Constant External Load Research Articles

Effect of temperature and geometrical nonlinearity on wave propagation characteristics of carbon nanotubes

Considering the effect of temperature and geometrical nonlinearity in the constitutive relation, the equation of motion for a carbon nanotube is obtained based on the Euler–Bernouli beam model. Also, the effect of van der Waals forces is taken into account in the formulation. The carbon nanotube is assumed to be under the application of a constant distributed external load. At any temperature, the equilibrium solutions of the governing equations for a single-walled carbon nanotube (SWCNT) and a double-walled carbon nanotube (DWCNT) are obtained. A small perturbation is assumed around the equilibrium solution. Using this perturbation, the nonlinear equations of motion are linearized. Using the linearized form of the equations of motion, the characteristic equations and dispersion relations are obtained. It is shown that in the linear case and for the case of high temperature there exists a temperature beyond which the phase velocity does not exist. It is shown that in the case of room or low temperature there is no critical value for temperature. Based on the dispersion equation, a relation for the critical value of temperature is obtained. It is found that when the large deformation effect is taken into account, the critical value for temperature does not exist. Also, the effect of large deformations on phase velocities and lateral deformations of single-walled and double-walled carbon nanotube beams are studied. It is found that unlike the linear theory, the nonlinear theory predicts a non-zero phase velocity at the temperature corresponding to linear critical temperature.

Steady-state multiplicity in a solid oxide fuel cell: Practical considerations

Steady-state multiplicity in a solid oxide fuel cell (SOFC) in three modes of operation, constant ohmic external load, potentiostatic and galvanostatic, is studied using a detailed first-principles lumped model. The SOFC model is derived by accounting for heat and mass transfer as well as electrochemical processes taking place inside the fuel cell. Conditions under which the fuel cell exhibits steady state multiplicity are determined. The effects of operating conditions such as convection heat transfer coefficient and inlet fuel and air temperatures and velocities on the steady state multiplicity regions are studied. Depending on the operating conditions, the cell exhibits one or three steady states. For example, it has three steady states: (a) at low external load resistance values in constant ohmic external load operation and (b) at low cell voltage in potentiostatic operation.

Microstructure of damage in thermally activated fracture of Lennard-Jones systems

We investigate the effect of thermal fluctuations on the critical stress and the microstructure of damage preceding macroscopic fracture of Lennard-Jones solids under a constant external load. Based on molecular dynamics simulations of notched specimens at finite temperatures, we show that the crystalline structure gets distorted ahead of the crack in the secondary creep regime. The damage profile characterizing the spatial distribution of lattice distortions is well described by an exponential form. The characteristic length of the exponential form provides the scale of damage, which is found to be an increasing function of the temperature: At low temperatures, damage is strongly localized to the crack tip, while at high temperatures, damage extends to a broader range, leading to more efficient relaxation of overloads. As a consequence, the stress intensity factor decreases with increasing temperature. The final macroscopic failure of the system occurs suddenly and is initiated by the creation of vacancies and voids. The creep strength exhibits inverse square root scaling with the notch size corrected by the extension of the process zone.

Two-way reversible shape memory effects in a free-standing polymer composite

Shape memory polymers (SMPs) have attracted significant research efforts due to theirease in manufacturing and highly tailorable thermomechanical properties. SMPs can betemporarily programmed and fixed in a nonequilibrium shape and are capable of recoveringthe original undeformed shape upon exposure to a stimulus, the most common beingtemperature. Most SMPs exhibit a one-way shape memory (1W-SM) effect since oneprogramming step can only yield one shape memory cycle; an additional shape memorycycle requires an extra programming step. Recently, a novel SMP that demonstrates both1W-SM and two-way shape memory (2W-SM) effects was demonstrated by one of theauthors (Mather). However, to achieve two-way actuation this SMP relies ona constant externally applied load. In this paper, an SMP composite where apre-stretched 2W-SMP is embedded in an elastomeric matrix is developed. Thiscomposite demonstrates 2W-SM effects in response to changes in temperaturewithout the requirement of a constant external load. A transversal actuation of ∼ 10% of actuator length is achieved. Cyclic tests show that the transversal actuation stabilizesafter an initial training cycle and shows no significant decreases after four cycles. A simpleanalytic model considering the programming stress and actuator dimensions is presentedand shown to agree well with the transverse displacement of the actuator. The model alsopredicts that larger actuation can be achieved when larger pre-stretch of 2W-SMP is used.The scheme used for this polymer composite can promote the design of new shape memorycomposites at micro- and nano-length scales to meet different application requirements.

Effects of bond crushing on the settlements of shallow foundations on soft rocks

The paper presents a model capable of describing the behaviour of shallow foundations on soft rocks when subjected to eccentric loading. The model is based on the concept of a ‘macroelement'. According to this concept, the foundation and the subsoil can be seen as a unique volume element. A mathematical relationship is established that links the force and moment acting on the foundation and the corresponding displacement and rotation, as if they were generalised stresses and strains, respectively. This relationship is based on the theory of hardening plasticity, with two hardening variables, both functions of the permanent generalised strains. It is assumed that the second of such variables, connected to the action of intergranular bonds, monotonically decreases as strains progress and bonds crush. It is shown that the model can reproduce quite well the observed displacement and rotation of a model circular foundation on a layer of calcarenite. A parametric study also highlights that the model can in principle predict the phenomenon of collapse, actually experienced by foundations on high-porosity rocks when a certain generalised stress or strain threshold is reached. It is suggested that such a phenomenon can take place when bonds are characterised by high strength that is rapidly decreasing with strain. It is finally shown that weathering-induced collapse under constant external loading can also, in principle, be modelled in this framework.

Three-phase elliptical inclusions with internal uniform hydrostatic stresses in finite plane elastostatics

We consider the internal stress field of a three-phase elliptical inclusion bonded to an infinite matrix through an interphase layer when the matrix is subjected to remote uniform stresses. The elastic materials comprising all the three phases belong to a particular class of harmonic materials, and the formed interfaces are two confocal ellipses. A condition leading to internal uniform hydrostatic stresses is derived. This condition relates the two remote principal stresses with the geometric parameters (the thickness of the interphase layer and the aspect ratio of the elliptical inclusion) of the three-phase elliptical inclusion. When this condition is met, the hoop stress in the interphase layer along the entire interphase/inclusion interface is also uniform. Five special situations of practical importance are discussed in considerable detail to demonstrate the unique phenomena inherent in harmonic materials. Our discussions indicate that when this condition is met, it is permissible for the two remote principal stresses to have opposite signs and that for given geometric and material parameters, the remote loading ratio is no longer constant and multiple external loading states exist leading to internal uniform hydrostatic stresses. It is found that this condition can be written into a hyperbola for the two remote principal stresses when the interphase layer is extremely compliant or relatively stiff or when the inclusion is almost rigid. When the magnitudes of the remote stresses are sufficiently large, this condition becomes a very simple one relating the remote loading ratio with the geometric parameters of the composite. Interestingly, it is clearly observed from the simple condition that for given geometric parameters of the three-phase elliptical inclusion, there exist two different values of the remote loading ratio, both of which lead to an internal uniform hydrostatic stress state.

Neuromuscular and muscle-tendon system adaptations to isotonic and isokinetic eccentric exercise

ObjectiveTo present the properties of an eccentric contraction and compare neuromuscular and muscle-tendon system adaptations induced by isotonic and isokinetic eccentric trainings. SynthesisAn eccentric muscle contraction is characterized by the production of muscle force associated to a lengthening of the muscle-tendon system. This muscle solicitation can cause micro lesions followed by a regeneration process of the muscle-tendon system. Eccentric exercise is commonly used in functional rehabilitation for its positive effect on collagen synthesis but also for resistance training to increase muscle strength and muscle mass in athletes. Indeed, eccentric training stimulates muscle hypertrophy, increases the fascicle pennation angle, fascicles length and neural activation, thus inducing greater strength gains than concentric or isometric training programs. Eccentric exercise is commonly performed either against a constant external load (isotonic) or at constant velocity (isokinetic), inducing different mechanical constraints. These different mechanical constraints could induce structural and neural adaptive strategies specific to each type of exercise. ConclusionThe literature tends to show that isotonic mode leads to a greater strength gain than isokinetic mode. This observation could be explained by a greater neuromuscular activation after IT training. However, the specific muscle adaptations induced by each mode remain difficult to determine due to the lack of standardized, comparative studies.

A unique crack growth rate curve method for fatigue life prediction of steel structures

In this paper, a unique crack growth rate curve method, which is based on the equivalent stress intensity factor range (ESIFR) as the driving force, has been proposed and examined with crack growth rate data of base metals and as welded joints of some structural steels under constant amplitude external loading. By expressing the crack growth rate data with ESIFR instead of stress intensity factor range (SIFR) make it possible to establish a concise model for crack growth data under different R-ratios to the curve corresponding to R=0 both for base metals and welded joints. The most commonly tested crack growth rate constants under R=0 ∼0.1 are sufficient in fatigue crack growth life prediction of components subjected to tensile-tensile, tensile-compressive loading. Only two equations, one for Mean curve, and the other for Mean + 2SD curve replace the recommended crack growth rate curves in BS7910 for most structural steels. The phenomena that crack growth rates of as-welded joints under different applied loading ratios behaves independent of the applied loading ratio can be explained and the crack growth rate in residual stress field can be predicted well by the present model. The unique crack growth rate curve method does not only allow us to estimate the fatigue life of specimens of base metal and weld joints, but also the fatigue life of structural components under complex loading conditions.

Robot-based methodology for a kinematic and kinetic analysis of unconstrained, but reproducible upper extremity movement

Although arm movements play an important role in everyday life, there is still a lack of procedures for the analysis of upper extremity movement. The main problems for standardizing the procedure are the variety of arm movements and the difficult assessment of external hand forces. The first problem requires the predefinition of motions, and the second one is the prerequisite for calculation of net joint forces and torques arising during motion. A new methodology for measuring external forces during prespecified, reproducible upper extremity movement has been introduced and validated. A robot-arm has been used to define the motion and 6 degrees of freedom (DoF) force sensor has been attached to it for acquiring the external loads acting on the arm. Additionally, force feedback has been used to help keeping external loads constant. Intra-individual reproducibility of joint angles was estimated by using correlation coefficients to compare a goal-directed movement with robot-guided task. Inter-individual reproducibility has been evaluated by using the mean standard deviation of joint angles for both types of movement. The results showed that both inter- and intra-individual reproducibility have significantly improved by using the robot. Also, the effectiveness of using force feedback for keeping a constant external load has been shown. This makes it possible to estimate net joint forces and torques which are important biomechanical information in motion analysis.

Crackling noise in sub-critical fracture of heterogeneous materials

We present a theoretical study of the sub-critical fracture of heterogeneous materials undera constant external load. A generic fiber bundle model is proposed, which provides a directconnection between the microscopic fracture mechanisms and the macroscopic timeevolution of the sub-critical system. In the model, material elements either fail due toimmediate breaking or undergo a damage accumulating ageing process. On themacrolevel the model reproduces the empirical Basquin law of rupture life, and itmakes it possible to derive a generic scaling form for the deformation histories ofdifferent load values. On the microlevel we found that sub-critical fracture isaccompanied by crackling noise, i.e. the competition of the two failure modes of fibersgives rise to a complex bursting activity, where slow damage sequences triggerbursts of breaking events. When the load is equally distributed over the fibers, thesize of damage sequences and of bursts, as well as the waiting times in between,are characterized by universal power law distributions, where only the cutoffshave material dependence. When stress concentrations arise in the vicinity offailed regions, the power law distributions of noise characteristics prevail butthe exponents are different from their equal load sharing counterparts. In thepresence of stress concentration the failure process accelerates resulting in a highervalue of the waiting time exponent compared to the case of homogeneous stressdistribution.

Numerical analysis and performance assessment of metal hydride-based hydrogen storage systems

This paper presents a detailed analysis of low-temperature, metal hydride-based hydrogen storage systems by means of a lumped model previously developed by the authors. In the first part of this study the model is applied to simulate experimental systems described in the literature, where the absorption and desorption of hydrogen under constant-pressure conditions are studied. The numerically evaluated time evolution of several key parameters (such as hydrogen mass flow, temperature and pressure) is reported and its accordance with experimental results available in the literature is discussed, in order to identify the model's strengths and limitations and its validity range, and to offer analytical insight into the behavior of the storage system as different operating parameters are varied. The model is then used as a tool to predict the performance of metal hydride hydrogen storage devices operating in FC-based energy systems. This is accomplished by analyzing the storage system response to a constant flow condition, i.e. when the FC operates under a constant external load.

Running in new and worn shoes: a comparison of three types of cushioning footwear

Objectives: In this study, the effect of shoe degradation on running biomechanics by comparing the kinetics and kinematics of running in new and worn shoes was investigated. Three types of...

Quantifying Energy Expenditure During Water-Immersion in Non-Trained Cyclists

This research project was designed to compare energy expenditures during water-immersion and ambient-air states at three external work rates of 50, 100, and 150 W. Eleven participants were tested on two separate occasions on a bicycle ergometer in water-immersion and ambient-air environments for 3 to 4 min at 50 rpm. Oxygen consumptions and heart rates were monitored continuously throughout both sessions. Perceived exertion was assessed at each work rate during the final 30 s of the sampling period. Water-immersion produced higher oxygen consumption values (2.21 0.20 vs. 1.00 0.14 L min-1 at 50 W; 2.64 0.33 vs. 1.45 0.12 L min-1 at 100 W; and 2.86 0.41 vs. 2.00 0.16 L min-1 at 150 W). Heart rates were greater in water (142.70 vs. 97.90 at 50 W; 152.73 vs. 113.14 at 100 W; and 163.82 vs. 135.09 at 150 W). Perceived exertion scores were higher in water (11.7 vs. 8.5 at 50 W; 15.0 vs. 10.6 at 100 W and 17.2 vs. 12.7 at 150 W). Energy expenditure rates were greater in the water-immersion environment (769 vs. 348 W at 50 W; 919 vs. 505 W at 100 W; 995 vs. 696 W at 150 W). It was concluded that at a constant external load, water-immersion produces greater oxygen consumption and heart rate responses compared to values assessed in an ambient-air state. This difference reflects the external load required to move the viscous liquid instead of air.

Effect of plastic deformation at the crack tip in a crystal on the direction of the tip growth under a mixed load

Evolution of plastic deformation at the tip of a wedge-shaped crack in a crystal under planar strain (modes I and II) was calculated for different cleavage planes, easy-slip systems, angles at the wedge tip, and ratios of the external extension and shear loads. Time distributions are obtained for the plastic deformation, the effective shear stress, the stress intensity factor, and the crack growth direction under monotonic load of the crystal up to a specified limit and further relaxation to establishment of equilibrium distributions under a constant external load. Numerical calculations were performed for an α-Fe crystal.

Nonlinear analysis of moderately thick reinforced concrete slabs at elevated temperatures using a rectangular layered plate element with Timoshenko beam functions

A simple displacement-based 4-node, 24-DOF rectangular layered plate element for nonlinear finite element analysis of thin-to-moderately-thick reinforced concrete slabs subjected to combined external and thermal loading is developed in this paper. Timoshenko’s composite beam functions are extended to form the basis of the bending strain of the layered plate element developed herein, so that the proposed model and approach not only provide a unified formulation for thin and moderately thick plates, but also avoid shear locking naturally. In addition to temperature-dependent material nonlinearity, geometric nonlinearity is accounted for to include the tension membrane effect, which has been identified as the major contributor to the load carrying capacity of RC slabs at elevated temperatures. Based on the proposed element model, the updated Lagrangian approach is employed to formulate the nonlinear finite element analysis and solution procedures for the nonlinear analysis of reinforced concrete slabs under constant external loading and thermal loading which are proposed in this paper. Comparison of the results from the proposed model and previous literatures for the numerical analysis of reinforced concrete slabs at ambient and elevated temperatures have demonstrated the effectiveness of the proposed model and solution procedures.

In situ characterization of a crack trajectory and shear bands interaction in metallic glasses

In situ SEM observations and atomic force microscopy measurements are carried out to follow changes in front of a mode I stationary crack tip initiated at the edge of thin ribbons of Cu 60Zr 30Ti 10, Zr 65Ni 10Cu 15Al 10 and Pd 42.5Cu 30Ni 7.5P 20 metallic glasses and maintained at constant external tensile loads. While the Cu- and Zr-based metallic glasses exhibit a nano-scale shear offset in the vicinity of the crack tip, Pd-based metallic glass showed a few regularly spaced shear bands prior to fracture away from the very near crack-tip region. We have found that the discreteness, the interactions of the shear bands and crack blunting in the three metallic glasses are different. AFM frictional analyses illustrate evidence of flow defects in the vein-like structure morphology of the fractured surfaces.

Insights into the Distribution of Water in a Self-Humidifying H2/O2Proton-Exchange Membrane Fuel Cell Using1H NMR Microscopy

Proton ((1)H) NMR microscopy is used to investigate in-situ the distribution of water throughout a self-humidifying proton-exchange membrane fuel cell, PEMFC, operating at ambient temperature and pressure on dry H(2)(g) and O(2)(g). The results provide the first experimental images of the in-plane distribution of water within the PEM of a membrane electrode assembly in an operating fuel cell. The effect of gas flow configuration on the distribution of water in the PEM and cathode flow field is investigated, revealing that the counter-flow configurations yield a more uniform distribution of water throughout the PEM. The maximum power output from the PEMFC, while operating under conditions of constant external load, occurs when H(2)O(l) is first visible in the (1)H NMR image of the cathode flow field, and subsequently declines as this H(2)O(l) continues to accumulate. The (1)H NMR microscopy experiments are in qualitative agreement with predictions from several theoretical modeling studies (e.g., Pasaogullari, U.; Wang, C. Y. J. Electrochem. Soc. 2005, 152, A380-A390), suggesting that combined theoretical and experimental approaches will constitute a powerful tool for PEMFC design, diagnosis, and optimization.

Open- and closed-circuit study of an intermediate temperature SOFC directly fueled with simulated biogas mixtures

An intermediate temperature solid oxide fuel cell (SOFC) based on a gadolinia doped ceria (GDC) solid electrolyte, a Ni(Au)-GDC cermet anode and a La 0.54Sr 0.46MnO 3 perovskite cathode was tested at 600 and 640 °C on direct feed of simulated biogas mixtures. The catalytic (open-circuit) rate of the methane dry (CO 2)-reforming reaction over Ni(Au)-GDC anode was found to be maximized at about equimolar CH 4/CO 2 feed ratio. Cell power density up to 60 mW cm −2, at a cell voltage of 445 mV and a current density of 135 mA cm −2 at 640 °C, has been obtained under closed-circuit cell operation at this optimal feed ratio. Carbon deposition was found not to downgrade cell output characteristics under closed-circuit conditions at constant external loads for ∼120 h, preceded by open- or closed-circuit operation for ∼100 additional hours.

Mechanism of Force Generation of a Viral DNA Packaging Motor

A large family of multimeric ATPases are involved in such diverse tasks as cell division, chromosome segregation, DNA recombination, strand separation, conjugation, and viral genome packaging. One such system is the Bacillus subtilis phage phi 29 DNA packaging motor, which generates large forces to compact its genome into a small protein capsid. Here we use optical tweezers to study, at the single-molecule level, the mechanism of force generation in this motor. We determine the kinetic parameters of the packaging motor and their dependence on external load to show that DNA translocation does not occur during ATP binding but is likely triggered by phosphate release. We also show that the motor subunits act in a coordinated, successive fashion with high processivity. Finally, we propose a minimal mechanochemical cycle of this DNA-translocating ATPase that rationalizes all of our findings.

Deformation of glassy polycarbonate and polystyrene: the influence of chemical structure and local environment

Understanding the mechanism of deformation is very important in various applications. Although the stress–strain behavior of crystals and glasses are similar, the mechanism of deformation is very different. We used molecular dynamics to study polycarbonate and polystyrene under constant external loads. The results indicate that high atomic/segmental mobility and low local density enable the formation (nucleation) of highly deformed regions that grow to form plastic defects, and the effect of chemical structure was found to dominate the deformation mechanism