Constant Loading Rate Research Articles

Hydrogen starvation mitigation strategies during the start-up of proton exchange membrane fuel cell stack

Hydrogen starvation during start-up loading and rapid load change is a severe threat to the life and durability of proton exchange membrane fuel cell. To mitigate and eliminate hydrogen starvation during start-up loading, a dual-path hydrogen supply strategy is proposed to improve hydrogen diffusion within the stack. Based on constant loading rate continuous loading, two optimized start-up strategies with variable loading rates are presented. Stack voltage, stack average temperature, and single-cell temperature uniformity under different start-up loading strategies are discussed in detail. Experimental results show that at the end of continuous loading with a loading rate of 0.67 A/s, the peak voltage uniformity rate of dual-path hydrogen supply mode is 12.67% lower than that of dead-ended anode mode. In start-up loading from open-circuit state (0 A) to nominal state (40 A), single-cell temperature uniformity index has a maximum reduction of 0.136 °C compared to dead-ended anode mode. Besides, the combination of dual-path hydrogen supply and gradual-decreasing loading rate eliminates voltage overshoot during start-up loading, and reduces the single-cell temperature uniformity index by a maximum of 15.8%. Results indicate that the proposed strategies can mitigate hydrogen starvation, and improve the stability of stack voltage and the temperature uniformity of single cells.

Experimental and calculation investigations of deformation and fracture processes of various types of steels during impact shear tests

The paper proposes the experimental technique for investigating the deformation and fracture of the materials under dynamic shear. The authors have tested the specimens with two shear zones. A series of numerical calculations are performed using the finite element method. The variation of the shear zone length in such specimens allows one to change the strain rate in these zones at a constant loading rate. The specimens are made of the following steels: steel 20, Hardox 400 high strength steel, and ARMOX 500T armored steel. The tests are made on the instrumented drop-weight impact testing machine with a high-speed registration system. The obtained results make it possible to assess the influence of many factors (strain rate, specimen geometry, material, and direction of the specimen cutting) on the deformation and fracture of materials under dynamic shear conditions. It is employed to specify the parameters of the material models for the numerical simulation of complex loading processes, as well as to develop the experimental method for the mechanical characteristics determination considering fracture during dynamic shear.

The relationship between the thickness and flexural strength of unidirectional carbon fiber reinforced polymers manufactured through VARI

The three-point bending tests at constant loading rates and constant span to thickness ratio were carried out to investigate the behavior and effect of thickness on unidirectional carbon fiber reinforced polymers (UD-CFRPs). The specimens were manufactured in different thicknesses through the vacuum assisted resin infusion (VARI) technique. Experiments were conducted at strain rate of 0.01 s−1 with constant span-to-thickness ratio of 40:1 according to ASTM D7264/D7264M-15 test standard. It was observed from the results that the thicker the laminate, the higher the peak force, and less strain the structure can withstand. The failure stress and strain curves revealed that the less thick the specimen is, the higher the stress and strain the specimen will endure. Failure modes and microstructures of specimens were observed to analyze and explain the underlying mechanism of reinforcing effect. It was observed that the specimens failed due to individual layer delamination and was found to be the main mechanism for avoiding catastrophic failure and interfacial damage of the sandwich specimen.

Toward accurate evaluation of bulk hardness from nanoindentation testing at low indent depths

Estimations of bulk hardness from nanoindentation are frequently subject to considerable uncertainties due to indentation size effects (ISE), pileup effects, and potential influence of surface quality or test methods. This study examined materials science principles of nanoindentation test methods to enable accurate prediction of bulk hardness for a series of high purity Fe and Fe-(3–25 wt%) Cr alloys. These materials were tested in as-annealed and thermally aged (100–900 h at 475 ℃ to produce Cr-rich α’ precipitates) conditions. Nanoindentation with a Berkovich indenter at constant strain rate (0.05–0.5 /s) and constant loading rate conditions provided comparable bulk equivalent hardness (H0) extracted by Nix-Gao model, indicating a weak strain rate sensitivity at room temperature. Results from electropolished and fine mechanically polished samples gave comparable measured hardness. Pileup corrections produced a 5–14% correction to H0 which agreed with the experimental bulk Vickers hardness within ∼10% for most tested materials. The microstructural model-predicted and measured strength values agreed for aged samples. A derived analytic expression demonstrates that an ISE error, associated with inappropriate methods such as hardness ratios or changes at a reference depth, would be as large as 60% in estimated bulk hardness for the investigated Fe-Cr alloys.

Consolidation testing of tailings in a slurry consolidometer using constant rate and accelerated loading

A high rate of rise of fine-grained mine tailings slurry on disposal in a tailings storage facility (TSF) may maintain their condition as underconsolidated. The proper simulation of such field deposition of tailings in conventional laboratory consolidation testing is challenging. A slurry consolidation test using a high constant rate of loading (CRL) simulates a faster rate of tailings disposal in a TSF. A better simulation is achieved, however, by applying accelerated loading in a slurry consolidometer to allow for the increased drainage path length as tailings are built up in the TSF. CRL and accelerated loading were used in a purpose-built, large, instrumented, stress-controlled slurry consolidometer to examine and compare the consolidation behaviour of coal tailings, red mud and gold tailings. At the end of the CRL and accelerated loading tests, the excess pore water pressures had not fully dissipated and the maximum applied stress was maintained to achieve full primary consolidation. The findings of this study showed that the consolidation behaviour of the tailings studied varied with the nature of the tailings and with the loading applied.

Effect of hydraulic selection pressure on the characteristics of partial nitritation/anammox granular sludge in a continuous-flow reactor

Strong hydraulic selection pressure is a control co-factor for maintaining the stability of granular sludge in a continuous-flow bioreactor. In this study, efficient nitrogen removal was achieved in a continuous partial nitritation/anammox (PN/A) granular reactor in a short hydraulic retention time (HRT), decreasing from 100 to 40 min, where more than 95% of ammonium and 80% of total inorganic nitrogen (TIN) were removed at a constant nitrogen loading rate of 2.0 kg (m 3 d) −1 . Results showed that the decreased HRT substantially influenced the observed biomass growth curve, with a trade-off between the growth and washout of microbial populations. Further, sludge characterization indicated that shorter HRT improved granular compactness with the accumulation of extracellular polymeric substances and increased the specific nitrogen removal rate, reaching q (TIN) = 39.9 mg N g −1 VSS ⋅ h −1 . These observations were consistent with shifts in the bacterial community structure of the PN/A sludge with different particle sizes. High-throughput pyrosequencing revealed that strong hydraulic selection pressure promoted the niche segregation of aerobic and anaerobic ammonium-oxidizing bacteria (AOB and AnAOB) and washout of nitrite-oxidizing species in the granules. The effective retention of k -strategists AnAOB (dominated by genus Candidatus Kuenenia) occupying the interior layer of the granules exhibited a solid retention time (SRT) 10-fold longer than that of AOB (identified as Nitrosomonas ) enriched in the aerobic zone. A strategy involving HRT and SRT coordination in operation can enhance the efficiency and stability of PN/A systems, considering the potential development of small aggregates into mature granules. • Strong hydraulic selection pressure was applied in a CSTR with PN/A granules. • NH 4 + -N removal ≥ 95% and TIN removal ≥ 80% were achieved at an HRT of 40 min. • HRT influenced the observed biomass growth curve and granular functional activity. • Shorter HRT promoted niche segregation of AOB and AnAOB in PN/A sludge.

Comment on “Berkovich nanoindentation of Zr55Cu30Al10Ni5 bulk metallic glass at a constant loading rate”, J. Non-Cryst. Solids, 561 (2021) 120750

It is argued that the decrease with increasing indenter load in the value of the elastic modulus [1] of a bulk metallic glass determined using a nanoindentation machine combined with the current methods of data analysis is not a manifestation of any damage in the metallic glass. Rather, the cause of the decrease of the elastic modulus is an error in the method of data analysis.

Indentation-derived creep response of cast and laser powder bed fused AlSi10Mg alloy: Air temperature

Compared with time-consuming conventional uniaxial tensile/compressive creep experiments, depth-sensing indentation testing is considered a reliable, and convenient testing technique to assess the time-dependent plastic deformation of materials in a reasonable time scale. In the present study, we report the ambient (room) temperature indentation creep properties of additively manufactured (i.e., laser-powder bed fused) and cast AlSi10Mg alloy at as-fabricated and different post-fabrication heat treatment states. The indentation creep testing parameters (i.e., dwell time, peak indentation load, and indenter shape) were optimized to adequately represent the creep response (time-displacement) variations for different material conditions. To this end, dual-stage, constant loading rate followed by constant-load holding, pyramidal indentation experiments were performed at a loading rate of 10 mN/s, a peak load of 200 mN, and a dwell time of 400 s. Besides, electron backscattered diffraction was performed to evaluate the manufacturing process (selective laser melted versus cast)/ post-fabrication heat treatment/ texture/ creep properties relationships for the studied AlSi10Mg alloy. Also, the indentation hardness, indentation strain rate sensitivity, indentation creep exponent, and activation volume were analyzed to study and confirm the mechanism of indentation creep. The calculated high values of creep stress exponents (i.e., >10) are attributed to dislocation-reinforcing particle interaction as the controlling mechanism of the creep which agrees with this assumption that AlSi10Mg is indeed an in-situ metal matrix composite with eutectic silicon as the reinforcing particles.

Effect of initial specimen height and solids concentration on the consolidation parameters of fine-grained coal tailings

ABSTRACT Proper understanding and analysis of the consolidation behaviour and parameters of tailings from a slurry-like to a soil-like state are vital for the design, management, and assessment of a tailings storage facility (TSF). Fine-grained coal tailings slurry can undergo relatively rapid and substantial settlement from a slurried state and can experience relatively slow and limited self-weight consolidation, remaining slurry-like and under-consolidated, rather than soil-like. A large purpose-built slurry consolidometer having a newly developed consolidometer cell, instrumented with load cells and pore water pressure transducers, is well-suited to test tailings from a slurry-like to a soil-like state and enables the constant rate of loading (CRL) to simulate the build-up of tailings. CRL is more representative of the rate of rise of tailings in a TSF than incremental step loading. By conducting consolidation tests on fine-grained coal tailings from a slurry-state under a loading rate of 0.12 kPa/min up to applied stress of 320 kPa, this research investigated the effect of initial specimen height (ISH) and initial solids concentration (ISC) on consolidation parameters and hydraulic conductivity. Using ISH and ISC as independent inputs, a statistical model has been proposed to estimate the consolidation settlement at the end of the slurry consolidation test.

Impacts of organic matter and loading methods on one-dimensional compression behavior of calcareous sand

To explore the effects of organic matter (OM) and loading methods on compression and particle breakage behaviors of calcareous sand (CS), a set of one-dimensional (1D) compression experiments were completed with two loading methods of constant rate of strain and incremental loading. Particle size distribution (PSD) analysis and three-dimensional (3D) modelling tests were performed to assess particle breakage degree and OM distribution form. Test results demonstrated that there existed an OM content of 25% in which the total vertical strain (TVS) in loading process declined first and then grew after that. While in unloading process, TVS decreased linearly from 16.1% to 4.2% approximately when OM content increased from 0 to 100%. In addition, there existed two loading stress turning points of 800 and 1600 kPa corresponding to vertical strain variation for CS-OM (A) and CS-OM (B) mixtures, respectively. The rebound index of mixtures increased from 0.002 to 0.107 when OM content increased to 100% which was determined collectively by the percentage of PCS and OM. OM can restrain particle breakage of CS to some degree and relative breakage ratio decreased from 0.24 to 0.03. Under the same condition, the relative breakage ratio (0.24) of CS-OM (B) mixtures with IL loading method were larger than that of CS-OM (A) mixtures or CRS loading method. Moreover, SEM and 3D models of CS-OM mixtures suggested that the OM are flocculent and distributed in surface or pores of CS particles.

Elastoviscoplastic modelling with distorted isotaches and swelling for constant strain rate and incremental loading

AbstractA novel elastoviscoplastic model is proposed to improve the modelling of the one‐dimensional unloading response of soils. Creep is modelled as an isotache process, but with the possibility to incorporate distortion of the isotaches after unloading. Upon unloading, swelling develops as a transient superimposed process that enables the sequential occurrence of swelling and creep as observed in experiments. The model features a flexible reference isotache to improve the fit to real laboratory tests. The model is implemented with full hydro‐mechanical coupling and enables simulation of both load‐controlled and strain rate‐controlled stress paths. The model is calibrated with a laboratory test with a complex stress path including constant rate of strain, incremental loading and relaxation stages.

Biogas production from anaerobic co-digestion using kitchen waste and poultry manure as substrate-part 1: substrate ratio and effect of temperature.

The rapidly declining fossil fuels are no longer able to meet the ever-increasing energy demand. Moreover, they are considered responsible for greenhouse gas (GHG) emission, contributing to the global warming. On the other hand, organic wastes, such as kitchen waste (KW) and poultry manure (PM), represent considerable pollution threat to the environment, if not properly managed. Therefore, anaerobic co-digestion of KW and PM could be a sustainable way of producing clean and renewable energy in the form of biogas while minimizing environmental impact. In this study, the anaerobic co-digestion of KW with PM was studied to assess the rate of cumulative biogas (CBG) production and methane percentage in four digester setups (D1, D2, D3, and D4) operated in batch mode. Each digester setup consisted of five parallelly connected laboratory-scale digesters having a capacity of 1 L each. The digester setups were fed with KW and PM at ratios of 1:0 (D1), 1:1 (D2), 2:1 (D3), and 3:1 (D4) at a constant loading rate of 300 mg/L with 50 gm cow manure (CM) as inoculum and were studied at both room temperature (28 °C) and mesophilic temperature (37 °C) over 24 days. The co-digestion of KW with PM demonstrated a synergistic effect which was evidenced by a 16% and 74% increase in CBG production and methane content, respectively, in D2 over D1. The D3 with 66.7% KW and 33.3% PM produced the highest CBG and methane percentage (396 ± 8 mL and 36%) at room temperature. At mesophilic condition, all the digesters showed better performance, and the highest CBG (920 ± 11 mL) and methane content (48%) were observed in D3. The study suggests that co-digestion of KW and PM at mesophilic condition might be a promising way to increase the production of biogas with better methane composition by ensuring nutrient balance, buffering capacity, and stability of the digester.

A simple approach to predict settlement due to constant rate loading in clays

Classical theory of consolidation was conceived considering loads instantaneously applied. Since then, researchers have addressed this issue by suggesting graphical and/or analytical solutions to incorporate different time-depending load schemes. The simplest alternative is to assume a linearly increasing load. Another approach to predict the average degree of consolidation caused by a constant rate loading is based on instantaneous excess pore pressures during and at the end of construction. This technical note explains why and how this approach leads to substantial errors after the end of construction. A corrected solution is then proposed, based on the concept of superposition of effects. The final set of equations agree with the theoretical ones. A new simple approximate methodology is also presented. Numerical examples using the proposed approach showed an excellent agreement with the analytical solution. The validity of this new approach was also proven by reproducing oedometer test results with a good agreement.

Investigation of Fats, Oils, and Grease Co-digestion With Food Waste in Anaerobic Membrane Bioreactors and the Associated Microbial Community Using MinION Sequencing.

Co-digestion of fats, oils, and grease (FOG) with food waste (FW) can improve the energy recovery in anaerobic membrane bioreactors (AnMBRs). Here, we investigated the effect of co-digestion of FW and FOG in AnMBRs at fat mass loading of 0.5, 0.75, and 1.0 kg m–3 day–1 with a constant organic loading rate of 5.0 gCOD L–1 day–1 in both a single-phase (SP) and two-phase (TP) configuration. A separate mono-digestion of FW at an identical organic loading rate was used as the benchmark. During co-digestion, higher daily biogas production, ranging from 4.0 to 12.0%, was observed in the two-phase methane phase (TP-MP) reactor compared to the SP reactor, but the difference was statistically insignificant (p > 0.05) due to the high variability in daily biogas production. However, the co-digestion of FW with FOG at 1.0 kg m–3 day–1 fat loading rate significantly (p < 0.05) improved daily biogas production in both the SP (11.0%) and TP (13.0%) reactors compared to the mono-digestion of FW. Microbial community analyses using cDNA-based MinION sequencing of weekly biomass samples from the AnMBRs revealed the prevalence of Lactobacillus (92.2–95.7% relative activity) and Anaerolineaceae (13.3–57.5% relative activity), which are known as fermenters and fatty acid degraders. Syntrophic fatty acid oxidizers were mostly present in the SP and TP-MP reactors, possibly because of the low pH and short solid retention time (SRT) in the acid phase digesters. A greater abundance of the mcrA gene copies (and methanogens) was observed in the SP and MP reactors compared to the acid-phase (AP) reactors. This study demonstrates that FW and FOG can be effectively co-digested in AnMBRs and is expected to inform full-scale decisions on the optimum fat loading rate.

Study of Surface Mechanical Characteristics of ABS/PC Blends Using Nanoindentation

Acrylonitrile butadiene styrene (ABS) and polycarbonate (PC) are considered a well-known class of engineering thermoplastics due to their efficient use in automotive, 3D printing, and electronics. However, improvement in toughness, processability, and thermal stability is achieved by mixing together ABS and PC. The present study focuses on the understanding of surface mechanical characterization of acrylonitrile butadiene styrene (ABS) and polycarbonate (PC) blends using nano-indentation. Polymer blends sheets with three different proportions of ABS/PC (75:25, 50:50, and 25:75) were fabricated via melt-processing and thermal press. Fourier transform infrared (FTIR) spectroscopy was performed to analyze the intermolecular interactions between the blends’ components. To understand the surface mechanical properties of ABS and PC blends, a sufficient number of nano-indentation tests were performed at a constant loading rate to a maximum load of 100 mN. Creeping effects were observed at the end of loading and start of unloading section. Elastic modulus, indentation hardness, and creep values were measured as a function of penetration displacement in the quasi-continuous stiffness mode (QCSM) indentation. Load-displacement curves indicated an increase in the displacement with the increase in ABS contents while a decreasing trend was observed in the hardness and elastic modulus values as the ABS content was increased. We believe this study would provide an effective pathway for developing new polymer blends with enhanced mechanical performance.

СВЯЗЬ МЕЖДУ СВОЙСТВАМИ ДРЕВЕСИНЫ И ДЕФОРМАТИВНОСТЬЮ ПРИ РАЗЛИЧНЫХ РЕЖИМАХ НАГРУЖЕНИЯ

Modification of wood by pressing makes it possible to obtain a durable material from low-quality wood species. The relationship between the structure of natural and pressed wood and deformability was investigated in the works of B.I. Ogarkov, P.P. Erinsh, V.A. Shamaev, G.K. Gavrilov. Meanwhile, at present wood is subjected to deformation in a plasticized state, the structure and properties of which are different from the structure of natural wood. In this work, an attempt to establish the relationship between the structure of natural and plasticized wood and its main, deformative indicators (stiffness, modulus of elasticity, Poisson's ratio, etc.) has been made . As a result of the experiments, it was found that (according to the results of processing empirical diagrams "deformation - stress") a three-phase nature of deformation of natural birch wood and plasticized with TCL (thermocatalytic cracking liquid) oil was established with the calculation of stiffness indicators; the dependences of the Punch ratio at variable humidity and temperature during the pressing process for constant and variable loading rates were obtained. Hysteresis in wood under tension and compression for natural and plasticized wood has been studied

Sustainable anaerobic digestion of euphorbiaceae waste for biogas production: Effects of feedstock variation

Anaerobic digestion (AD) refers to the biological process which can convert organic substrates to biogas in the absence of oxygen. The aim of this study was to determine the capability of feedstock to produce biogas and to quantify the biogas yield from different feedstocks. A co-digestion approach was carried out in a continuous stirred tank reactor operated under mesophilic conditions and at a constant organic loading rate of 0.0756 g COD/ L.day, with a hydraulic retention time of 25 days. For comparison, mono-digestion was also included in the experimental work. 2 L working volumes were used throughout the experimental work. The seed culture was obtained from composting as substrate digestion. When the feedstock was added to seeding, the biogas started to emit after three days of retention time. The highest volume of biogas was observed when the seeding volume used for 1000 mL. However, the lowest volume of biogas yield was obtained from both co-digestion reactors, with a value of 340 mL. For methane yield, the highest methane production rate was 0.16 L CH4/mg. The COD with yield was at 8.6% and the lowest was at 0.5%. The highest quantity of methane was obtained from a reactor of Euphorbiaceae peel with added seeding, while the lowest methane yield came from a reactor of Euphorbiaceae stems with added seeding. In this study, sodium bicarbonate (NaHCO3) was used as a buffering solution to correct the pH in the reactor if the reactor condition was found to be in a souring or acidic condition.

Berkovich nanoindentation of Zr55Cu30Al10Ni5 bulk metallic glass at a constant loading rate

Berkovich nanoindentation experiments were conducted on Zr55Cu30Al10Ni5 bulk metallic glass under a constant loading rate. Indentation hardness decreased linearly with contact depth, which defied the direct application of conventional indentation size effect model for crystalline material. Elastic modulus remained invariant under small loads, but decreased linearly under large loads. The scaling relationships between indentation variables such as contact depth, permanent depth, the maximum indentation displacement, plastic energy, the total work of indentation and their ratios were investigated. The area of shear band circle, the projected contact area at the maximum load, and the residual projected area were all found to be proportional to one another. A new methodology to determine fracture toughness was proposed based on the total work of indentation, provided that the critical indentation displacement at the onset of fracture corresponds to zero value of elastic modulus extrapolated from curve fitting.

Modeling of the attached and suspended biomass fractions in a moving bed biofilm reactor

The performance, kinetics, and stoichiometry of three high-rate moving bed biofilm reactors (MBBRs) were evaluated. A constant surface area loading rate (SALR) and three different hydraulic retention times (HRTs) were utilized to create scenarios where the attached and suspended biomass fractions would differentiate, despite the main design parameter remaining constant. Performance was simulated using BioWin™ 6.0 software. The objective was to evaluate whether a calibrated/validated model could accurately predict experimental results. Initially, a sensitivity analysis was performed to determine influential parameters. The calibration/validation of influential parameters was then conducted via steady-state simulations for two base cases: 1) highest HRT; and 2) lowest HRT. Both sets of calibrated/validated parameters were substantiated using: 1) steady-state simulations at the other HRTs; and 2) dynamic simulations to evaluate the kinetic rates of attached and suspended biomass fractions at all HRTs. Results demonstrated that the model could be calibrated/validated for a single HRT, but could not accurately predict the performance, kinetics, or stoichiometry at other HRTs.

Performance Analysis and Microbial Community Evolution of In Situ Biological Biogas Upgrading with Increasing H2/CO2 Ratio.

The effect of the amount of hydrogen supplied for the in situ biological biogas upgrading was investigated by monitoring the process and evolution of the microbial community. Two parallel reactors, operated at 37°C for 211 days, were continuously fed with sewage sludge at a constant organic loading rate of 1.5 gCOD∙(L∙d)−1 and hydrogen (H2). The molar ratio of H2/CO2 was progressively increased from 0.5 : 1 to 7 : 1 to convert carbon dioxide (CO2) into biomethane via hydrogenotrophic methanogenesis. Changes in the biogas composition become statistically different above the stoichiometric H2/CO2 ratio (4 : 1). At a H2/CO2 ratio of 7 : 1, the methane content in the biogas reached 90%, without adversely affecting degradation of the organic matter. The possibility of selecting, adapting, and enriching the original biomass with target-oriented microorganisms able to biologically convert CO2 into methane was verified: high throughput sequencing of 16S rRNA gene revealed that hydrogenotrophic methanogens, belonging to Methanolinea and Methanobacterium genera, were dominant. Based on the outcomes of this study, further optimization and engineering of this process is feasible and needed as a means to boost energy recovery from sludge treatment.