Specific Loading Rate Research Articles

Effect of nitrate on acetate degradation in a sulfidogenic staged reactor

The optimisation of acetate removal in sulfidogenic reactors by adding nitrate to the last compartment of a baffled reactor was evaluated. In a 5.4 l baffled reactor, containing three equal compartments, a VFA mixture (acetate:propionate:butyrate ratio 1:2:2 on COD basis; pH 8) was treated under mesophilic (30°C) and sulfidogenic (COD:SO 2− 4 ratio: 0.5) conditions. At a specific sludge loading rate of 1.1 g COD gVSS −1 d −1, a COD and sulfate removal of 80 and 35%, respectively, was obtained. In the baffled reactor, staging of the sulfidogenic VFA degradation occurred. Propionate and butyrate were mainly degraded in the first compartment. Their degradation was incomplete, resulting in elevated acetate concentrations in Compartment I. In the second and third compartment of the baffled reactor, a net degradation of acetate took place. Acetate was the sole substrate present in Compartment III and residual acetate concentrations of about 600 mg/l were present in the effluent at the highest specific sludge loading applied. In the three compartments, development of sludges with different maximum specific acetate degrading activities was observed. The maximal specific activity with acetate was three times higher in sludge growing in Compartment III as compared to sludge growing in Compartment I. Batch experiments showed that the addition of about 500 mg/l nitrate (COD/nitrate ratio of 2.5) increased the specific acetate degradation rate by 300%. Nitrate was denitrified (pH optimum of 7.0–8.0) with a temporary accumulation of the intermediate nitrite. When 2.3 g/l nitrate (COD/nitrate ratio of 0.5) was dosed in compartment III of the baffled reactor, the effluent acetate concentration could be decreased from 600 to 100 mg/l.

Anaerobic sequencing batch reactor treatment of landfill leachate

Anaerobic treatability of municipal landfill leachate was evaluated using lab-scale anaerobic sequencing batch reactors (ASBR) at 35°C. Experimental studies were conducted at wide-range of volumetric (0.4–9.4 g COD l −1 d −1) and specific (0.2–1.9 g COD g −1 VSS d −1) loading rates by varying hydraulic retention times (HRT) (10–1.5 days) and influent wastewater COD's (3800–15900 mg l −1). The COD removal efficiencies were in the range of 64–85% depending on volumetric and specific loading rates applied. The net daily biomass conversion yields were about 0.1 g volatile suspended solids (VSS) per g of COD removed. Bacterial decay rate constant was 0.01 d −1. The maximum volumetric methane production rate (VMPR) of 1.85 l CH 4 l −1 d −1 was achieved at the volumetric loading rate of 9.4 g COD l −1 d −1. The specific methane production rates in terms of COD were about 1.06 g CH 4–COD g VSS −1 d −1 in the reactor. The results have shown that about 83% of COD removed during the treatment was converted to methane. The average biomass yield was determined as 0.12 g VSS per gram of COD removed.

Effect of staging on volatile fatty acid degradation in a sulfidogenic granular sludge reactor

The use of an upflow staged sludge bed (USSB) reactor for the optimization of acetate removal during treatment of sulfate rich wastewaters was investigated. The USSB treated a VFA mixture (acetate:propionate:butyrate ratio 1:2:2 on COD basis; pH 8) under mesophilic (30°C) and sulfidogenic (COD:SO 4 2− ratio 0.5) conditions. Its performance was compared to that of an upflow anaerobic sludge bed (UASB) reactor operating at the same operational conditions. The hydrodynamic conditions were clearly different in both reactor types. The USSB reactor (number of ideal mixed reactors N th=10) had a more plug-flow pattern than the UASB reactor (N th=3). Both reactors reached a specific sludge loading rate of 1.20 gCOD gVSS −1 d −1, corresponding to a volumetric loading rate of 20 and 30 gCOD l −1 d −1 for the UASB and USSB reactor, respectively. At these loading rates, a maximum COD removal of 80% was obtained by both reactors and the average %COD used by sulfate reducing bacteria was 74(±7)% and 80(±9)% for the UASB and USSB reactor, respectively. The UASB and USSB reactor reached a sulfate loading rate of 40 and 60 g SO 4 2− l −1 d −1, respectively, with an average sulfate removal of 45(±19)% and 35(±24)%, respectively. The USSB reactor showed more oscillations in its performance than the UASB reactor, already at lower loading rates. Staging of sulfidogenic treatment in the USSB reactor resulted in development of sludges with different maximum specific microbial activities, but with similar physico-chemical sludge characteristics. Activity of the USSB sludge on a VFA mixture or with acetate as the sole substrate increased from the bottom to the top compartments. Sludge in the top compartments adapted only slowly to acetate, as indicated by the minor increase (13.5% in 138 days) of the activity on acetate and sulfate in those compartments.

Acetamide degradation by a continuous-fed batch culture ofBacillus sphaericus

The methanogenesis of acetamide occurs through a two-step reaction in methanogenic sludges. First, acetamide is hydrolyzed to acetate and ammonia by a strict aerobic bacterium (Bacillus sphaericus), then acetate is used by Bacillus as carbon source or converted to methane by methanogens. In this work, the kinetics of acetamide degradation by B. sphaericus was studied in a continuous reactor with biomass accumulation, fed with acetamide. The oxygen supplied was dissolved in the feed (6.4 mg/L) to resemble conditions in an anaerobic wastewater treatment reactor. A reaction in series model (acetamide --> acetate --> biomass) was used to find the kinetic parameters. Results show that B. sphaericus can hydrolyze acetamide in a second-order reaction with K1 = 1.1 L/g/d, implying that the amount of biomass determines the rate and that no reaction will take place at specific loading rates greater than 35 gAm/gX/d. Growth parameters on acetate, as carbon source, under limiting O2 conditions, are mu(max) = 0.102/d,Ks = 37 mg/L, Y = 0.081 gX/gAm.

Anaerobic treatment of leachate using sequencing batch reactor and hybrid bed filter

Landfill leachate taken from a young municipal landfill site (≈3.5 years old) containing high organic contaminants (Total Organic Carbon -TOC of about 5000 mg l−1) was treated in bench-scale Anaerobic Sequencing Batch Reactors (ASBR) and an Anaerobic Hybrid Bed Filter (AHBF) at mesophilic conditions. Twenty months of testing has been conducted at varied influent leachate concentration of 546–5770 mgTOC l−1, Hydraulic Retention Time (HRT) of 10–1.5 days and Solid Retention Time (SRT) of 40–9 days in ASBR's, and influent leachate concentration of 1250–4490 mgTOC l−1 and HRT of 5.1–0.9 days in AHBF. ASBR achieved 73.9% TOC removal at maximum organic loading rate of 2.8 kgTOC m−3 d−1 at 1.5 days of HRT, and 65.3% at 0.561 kgTOC kgVSS−1 d−1 of maximum specific loading rate and 2 days of HRT. The AHBF maintained 81.4% TOC removal at 1.2 kgTOC m−3 d−1 of loading and 2.4 days of HRT. Methane conversion ratio averaged 0.742 m3CH4 kgTOC−1 removed at (STP).

Membrane Bioreactor for Control of Volatile Organic Compound Emissions

A membrane bioreactor system that overcomes many of the limitations of conventional compost biofilters is described. The system utilizes microporous hydrophobic hollow fiber membranes for mass transfer of volatile organic compounds (VOCs) from the gas phase to a microbially active liquid phase. The reactor design provides a high biomass concentration, a method for wasting biomass, and a method for addition of pH buffers, nutrients, cometabolites, and/or other amendments. A theoretical model is developed, describing mass transfer and biodegradation in the membrane bioreactor. Reactor performance was determined in a laboratory scale membrane bioreactor over a range of gas loading rates using toluene as a model VOC. Toluene removal efficiency was greater than 98% at an inlet concentration of 100 ppm\dv and a gas residence time of less than 2 s. Factors controlling bioreactor performance were determined through both experiments and theoretical modeling to include: compound Henry’s law constant, membrane specific surface area, gas and VOC loading rates, liquid phase turbulence, and biomass substrate utilization rate.

Anaerobic degradation kinetics of 2,4-dichlorophenol (2,4-DCP) with linear sorption

Degradation of 2,4-DCP by acclimated anaerobic granules enriched at two specific loading rates was examined experimentally in batch mode. The data from process curves were analyzed by nonlinear regression to estimate parameters in the Haldane inhibition equation and one of its modified forms to obtain quantitative information on degradation kinetics. Linear sorption kinetics of 2,4-DCP were employed in data treatment to discriminate between the effects of sorption and biodegradation. It was observed that the modified inhibition equation described experimental data better than the Haldane inhibition equation since the later failed to predict the trend in experimental data. Parameter values estimated with the modified inhibition equation for two types of anaerobic granule, were 0.14 and 0.17 mg/gVSS.h for maximum degradation rate, k; 2.07 and 2.06 mg/L for saturation coefficient, Ks and 65.1 and 67.5 mg/L for inhibition coefficients, respectively. On the basis of 2,4-DCP toxicity effects on the two anaerobic granule types, it was found that the liquid phase concentration determined toxicity and degradation of the chemical.

Anaerobic degradation kinetics of 2,4-dichlorophenol (2,4-DCP) with linear sorption

Degradation of 2,4-DCP by acclimated anaerobic granules enriched at two specific loading rates was examined experimentally in batch mode. The data from process curves were analyzed by nonlinear regression to estimate parameters in the Haldane inhibition equation and one of its modified forms to obtain quantitative information on degradation kinetics. Linear sorption kinetics of 2,4-DCP were employed in data treatment to discriminate between the effects of sorption and biodegradation. It was observed that the modified inhibition equation described experimental data better than the Haldane inhibition equation since the later failed to predict the trend in experimental data. Parameter values estimated with the modified inhibition equation for two types of anaerobic granule, were 0.14 and 0.17 mg/gVSS.h for maximum degradation rate, k; 2.07 and 2.06 mg/L for saturation coefficient, Ks, and 65.1 and 67.5 mg/L for inhibition coefficients, respectively. On the basis of 2,4-DCP toxicity effects on the two anaerobic granule types, it was found that the liquid phase concentration determined toxicity and degradation of the chemical.

Anaerobic treatment of leachate using sequencing batch reactor and hybrid bed filter

Landfill leachate taken from a young municipal landfill site (≈3.5 years old) containing high organic contaminants (Total Organic Carbon -TOC of about 5000 mg 1−1) was treated in bench-scale Anaerobic Sequencing Batch Reactors (ASBR) and an Anaerobic Hybrid Bed Filter (AHBF) at mesophilic conditions. Twenty months of testing has been conducted at varied influent leachate concentration of 546–5770 mgTOC 1−1, Hydraulic Retention Time (HRT) of 10–1.5 days and Solid Retention Time (SRT) of 40–9 days in ASBR's, and influent leachate concentration of 1250–4490 mgTOC 1−1 and HRT of 5.1–0.9 days in AHBF. ASBR achieved 73.9% TOC removal at maximum organic loading rate of 2.8 kgTOC m−3 d−1 at 1.5 days of HRT, and 65.3% at 0.561 kgTOC kgVSS−1 d−1 of maximum specific loading rate and 2 days of HRT. The AHBF maintained 81.4% TOC removal at 1.2 kgTOC m−3 d−1 of loading and 2.4 days of HRT. Methane conversion ratio averaged 0.742 m3CH4 kgTOC−1 removed at (STP).

Feasibility of methanolic waste treatment in UASB reactors

Feasibility of methanolic waste treatment in an upflow anaerobic sludge blanket (UASB) reactor operating continuously for a period of more than 400 days was examined. Optimum pH was found to be between 7.0–7.3. In this pH range, there was no excessive build-up of volatile fatty acids and no upset or reactor failure was observed. Elementary pathways of degradation of methanol to methane were shown to be governed by the operating pH. Permissible volumetric loading rate to achieve 80% of TOC removal efficiency was determined to be 8 g-TOC/1·d. Below this value, the effect of hydraulic retention time or loading rate on TOC removal efficiency was not pronounced. Kinetic coefficients of k, Ks, Y and b were determined to be 1.11 g-TOC/g-VSS·d, 0.385 g-TOC/1, 0.213 g-VSS/g-TOC and 0.005 d −1, respectively. Maximum specific methanogenic activity was experimentally found to be 2 g-CH 4-COD/g-VSS·d while maximum specific loading rate to achieve 80% of TOC removal efficiency was determined to be 2 g-TOC/g-VSS·d. The results of this study suggest that single-step anaerobic treatment of methanolic waste is highly feasible provided that the pH is maintained close to 7.0.

Long-term change in Lake Tahoe (California-Nevada, U.S.A.) and its relation to atmospheric deposition of algal nutrients

Measurements at Lake Tahoe since 1967 have documented an increase in primary productivity and a shift from colimitation by nitrogen and phosphorus to consistent phosphorus limitation. Based on studies of annual nutrient loading, atmospheric deposition of nitrogen has been hypothesized to underlie these changes. The water chemistry data are consistent with this hypothesis. Specific loading rates are 0.032 ± 0.004 yr -1 for total nitrogen (TN) and 0.022±0.005 yr -1 for total phosphorus (TP). Bounds for the doubling times of the TN pool and the TN: TP ratio demonstrate that loading is fast enough to cause the observed changes. Based on cross-sectional studies of nutrient loss rates at other lakes, the doubling time for the TN pool was estimated as 52 yr and the doubling time for TN: TP as 30 yr. In the absence of atmospheric sources, the doubling time for TN would have been 220 yr and the TN: TP ratio would have halved in 150 yr. Long-term series for nitrate and total acid-hydrolyzable phosphorus exhibit, at best, only weak trends, but interannual variability is large enough to disguise trends of the expected magnitude. Much of the year-to-year variability in nitrate is due to the variable depth of spring mixing. One possible consequence of a shift in nutrient limitation is a reversal of eutrophication, although such a reversal has not been observed to date for Lake Tahoe. Mountain lakes near urban or agricultural areas have characteristics predisposing them to the enrichment effects of atmospheric deposition.

Dynamic changes in spatial microbial distribution in mixed-population biofilms: experimental results and model simulation

Dynamic changes in spatial microbial distribution in mixed-population biofilms were experimentally determined using a microslicer technique and simulated by a biofilm accumulation model (BAM). Experimental results were compared with the model simulation. The biofilms cultured in partially submerged rotating biological contactors (RBC) with synthetic wastewater were used as test materials. Experimental results showed that an increase of substrate loading rate (i.e., organic carbon and NH4-N) resulted in the microbial stratification in the biofilms. Heterotrophs defeated nitrifiers and dominated in the outer biofilm, whereas nitrifiers were diluted out in the outer biofilm and forced into the inner biofilm. At higher organic loading rates, a stronger stratified microbial spatial distribution was observed, which imposed a severe internal oxygen diffusion limitation on nitrifiers and resulted in the deterioration of nitrification efficiency. Model simulations described a general trend of the stratified biofilm structure. However, the actual stratification was stronger than the simulated results. For implication in the reactor design, when the specific carbon loading rate exceeds a certain limit, nitrification will be deteriorated or require a long start-up period due to the interspecies competition resulting in oxygen diffusion limitation. The extend of microbial stratification in the biofilm is especially important for determination of feasibility of nitrification in the presence of organic matters.

Dynamic changes in spatial microbial distribution in mixed-population biofilms: Experimental results and model simulation

Dynamic changes in spatial microbial distribution in mixed-population biofilms were experimentally determined using a microslicer technique and simulated by a biofilm accumulation model (BAM). Experimental results were compared with the model simulation. The biofilms cultured in partially submerged rotating biological contactors (RBC) with synthetic wastewater were used as test materials. Experimental results showed that an increase of substrate loading rate (i.e, organic carbon and NH4-H) resulted in the microbial stratification in the biofilms. Heterotrophs defeated nitriflers and dominated in the outer biofilm, whereas nitrifiers were diluted out in the outer biofilm and forced into the inner biofilm. At higher organic loading rates, a stronger stratified microbial spatial distribution was observed, which imposed a severe internal oxygen difuitsion limitation on nitrifiers and resulted in the deterioration of nitrification efficiency. Model simulations described a general trend of the stratified biofilm structure. However, the actual stratification was stronger than the simulated results For implication in the reactor design, when the specific carbon loading rate exceeds a certain limit, nitrification will be deteriorated or require a long start-up period due to the interspecies competition resulting in oxygen diflijsion limitation The extend of microbial stratification in the biofilm is especially important for determination of feasibility of nitrification in the presence of organic matters

Impact of the reactor hydrodynamics and organic loading on the size and activity of anaerobic granules

Wastewater treatment processes based on the upflow anaerobic sludge bed design are strongly dependent on the aggregation of biomass into macroscopic granules (1–3 mm) which settle well. The reactor hydrodynamics is of importance in the granulation process. The effect of the liquid upflow velocity υ UP associated with the operating time on the mean granule size and on the hydrogen, formate, acetate, propionate and glucose specific activities was studied, at various specific loading rates, in upflow sludge bed and filter reactors of 13 l fed with sugar wastewater. Reactors which were operated at 0.9 m h −1 behaved as fixed beds while those run at 2.2, 4.4 and 6.6 m h −1 were fluidized, because an immediate spatial gradient of the sludge particle sizes was induced. The υ UP had a significant positive effect on mean granule size. A specific loading rate increase from 0.5 to 1.5 g chemical oxygen demand per gram of volatile suspended solids per day raised proportionally the biomass growth rate, but had no positive effect on the granule development in size. Moreover, the υ UP had little effect on the specific wash-out rate of the smaller particles. Henceforth the resulting final size of granules is essentially a function of the hydrodynamic regime. Major impact on granule net steady size is attributed to several mechanisms related to fluidization: improved penetration of substrates into biofilm; insignificance of liquid shear relative to the shear of gas; reduction of particle friction and attrition with the bed voidage. Acidogenic (glucotrophic) activity decreased with υ UP increased yielding minimum values at intermediate υ UP, between 2 and 5 m h −1. Postacidogenic activities (propionate, acetate, formate, H 2) were positively influenced by υ UP to a slight extent. Glucose activity gradient within the granule bed was highly and inversely correlated to the granule size, while for acetate activity gradients, the correlation was direct, although less strong. These observations are discussed in detail with regard to an ordered distribution of the consortium populations within the granule spatial structure.

Toxicity of Sulfite and Cadmium to Anaerobic Granular Sludge

Abstract Toxicity of sulfite (Na2SO3) and cadmium (CdCl2) ions to anaerobic granular sludge was investigated in 1.2 litre bench-scale upflow anaerobic sludge blanket (UASB) reactors during process acclimation and shock load conditions. Minimal sulfite toxicity was observed under gradual and shock load conditions at sulfite concentrations of up to 1000 mg S/L if proper acclimation was allowed to occur. No long-term toxic effects were observed although the COD digestion rate was temporarily inhibited by shock load of sulfite. Scanning electron micrographs indicated that more sulfate-reducing bacteria were present in the granules developed in the reactors with sulfite supplement although rod-shaped Methanosaeta-like bacteria were still dominant. High bacterial growth rate was observed in the reactors which were supplied with the feed containing sulfite. The COD digestion rate was inhibited at a cadmium loading rate of 2.4 g Cd per day under both acclimation and shock load conditions. Acclimation did not seem to improve the bacteria to tolerate the toxicity of cadmium. The concentration of free cadmium was very low in the reactors under normal conditions, but increased rapidly when the COD digestion in the reactors ceased. The bacteria could not be reactivated after inhibited by cadmium. When reactors were operated at low specific COD loading rates, more inorganic precipitates were formed inside the granules which consequently settled faster.

Advantages of Fluidization on Granule Size and Activity Development in Upflow Anaerobic Sludge Bed Reactors

Wastewater treatment processes such as upflow anaerobic sludge bed (UASB)-like reactors are strongly dependent on the aggregation of biomass into macroscopic granules (1-3 mm) which settle well. The effect of the liquid upflow velocity (vUP) on the mean granule size and on the hydrogen, acetate, propionate and glucose specific activities was studied, at various specific loading rates, using sugar wastewater. Reactors operated at 0.9 m h-1 behaved as fixed beds while those run at 2.2,4.4 and 6.6 m h-1 were fluidized. The vUP had a significant effect on granule mean geometric diameter (Dm) and on the specific activities. The Dm increased with the vUP, as a direct function of the hydrodynamic regime, rather than as a result of a specific wash-out of smaller particles at higher vUP. Acidogenic (glucotrophic) activity decreased when vUP augmented. All other post-glucotrophic activities (propionate, acetate, H2) were positively influenced by the vUP. The positive effect of vup increased with the specific loading rate (SLR). Glucose activity gradient within the granule bed was highly and inversely correlated to that of the granule size, while for propionate and acetate activity gradients, the correlation was direct, although less strong. No gradient was observed for H2 activity. These observations are discussed in details with regard to an ordered distribution of the consortium populations within the granule spatial structure. They support the so-called multilayer arrangement of anaerobic granules. These results are encouraging for environmental industries which are promoting UASB-like designs handling anaerobic natural granules under fluidization.

Anaerobic butyrate degradation in a fluidized-bed reactor: effects of increased concentrations of hydrogen and acetate

The microbial degradation of butyrate was studied in a continuous-flow anaerobic fluidized-bed reactor. The reactor characteristics at steady-state butyrate loading, and in response to transient loadings of butyrate, hydrogen, acetate, and formate, were determined using an on-line computerized data acquisition system. The fluidized-bed reactor operated at a steady-state butyrate loading of 10 g of chemical oxygen demand (COD)/L-day and at an unusually high specific loading rate of 8-10 g of COD/g of volatile solids per day, achieving >97% COD removal

Effect of NSSC Spent Liquor on Granule Formation and Specific Microbial Activities in Upflow Anaerobic Reactors

This study was conducted to evaluate the capability of sulfonated lignin (S-lignin) to induce the flocculation of anaerobic sludge in upflow wastewater treatment. Two upflow anaerobic sludge bed-filter (UBF) reactors fed with a synthetic sugar waste were operated at a specific loading rate of 0.6 to 1.8 g of chemical oxygen demand (COD) per g of volatile suspended solid (VSS) per day (d). One of these reactors was stepwise supplemented with a spent liquor rich in S-lignin from a neutral sulfite semi-chemical (NSSC) pulping plant to reach an addition rate of 0.17 g S-lignin/g VSS d; 50 % of the S-lignin was adsorbed onto the biomass. This resulted in pre-granulation of the anaerobic sludge: 50 to 60% of the biomass was in particles > 0.3 mm versus 70 to 80% with a size < 75 µm in the control reactor. In addition, the S-lignin supplementation slightly enhanced the specific metabolic activities of the biomass. Calcium ion addition following the S-lignin treatment enhanced the stability and the density of the aggregates.

Anaerobic (UASB) Treatment of Pulp (CTMP) Wastewater and the Toxicity on Granules

A chemi-thermomechanical pulp (CTMP) effluent was treated by upflow anaerobic sludge bed (UASB) reactors. The purpose of the 254 day experiment was to evaluate the acclimatization process for two types of anaerobic granules to CTMP wastewater. One type was an acetic acid enriched culture composed mainly of methanogenic bacteria (reactor I) and the other was a mixed anaerobic culture containing both methanogenic and acidogenic bacteria (reactor II). Initially the reactors were maintained on synthetic feed with acetic acid (reactor I) or sucrose (reactor II) as the carbon source. In order to acclimatize the granules to the wastewater, CTMP wastewater was introduced in 3 stages (10, 50, and 100%). The wastewater contained 8200 mg/l COD, 30-40% of which was non-biodegradable. Both reactors were operated at a specific loading rate of 0.16 g COD/g VSS/day throughout the experiment. Supplementary batch tests using serum bottles were conducted to evaluate toxicity of the CTMP wastewater and two known toxic compounds, pentachlorophenol and mercuric chloride, on mixed anaerobic culture granules. Reactor II acclimatized to the CTMP wastewater better than reactor I. At 50% CTMP wastewater, a COD removal rate of 80% was attained for reactor II, while reactor I removed only 55% of the influent COD. Coincidentally, acetic acid consumption in reactor I was also inhibited after the introduction of 50% CTMP wastewater. It took reactor I 60 days to recover from the CTMP toxicity and to achieve the level of reactor II performance. An unexpected discovery was that the granules from both reactors were completely inhibited 2 weeks after the completion of the 254 day experiment. The granules were stored in a cold room (4°C) for two weeks, during this period, the granules lost the ability to degrade acetic acid. These sick (inhibited) granules re-gained acetoclastic activity when sucrose feed was provided as a carbon source. The results of batch tests showed a dramatic reduction in granule biogas production after exposure both to starvation and to toxicants at levels higher than a certain threshold concentration. The biogas production ratio between the starved and the fed granules was decreased from 89.9% to 43.0% when they were exposed to 50% and 300% CTMP wastewater, respectively. The toxicity of 100% CTMP was equivalent to 7 ppm of pentachlorophenol and 25 ppm of mercuric chloride.

Effect of Subcritical Crack Growth on Fracture Toughness and Work‐of‐Fracture Tests Using Chevron‐Notched Specimens

A correlation between the plane strain stress intensity factor KI, load, and crack extension has been analyzed for constant displacement and constant loading rate experiments, using chevron‐notched, four‐point‐bend specimens. It is assumed that at the beginning of the experiment the chevron triangle tip is not ideally sharp. As loading continues, the crack initially moves with velocity vt at KI equal to a threshold value Kt. Maximum crack velocity is reached at KI= KIC, the fracture toughness. Depending on the type of material tested, a specific displacement or loading rate must be used to correlate the maximum load with KIc. An error in KIC calculation is estimated if different displacement rates are applied. Repeated loading‐unloading work‐of‐fracture (WOF) experiments generate values related to the resistance of the material to fracture initiation, Kt, only when the crack length approaches 100% of the specimen width. Values related to material's fracture toughness, KIC are not generated in WOF tests.