Elevated Loading Rates Research Articles

Impact of elevated loading rates on the shape of the Master Curve (ASTM E1921) for a German RPV steel

The Master Curve Methodology (ASTM E1921) experimentally assesses a materials temperature-dependent fracture toughness, predominantly for quasi-static testing conditions. The treatment of elevated loading rates is described by the annex A1 of ASTM E1921 and A14 of ASTM E1820. This paper presents results of the evaluation of a large and standard-conforming database in order to verify the procedures recommended by the standard for elevated loading rates. Testing involved C(T)- and SEN(B)-specimens of the RPV-steel 22NiMoCr3-7 (A508 Grade 2) for loading rates of 100 MPa√m/s ≤ K̇ ≤ 104 MPa√m/s in the ductile to brittle transition region. While valid T0-values were found, single-temperature T0-values were observed to differ more than expected from multi-temperature T0-values, which cannot be explained by the Master Curve uncertainty. The shape and underlying distribution of the Master Curve show deviations with increased loading rate. The shape factor p is optimized with respect to the individual data, and it increases with K̇, but deviations are not completely overcome. This can be linked to a change in distribution, which was demonstrated by an optimization of minimum fracture toughness Kmin, which increases with temperature. It is argued that the cause for the observations is linked to both heating processes and local crack arrest that severely influence macroscopic fracture behavior. Also, an individual adjustment of p or Kmin is not helpful due to the material-dependency in practice. It is recommended that fracture mechanics testing at elevated loading rates is performed close to or below T0 in order to minimize the influence of dynamic loading conditions on the assessment.

Tunable and recoverable energy absorption of foam-embedded architected cellular composite material at multiple strain rates

Cellular multistable architected material has been widely studied for its promising energy absorption applications for personnel protection, protective packaging, and crash mitigation. Despite the tailorable properties enabled by their geometric features, a major issue in the actual application is their lower strengths in the elastic behavior of the multistable architected material. Herein, a novel design strategy of strain rate-dependent architected cellular composite material (FACCM) is proposed, aiming to achieve enhanced energy-absorbing properties with the proposed composite system than the single material constituent. Guided by numerical simulations and experimental tests, the compressive behaviour of FACCM at multiple strain rates and the effect of filled foam on it with specific geometric parameters are characterized. The quasi-static performances of the FACCM specimens were evaluated both at the cell level and planar array level. Our results indicate that the strength, stiffness, and snap-through behaviour of multistable architected material can be significantly improved by the filled foam at elevated loading rates regardless of quasi-static loading or impact tests. The current study offers a new strategy for developing novel packing, shock absorption, and impact protection systems.

Design strategy and mechanism of nitrite oxidation suppression of elevated loading rate partial nitritation system.

There is a current need for a low operational intensity, effective and small footprint system to achieve stable partial nitritation for subsequent anammox treatment at mainstream municipal wastewaters. This research identifies a unique design strategy using an elevated total ammonia nitrogen (TAN) surface area loading rate (SALR) of 5 g TAN/m2.d to achieve cost-effective, stable, and elevated rates of partial nitritation in a moving bed biofilm reactor (MBBR) system under mainstream conditions. The elevated loaded partial nitritation MBBR system achieves a TAN surface area removal rate (SARR) of 2.01 ± 0.07 g TAN/m2.d and NO2 --N: NH4 +-N stoichiometric ratio of 1.15:1, which is appropriate for downstream anammox treatment. The elevated TAN SALR design strategy promotes nitrite-oxidizing bacteria (NOB) activity suppression rather than a reduction in NOB population as the reason for the suppression of nitrite oxidation in the mainstream elevated loaded partial nitritation MBBR system. NOB activity is limited at an elevated TAN SALR likely due to thick biofilm embedding the NOB population and competition for dissolved oxygen (DO) with ammonia-oxidizing bacteria for TAN oxidation to nitrite within the biofilm structure, which ultimately limits the uptake of DO by NOB in the system. Therefore, this design strategy offers a cost-effective and efficient alternative for mainstream partial nitritation MBBR systems at water resource recovery facilities.

Analysis and improvement of local cleavage fracture models at elevated loading rates

Brittle fracture has to be excluded in safety-relevant components such as nuclear reactor pressure vessels. Recent work has indicated that brittle failure under dynamic loading cannot be assessed reliably by macroscopic methods such as the Master Curve concept (ASTM E1921). This finding is strongly connected to adiabatic heating processes and local crack arrest events in the crack tip region. However, a detailed study involving the assessment at elevated loading rates with local cleavage fracture models is not available to this point. The present work deals with the application of a local model to various dynamic loading conditions, as well as a proposed model adjustment to improve the analysis results. It was found that existing local concepts are not capable of describing fracture behavior adequately. Though the explicit numerical consideration of heat generation and conduction at elevated loading rates allows a better estimation of failure compared to the Master Curve concept, this still poor assessment quality can also be obtained by simply utilizing the adjusted “dynamic” Master Curve with an increased exponent of p = 0.030 /°C. It is therefore concluded that this exponent adjustment approximates the impact of heat effects quite well. The shortcomings of the numerical model were observed to correlate strongly with the documented amount of local crack arrest events on the fracture surface. This leads to a micromechanically-motivated adjustment of a local approach model to incorporate this mechanism. This modification ensures a more satisfying physical background of the local approach model. Moreover, it significantly improves model analysis accuracy for all examined testing conditions.

Numerical modelling of quasi-static and dynamic compact tension tests for obtaining the translaminar fracture toughness of CFRP

In the recent past, several studies have been undertaken to characterise the loading rate dependency of the translaminar fracture toughness GIcf+, however, no consensus has yet been reached. In an attempt to make a first step towards resolving this inconsistency, this work employed a modelling campaign based on the experimental data generated in Hoffmann et al. [Compos Sci Technol 164 (2018) 110–119] in order to identify the driving mechanism behind the negative loading rate trend observed in said study. The numerical analysis employed detailed FE models of the quasi-static and high-rate experiments, as well as a high-fidelity strain-rate- and pressure-dependent user material model to represent the intralaminar behaviour of the composite material. This study found the energy release rate along the macroscopic crack to be practically rate-insensitive. Loading rate, however, was shown to have a pronounced effect on the extent of “secondary” energy dissipation mechanisms away from the macroscopic crack in the form of matrix damage and plasticity. This damage zone was significantly larger under quasi-static than under high-rate loading, thus providing an explanation for the observed drop of GIcf+ under elevated loading rates in Hoffmann et al. (2018).

Autotrophic denitrification of nitrate rich wastewater in fluidized bed reactors using pyrite and elemental sulfur as electron donors

This study compared denitrification performances and microbial communities in fluidized bed reactors (FBRs) carrying out autotrophic denitrification using elemental sulfur (S 0 ) and pyrite (FeS 2 ) as electron donors. The reactors were operated for 220 days with nitrate loading rates varying between 23 and 200 mg N-NO 3 − /L ⋅ d and HRT between 48 and 4 h. The highest denitrification rates achieved were 142.2 and 184.4 mg N-NO 3 − /L ⋅ d in pyrite and sulfur FBRs, respectively. Pyrite-driven denitrification produced less SO 4 2 − and no buffer addition was needed to regulate the pH. The sulfur FBR needed instead CaCO 3 to maintain the pH neutral and consequentially more sludge was produced (CaSO 4 precipitation). The active community of pyrite-based systems was investigated and Azospira sp ., Ferruginibacter sp. , Rhodococcus sp. and Pseudomonas sp. were the predominant genera, while Thiobacillus sp. and Sulfurovum sp. dominated the active community in the sulfur FBR. However, Thiobacillus sp. became more dominant when operating at elevated nitrogen loading rate. Patterns of diversity and microbial community assembly were assessed and revealed three distinct stages of microbial community succession which corresponded with the operation of a period of high influent nitrate concentration (135 mg N-NO 3 − /L). It is proposed that a high degree of functional redundancy in the initial microbial communities may have helped both reactors to respond better to such high influent nitrate concentration. • Max denitrification rates were 142.2 and 184.4 mgN-NO 3 − / L ⋅ d in the FeS 2 and S 0 FBRs. • FeS 2 -driven denitrification produced low SO 4 2 − and no buffer addition was needed. • Microbial community succession was observed in response to NLR increased. • Functional redundancy in the AD microbiome resulted in resilience to increased NLRs. • The active community of FeS 2 FBR had Azospira and Pseudomonas as dominant genera.

Nitrification performance and bacterial community dynamics in a membrane bioreactor with elevated ammonia concentration: The combined inhibition effect of salinity, free ammonia and free nitrous acid on nitrification at high ammonia loading rates

The responses of the operational performance and bacterial community structure of a nitrification membrane bioreactor (MBR) to elevated ammonia loading rate (ALR) were investigated. Effective nitrification performance was achieved at high ALR up to 3.43 kg NH4+-N/m3·d, corresponding to influent NH4+-N concentration of 2000 mg/L. Further increasing influent NH4+-N concentration to 3000 mg/L, the MBR system finally became completely inefficient due to the combined inhibition effect of salinity, free ammonia and free nitrous acid on nitrification. Ammonia-oxidizing bacteria (AOB) Nitrosomonas were enriched with the increase of ALR. The relative abundance of Nitrosomonas in the sludge with ALR of 2.57 kg NH4+-N/m3·d was up to 14.82%, which were 9-fold and 53-fold higher than that in seed sludge and the sludge with ALR of 0.10 kg NH4+-N/m3·d, respectively. The phylogenetic analysis of AOB amoA genes showed that Nitrosomonas europaea/mobilis lineage are chiefly responsible for catalyzing ammonia oxidation at high ALRs.

Mode III testing of structural adhesive joints at elevated loading rates

The Out-of-plane loaded Double Cantilever Beam (ODCB) test has proven to be a well-suited test method for determining both energy release rate and cohesive law of adhesive layers in mode III under quasi-static conditions. As the rate-dependency of the fracture behaviour of adhesive layers in shear is still a topic of ongoing research, we aim to investigate, whether the ODCB test can be modified to be applicable at elevated loading rates. For this purpose, the experimental setup of the ODCB test was modified to allow testing at elevated loading rates. Tests were performed on an epoxy adhesive at outer loading rates over several orders of magnitude, ranging from quasi-static conditions up to the maximum possible test speed of the used servo-hydraulic test machine. The results of these experiments are discussed thoroughly to gain insight into the applicability of the ODCB test at moderate loading rates. Furthermore, the experimental challenges associated with an increase in loading rate are highlighted to identify limitations of the experimental setup that need to be addressed when transitioning to impact testing.

Foreshock Activity Promoted by Locally Elevated Loading Rate on a 4‐m‐Long Laboratory Fault

AbstractWe report laboratory experimental results to reveal the factors that control the occurrence and magnitude of foreshocks. We conducted rock friction experiments using an apparatus that can shear 4‐m‐long rock specimens. We observed many stick‐slip events, as well as very slow (several tens of μm/s) but long‐lasting slips between those main events. These long‐term slow slips individually initiated from both the leading and trailing edges of the fault, and kept propagating steadily toward each other. Such steady slips did not immediately trigger any seismic events, regardless of the accumulated slip amount. After the coalescence of the two long‐term slow slip fronts, a second phase of slow slip with higher slip rate (several hundreds of μm/s)—called precursory slow slip, began at the central area and was accompanied by the occurrence of small seismic events (foreshocks). Subsequently, the main fast rupture eventually developed. We propose that the asperities that hosted foreshocks had similar size to the local critical nucleation length; the asperities slipped stably when the local loading rate was low, but could also slip unstably and radiated seismic waves when the local loading rate became high. We further found a clear positive correlation between foreshock magnitude and local slip rate. These results suggest that local loading rate has a significant influence on the occurrence and magnitude of foreshocks. Therefore, its effect should be taken into account during the studies of earthquake nucleation process and other similar phenomena such as Episodic Tremor and Slip (ETS), icequakes, and repeating earthquakes.

Effect of collagen damage induced by heat treatment on the mixed-mode fracture behavior of bovine cortical bone under elevated loading rates

Effect of collagen damage induced by heat treatment on the mixed-mode fracture behavior of bovine cortical bone under elevated loading rates

Elevated loading rates as a low operational intensity and small land footprint design strategy to achieve partial nitritation

An urgent need exists for cost-effective, low operational intensity and small land footprint ammonia treatment systems to meet stringent wastewater discharge regulations. A moving bed biofilm reactor system operating under a novel design strategy using elevated total ammonia nitrogen (TAN) loading rates ranging from 3 to 6.5 g TAN/m2·d has shown promise to achieve robust partial nitritation (PN) and the oxidation of TAN with limited oxidation of nitrite without the need for intense operational measures. An investigation of the biofilm, the viability of the embedded cells and the microbial ecology is necessary to fully understand this low operational intensity design. The investigation revealed that, at a loading rate of 6.5 g TAN/m2·d, the abundance of nitrite oxidizing bacteria (NOB) was 2 ± 0.46%; however, the NOB activity was negligible with 99 ± 0.2% nitrite accumulation; thus, the mechanism to attain robust PN is shown to be the suppression of NOB activity as opposed to NOB population suppression. This decrease in NOB activity is associated with an increase in thickness and mass of the biofilm, as well as the relative increase in ammonia oxidizing bacteria (AOB) that preferentially uptake oxygen compared to NOB. As such, this design strategy results in the suppression of the NOB activity due to limited access to oxygen via mass transfer limitations through thick biofilm and competition with an augmented AOB population in the biofilm. This research presents a low operational intensity and small footprint PN strategy that can be an effective alternative to more intensive strategies.

A Metric for Identifying Stress Fractures in Runners.

Stress fractures are common overuse running injuries. Individuals with stress fractures exhibit running biomechanics characterized by elevated impact peak and loading rate. While elevated impact peak and loading rate are associated with stress fractures, there are few established metrics used to identify the presence of stress fractures in individuals. Here this study aims to exploit the linear relationship between the impact peak and loading rate to establish a metric to help identify individuals with stress fractures. We hypothesize that the ratio between the impact peak and loading rate will serve as a metric to delineate between healthy controls and those with stress fractures. Fifteen healthy controls and 11 individuals with stress fractures performed a running protocol. A linear regression model fit to the stress fracture impact peak and loading rate data produced a lower 95% confidence limit boundary that served as the demarcation line between the two groups. Individuals with stress fractures tended to reside above the line with the line accurately classifying 82% of the individuals with stress fractures. The analysis supported the hypothesis and demonstrated how the relationship between impact peak and loading rate can help identify the presence of stress fractures in individuals.Clinical Relevance- The relationship between impact peak and loading rate has the potential to serve as clinically useful metric to identify stress fractures during running.

Consideration of the Heating of High-Performance Concretes during Cyclic Tests in the Evaluation of Results

Material-efficient, highly load-bearing members made of high-performance compressive concretes are often exposed to cyclical loads because of their slender construction, which can be relevant to the design. When investigating the fatigue behaviour of high-performance concretes in pressure swell tests, however, the specimen temperature rises strongly owing to the elevated loading rate at frequencies higher than 3 Hz. This leads to a negative influence on the achieved number of load cycles compared to tests carried out at slow speeds and calculated values, for example, according to fib Model Code 2010. This phenomenon, which was already observed, must be considered when generating design formulae or Wöhler lines for component design, as the test conditions with high constant load frequencies as well as sample storage in a climate chamber at constant conditions are prerequisites that cannot be expected in real material applications. Therefore, laboratory testing influences must be eliminated in order to avoid underestimating the material. Instead of adjusting the test conditions to prevent or control temperature development, as was the case in previous approaches, this article shows how the temperature effects can be corrected when analysing the results, considering both the applied stress and the maximum temperature reached. For this purpose, a calculation method was developed that was validated on the basis of a large number of fatigue tests. Thus, in the future, the application of one temperature sensor to the test specimen can effectively advance the extraction of values for Wöhler curves, even with high test frequencies.

Foreshock Activity Promoted by Locally Elevated Loading Rate on a 4-meter-long Laboratory Fault

Foreshock Activity Promoted by Locally Elevated Loading Rate on a 4-meter-long Laboratory Fault

Comparison between Loading Rate and Local Stress Rate When Applying ASTM E1921 at Elevated Loading Rates

Abstract ASTM E1921-2019b, Standard Test Method for Determination of Reference Temperature, T0, for Ferritic Steels in the Transition Range, Annex 1, describes the determination of the reference temperature T0,X for a loading rate X for ferritic steels in the transition range, where X corresponds to the logarithm of the average loading rate for the tests performed, rounded to the nearest integer. Within the framework of three research projects on dynamic crack initiation and crack arrest, funded by the German government, fracture mechanics tests using specimens of 22NiMoCr3-7 steel (A 508 Grade 2 Cl.1) were performed in a range from 102 MPa√m s−1 to 106 MPa√m s−1 at different temperatures in the ductile to brittle transition region. The Weibull stress rate was calculated from the results of a numerical analysis of tests with 1-inch compact tension (1T C(T)) specimens. The modified three-parameter Beremin model of Petti and Dodds was used, since this approach is congruent with the Master Curve method. In addition to the stress parameters corresponding to Kmin and K0, the Weibull exponent can be calibrated using the experimental results. The analysis shows the effect of the Weibull exponent on the time-dependent Weibull stress rate for a dynamic 1T C(T) test. Contrary to the loading rate calculated from KJc divided by the time to cleavage according to ASTM E1921, the Weibull stress rate decreases with increasing plastic behavior. This has to be considered when the dynamic reference temperature T0,X is determined using results from tests with large plastic contribution.

Exploring the novel PES/malachite mixed matrix membrane to remove organic matter for water purification

This study presents a greener approach to fabricate mixed matrix membranes (MMMs) by introducing malachite nanoparticles (MLC NPs) synthesized from copper sulfate, a toxic waste from many industrial applications, into polyethersulfone (PES) membrane to remove organic matter for water reclamation. The PES/MLC MMMs were characterized by XRD, FTIR, SEM(EDX), Raman Spectroscopy, Photoluminescence Spectrometry, and Contact angle measurements. Results showed that the incorporation of MLC NPs into PES altered the membrane morphology, and made the membrane more hydrophilic. At the optimal amount of 0.1wt.% MLC NPs, PES/MLC showed the best water flux of 931.1L/(m2h) (LMH) with the highest breaking stability of 5.06±0.33MPa. The membrane also exhibited enhanced adsorption of organic foulants with a corresponding flux recovery rate of 0.55±0.03%. The inevitable agglomeration of MLC NPs witnessed at MLC NP dosage of 1wt.% compromised either the throughput or the mechanical integrity. The elevated NP loading rate from 0 to 1wt.% enhanced the removal rate of organic carbon (C) from 20.8±1.7% to 26.3±2.3%, respectively. Optical methods demonstrated the preferential rejection of aromatic constituents by the fabricated membranes suggests their potential applicability of optical methods, particularly the fluorescence spectrometry for better prediction of treated water quality by membrane filtration.

Loading rate effects on mode-I delamination in glass/epoxy and glass/CNF/epoxy laminated composites

The main objective of the present study is to assess the loading rate effects on mode-I delamination growth in glass/epoxy laminated composites. Furthermore, hybrid reinforcement of these composites by incorporating carbon nanofibers (CNFs) was followed to enhance the interlaminar fracture resistance and affect its loading rate sensitivity. Experiments on DCB specimens made of glass/epoxy and glass/CNF/epoxy laminated composites were conducted by varying the loading rate from standard quasi-static testing up to 200 mm/sec crosshead speed in a servo-hydraulic test machine. More than 21% decrease was observed in the propagation fracture toughness of the glass/epoxy samples due to loading rate elevation in the studied range. Moreover, the results of the present study clearly show the benefits of CNF modification, not only in enhancing the fracture toughness but also in reducing the loading rate dependency. Adding CNFs to glass/epoxy composites caused 32.8% and 13.5% increase in the quasi-static values of the initiation- and propagation-interlaminar fracture toughness (GIC), respectively. Also, owing to CNF incorporation, the maximum drop in the propagation fracture toughness at elevated loading rates was decreased to 8%. Fractography inspections were performed to provide an in-depth explanation for the observed loading rate effects and the advantages of CNF reinforcement.

Partial nitritation at elevated loading rates: design curves and biofilm characteristics.

There is a need to develop low operational intensity, cost-effective, and small-footprint systems to treat wastewater. Partial nitritation has been studied using a variety of control strategies, however, a gap in passive operation is evident. This research investigates the use of elevated loading rates as a strategy for achieving low operational intensity partial nitritation in a moving bed biofilm reactor (MBBR) system. The effects of loading rates on nitrification kinetics and biofilm characteristics were determined at elevated, steady dissolved oxygen concentrations between 5.5 and 7.0mg O2/L and ambient temperatures between 19 and 21°C. Four elevated loading rates (3, 4, 5 and 6.5g NH4+-N/m2days) were tested with a distinct shift in kinetics being observed towards nitritation at elevated loadings. Complete partial nitritation (100% nitrite production) was achieved at 6.5g NH4+-N/m2days, likely due to thick biofilm (572µm) and elevated NH4+-N load, which resulted in suppression of nitrite oxidation.

Performance of Anaerobic Digestion of Chicken Manure Under Gradually Elevated Organic Loading Rates.

Poultry manure is the main source of agricultural and rural non-point source pollution, and its effective disposal through anaerobic digestion (AD) is of great significance; meanwhile, the high nitrogen content of chicken manure makes it a typical feedstock for anaerobic digestion. The performance of chicken-manure-based AD at gradient organic loading rates (OLRs) in a continuous stirred tank reactor (CSTR) was investigated herein. The whole AD process was divided into five stages according to different OLRs, and it lasted for 150 days. The results showed that the biogas yield increased with increasing OLR, which was based on the volatile solids (VS), before reaching up to 11.5 g VS/(L·d), while the methane content was kept relatively stable and maintained at approximately 60%. However, when the VS was further increased to 11.5 g VS/(L·d), the total ammonia nitrogen (TAN), pH, and alkalinity (CaCO3) rose to 2560 mg·L−1, 8.2, and 15,000 mg·L−1, respectively, while the volumetric biogas production rate (VBPR), methane content, and VS removal efficiency decreased to 0.30 L·(L·d)−1, 45%, and 40%, respectively. Therefore, the AD performance immediately deteriorated and ammonia inhibition occurred. Further analysis demonstrated that the microbial biomass yield and concentrations dropped dramatically in this period. These results indicated that the AD stayed steady when the OLR was lower than 11.5 g VS/(L·d); this also provides valuable information for improving the efficiency and stability of AD of a nitrogen-rich substrate.

The Influence of Concordant Complex Posture and Loading Rate on Motion Segment Failure

Microstructural investigation of compression-induced herniation of a lumbar disc held in a concordant complex posture. To explore the significance of loading rate in a highly asymmetric concordant posture, comparing the mechanisms of failure to an earlier study using a nonconcordant complex posture. A recent study with a nonconcordant complex posture (turning in the opposite direction to that which the load is applied) demonstrated the vulnerability of the disc to loading that is borne by one set of oblique-counter oblique fiber sets in the alternating lamellae of the annulus, and aggravated by an elevated loading rate. Given the strain rate-dependent properties of the disc it might be expected that the outcome differs if the posture is reversed. Forty-one motion segments from ovine 16 spines were split into two cohorts; adopting the previously employed low rate (40 mm/min) and surprise rate (400 mm/min) of loading. Both groups of damaged discs were then analyzed microstructurally. With the lower rate loading the concordant posture significantly reduced the load required to cause disc failure than earlier described for nonconcordant posture (6.9 vs. 8.4 kN), with more direct tears and alternate lamella damage extending to the anterior disc. Contrary to this result, with a surprise rate, the load at failure was significantly increased with the concordant posture (8.08 vs. 6.96 kN), although remaining significantly less than that from a simple flexed posture (9.6 kN). Analysis of the damage modes and postures suggest facet engagement plays a significant role. This study confirms that adding shear to the posture lowers the load at failure, and causes alternate lamella rupture. Load at failure in a complex posture is not determined by loading rate alone. Rather, the strain rate-dependent properties of the disc influence which elements of the system are brought into play. N/A.