Role Of Strain Rate Research Articles

Quasi-static fracturing in double-flawed specimens under uniaxial loading: the role of strain rate

Mechanical properties and cracking behaviors of flaw-weakened rock mass are distinguishably influenced by the quasi-static strain rate. Such advanced knowledge is mainly obtained through the numerical simulations, which are rarely verified experimentally. This paper carries out a coupled experimental-numerical investigation on double-flawed rock-like specimens at different quasi-static strain rates, with consideration given to the macro-mechanical properties, the quasi-static cracking behaviors and the underlying fracture mechanism. Three ordinary arrays of double pre-existing flaws, namely, the coplanar array, the vertical non-overlapping array and the vertical aligning array, are analyzed experimentally. The obtained results show that the crack initiation stress and the peak stress of double flaw-contained specimens generally increase with increasing the strain rate. The extension length of the first macroscopic crack, the crack initiation mode, the amount of far-field cracks, and the degree of ultimate fragmentation in double flaw-contained specimens are closely related to the strain rate. Furthermore, the vertical non-overlapping array of double pre-existing flaws is studied numerically by using peridynamics. The quasi-static fracture behaviors obtained by peridynamics are in good agreement with the experimental observations. The stress concentrates around the tips of flaws when the strain rate is relatively low, while apart from around the tips of flaws, the stress concentration also occurs in the interior of specimens when the strain rate is relatively high. This study provides the better understanding of the quasi-static strain rate effects on mechanical properties and fracture behaviors of rock mass, in particular those containing natural flaws that appear in sets or groups with similar orientation and characteristics.

Role of Strain Rate in the Pathogenesis of Ventilator-Induced Lung Edema.

Lungs behave as viscoelastic polymers. Harms of mechanical ventilation could then depend on not only amplitude (strain) but also velocity (strain rate) of lung deformation. Herein, we tested this hypothesis. Laboratory investigation. Animal unit. Thirty healthy piglets. Two groups of animals were ventilated for 54 hours with matched lung strains (ratio between tidal volume and functional residual capacity) but different lung strain rates (ratio between strain and inspiratory time). Individual strains ranged between 0.6 and 3.5 in both groups. Piglets ventilated with low strain rates had an inspiratory-to-expiratory time ratio of 1:2-1:3. Those ventilated with high strain rates had much lower inspiratory-to-expiratory time ratios (down to 1:9). Respiratory rate was always 15 breaths/min. Lung viscoelastic behavior, with ventilator setting required per protocol, was "quantified" as dynamic respiratory system hysteresis (pressure-volume loop [in Joules]) and stress relaxation (airway pressure drop during an end-inspiratory pause [in cm H2O]). Primary outcome was the occurrence of pulmonary edema within 54 hours. On average, the two study groups were ventilated with well-matched strains (2.1 ± 0.9 vs 2.1 ± 0.9; p = 0.864) but different strain rates (1.8 ± 0.8 vs 4.6 ± 1.5 s; p < 0.001), dynamic respiratory system hysteresis (0.6 ± 0.3 vs 1.4 ± 0.8 J; p = 0.001), and stress relaxation (3.1 ± 0.9 vs 5.0 ± 2.3 cm H2O; p = 0.008). The prevalence of pulmonary edema was 20% among piglets ventilated with low strain rates and 73% among those ventilated with high strain rates (p = 0.010). High strain rate is a risk factor for ventilator-induced pulmonary edema, possibly because it amplifies lung viscoelastic behavior.

An analytical model for predicting counterflow flame propagation through premixed dust micro particles with radiative heat loss

In this paper, an analytical model has been performed to scrutinize the structure of the flame propagation in counterflow configuration where the mixture of solid fuel particles and air are injected as opposed streams. The structure of counterflow premixed flame in a symmetric configuration, containing uniformly distributed volatile fuel particles, with nonunity Lewis number is examined with considering radiative heat loss effect in counterflow configuration with strain rate issue. The flame structure governing equations, required boundary conditions, and matching conditions are applied for each zone in order to solve the differential equations. The flame position is determined, mass fraction of solid particles and gaseous phases, effect of Lewis number change on the gaseous and solid fuel mass fraction distribution, and the role of strain rate, and different particle diameters are investigated with and without considering thermal radiation effect. In addition, the effect of equivalence ratio on the flame temperature, mixture temperate and non-dimensional flame position is investigated in counterflow flame propagation. According to our finding, the burning velocity of counterflow flame remarkably increases as a function of vaporization Damköhler number as well as non-dimensional vaporization temperature with considering thermal radiation effect in counterflow domain.

Strain rate-dependent tensile properties and dynamic electromechanical response of carbon nanotube fibers

This investigation into the rate-dependent tensile behavior of carbon nanotube (CNT) fibers provides insight into the role of strain rate and specimen gage length on tensile strength. Chemical vapor produced CNT continuous fibers made of single and dual wall CNTs are evaluated and the potential for fiber improvement by post-process stretching to improve alignment is explored. Post-processed CNT fibers exhibit significantly higher strengths (3–5GPa) and moduli (80–200GPa) than untreated fibers. During dynamic tension evaluation, real-time electrical measurements provide correlations between high rate deformation/damage mechanical behavior and electrical resistance of the fiber specimens. Furthermore, this first look into the dynamic tensile behavior of CNT fibers demonstrates their potential to serve as sensors in high rate applications.

Influence of stress state and strain rate on structural amorphization in boron carbide

The reduced performance of B4C armor plate for impact against tungsten carbide penetrators beyond a critical velocity has been attributed in the literature to localized amorphization. However, it is unclear if this reduction in strength is a consequence of high pressure or high velocity. Despite numerous fundamental studies of B4C under indentation and impact, the roles of strain rate and pressure on amorphization have not been fully established. Toward this end, rate dependent uniaxial compressive strength and rate dependent indentation hardness, along with Raman spectroscopy, have been employed to show that high strain rate deformation alone (without concurrent high pressure) cannot trigger localized amorphization in B4C. Based on our analysis, it is also suggested that rate dependent indentation hardness can be used to reveal if a given B4C ceramic exhibits amorphization under high pressure and high strain rate loading. It is argued that when amorphization does occur in B4C, its dynamic inelastic properties degrade more severely than its static properties. Finally, it is suggested that dynamic hardness, in conjunction with static hardness, can be used as a measurable mechanical property to reveal the incidence of amorphization in B4C without the need for postmortem TEM or Raman spectroscopy analyses.

The role of strain rate during deposition of CAP on Ti6Al4V by superplastic deformation-like method using high-temperature compression test machine

This paper describes an implementation of superplastic deformation method for the deposition of carbonated-apatite (CAP) on the well-know titanium alloy, Ti6Al4V. This deposition process was carried out using high-temperature compression test machine, at temperature of 775 °C, different strain rates, and conducted along the elastic region of the sample. Before the process, titanium substrate was cryogenically treated in order to approach superplastic characteristic during the process. After the process, thin film of CAP was created on the substrate with the thickness from 0.71 μm to 1.42 μm. The resulted film has a high density of CAP that covered completely the surface of the substrate. From the stress–strain relation chart, it can be observed that as the strain rate decreases, the area under stress–strain chart also decreases. This condition influences the density of CAP layer on the substrate that as this area decreases, the density of CAP layer also decreases as also confirmed by X-ray diffraction characterization. In addition, since the resulting layer of CAP is in the form of thin film, this layer did not alter the hardness of the substrate as measured by Vickers hardness test method. On the other hand, the resulting films also show a good bonding strength properties as the layer remain exist after friction test against polishing clothes for 1 h.

Microstructure and dynamic material performance of high strength and ultra high strength strip steels

The combined need for weight reduction and increased safety in modern vehicles has led automobile manufacturers to utilise the unique properties of high strength steels in their vehicles. Standard quasi-static tensile tests allow the static mechanical properties to be evaluated for any given steel grade. However, automobile designers require more advanced data, including high strain rate tensile data to fully understand the performance of a steel grade during deformation. Knowledge of the role of strain rate on a given strip steel grades deformation characteristics is essential to fully predict the behaviour of the steel during a crash event to help ensure the safety of the vehicle passengers. In the present study, three commercial strip steel grades were subjected to room temperature deformation at variable strain rates, ranging from quasi-static to dynamic loading conditions, where their obtained mechanical properties were measured. The steel grades studied were a HSLA grade (XF450), a carbon manganese grade (CMn800) and an advanced high strength steel grade (DP1400), each with very different microstructures. The present study showed that the XF450 with a ferrite (97%)-pearlite microstructure exhibited superior elongation and was the most strain rate sensitive grade. This was the only grade where energy absorption until 10% elongation could be measured. The DP1400 with a mainly martensitic microstructure showed very little change in strength or ductility properties over the tested strain rates.

On the role of strain rate and vorticity in plasmas

It is shown that the magnetic field's local evolution in plasmas is directly affected by an interplay between the deformation (strain rate) and the self-rotation (vorticity) of the elementary plasma fluid volumes. At regions of strong strain rate, the fast local convergence or divergence of the flow causes exponential increase or decrease of the field's components, respectively. At regions of strong vorticity, faster directional than magnitude variations of the field occur, explaining the field alignment of the field minimum-variance direction and the random wandering of the field's tip on a sphere, both observed in the solar wind plasma, even in the absence of Alfvén waves. We further investigate the coupling between the maximum strain-rate direction and the local magnetic field, that was previously deduced from magnetic field measurements in the outer heliosphere. Cases of long-lasting, non-Parkerian, radial heliospheric magnetic field are also shown to be periods of field-aligned strain rate. A statistical proof for this alignment is given, assuming that the small-scale field fluctuations are weakly stationary and time reversible. We further propose a generalization of the Stokesian fluid stress-strain relation to the case of one-fluid, collisionless MHD plasmas, including the effects of turbulent viscosity and magnetically induced shearing motion. For a negligible or isotropic or `field-aligned' thermal pressure tensor, the proposed `Stokesian plasma' relation implies the field alignment of the solar wind plasma strain-rate direction and leads to anisotropic stress-strain balance equations, related to those of `firehose' plasma instability. The field alignment of a principal strain-rate direction leads to simplifications in the magnetic-induction equation, especially in the case of force-free fields. For the simplified case of homogeneous, isotropic and incompressible plasma turbulence, the proposed stress-strain-rate relation implies that the velocity and magnetic field inertial range spectra should be identical, further reducing to the Kolmogorov-like k-5/3 law for scale-invariant eddy cascade at constant energy-dissipation rate.

Mechanotransduction in bone: role of strain rate

Bone tissue can detect and respond to its mechanical environment, but there is no consensus for how bone cells detect mechanical loads. Some think that cells sense tissue deformation (strain) and respond when strain is abnormally high. However, strains in bone tissue are usually very small, and it is questionable whether bone cells are sensitive enough to detect them. Another theory suggests that mechanical loads are coupled to the bone cells by stress-generated fluid flow within the bone tissue, which is dependent on the rate of change of bone strain. We applied bending loads to the tibiae of adult rats to create equivalent peak strains in the bone tissue but with varied rates of strain. Bone formation was significantly increased in the two experimental groups when the highest strain rates were compared with lower strain rates (P < 0.01), and the amount of new bone formation was directly proportional to the rate of strain in the bone tissue. These results suggest that relatively large strains alone are not sufficient to activate bone cells. High strain rates and possibly stress-generated fluid flow are required to stimulate new bone formation.

An Incremental Damage Formulation for Stress Corrosion Cracking and Its Application to Crack Growth Interpretation Based on CERT Data

Abstract In recent years the role of strain rate as a key mechanics variable has been well accepted in the quantitative evaluation of certain environmentally influenced cracking processes of ductile alloys—especially those involving the conjoint action of stress and corrosion. The author had developed one such evaluation model for the intergranular stress corrosion cracking (IGSCC) in aqueous environments that addressed the question of time-to-failure in the prevalent sense of time to initiate an observable crack. The main objective of this paper is to extend the same underlying basic modeling concepts to address the crack growth versus time relation starting with a smooth surface. In particular, it is shown that the extension to analyze crack growth, at least until a sizable crack is grown, can be based on a consistent and unified implementation of the deformation mechanics relations in the form of an incremental damage formulation After proposing the formulation, its application is presented for the com...

The Role of Strain Rate in the Formation of Dislocation Structure and Its Influence on the Mechanical Properties of Aluminium

The relationship between mechanical properties and dislocation substructure in aluminium after deformation at different strain rates has been studied. It was established that increase in strain rate increases flow stress and decreases the cell size of the dislocation substructure. From measurements of the misorientation angle between subgrains it was possible to determine the dislocation density and to present it as a function of the strain rate (at constant strain). It was also found that the flow stress is related to the cell size by the Hall–Petch equation, and that the proportionality factor k in this equation is independent of the misorientation angle.