Constant Loading Rate Research Articles

Theoretical approach to determine dynamic fatigue strength characteristics of ceramics under variable loading rates on the basis of SCG concept

This paper presents a theoretical approach to determine the dynamic fatigue strength characteristics of ceramics under variable loading rates on the basis of the slow crack growth (SCG) concept. First, a probabilistic effective inert strength model was derived on the basis of the SCG concept in conjunction with the Weibull distribution for ceramics subjected to multi-stage loading. Second, a four-point bending test was conducted on $$\hbox {Al}_{{2}}\hbox {O}_{{3}}$$ under constant and two-stage variable loading rates, and the fracture surface was then observed. The experimental data that depend on loading rates can be unifiedly evaluated after converting the data to the effective inert strength, obeying the three-parameter Weibull distribution. In addition, the Weibull plots of the inert strength, which were calculated from the inclusion size on the fracture surface using the grain fracture model, showed good agreement with the three-parameter Weibull distribution for the converted effective inert strength. These analytical results theoretically indicate that dynamic fatigue under variable loading rates occurs by obeying SCG at the inclusion. Further, the inert strength and its scatter depend on the size and distribution of inclusions.

High-rate Nitrogen Removal in a Two-stage Partial Nitritation-ANAMMOX Process Under Mainstream Conditions

A two-stage partial nitritation (PN)-ANAMMOX process was successfully carried out for low-strength NH4+-N (50 mg·L-1) wastewater treatment at ambient/low temperatures. The results show that an average total nitrogen removal rate and removal efficiency above 0.6 kg·(m3·d)-1and 80% could be maintained, respectively, at temperatures between 20℃ and 14℃. The two-stage PN-ANAMMOX process was successful both under NO2--N-limited and NH4+-N-limited conditions. When the two-stage PN-ANAMMOX process was operated under NH4+-N-limited conditions, the "limit of technology" for nitrogen removal (TN<3 mg·L-1) could be easily achieved by following anoxic denitrification. Lowering the temperature to 12℃ resulted in the reduction of the total nitrogen removal rate to~0.5 kg·(m3·d)-1. Due to the low temperature, the anammox reaction became the rate-limiting step for nitrogen removal, while the PN reaction was not impacted. In the temperature range of 10-20℃, the activity-temperature coefficients (θ) of the PN granules and ANAMMOX sludge were 1.056 and 1.172, respectively, suggesting that the anammox bacteria have a higher temperature sensitivity than the ammonium oxidizing bacteria (AOB). Overall, the results clearly indicate that the nitrogen removal loading rate of the two-stage PN-ANAMMOX process is mainly controlled by the activity and quantity of anammox biomass. In contrast, the process nitrogen removal efficiency mainly depends on the performance of the first-stage PN (i.e., effluent NO2--N/NH4+-N ratio and NO3--N concentration) under a constant nitrogen removal loading rate (no overload). Based on these results, a hierarchically separate control strategy was proposed to obtain a high-rate nitrogen removal during the two-stage mainstream PN-ANAMMOX process.

Strain rate sensitivity behavior of a structural steel during low-cycle fatigue investigated using indentation

In this study, a series of experiments consisting of low-cycle fatigue, constant loading rate indentation, creep indentation experiments, optical microscopy examination, and transmission electron microscopy were performed to investigate the strain rate sensitivity behavior of SS400 structural steel under cyclic loading conditions. 400 indentations were conducted in the displacement control mode at different strain rate indentations varied from 0.02 s−1 to 0.2 s−1 and the effects of strain rate indentation on mechanical properties were investigated on four fatigue failure specimens. An analytical method was used to estimate mechanical properties of the structural steel from the loading/unloading curves. The results indicated that the yield strength and the strain hardening exponent strongly depended on both the strain rate indentation and the strain amplitude. Yield strength and strain hardening exponent tended to increase when the strain amplitude increased. The strain rate sensitivity (SRS) was also determined from the yield strength and the creep indentation data, and the dependency of the strain rate sensitivity on the strain amplitude level was examined. The results showed that the strain rate sensitivity exhibited decrease almost linearly when the strain amplitude increased from 0.4% to 1.0%. The results of the present study were used to assess and understand the strain rate sensitivity behavior of SS400 structural steel during low-cycle fatigue.

A study on stress corrosion cracking and hydrogen embrittlement of Jethete M152 martensitic stainless steel

Stress corrosion cracking and hydrogen embrittlement of Jethete M152 martensitic stainless steel were investigated by using constant load and slow strain rate tension test, optical microscope, transmission electron microscope and scanning electron microscope. The results show that lath martensite was formed and no austenite was observed. Stress corrosion cracking and hydrogen embrittlement were intensified with enhancing stress level and increasing hydrogen charging current density. Fractographic analysis revealed that corrosive environment resulted in the distinct change of fracture morphology from dimple to intergranular. It is indicated that undeformable Al2O3 inclusion is an important factor in stress corrosion cracking and hydrogen embrittlement for Jethete M152 steel. Meanwhile, it is beneficial to enhance resistance to stress corrosion cracking and hydrogen embrittlement by decreasing the inclusion size.

Soil–Geogrid Interaction at Various Influencing Factors by Pullout Tests with Applications of FBG Sensors

The behavior of soil–geogrid interaction is a dominant factor in the long-term performance of geogrid-reinforced earth structures. This study investigates the interaction of sandy soil and the embedded geogrid by pullout tests with the application of a novel testing method using fiber Bragg grating (FBG) sensors. Three influencing parameters are considered: dry density of soil, initial normal stress, and fixing condition of the pullout back end. A displacement-controlled mode is used for the normal stress, and the pullout load is applied at a constant loading rate. The results indicate that for the case with a free back end, the peak shear stress mobilizes from the front face toward the back end with the elongation of the geogrid during the pullout process, before the entire slippage of the geogrid. With higher dry density or higher initial normal stress, the mobilization of the peak shear stress transmits more slowly and the distribution area of shear stress along the geogrid is accordingly narrower due to the corresponding higher resistance by the soil particles. However, the values of the peak shear stress in these two cases are still higher than in cases with lower dry density or lower initial normal stress. By contrast, a different distribution mode of shear stress along the soil–geogrid interface is identified for geogrid with a fixed back end, with maximum shear stress developing near the front face for all pullout displacements.

Depth sensing indentation of magnesium/boron nitride nanocomposites

Magnesium nanocomposites, considered as energy-saving lightweight materials of future, are a fairly new family of composite materials with enhanced specific strength and ductility compared to pure magnesium and/or magnesium alloys. In the present study, time-dependent plastic deformation of novel light-weight magnesium/boron nitride nanocomposites containing 0.5, 1.5 and 2.5 vol% of nano-boron nitride particulates is studied through a depth-sensing indentation approach against monolithic pure magnesium. The synthesis of magnesium–boron nitride nanocomposites was accomplished using powder metallurgy technique coupled with microwave sintering, followed by hot extrusion (end products are 8-mm diameter rods). The depth sensing indentation creep tests were conducted at room temperature (∼0.32Tm of magnesium) using an instrumented indentation platform via a self-similar pyramidal (Berkovich) indenter. To assess the influence of loading rate on the indentation-induced deformation behavior of the materials, a dual stage indentation creep including a constant loading rate followed by a constant load-holding scheme was used; indentation tests were performed on the specimens. The specimens were loaded at rates of 0.05, 0.5, 5, and 50 mN/s to a peak load of 50 mN then force was held constant for 400 s while load/displacement/time data were recorded continuously. The results of the depth sensing indentation tests were correlated and explained using the microstructural characteristics placing special emphasis on the volume fraction of reinforcement and the indentation loading rate. Finally, the controlling creep mechanisms of the magnesium–boron nitride nanocomposites and the base metal (pure magnesium) were discussed in the present paper. The results of this paper can be used as a baseline for high-temperature creep analysis of magnesium–boron nitride nanocomposites which is of engineering significance.

Failure Law and Mechanism of the Rock-Loose Coal Composite Specimen under Combined Loading Rate

The surrounding rock deformation in the underground mining roadway increases rapidly during excavation and mining disturbance. The semirock and coal roadway, which is formed by the rock and coal composite system, will show different mechanical properties. Therefore, loading rate and anchoring or not are critical to grasp the stability law of the rock-coal combination system under different conditions. The uniaxial mechanical test under the constant loading rate and combined loading rate is carried out in the static loading range (0.01∼10 mm·min−1) of the rock-loose coal composite specimen (RCCS). The test results show that the rock and loose coal composite specimen without bolt (RCB(0) specimen) are abnormal, and the uniaxial compressive strength (UCS) and residual strength (RS) of the specimen do not increase but decrease with the increase of loading rate. In contrast, the UCS of the rock-loose coal composite specimen with the bolt (RCB(1∼2) specimen) is consistent with that of the ordinary hard and brittle rock, which increases with the increase of loading rate. To a certain extent, the initial damage and the development of microcracks in loose coal bodies are limited by bolts. Finally, the deformation mechanism and constitutive equation of the rock-loose coal composite system are discussed.

Clustering effect on damage mechanisms in open-hole laminated carbon/epoxy composite under constant tensile loading rate, using acoustic emission

In the present research, damage mechanisms in the open-hole laminated carbon/epoxy composite have been investigated using the acoustic emission method. For such objective, standard specimens, made of the pure resin, the pure fiber and composites, were tested under tensile conditions, at a constant loading rate. Then, the clustering effect was studied in order to know which damages (debonding, fiber pull-out, delamination, matrix cracking and fiber breakage defects) occurred in the material. For the analysis of acoustic emission signals, wavelet packet transform and fuzzy C-means methods were applied to data. Obtained results indicated that the frequency range for matrix cracking, the fiber breakage, the fiber pull-out, debonding of fibers from the matrix and the delamination was obtained as 100–250 kHz, 420–500 kHz, 250–320 kHz, 320–380 kHz and 380–420 kHz, respectively. Scanning electron microscopy images showed that the dominant damage was qualitatively related to debonding, the fiber pull-out and the delamination, which had a proper agreement with obtained results from the analysis of acoustic emission signals.

A Novel Approach to Modelling Nanoindentation Instabilities

We review the recently developed models for load fluctuations in the displacement controlled mode and displacement jumps in the load controlled mode of indentation. To do this, we devise a method for calculating plastic contribution to load drops and displacement jumps by setting-up a system of coupled nonlinear time evolution equations for the mobile and forest dislocation densities by including relevant dislocation mechanisms. These equations are then coupled to the equation defining constant displacement rate or load rate. The model for the displacement controlled mode using a spherical indenter predicts all the generic features of nanoindentation such as the elastic branch followed by several force drops of decreasing magnitudes and residual indentation depth after unloading. The stress corresponding to the elastic force maximum is close to the yield stress of an ideal solid. The predicted numbers for all the quantities match experiments on single crystals of Au using a spherical indenter. We extend the approach to model the load controlled nanoindentation experiments that employ a Berkovich indenter. We first identify the dislocation mechanisms contributing to different regions of the F − z curve as a first step for obtaining a good fit to a given experimental F − z curve. This is done by studying the influence of the parameters associated with various dislocation mechanisms on the model F − z curves. The study also demonstrates that the model predicts all the generic features of nanoindentation such as the existence of an initial elastic branch followed by several displacement jumps of decreasing magnitudes and residual plasticity after unloading for a range of model parameter values. Furthermore, an optimized set of parameter values can be easily determined that give a good fit to the experimental load–displacement curves for Al single crystals of ( 110 ) and ( 133 ) orientations. Our model also predicts the indentation size effect in a region where the displacement jumps disappear. The good agreement of the results of the models with experiments supports our view that the present approach can be used as an alternate method to simulations. The approach also provides insights into several open questions.

Analysis of Solvent Effect on Mechanical Properties of Poly(ether ether ketone) Using Nano-indentation

Poly(ether ether ketone) (PEEK) is a high-performance semi-crystalline thermoplastic polymer. Exposure of the polymeric surface to solvents can have a strong effect like softening/swelling of polymeric network or dissolution. In this study, nano-indentation analysis was performed to study the effect of acetone on the surface mechanical properties of PEEK using different exposure time. The experiments were performed with a constant loading rate (10 nm/s) to a maximum indentation displacement (1000 nm). A 30-second hold segment was included at the maximum load to account for any creep effects followed by an unloading segment to 80% unloading. The indentation hardness and the elastic modulus were computed as a continuous function of the penetration displacement in the continuous stiffness mode (CSM) indentation. The experimental data showed that the peak load decreased from ∼5.2 mN to ∼1.7 mN as exposure time in solvent environment increased from 0 to 18 days. The elastic modulus and the hardness of PEEK samples also displayed a decreasing trend as a function of exposure time in the solvent environment. Two empirical models were used to fit the experimental data of hardness as a function of exposure time which showed a good agreement with the experimental values.

Characterisation of time-dependent, statistical failure of cellulose fibre networks

Cellulosic materials have special advantages for transport packaging, because of their light-weight and recyclable natures and also relatively high specific strength. The strength of such materials is normally evaluated by applying monotonically increasing, quasi-static displacement (or load). However, in real circumstances, the material is subjected to far more complex loading histories, such as creep, fatigue, and random loading. Failures under such circumstances are, not only time-dependent, but also notoriously variable. For example, the coefficient of variation for creep lifetime reaches or even exceeds 100%. The objective of this study is to develop a method to characterise both time-dependent and statistical natures of failures of cellulosic materials. We have used a general formulation of time-dependent, statistical failure, originally proposed by Coleman (J Appl Phys 29(6):968–983, 1958). We have identified three material parameters: (1) characteristic strength, representing short term strength, (2) brittleness parameter (or durability), and (3) Weibull shape parameter related to long-term reliability. These parameters were determined by special protocols of creep and constant loading-rate (CLR) tests for a series of containerboards. Results have shown that these two test methods yield comparable values for the materials parameters. This implies the possibility of replacing extremely time-consuming creep tests with the more time-efficient CLR tests. Comparing the cellulose fibre networks with fibres and composites used for advanced structural applications, we have found that they are very competitive in both reliability and durability aspects with Kevlar and glass-fibre composites.Graphical abstract

Effects of bondline discontinuity during growth of interface cracks including stability and kinetic considerations

The effects of a single discontinuity/void on the mode I interface crack growth under a constant loading rate are investigated experimentally and analytically. To account for a coexistence of the void and the crack tip process zone, a new unit pattern model is derived. The crack growth path is composed from the three distinctive zones: (1) The initially bonded crack front vicinity region––carrying the majority of the loading––is of finite length and modelled as a series of elastic springs. (2) The discontinuity region is modelled as a simple beam not carrying any surface tractions. (3) The far field is of infinite length and represented with elastic springs. In this model, the constant loading rate boundary conditions are fully considered. Subsequently, the kinetic effects associated with the specimen geometry and the presence of the discontinuity are attempted using the generalized Griffith's theory. A very good agreement between the experimental and the analytical results is observed for both the load response and the R curves for all void-to-process zone length ratios. A new light and a more profound appraisal of the equilibrium paths are gained.

Forced-rupture of cell-adhesion complexes reveals abrupt switch between two brittle states.

Cell adhesion complexes (CACs), which are activated by ligand binding, play key roles in many cellular functions ranging from cell cycle regulation to mediation of cell extracellular matrix adhesion. Inspired by single molecule pulling experiments using atomic force spectroscopy on leukocyte function-associated antigen-1 (LFA-1), expressed in T-cells, bound to intercellular adhesion molecules (ICAM), we performed constant loading rate (rf) and constant force (F) simulations using the self-organized polymer model to describe the mechanism of ligand rupture from CACs. The simulations reproduce the major experimental finding on the kinetics of the rupture process, namely, the dependence of the most probable rupture forces (f*s) on ln rf (rf is the loading rate) exhibits two distinct linear regimes. The first, at low rf, has a shallow slope, whereas the slope at high rf is much larger, especially for a LFA-1/ICAM-1 complex with the transition between the two occurring over a narrow rf range. Locations of the two transition states (TSs) extracted from the simulations show an abrupt change from a high value at low rf or constant force, F, to a low value at high rf or F. This unusual behavior in which the CACs switch from one brittle (TS position is a constant over a range of forces) state to another brittle state is not found in forced-rupture in other protein complexes. We explain this novel behavior by constructing the free energy profiles, F(Λ)s, as a function of a collective reaction coordinate (Λ), involving many key charged residues and a critical metal ion (Mg2+). The TS positions in F(Λ), which quantitatively agree with the parameters extracted using the Bell-Evans model, change abruptly at a critical force, demonstrating that it, rather than the molecular extension, is a good reaction coordinate. Our combined analyses using simulations performed in both the pulling modes (constant rf and F) reveal a new mechanism for the two loading regimes observed in the rupture kinetics in CACs.

The influence of the total solid content on the stability of dry-thermophilic anaerobic digestion of rice straw and pig manure

Dry anaerobic digestion is a promising technology for the recycling of agricultural waste to produce energy and fertilizer. Adding water to the substrate enables better handling and avoid inhibition caused by high total solid (TS) content in the reactor; however, it also increases leachate and operational costs. To assess the extent to which the amount of water added can be reduced, it was examined how the TS content in the reactor influenced the production of biogas. A semi-batch dry thermophilic anaerobic digester was fed with substrate (rice straw and pig manure) at a constant organic loading rate, and varied the TS contents (27%, 32%, 37%, and 42%) of the substrate by adding different amounts of water (representing 0–36% of the total substrate). During incubation, the TS content in the reactor gradually increased from 18% to 31%. Biogas production was stable and high (564 ± 13–580 ± 36 N m3 t−1 VS), and there was no accumulation of volatile fatty acids when the TS content of the reactor was between 18% and 27%. However, the rate decreased sharply and propionate and acetate were also produced when the TS content of the reactor exceeded 28%. By applying a simple TS balance model, it was found that stable biogas production could be achieved at a substrate TS content of 32%, at which reactor TS content reached 23% at steady-state condition.

Ultrasonic investigation of granular materials subjected to compression and crushing

Ultrasonic wave propagation measurement has been used as a suitable technique for studying the granular materials and investigating the soil fabric structure, the grain contact stiffness, frictional strength, and inter-particle contact area. Previous studies have focused on the variations of shear and compressional wave velocities with effective stress and void ratio, and lesser effort has been made in understanding the variation of amplitude and dominant frequency of transmitted compressional waves with deformation of soil packing. In this study, continuous compressional wave transmission measurements during compaction of unconsolidated quartz sand are used to investigate the impact of soil layer deformation on ultrasonic wave properties. The test setup consisted of a loading machine to apply constant loading rate to a sand layer (granular quartz) of 6 mm thickness compressed between two forcing blocks, and an ultrasonic wave measurement system to continuously monitor the soil layer during compression up to 48 MPa normal stress. The variations in compressional wave attributes such as wave velocity, transmitted amplitude, and dominant frequency were studied as a function of the applied normal stress and the measured normal strain as well as void ratio and particle size. An increasing trend was observed for P-wave velocity, transmitted amplitude and dominant frequency with normal stress. In specimen with the largest particle size (D50 = 0.32 mm), the wave velocity, amplitude and dominant frequency were found to increase about 230%, 4700% and 320% as the normal stress reached the value of 48 MPa. The absolute values of transmitted wave amplitude and dominant frequency were greater for specimens with smaller particle sizes while the normalized values indicate an opposite trend. The changes in the transmitted amplitude were linked to the changes in the true contact area between the particles with a transitional point in the slope of normalized amplitude, coinciding with the yield stress of the granular soil layer. The amount of grain crushing as a result of increase in the normal stress was experimentally measured and a linear correlation was found between the degree of grain crushing and the changes in the normalized dominant frequency of compressional waves.

Investigation of Sectional-Stage Loading Strategies on a Two-Stage Turbocharged Heavy-Duty Diesel Engine under Transient Operation with EGR

The influence of loading strategies on combustion and emissions parameters is experimentally and numerically studied under typical 5 s transient conditions of constant speed and increasing torque. The experiment is conducted on a two-stage turbocharged heavy-duty diesel engine with a constant opening valve high-pressure exhaust gas recirculation (EGR) system. The test results show that: compared with the full-stage loading (FSL) strategy (constant loading rate during the entire transient process), the sectional-stage loading (SSL) strategies (holding a certain time at 50% load) can significantly reduce soot emissions (by 41.3%); the greater the first-stage loading rate, the better the torque response performance, which maximally increases by 56.7%. Besides, longer loading holding time can effectively restrain the overshoot of EGR rate and advance the combustion phase (CA10, CA50) at medium and large loads. However, the larger second-stage loading rate slightly deteriorates the combustion and emission performance. This deterioration situation can be markedly suppressed by adopting a suitable loading hold time.

Numerical Simulation to Select Proper Strain Rates during CRS Consolidation Test

Constant rate of strain (CRS) and incremental loading (IL) consolidation tests are extensively used to measure consolidation properties of clayey soils. However, results of CRS test are usually strain rate dependent. In this study, a finite differences simulation of CRS test based on Terzaghi’s theory of consolidation is performed. The numerical simulation permits to evaluate a strain rates range satisfying a preset relative pressure criterion. Results of simulation show that the required relative pressure criterion can be verified, during the steady state stage of CRS test, only for particular types of soils and particular range of strain rates. Subsequently, a simple method is proposed to select, for a soil sample defined by its initial height, initial void ratio and liquid limit, an appropriate strain rates range satisfying the ASTM 4186-06 criterion. Comparison of previsions and experimental results reported in literature shows good agreement.

An Investigation into the Effect of Normal Load Frequency on Fretting Fatigue Behavior of Al7075-T6

ABSTRACTMost previous studies on fretting fatigue have been accomplished under constant normal loading and less attention has been paid to cyclic normal loading. An innovative test apparatus was specially designed and manufactured for fretting fatigue tests under cyclic loading in this work and the fretting fatigue behavior of Al7075-T6 was investigated at different normal load frequencies. A finite element model was developed to study the effect of normal load frequency on the contact stress distribution. It was found that the cyclic normal load has a more damaging effect on fretting fatigue life compared to constant normal load, particularly at lower frequencies. The results showed that at the normal load frequency of f = 1 Hz, fatigue life decreased by 52% in the high cycle fatigue regime and 28% in the low cycle fatigue regime. The experimental results also indicated that at the normal load frequency of 80 Hz, the fretting fatigue life converged to its corresponding life under constant normal load condition. The fracture surface and the fretting area of the specimens were examined using both optical and scanning electron microscopy (SEM). The experimental observations showed that the dominant partial slip condition with a wider slip region compared to constant normal loading, severe delamination, and higher oxidation rate due to the normal load release at each cycle, are the most important reasons for significant reductions in fretting fatigue life, under cyclic normal loading, especially for low normal load frequencies.

Strain rate effect on fault slip and rupture evolution: Insight from meter-scale rock friction experiments

We conduct meter-scale rock friction experiments to study strain rate effect on fault slip and rupture evolution. Two rock samples made of Indian metagabbro, with a nominal contact dimension of 1.5m long and 0.1m wide, are juxtaposed and loaded in a direct shear configuration to simulate the fault motion. A series of experimental tests, under constant loading rates ranging from 0.01mm/s to 1mm/s and under a fixed normal stress of 6.7MPa, are performed to simulate conditions with changing strain rates. Load cells and displacement transducers are utilized to examine the macroscopic fault behavior, while high-density arrays of strain gauges close to the fault are used to investigate the local fault behavior. The observations show that the macroscopic peak strength, strength drop, and the rate of strength drop can increase with increasing loading rate. At the local scale, the observations reveal that slow loading rates favor generation of characteristic ruptures that always nucleate in the form of slow slip at about the same location. In contrast, fast loading rates can promote very abrupt rupture nucleation and along-strike scatter of hypocenter locations. At a given propagation distance, rupture speed tends to increase with increasing loading rate. We propose that a strain-rate-dependent fault fragmentation process can enhance the efficiency of fault healing during the stick period, which together with healing time controls the recovery of fault strength. In addition, a strain-rate-dependent weakening mechanism can be activated during the slip period, which together with strain energy selects the modes of fault slip and rupture propagation. The results help to understand the spectrum of fault slip and rock deformation modes in nature, and emphasize the role of heterogeneity in tuning fault behavior under different strain rates.

Modeled Response of Wastewater Nutrient Treatment to Indoor Water Conservation

Abstract Effect of residential water conservation practices on wastewater treatment was modeled for influent flow reductions up to 54% at constant mass loading rates of COD, TKN, and total phosphor...