Rate Loading Conditions Research Articles

The role of adhesive in the ballistic/structural performance of ceramic/polymer–matrix composite hybrid armor

Transient non-linear dynamics computational finite-element analyses are employed to investigate the role of the adhesive used in ceramic strike-face/composite back-face hybrid armor. The role of the adhesive is investigated under: (a) high loading-rate conditions accompanying impact of an armor-piercing projectile onto the armor structure; and (b) low loading rate conditions associated with the ingress of the loads which are generated at the road/tire contact interfaces and transmitted into the armor structure through the vehicle frame. The high loading-rate conditions were analyzed since they reveal the role of the adhesive under ballistic impact scenarios which controls the overall penetration resistance of the armor. On the other hand, the low loading-rate conditions were analyzed since they may reveal the potential structural damage that armor may experience due to sustained in-service loads. Since structural thermoset-polymer-based adhesives suffer from inadequate ductility, only the case of flexible (elastomer-based) adhesives was considered. The following two specific aspects of the adhesive mechanical response are given special attention: (a) strain rate sensitivity and (b) the degree of volumetric compressibility. The results obtained show that while significant improvements in the ballistic protection performance and durability of hybrid armor can be attained by proper modifications in the adhesive layer mechanical properties, there is no single combination of these properties which simultaneously optimizes all the performance requirements.

Dragon-Kings in rock fracturing: Insights gained from rock fracture tests in the laboratory

In order to shed some lights to the “dragon-kings” concept, this paper re-examines experimental results on rock fracture tests in the laboratory, obtained from acoustic emission monitoring. The fracture of intact rocks as well as rocks containing natural structures (joints, faults, foliations) under constant stress rate loading or creep conditions is generally characterized by typical stages with different underlying physics. The primary phase reflects the initial rupture of pre-existing microcrack population in the sample or in the fault zone. Sub-critical growth dominates the secondary phase. The third phases termed nucleation phase corresponds to the initiation and accelerated growth of the ultimate fracture. The secondary and nucleation phases in both intact rock and faulted rock show power-law (of time-to-failure) increasing event rate and moment release. Samples containing planar structures such as foliations and faults demonstrate very similar features to natural earthquakes including: 1) small number of immediate foreshocks by which fault nucleation zones could be mapped; 2) the critical nucleation zone size is normally a fraction of the sample dimension; 3) a lot of aftershocks concentrated on the fault ruptured during the main event; 4) stress drop due to the main rupture is of the order from a few tens to a few hundreds MPa; 5) b-value drops during foreshocks and recovers during the aftershocks. All these results agree with the suggestion that laboratory measurements require no scaling but can be applied directly to the Earth to represent local fault behavior. The ultimate failure of the sample, or fracture of major asperities on the fault surface, normally lead to extreme events, i.e., dragon-kings, which has a magnitude significantly greater than that expected by the Gutenberg-Richter power-law relation in the magnitude-frequency distribution for either foreshocks or aftershocks. There are at least two mechanisms that may lead to dragon-kings: 1) The power-law increasing event rate and moment release; and 2) Hierarchical fracturing behavior resulting from hierarchical inhomogeneities in the sample. In the 1st mechanism, the final failure corresponds to the end point of the progressive occurrence of events and thus the resulted dragon-king event can be interpreted as a superposition of many small events. While for the 2nd mechanism an event of extreme size is the result of fracture growth stepping from a lower hierarchy into a higher hierarchy on fault surface having asperities characterized by hierarchical distribution (of size or strength) rather than simple fractal distribution. In both mechanisms the underlying physics is that fracture in rocks is hard to stop beyond certain threshold corresponding to the critical nucleation zone size.

Mechanisms of dynamic deformation and dynamic failure in aluminum nitride

Uniaxial quasi-static, uniaxial dynamic and confined dynamic compression experiments have been performed to characterize the failure and deformation mechanisms of a sintered polycrystalline aluminum nitride using a servohydraulic machine and a modified Kolsky bar. Scanning electron microscopy and transmission electron microscopy (TEM) are used to identify the fracture and deformation mechanisms under high rate and high pressure loading conditions. These results show that the fracture mechanisms are strong functions of confining stress and strain rate, with transgranular fracture becoming more common at high strain rates. Dynamic fracture mechanics and micromechanical models are used to analyze the observed fracture mechanisms. TEM characterization of fragments from the confined dynamic experiments shows that at higher pressures dislocation motion becomes a common dominant deformation mechanism in AlN. Prismatic slip is dominant, and pronounced microcrack–dislocation interactions are observed, suggesting that the dislocation plasticity affects the macroscopic fracture behavior in this material under high confining stresses.

The Start-Up of Aerated Submerged Fixed-Bed Biofilm Reactors for Steady State Nitrification

The effects of two different start-up regimes on the operation performance and population dynamics of nitrification were evaluated in aerated submerged fixed-bed biofilm reactors (SFBBRs) operated at 20 °C on a mediumstrength ammonium wastewater at the bench scale. The reactors were operated in parallel at start-up with two reactors under the maximum nitrogen loading rate condition (NLR) designed for the system, and for the other two reactors, the NLR was increased stepwise over a period of 22 days. The dominant microorganism among the ammonia-oxidising bacteria (AOB) in the seeding sludge was Nitrosomonas oligotropha. There was an observed growth of Nitrosospira and Nitrosomonas europaea/eutropha when the reactors were started up. Nitrosospira vanished during the course of reactor operations, while N. communis was observed to flourish in the second low-load stage of the SFBBR cascade. Nitrospira was the dominant nitrite-oxidising bacteria (NOB) genus present throughout the investigation. Reactors gradually loaded showed higher nitrification rates during the start-up. However, there was no significant difference in the total time required to reach steady state between the two start-up regimes. The AOB required 30 days and the NOB required an additional 8 days on average to reach steady state because NOB development was inhibited by free ammonia (FA). Proposals with respect to pH control, seeding sludge and sludge recycling were developed to shorten the start-up period in SFBBRs. The specific nitrification rates of the attached biomass in SFBBRs were compared to previous studies as well. The results from these experiments will aid in process design.

English

The dry anaerobic digestion process is an innovative waste-recycling method to treat high-solid-content bio-wastes. This can be done without dilution with water by microbial consortia in an oxygen-free environment to recover potential renewable energy and nutrient-rich fertilizer for sustainable solid waste management. It generally takes place at solid concentrations higher than 10% and enables a higher volumetric organic loading rate, minimal material handling, lower energy requirements for heating, limited environmental consequences and energetically effective performance. The long retention time, poor startup performance, incomplete mixing and the accumulation of volatile fatty acids (VFAs) are considered as the main disadvantages for the solid-state fermentation process. In order to develop feasible dry anaerobic digestion processes, it is important to review the optimization techniques and suggest possible areas where improvements could be made. These include reactor configuration, mixing, solid retention time, feedstocks, organic loading rate, inoculation, co-digestion, pretreatment, percolation, additives and environmental conditions within the digester such as temperature, pH, buffering capacity and VFAs concentration.   Key words: Solid organic wastes, dry anaerobic digestion process, biogas, optimization.

High temperature deformation and fracture behaviour of 316L stainless steel under high strain rate loading

The high temperature deformation and fracture behaviour of 316L stainless steel under high strain rate loading conditions are investigated by means of a split Hopkinson pressure bar. Impact tests are performed at strain rates ranging from 1 × 10 3 s −1 to 5 × 10 3 s −1 and temperatures between 25 °C and 800 °C. The experimental results indicate that the flow response and fracture characteristics of 316L stainless steel are significantly dependent on the strain rate and temperature. The fracture analysis results indicate that the 316L specimens fail predominantly as the result of intensive localised shearing. Furthermore, it is shown that the flow localisation effect leads to the formation of adiabatic shear bands. The fracture surfaces of the deformed 316L specimens are characterised by a dimple-like structure with knobby features. The knobby features are thought to be the result of a rise in the local temperature to a value greater than the melting point.

The Dynamic Response of Q345 Steel at High Strain Rates and High Temperature

Mechanical properties of Q345 steel used for industrial structure under high strain rate and high temperature loading conditions such as rocket launching are required to provide appropriate safety assessment to these mechanical structures. The split Hopkinson pressure bar (SHPB) technique with a special experimental apparatus can be used to obtain the material behavior under high strain rate loading conditions. In this paper, dynamic deformation behaviors of Q345 steel under both high strain rate compressive and high temperature loading are determined using the SHPB technique.

Characterization of cavitation development while tensile testing PVF2 using 3D X-ray microtomography

Abstract Fast X-ray microtomography, coupled with a specially designed tensile cell, has been used to investigate in situ spatio-temporal development of cavitation in PVF2. Experiments have been performed during a monotonic loading up to fracture. The influence of the root stress triaxiality and of the loading rate conditions have also been investigated through non-monotonic tensile tests including load reversal and relaxation sequences. The analysis of the monotonic tests has revealed two types of cavitation: (subvoxel) diffuse cavitation and (resolved) macro-cavitation. The presence of particles in the high stress triaxiality areas of the material have been reported as potential initiation site of macro-cavities. Macro-cavity growth is shown to be responsible for the fracture of the high root stress triaxiality specimen. In the low root stress triaxiality specimen, both macro-cavity growth and microstructural reorganization are involved in the fracture mechanisms. The non-monotonic tests have revealed the viscoplastic feature of diffuse cavitation. Porosity can be divided into a reversible component and a residual component. The residual porosity value is not affected by the loading rate conditions and only depends on the root stress triaxiality ratio. The higher the root stress triaxiality, the higher the residual porosity. This study forms a complete experimental data set that can be used to improve the predictability of the mechanical models in terms of alteration distribution, and help in understanding the fracture mechanisms to define relevant fracture criteria for the polymer materials under service conditions.

A Test Protocol to Quantify the Uplift Resistance of Adhesive Applied Roofing Specimens Subjected to Tensile Loading

Abstract Adhesive applied roofing systems (AARS), a new generation of built-up roofs, are gaining popularity in North American low slope application. AARS uses no fasteners and all components (e.g., deck, vapour barrier, insulation board, and membrane) are integrated by the use of adhesives. As there are no metal fasteners, AARS can offer an advantage of reduction in condensation and thermal bridges for the roof assemblies. Even though, AARS have been in use, there have been no standard that exists to quantify its wind uplift performances. An industries-university-Canadian government collaborative research project, “Development of Wind Uplift Standard for Adhesive Applied Low Slope Roofing System,” has been initiated with three major tasks: Experimental investigation, formulation of a numerical model and development of wind design guide and standards. This paper documents a standardized small scale laboratory test method for the quantification of uplift resistance subject to tensile loading. As part of Task 1 of the project, this systematic investigation focuses on three key parameters: Loading rate, specimen size and end condition. The investigations were completed by constructing over 400 specimens. Both the maximum uplift resistance and the consistence in the failure plane are equally considered during the test method standardization. The data from this small scale testing can facilitate roofing industries to optimize the material combinations such that the uplift resistance data can be used as an indicator before performing a full-scale wind uplift testing.

Coupled experiment/finite element analysis on the mechanical response of porcine brain under high strain rates

This paper presents a coupled experimental/modeling study of the mechanical response of porcine brain under high strain rate loading conditions. Essentially, the stress wave propagation through the brain tissue is quantified. A Split-Hopkinson Pressure Bar (SPHB) apparatus, using a polycarbonate (viscoelastic) striker bar was employed for inducing compression waves for strain rates ranging from 50 to 750 s −1. The experimental responses along with high speed video showed that the brain tissue’s response was nonlinear and inelastic. Also, Finite Element Analysis (FEA) of the SHPB tests revealed that the tissue underwent a non-uniform stress state during testing when glue is used to secure the specimen with the test fixture. This result renders erroneous the assumption of uniaxial loading. In this study, the uniaxial volume averaged stress–strain behavior was extracted from the FEA to help calibrate inelastic constitutive equations.

Mesoscale modelling of concrete for static and dynamic response analysis -Part 2: numerical investigations

As a brittle and heterogeneous material, concrete behaves differently under different stress conditions and its bulk strength is loading rate dependent. To a large extent, the varying behavioural properties of concrete can be explained by the mechanical failure processes at a mesoscopic level. The development of a computational mesoscale model in a general finite element environment, as presented in the preceding companion paper (Part 1), makes it possible to investigate into the underlying mechanisms governing the bulk-scale behaviour of concrete under a variety of loading conditions and to characterise the variation in quantitative terms. In this paper, we first present a series of parametric studies on the behaviour of concrete material under quasi-static compression and tension conditions. The loading-face friction effect, the possible influences of the non-homogeneity within the mortar and ITZ phases, and the effect of randomness of coarse aggregates are examined. The mesoscale model is then applied to analyze the dynamic behaviour of concrete under high rate loading conditions. The potential contribution of the mesoscopic heterogeneity towards the generally recognized rate enhancement of the material compressive strength is discussed.

Variations in methanogenic population structure under overloading of pre-acidified high-strength organic wastewaters

Methanogenic populations were quantitatively investigated in two separate anaerobic reactors, which received effluents from acidogenesis of either food waste-recycling wastewater or swine wastewater. Each set of the anaerobic digesters was operated under high organic loading rate (OLR) conditions (8.3–39.8 g chemical oxygen demand (COD)/L/d) with four different hydraulic retention times (10.4–3.0 days). Real-time PCR analysis demonstrated that the order Methanosarcinales was the most abundant (>55%) methanogenic group in the six runs with >50% COD removal, while the dominance (>70%) of Methanobacteriales was observed in the other two runs (<20% COD removal). Methanomicrobiales constituted a minor (<30%) population in all eight runs. The two reactors with relatively low performance were the only trials which contained less than 5 × 10 7 copies/mL of 16S rRNA gene of Methanosarcinales. These results imply that a minimum level of Methanosarcinales is important for achieving even moderate levels of process efficiency under high OLR.

Determination of The Dynamic Response of Q345 Steel Materials by Using SHPB

In this paper a split Hopkinson pressure bar (SHPB) setup was used to test mechanical properties of Q345 steel materials used for industrial structure under high strain rate and high temperature loading conditions such as rocket launching. To provide a consistent and accurate safety assessment to these mechanical structures, the mechanical dynamic response of Q345 steel under high strain rate loading conditions in the same high temperature in different shock times have been developed numerically and experimentally. Special emphasis is placed on determining the experimental relationship between stress and strain compared with different shock times, which is used as a basis for the data analysis procedure. Experimental results for mechanical properties of Q345 steel materials under the extreme conditions from SHPB tests are provided.

Influence of Temperature on Dynamic Behavior of Rubber Materials by Taylor Impact Test

The understanding of the deformation behavior of rubber materials under high strain-rate or high loading-rate conditions will be important in their impact applications adopting significant viscoelastic behavior. Taylor impact test has originally used to determine the average dynamic yield strength of metallic materials at high strain rates, but it also can be used to examine the overall deformation behavior of rubbers representing large elastic deformation by using a high-speed photography technique. Taylor impact tests of rubber materials were carried out in the velocity range between 100~250 m/s using a 20 mm air gun. In order to investigate the overall dynamic deformation behavior of rubber projectiles during Taylor impact test, a 8-Ch high-speed photography system which provides a series of images at each elapsed time was incorporated. Three kinds of rubber materials with different Tg (glass transition temperature) were supplied. The bulging behavior of rubber projectile could be evaluated quantitatively by digitizing images taken. Taylor impact tests at various temperature levels were conducted to predict the bulging behavior of rubbers at high strain rate.

Nitrogen transformations under different conditions in open ponds by means of microalgae–bacteria consortium treating pig slurry

Four open ponds inoculated with microalgae-bacteria consortium treating different swine slurries (fresh and anaerobically digested) were evaluated in terms of nitrogen transformation under optimal and real conditions of temperature and illumination. Ammonium complete depletion was not achieved. Ponds operated under real conditions presented lower ammonium removal. Elimination capacities were around 26 mg N/Ld and were subsequently increased with increasing inlet ammonium loading rate. Different nitrogen transformation was observed depending on substrate source. When anaerobically digested slurry was fed to the ponds, nitrification followed by biomass uptake and denitrification were the main nitrogen transformation taking place depending on inlet ammonium loading rate and operational conditions. Ponds fed with fresh slurry exhibited denitrification as the main nitrogen removal mechanism for the pond operated under real conditions while under optimal conditions stripping, denitrification and biomass uptake contributed similarly. Therefore, this study confirmed that the so-claimed nitrogen recovery by microalgae biomass is frequently overestimated.

High strain rate response of rabbit femur bones

Strain rate dependence of the mechanical response of hard tissues has led to a keen interest in their dynamic properties. The current study attempts to understand the high strain rate characteristics of rabbit femur bones. The testing was conducted using a split-Hopkinson pressure bar equipped with a high speed imaging system to capture the fracture patterns. The bones were also characterized under quasi-static compression to enable comparison with the high strain rate results. The quasi-static compressive moduli of the epiphyseal and diaphyseal regions were measured to be in the range of 2–3 and 5–7 GPa, respectively. Under high strain rate loading conditions the modulus is observed to increase with strain rate and attains values as high as 15 GPa for epiphyseal and 30 GPa for diaphyseal regions of the femur. The strength at high strain rate was measured to be about twice the quasi-static strength value. A large number of small cracks initiated on the specimen surface close to the incident bar. Coalescence of crack branches leading to fewer large cracks resulted in specimen fragmentation. In comparison, the quasi-static failure was due to shear cracking.

Loading Rate-Induced Transition in Toughening Mechanism of Rubber-Modified Epoxy

The effect of loading rate on toughening mechanisms of rubber-modified epoxy resin was investigated using amine-terminated butadiene acrylonitrile (ATBN) rubber. Rubber-modified samples were tested at various loading rates in the range of 1–1000 mm/min. At low loading rates, modified resin exhibited a high fracture toughness, which decreased by increasing the rate of loading. At higher loading rate a transition region was observed, where the fracture toughness did not change noticeably with increasing the loading rate. Beyond the transition region, the resin fracture toughness dropped dramatically to the level of unmodified epoxy. Fractography of the damage zone showed different toughening mechanisms for the various loading rate regions, i.e., shear yielding and crack branching. It is believed that the interaction between these two toughening mechanisms, each dominating in one extreme loading rate condition, is responsible for the intermediate transition region, where toughness is virtually independent from loading rate.

Effect of Loading Rate and Surface Conditions on the Flexural Strength of Borosilicate Glass

This study evaluates the loading rate and surface condition dependence of the flexural strength of a borosilicate glass. The glass specimens are subjected to three different surface treatments before four‐point bending tests to study the effect of surface flaws. Quasistatic (Material Test System 810) and dynamic (Kolsky bar) experiments are performed at loading rates ranging from 0.7 to 4 × 106MPa/s. The results show that the flexural strength of the borosilicate glass has a strong dependence on the loading rate. A chemically etched surface produces an enhanced flexural strength by about an order of magnitude. Scanning electron microscopy images on fracture surfaces indicate that the failure is governed by different types of flaws under different surface treatment conditions. Edge failure is also identified for samples possessing high flexural strength.

Compression properties of individual micron-sized acrylic particles

Compression behaviours of individual micron-sized acrylic particles have been studied by using a nanoindentation-based method with a flat-end tip, instead of a commonly used sharp tip. The effect of loading rate on the mechanical properties of the high monodisperse acrylic particles, such as K-value, breaking force and breaking displacement, has been investigated. A wide range of loading rate conditions has been applied to examine the sensitivity of selected mechanical properties. It has been observed that the loading rate has a significant effect on K-value and breaking force whereas breaking displacement is independent of loading rate. These results facilitate the industrial applications of acrylic particles.

Modelling the steady state deformation stress under various deformation conditions using a single irreversible thermodynamics based formulation

A new unified description for the steady state deformation stress in single and polycrystalline metals and for various deformation conditions is presented. The new formulation for dislocation controlled deformation stems from the field of irreversible thermodynamics. The model applies to conditions of dynamic recovery as well as dynamic recrystallization and has been validated for constant strain rate and creep loading conditions. Unlike existing approaches, the new model captures transitions between deformation mechanisms within a single formulation. For conditions of dynamic recrystallization, the average dislocation density is found to be a function of the shear strain rate and a term combining the dislocation climb velocity and the grain boundary velocity.