodology is used to investigate the behavior of an all-composite wing-box under a compressive load taking into account the presence of concurrent inter-laminar damages such as delaminations and skin-stringer debonds and their propagation. The proposed numerical methodology, based on linear structural analyses, is computationally inexpensive if compared to the VCCT and CZM based numerical techniques, hence, it appears to be particularly suitable for the preliminary design phase of complex structure. The presented linear approach is an evolution of a previous technique developed specifically for delaminations. Indeed, in this paper the linear approach has been enhanced to predict the propagation of inter-laminar damages with open crack surfaces such as skin-stringer debonds and it has been implemented in a commercial Finite Element platform. The effectiveness (in terms of computational cost and accuracy of results) of the suggested linear numerical methodology, in predicting the compressive behavior of complex composite structures with inter-laminar damages, has been confirmed by cross-comparisons with the standard non-linear VCCT technique. Finally, comparisons in terms of critical propagation load and Energy Release Rate distribution along the damage tip predictions, for several examined structural configurations with different damage extension and depth, have allowed to assess the field of applicability of the propose novel numerical methodology.

Analysis of strength and response of polymer nano thin film interfaces applying nanoindentation and nanoscratch techniques

Polymer and hydrogel thin films on a compliant substrate are gaining significant attention these days in the fields of soft electronics, digital display, biomedical devices and sensors to name a few. The functional characteristics of these thin films depend on the performance of its interface with the substrate. The interface response is a non-linear function of surface energy and stiffness of a substrate beside other factors. The deviation of thin film properties under confinement from its bulk counterparts and its ultra-low physical dimension makes characterization of an interface extremely challenging. Probing and shearing of these interfaces with appropriate load value and up to the critical load rate can provide insight into the integrity of these interfaces under different service conditions. We have studied the response of PMMA thin film (150 nm) on a softer epoxy substrate and a stiffer glass substrate of increasing interfacial strength applying nanoindentation (NI) and nanoscratching. Pile up behaviour of the film material at confined state, the critical load at the interface proximity and the plastic work done during indentation as determined in this work can be successfully applied to characterize interface behaviours. These parameters also help in developing interface models and build qualitative correlations between thermodynamic work of adhesion and true mechanical work of adhesion at the interface. We hypothesize that the elasto-plastic region around load tip plays a significant role in determining pile up above film surface. We have discussed the possible molecular mechanisms in this region under stress leading to pile up.

Boudinage as a material instability of elasto-visco-plastic rocks

Pinch-and-swell structures are commonly interpreted to evolve out of viscosity contrasts, which are induced by geometric interactions and material imperfections. From materials science an additional localization phenomenon is well established, where localization emerges out of steady state for critical material parameters and/or loading rates. Here we investigate the conditions under which this second type of instabilities prevails and whether geological materials necessarily require a trigger by imperfections in order to generate instability. We focus on imperfections in terms of grain size variations embedded in a deformation environment controlled by thermo-mechanical feedbacks. We introduce a random distribution of grain sizes over two orders of magnitude in a central layer embedded in a matrix with a diffusion creep rheology. The rheology of the layer evolves with dislocation and diffusion creep as end-member deformation mechanisms. Applying pure shear extension, the 3-layer model is subjected to natural deformation conditions. The central layer quickly establishes a viscous steady state as a natural response of the system due to relaxation and energy optimization. Upon continued loading, localization then intriguingly arises out of a homogeneous state. We present an analysis which confirms that this type of instability is indeed physically admissible. Using vibration analysis, we verify the robustness of the numerical solution by first identifying the natural mode shapes and frequencies of the simulated structure and material parameters, including geometric imperfections. In a second step, the eigenmodes are perturbed and superposed to the initial conditions. We conclude that this pattern of perturbations guides the onset of strain localization. Boudinage can therefore be seen both as a geometric problem and/or a material bifurcation, which evolves out of homogeneous state. The latter class offers the great possibility of extracting fundamental material parameters out of localized structures directly from field observations.

The biodegradation of petroleum hydrocarbons in an upflow sludge-blanket/fixed-film hybrid bioreactor under nitrate-reducing conditions: Performance evaluation and microbial identification

The effects of inlet concentrations of total petroleum hydrocarbons (TPH), hydraulic retention time (HRT), salinity, and the presence/absence of nitrate on TPH biodegradation in a continuous upflow sludge-blanket/fixed-film hybrid bioreactor (UAnSFB) operated under nitrate-reducing conditions were investigated. The critical TPH loading rate was around 78g/m3h based on changes in TPH inlet concentration and around 60.5g/m3h based on changes in HRT. The TPH biodegradation was not significantly affected by amounts of NaCl up to 30g/L. TPH biodegradation sharply decreased in UAnSFB from 99.7% in the presence of nitrate to 37.6% only 3days after the nitrate was withdrawn from the influent. 16S rDNA fragment and phylogenetic analysis identified Stenotrophomonas maltophilia and Stenotrophomonas acidaminiphila in biofilm and Staphylococcus warneri and Bacillus cereus in the sludge blanket as the most abundant species. Accordingly, the UAnSFB is a promising bioreactor for the efficient biodegradation of TPH in saline media.

Tribological properties of CrCN/CrN multilayer coatings

The wear of the coating is one of the most important parameters determining its applications. The quality of the coating is highly influenced by the adhesion. An improvement of these properties can open new areas of applications. In this paper, the adhesion and wear results for CrCN/CrN multilayer coatings are presented. The coatings were deposited on HS6-5-2 steel substrate using cathodic arc evaporation. Two series of CrCN/CrN coatings were tested: (1) with fixed bilayer thickness and various thickness ratios of CrCN and CrN layers in the bilayer and (2) with fixed thickness ratios of CrCN and CrN layers in the bilayer and various numbers of bilayers (Λ).The adherence of the coatings to the steel substrate was evaluated using a scratch test and the Daimler–Benz test. In the scratch test, the critical force as the beginning of spalling or delamination of the coating was determined. The tribological properties were defined using the ball-on-disc method with an Al2O3 ball as the counterpart.All the coatings show Lc2 values higher than 80N. The critical load and wear rate depend on the thickness ratio of the CrCN and CrN layers in the bilayer. The increase in the CrCN layer thickness leads to the critical load decrease.

Determination of the critical load and energy release rate in mode II delamination using a meshfree method

AbstractSimulation of fracture by using numerical methods is important to treat geometries that change in time. In this study, both numerical and experimental investigations are presented for the delamination under mode II loading, detailing the derivation of the formulations in numerical simulations of fracture. The simulation of the delamination under mode II loading based on the cohesive segments model was investigated by using a meshfree method. Then, an experimental investigation was used to verify the meshfree method’s results. For tests under mode II loading, three-point end-notched flexure specimens, which are made of carbon/epoxy laminate (AS4/3501-6) which consists of 10 plies in [0]10 and [0/90/0/90/0]s lay-up with delamination inserted in the middle of the laminate, were used for the interlaminar fracture toughness tests. The problem was solved for [0]10, [0/45/-45/90/0]s, [0/90/0/90/0]s, [0/90/0/90/30]s, [0/90/0/90/45]s and [0/90/0/90/60]s laminates with mid-plane delaminations, and the results were verified for different composite materials. The critical fracture force, which can be experimentally measured, was used to calculate the mode II delamination fracture toughness of the carbon/epoxy laminate. In addition, values of the integral for 209 (11×19) and 253 (11×23) background meshes with equivalent interval sizes were compared. For a relatively fine background mesh, the critical load was converged. Results obtained from the meshfree element-free Galerkin method showed very good agreement with experimental data for single-mode delamination under mode II loading. The results presented will help in the implementation of mesh design techniques that protect numerical accuracy while minimizing computational expense.

On the role of spatial dynamics and topology on network flows

Particle flows in spatial networks are susceptible to congestion. In this paper, we analyze the phase transitions of these networks to a state of congested transport and the influence of both topology and spatial dynamics on its emergence. We systematically show that the value of the critical loading rate at which congestion emerges is affected by the addition of spatial dynamics, changing the nature of this transition from a continuous to a discontinuous one. Our numerical results are confirmed by introducing an analytical solvable framework. As a case of study, we explore the implications of our findings in the San Francisco road network where we can locate the roads that originate the congested phase. These roads are spatially constrained, and not necessarily those with high betweenness as predicted by models without spatial dynamics.

A novel bench-scale column assay to investigate site-specific nitrification biokinetics in biological rapid sand filters

A bench-scale assay was developed to obtain site-specific nitrification biokinetic information from biological rapid sand filters employed in groundwater treatment. The experimental set-up uses granular material subsampled from a full-scale filter, packed in a column, and operated with controlled and continuous hydraulic and ammonium loading. Flowrates and flow recirculation around the column are chosen to mimic full-scale hydrodynamic conditions, and minimize axial gradients. A reference ammonium loading rate is calculated based on the average loading experienced in the active zone of the full-scale filter. Effluent concentrations of ammonium are analyzed when the bench-scale column is subject to reference loading, from which removal rates are calculated. Subsequently, removal rates above the reference loading are measured by imposing short-term loading variations. A critical loading rate corresponding to the maximum removal rate can be inferred. The assay was successfully applied to characterize biokinetic behavior from a test rapid sand filter; removal rates at reference loading matched those observed from full-scale observations, while a maximum removal capacity of 6.9 g NH4+–N/m3 packed sand/h could easily be determined at 7.5 g NH4+–N/m3 packed sand/h. This assay, with conditions reflecting full-scale observations, and where the biological activity is subject to minimal physical disturbance, provides a simple and fast, yet powerful tool to gain insight in nitrification kinetics in rapid sand filters.

Compressive mechanical properties of bovine cortical bone under varied loading rates

Bone tissue functions in varied mechanical systems of the body under static and dynamic conditions. Therefore, it is essential to understand the mechanical responses of bone at varied loading rates, especially those at fast loading rates. This study has investigated the effect of loading rate on the compressive mechanical properties of bovine cortical bone. Bone specimens of 3.85 mm in diameter and 7.7 mm in length were compressed longitudinally with the loading rates of 2 to 2000 mm/s (corresponding strain rates of 0.26 to 260 s(-1)). As a result, bovine cortical bone showed high linear elasticity when the loading rate was slow, and exhibited three definite regions of linear elasticity, plastic deformation, and densification at faster loading rates. The elastic modulus showed no dependency on the loading rate. Compressive strength, strain at fracture, and toughness increased as the loading rate increased under the condition that the loading rates were slower than each critical loading rate of 1000, 100, and 1500 mm/s, respectively. However, all showed no significant changes when the loading rates were faster than the corresponding critical loading rates. In conclusion, as the loading rate increased, changes in the compressive mechanical parameters were different depending on the parameter and the loading rate range. Compressive mechanical behaviour of bovine cortical bone showed a brittle nature under high strain rates (strain rates > 13 s(-1)). These findings should be reflected in the biomimetic simulation of biomaterials for bone tissue repair and engineering.

Dynamic Fracture of Concrete–3D Numerical Study of Compact Tension Specimen

The behavior of concrete structures is strongly influenced by the loading rate. Compared to quasi-static loading concrete loaded by impact loading acts in a different way. First, there is a strain-rate influence on strength, stiffness, and ductility, and, second, there are inertia forces activated. Both influences are clearly demonstrated in experiments. For concrete structures, which exhibit damage and fracture phenomena, the failure mode and cracking pattern depend on loading rate. Moreover, theoretical and experimental investigations indicate that after the crack reaches critical speed of propagation there is crack branching. The present paper focuses on 3D finite-element study of the crack propagation of the concrete compact tension specimen. The rate sensitive microplane model is used as a constitutive law for concrete. The strain-rate influence is captured by the activation energy theory. Inertia forces are implicitly accounted for through dynamic finite element analysis. The results of the study show that the fracture of the specimen strongly depends on the loading rate. For relatively low loading rates there is a single crack due to the mode-I fracture. However, with the increase of loading rate crack branching is observed. Up to certain threshold (critical) loading rate the maximal crack velocity increases with increase of loading rate, however, for higher loading rates maximal velocity of the crack propagation becomes independent of the loading rate. The critical crack velocity at the onset of crack branching is found to be approximately 500 to 600 m/s.