Regional Strain Rate Research Articles

Numerical Study On Impulsive Local Damage OfReinforced Concrete Structures By A SophisticatedConstitutive And Failure Model

This study offers a new constitutive and failure model for the concrete which covers a wide range of the impact phenomena. The authors extended the static non-uniform hardening plasticity model developed by Han and Chen, mainly to the high strain rate region. The model, called 'Dynamic Drucker-Prager CAP Model', was implemented in a hydrocode AUTODYNTM through the subroutines which are open to the user. Then, a number of numerical analyses were carried out by using the material model, which simulate the impact tests on reinforced concrete structures by the aircraft. In the experiment, the engine was modeled by a rigid or a deformable missile on three scales: full, 1/2.5 and 1/7.5. The penetration, perforation and scabbing modes of the reinforced concrete were numerically simulated in good agreement with the experiment. The numerical results were discussed over the comparison with the Degen's damage equation.

Assessment of Regional Longitudinal Myocardial Strain Rate Derived from Doppler Myocardial Imaging Indexes in Normal and Infarcted Myocardium

Myocardial deformation properties may be characterized by regional strain rates (SRs) calculated from Doppler myocardial velocity data. In 10 control subjects and 12 patients with established transmural infarcts, longitudinal median segmental SR, strain, and myocardial velocity were analyzed and compared with the corresponding wall motion score. All segments in control subjects and normal segments in infarct patients showed no significant difference in either systolic or diastolic SR (systolic: –1.27 ± 0.39 s–1 versus –1.23 ± 0.24 s–1, not significant [NS]; and isovolumic relaxation [IVR]: 1.23 ± 0.38 s–1 versus 1.95 ± 0.62 s–1, NS; respectively) and strain (–0.21 ± 0.06 versus –0.19 ± 0.06, NS). In infarcted segments, peak systolic SR, systolic strain, and early diastolic SR showed the most pronounced reduction (hypokinetic and akinetic) or even inversion (dyskinetic segments: 0.10 ± 0.26 s–1, 0.00 ± 0.03, and –1.78 ± 0.67 s–1, respectively; P <.001). In this study, new myocardial deformation indexes were shown to quantitatively describe the function of normal and chronically infarcted regions. (J Am Soc Echocardiogr 2000;13:588-98.)

On the measurement of strain rate in an oscillatory baffled column using particle image velocimetry

We report, for the first time, our direct experimental measurement of velocity vectors and strain-rate distributions in an oscillatory baffled column (OBC) using time-resolved particle image velocimetry (PIV). The technique allowed a time series of spatial velocity maps to be obtained for several phases per oscillation cycle and the results, comprising several thousand such maps, illustrate in detail the variations of velocity and strain rate in a baffled region over a range of oscillation amplitudes and oscillation frequencies and provide a deep insight into the mixing, vortex convection and transport mechanism in such a device. The results show that an OBC not only provides enhanced mixing, but also offers low strain rates, which are lower than those in stirred tank vessels. We have also reported for the first time the quantitative correlation between the strain rate and the power dissipation in an OBC. Using the PIV technique we are able to quantify the strain rates experienced in an OBC.

Grain boundary-related high temperature plasticity in oxide ceramics : The effect of cation doping

High-temperature plastic flow in oxide ceramics is controlled by grain boundary diffusion of the constituent ions in a fairly wide temperature and strain rate region. In such deformation conditions, the plastic flow is very sensitive to the grain boundary segregation of a dopant cation. Two typical examples of the dopant effect, high-temperature creep resistance in high-purity, polycrystalline A1203 and tensile ductility in SiO 2 -doped tetragonal zirconia polycrystal are briefly introduced in this paper.

Superplastic Deformation Mechanisms of Alumina Zirconia Two Phase Ceramics

Deformation behavior at high temperature has been investigated for fine-grained Al 2 O 3 -20%ZrO 2 (3 mol%Y 2 O 3 doped ZrO 2 ) ceramics prepared with no additives in order to characterize the superplastic deformation mechanisms. The ceramics can be deformed superplastically in compression tests in the temperature range from 1400 to 1600°C. Grain boundary sliding clearly is found to take place during the superplastic deformation. The strain rate dependence of the flow stress and the apparent activation energy are different between the lower and higher strain rate regions. The deformation mechanisms in the lower strain rate region are considered to be grain boundary sliding, intragranular strain and interface-controlled diffusion creep. The rate controlling mechanism in the lower strain rate region is the interface-controlled diffusion creep. The deformation mechanisms in the higher strain rate region are grain boundary sliding, intragranular strain and grain boundary diffusion creep. Among them, the grain boundary diffusion creep is the rate controlling mechanism and hardly contributes to the total strain.

Amplitude Equation for a Dynamic Strain Aging Model: Beyond Linear Stability Analysis of Serrated Flow in Metallic Alloys

ABSTRACTA method for construction of “processing windows” to avoid negative strain rate sensitivity and associated serrated flow in some aluminum alloys is described. The method is based on the amplitude Ginzburg-Landau (GL) equations and analysis of bifurcation diagrams. The mathematical technique developed in the present work was applied to a specific aluminum alloy, Al-0.4%Mg-0.2%Si considered earlier in the literature [1-3], and yielded good results in terms of predicting the negative strain rate sensitivity regions in the “strain rate “temperature” parameter space. Using the GL-analysis it was demonstrated that even though the instability area is located in the region of intermediate strain rates, a qualitative difference exists between the areas of (relatively) fast and (relatively) slow strain rates. In the first case the dynamic behavior of the system is supercritical, in the second case it is subcritical. The second case is highly undesirable because it causes a sudden onset of stable stress serrations that are difficult to suppress, while in the first case the development of instability is gradual and, consequently, more easily controllable.

Effect of Reinforcement Orientation on the Impact Fracture of Carbon Fiber Reinforced 7075-T6 Aluminum Matrix Composite

The impact response and fracture characteristics of 7075-T6 aluminum alloy reinforced with 10% V f laminated carbon fiber are studied experimentally with regard to the relations between mechanical properties, reinforcement orientation, strain rate and temperature. Cylindrical specimens prepared with two different fiber layer orientations, transverse and longitudinal, are deformed over a strain rate range of 10 -2 to 5 × 10 3 s -1 at temperatures from 25 to 300°C using a Saginomia 100 metal forming machine for low strain rates and a compressive split-Hopkinson bar for high strain rates. The resulting data indicate that the strength of the composite is strongly strain rate and temperature sensitive, and that the strength is considerably improved if the fiber layers are aligned in the transverse orientation. In the high strain rate region, the transverse composite displays a stronger rate sensitivity than the longitudinal composite. The activation volume of the composite changes significantly with variation of temperature and work hardening stress, but changes only slightly with fiber layer orientation. Fracture feature observations reveal that the transverse specimens tailed mainly by shear along a plane of maximum shear stress, while the longitudinal specimens failed by longitudinal interfacial splitting due to relatively weak fiber/matrix bonding and lower matrix constraints. Under high temperature conditions, all specimens showed extensive plastic matrix flow and exhibited a pronounced tendency for the adhesion of matrix to the fiber surface.

Anomalous Strain Rate Dependence of Tensile Elongation in Moisture-Embrittled L12 Alloys

ABSTRACTThe effect of strain rate on tensile ductility of moisture-embrittled L12-type Co3Ti and Ni3(Si, Ti) ordered alloys was investigated at ambient temperatures (298K∼423K) by tensile test and SEM fractography. The anomalous increase of tensile elongation and ultimate tensile stress was observed in a low strain rate region and also at high temperatures, accompanied with an increased area fraction in ductile transgranular fracture pattern. The anomalous strain rate dependence of tensile ductility was shown to become more evident with decreasing grain size. As a process counteracting to the hydrogen decomposition from moisture in air, oxidation process on the alloy surface was suggested.

Global finite-element model makes a small contribution to intraplate seismic hazard estimation

Abstract Seismic-hazard estimates require merging datasets from seismology, geodesy, and geology that have different footprints, which works best when there is a global model that relates them. The rigid-plate model serves this purpose for plate-boundary hazard estimation but fails for intraplate hazard. The global thin-shell finite-element model of the lithosphere by Bird (1998) is a first attempt at a model with realistic intraplate regions. It is based on stress equilibrium and assumed flow laws for the lithosphere, incorporates topographic and geothermal variations and plate-boundary faults and predicts long-term anelastic strain rates in all plate interiors. For comparison, the historical seismic catalog JISSIG (Utsu, 1996) and the instrumental catalog CMT (Dziewonski et al., 1997) are converted to seismic strain rates in intraplate regions corresponding to the finite elements. Spatial correlations are tested between the logarithms of strain rates in order to reduce the dominance of the greatest earthquakes. The finite-element model has correlations of 0.391 with JISSIG and 0.338 with CMT, neither of which is as high as the correlation of JISSIG with CMT (0.476). This may mean that the model is defective or that intraplate seismicity contains a large fraction of aftershocks and other triggered events. If we take the CMT dataset as representative of the short-term intraplate seismicity that we might try to forecast, then multiple regression shows that this is best correlated (0.504) with a 56/44% mixture of the JISSIG and finite-element model strain rates. The contribution of the finite-element model, though small, is significant with 99.9% confidence.

Crustal deformation during 1994–1998 due to oblique continental collision in the central Southern Alps, New Zealand, and implications for seismic potential of the Alpine fault

The positions of 115 ground marks in a 150 × 100 km area of oblique continental collision in the central Southern Alps, New Zealand, have been measured by Global Positioning System (GPS) two to four times between 1994 and 1998. Contemporary velocity and strain rate fields derived from these observations are largely invariant along the northeasterly strike of the mountains and Alpine fault. Across strike, more than 60% of the strain occurs within a band from 5 km NW to 20 km SE of the Alpine fault, but significant strain continues at least a further 60 km SE to near the edge of the Southern Alps foothills. Projections of the fault‐parallel and fault‐normal components of velocity onto an Alpine faultnormal profile show that about 85% of the NUVEL‐1A model relative plate motion is observed within the GPS network. The surface displacements in the high strain rate region are well fit by a model in which stable slip or shearing is occurring at 50–70% of the relative plate rate in a region deeper than about 5–8 km on the down‐dip extension of the SE dipping Alpine fault. Material shallower than this is behaving elastically and thus storing elastic strain in the region of the Alpine fault. The longer‐wavelength displacements can be modeled either as distributed deformation beneath the Southern Alps, or by localization of elastic strain around the upper end of a discrete NW dipping fault or shear zone that is slipping stably below about 30 km depth and would outcrop near the SE boundary of the mountains if extrapolated to the surface. Strain determined from a small‐scale survey network crossing the Alpine fault indicates no significant near‐surface aseismic fault slip on the central Alpine fault over the past 25 years. Our results are consistent with independent geological evidence that the central section of the Alpine fault is capable of producing large to great earthquakes.

Abnormal Postsystolic Thickening in Acutely Ischemic Myocardium During Coronary Angioplasty: A Velocity, Strain, and Strain Rate Doppler Myocardial Imaging Study

We report a case in which the combination of gray scale imaging of wall thickness changes allied to color DMI regional velocity, strain, and strain rate data identified the development and regression of diastolic thickening in the acute ischemic segment during a right coronary artery percutaneous transluminal coronary angioplasty (PTCA). We also discuss the possible mechanisms and potential clinical implications of this finding. (J Am Soc Echocardiogr 1999;12:994-6.)

The effect of grain size and temperature on the superplastic deformation behavior of a 7075 Al alloy

The high-temperature deformation behavior of a 7075 Al alloy has been investigated within the framework of a recently proposed internal-variable theory for structural superplasticity (SSP). The flow curves were obtained by performing a series of load relaxation tests for specimens with various grain sizes, at temperatures ranging from 445 °C to 515 °C. The overall flow curves were then separated into two parts, according to the respective physical mechanisms, viz., the grain-boundary sliding (GBS) and the accommodating dislocation glide processes, contrary to the conventional approach which uses a single power-law relation. These individual curves were then analyzed based on the internal-variable theory. Much valuable information has been obtained in this way, providing new physical insight as well as a more comprehensive understanding of SSP. The GBS curve could be described as a Newtonian viscous flow, signified by the power-index value of Mg=1.0 for this alloy. The unresolved issue of threshold stress is also clarified and identified as a critical stress required for the GBS. The role of grain refinement is found to shift the grain-matrix deformation (GMD) curve into a higher stress and strain-rate region, while the GBS curve into a lower stress and higher strain-rate region along the respective characteristic scaling line to bring both curves into a common flow-stress region, in which the GMD and GBS can operate simultaneously, resulting in the usual superplastic deformation behavior.

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.

Tissue Doppler Echocardiography: Historical Perspective and Technological Considerations.

Tissue Doppler echocardiography is a new ultrasonographic approach to the quantification of myocardial function. It is based on the interrogation of the high amplitude, low velocity reflected ultrasound signals from the myocardium. Velocity data can be obtained during pulsed- or color Doppler methodologies. Color Doppler data can be processed to determine regional acceleration, strain, strain rate, delay, and amplitude of motion. Color myocardial Doppler frame rates of up to 200 s(-1) can be implemented by parallel processing of the data, thus overcoming the early temporal resolution limitations of the technique.

Strain Rate Imaging by Ultrasound in the Diagnosis of Regional Dysfunction of the Left Ventricle

BACKGROUND: Regional strain rate in the left ventricle can be assessed by tissue Doppler velocity gradient and color mapped in real time. Regional contractility thus can be visualized and graded. To validate the method, we made a comparison with standard echocardiography. METHODS AND RESULTS: Fifteen patients with recent myocardial infarction were examined with the use of strain rate imaging (SRI). Velocity gradients were mapped by color. Gray-scale imaging was performed using the second harmonic mode. Cine loops of two-dimensional echocardiography (2-D echo) and SRI images from three standard apical planes were analyzed off line. A four-grade scale in 16 segments was used to score wall motion by 2-D echo and by SRI. Of a total of 236 segments, 235 segments were analyzable by 2-D echo and 218 segments were analyzable by SRI. Correlation of wall motion score index with ejection fraction was - 0.84 by 2-D echo and - 0.92 by SRI. One hundred fourteen segments had an equal score by the two methods: 51 segments differed by 1 degree and 14 segments differed by 2 degrees (kappa = 0.45). CONCLUSIONS: SRI agrees well with echocardiography in grading regional wall function, and the method can be seen as validated in a clinical setting for assessment of regional systolic wall function and is demonstrated to be applicable for semiquantitative wall motion assessment. SRI has theoretical advantages and may be a valuable addition to standard echocardiography, especially in the field of stress echocardiography.

Depositional features and transportation mechanism of valley-filling Iwasegawa and Kaida debris avalanches, Japan

The depositional features of two valley-filling debris avalanche deposits were studied to reveal their transportation and depositional mechanisms. The valley-filling Iwasegawa debris avalanche deposit (ca. 0.1 km3) is distributed along the valleys at the southeastern foot of Tashirodake Volcano, northern Honshu, Japan. Debris-avalanche blocks range in size from 0.3 km3) is distributed along the valleys at the eastern-southeastern foot of Ontake Volcano, central Japan. Debris-avalanche blocks range in size from <25 m proximally to <7 m in the medial zone. Debris-avalanche matrix percentages increase from 50–70% in the proximal zone to 80% in the distal zone. The debris-avalanche matrix is more abundant (80–90%) at the bottom part of the deposit. Deformation structures observed in the debris-avalanche blocks include elongation, folding, conjugate reverse faults, and numerous minor faults in unconsolidated materials. Lithic components within the debris-avalanche matrix tend to have a higher percentage of plucked clasts from the adjacent underlying formations. A Bingham "plug flow" model is consistent with the transportation and depositional mechanisms of the valley-filling debris avalanches. In the plug of the debris avalanche, fragile blocks were transported without major rupturing due to relatively small shear stresses in regions of small strain rate. The debris-avalanche matrix was mainly produced by shearing at the bottom and margins of the avalanche. Valley-filling debris avalanches tend to have smaller debris-avalanche blocks and larger amounts of debris-avalanche matrix than do unconfined debris avalanches. These differences may be due to disaggregation of debris-avalanche blocks by shearing against valley walls and interaction between debris-avalanche blocks and valley walls. Oriented wood logs and branches, reverse grading of clasts at the base, and a higher proportion of incorporated clasts at the base are interpreted to result from shearing along the bottom and valley walls.

Thermal explosion theory for shear localizing energetic solids

The behaviour of energetic solids subjected to simple shear loading is modelled to predict ignition. The model is transient, one dimensional and includes effects of thermal diffusion, plastic work and exothermic reaction with one-step irreversible Arrhenius kinetics. A common power-law constitutive model for shear stress which accounts for strain hardening, strain-rate hardening and thermal softening is used. For the energetic solid composite LX-14 subjected to an average strain rate of , the model predicts reactive shear localization after an induction time of approximately t=5 ms, in which regions of high strain rate () are confined to thin spatial zones. An approximate thermal explosion theory which enforces spatially homogeneous stress, temperature and reaction progress as well as accounting for the early-time dominance of plastic work over exothermic reaction allows the development of simple analytic expressions for the temporal evolution of all variables. The simple expressions give predictions which agree well with both: (a) numerical predictions of a spatially homogeneous theory which allows simultaneous influences of reaction and plastic work and (b) numerical predictions of a spatially inhomogeneous theory which accounts for spatial stress distributions, thermal diffusion and spatially localized reaction. In particular, the induction time to ignition is captured accurately by the approximate theory; hence, while reactive shear localization may accompany an ignition event, ignition causality is most dependent on plastic work done during the time of spatially homogeneous shear.

Relationships between strong internal waves in the coastal zone and their radar and radiometric signatures

Strong internal waves (IW) in the form of soliton groups were observed off the Oregon coast with in situ and remote sensors, including shore‐based X band and Ka band Doppler radars and an airborne microwave radiometer operating at 37 GHz. Here we analyze the relationships between oceanic isotherm vertical displacements, internal currents, and radar backscatter cross sections, along with Doppler velocity signals at horizontal (HH) and vertical (VV) polarizations, and 37‐GHz brightness temperatures measured at intermediate incidence angles from an airship. Analysis of these observations shows that (1) the horizontal spatial structure of the IW field depends on whether it is forced by a strong (spring) tide or weak (neap) tide; (2) while both HH and VV are strongly modulated by IWs, the modulation depth of HH always exceeds that of VV, and they both increase with the IW amplitude; (3) 37‐GHz brightness temperature modulations were in phase with radar signal modulations; and (4) the phase of radar signal modulation with respect to the IW is such that the minimum radar signal intensity and lowest microwave brightness temperature lie close to the maximum of the IW thermocline depression or, equivalently, the horizontal current excursion near the surface. This last observation conflicts with the expectation that the highest backscattered signal would be found near the region of greatest surface strain rate (the surface current gradient). Existing theoretical models are briefly reviewed for interactions between the IW field and the intensity of short gravity‐capillary waves that might be responsible for this behavior.

Invesigation of Superplasticity in 3Y-TZP by Stress Relaxation

Stress relaxation testing was used to study superplasticity in tetragonal zirconia polycrystals stabilized with 3% mol fraction yttria (3Y-TZP). The stress - strain rate behavior measured by this technique compared favorably with data obtained by more traditional techniques, and it is suggested that stress relaxation is an ideal method for studying superplasticity. The range of data obtained provides evidence for three distinct regions of deformation for 3Y-TZP. The action of two deformation mechanisms operating in parallel connected by a threshold stress associated with the high strain rate region can suitably explain the observed behavior of 3Y-TZP.

アルミナ・ジルコニア二相セラミックスの超塑性変形機構

Deformation behavior at high temperature has been investigated for fine-grained Al2O3-20%ZrO2 (3 mol%Y2O3 doped ZrO2) ceramics prepared with no additives in order to characterize the superplastic deformation mechanism. Large strains are obtained by compression tests in the temperature range from 1400 to 1600°C. Grain boundary sliding is clearly found to take place during the superplastic deformation. The strain rate dependence of the flow stress and the apparent activation energy are differ between the lower and higher strain rate regions. The deformation mechanisms contributing to the total strain in the lower strain rate region are grain boundary sliding, intragranular strain and grain boundary diffusion creep. The rate controlling mechanism in the lower strain rate region is interface-controlled diffusion creep. The deformation mechanisms in the higher strain rate region are grain boundary sliding, intragranular strain and grain boundary diffusion creep. Among them, the grain boundary diffusion creep is the rate controlling mechanism and has almost no contribution to the total strain.