Estimation of conduction velocity vector fields from epicardial mapping data

An automated method to estimate vector fields of propagation velocity from observed epicardial extracellular potentials is introduced. The method relies on fitting polynomial surfaces T(x, y) to the space-time (x, y, t) coordinates of activity. Both speed and direction of propagation are computed from the gradient of the local polynomial surface. The components of velocity, which are total derivatives, are expressed in terms of the partial derivatives which comprise the gradient of T. The method was validated on two-dimensional (2-D) simulations of propagation and then applied to cardiac mapping data. Conduction velocity was estimated at multiple epicardial locations during sinus rhythm, pacing, and ventricular fibrillation (VF) in pigs. Data were obtained via a 528-channel mapping system from 23 x 22 and 24 x 21 arrays of unipolar electrodes sutured to the right ventricular epicardium. Velocity estimates are displayed as vector fields and are used to characterize propagation qualitatively and quantitatively during both simple and complex rhythms.     

Bulk strain and grain strain in axisymmetric deformation

Bulk strain, defined in terms of original and final dimensions of a specimen, correlates well with the logarithm of the average number of intercepts per unit length on test lines parallel to axes of symmetry after uniaxial deformation of an axisymmetric specimen. This suggests a definition of principal grain strain based on the mean linear intercepts before and after deformation, measured along principal directions. Results of type 304 stainless steel strained to varying amounts between necking and fracture support the hypothesis that the axial gradient of the total grain boundary area projected onto any cylindrical surface coaxial with the axis of deformation remains unchanged as deformation progresses. Although the behavior of the grain boundary network leads to simple correspondences between grain and bulk strains, they are not numerically equal for this type of deformation.     

Estimation of growth velocities from individual longitudinal data

Since the construction of growth velocity curves using the simple increment method (i.e., plotting the increments deltaxt vs, time, t) involves a number of methodological problems, when mathematical curves are fit, they are generally fit to the "distance traveled", plot (of xt vs. t) and the corresponding velocity curve is then obtained by differentiation. However, most of the traditional methods for accomplishing this (e.g., fitting the Gompertz or logistic curves) require that the course of growth be studied over a sufficiently long time period to allow accurate determinations of the upper and lower asymptotes of these S-shaped functions. This is not always feasible in practice: a case in point being the mixed-longitudinal Nijmegen Growth Study where each of the cohorts comprising the sample is followed for but a five-year period. In such situations, alternative approaches to the problem of constructing growth velocity curves may be of considerable practical as well as theoretical interest. One such approach is developed in this paper and its use is illustrated on some data collected as part of the Nijmegen Growth Study. These results are then compared with those obtained using the increment method on the same data.     

Estimation of individual and population vaccination effectiveness from time-to-event data

Longini and Halloran and Halloran et al. recently developed a method for estimation of parameters related to the efficacy of a vaccine and to the transmission of an infectious disease from time-to-event (disease or infection) data. This work uses their method to evaluate the individual and population effectiveness of a vaccination programme (Haber et al.). Data from an outbreak of measles in Chad illustrates the new methods.     

Local strain hardening and nonuniformity of plastic deformation

Local strain hardening in a cross-section of a deforming crystal has been related to the evolutionary behavior of the coupled densities of the mobile and (relatively immobile) forest dislocations. The local mechanical properties thus calculated differ from the bulk properties as obtained by the application of the Orowan relation on the bulk scale or by measuring the overall strain hardening in a macroscopic specimen. This difference is especially pronounced at small strains, and it is attributed to the inherently nonuniform character of plastic deformation by slip. The intrinsic stability of plastic deformation on the local scale has been examined with the help of standard methods of nonlinear analysis. In addition to such nonuniformities as yield points and the occurrence of necking (Considère instability), slip lines and slip bands also appear to be interprétable in terms of local instability of plastic flow.     

Estimation of plastic deformation in the technologies of bulk cold deformation

A technique for determining the degree of reduction upon forming by cold bulk shaping has been proposed. The technique is based on a comparison of the measurement data on the hardness of a cold-worked metal with that of the same metal subjected to a compression test at a certain strain intensity. The technique has been tested in technological operations of reduction and thread rolling.     

基于应变梯度理论的假设应变有限元方法

采用基于应变梯度理论的假设应变有限元方法研究了微尺度梁弯曲的尺寸效应.在假设应变单元设计中,非局部的应变梯度项通过围绕高斯点的胞元进行数值积分得到.在本构方程中引入等效应变梯度项,在积分本构方程时就可以反映材料在微细变形时的尺寸效应.为了验证本方法的正确性,对微尺度下的悬臂梁进行了模拟计算.计算结果与已发表的实验结果比较吻合,表明可以模拟出材料微细变形的尺寸效应,具有较好的计算精度.     

In vitro verification of myocardial motion tracking from phase-contrast velocity data

The ability to track motion from cine phase-contrast (PC) magnetic resonance (MR) velocity measurements was investigated using an in vitro model. A computer-controlled deformable phantom was used for the characterization of the accuracy and precision of the forward-backward and the compensated Fourier integration techniques. Trajectory accuracy is limited by temporal resolution when the forward-backward technique is used. With this technique the extent of the calculated trajectories is underestimated by an amount related to the motion period and the sequence repetition time, because of the band-limiting caused in the cine interpolation step. When the compensated Fourier integration technique is used, trajectory accuracy is independent of temporal resolution and is better than 1 mm for excursions of less than 15 mm, which are comparable to those observed in the myocardium. Measurement precision is dominated by the artifact level in the phase-contrast images. If no artifacts are present precision is limited by the inherent signal-to-noise ratio of the images. In the presence of artifacts, similar in magnitude to those observed in vivo, the reproducibility of tracking a 2.2 x 2.2 mm2 region of interest is better than 0.5 mm. When the Fourier integration technique is used, the improved accuracy is accompanied by a reduction in precision. We verified that tracking three-dimensional (3D) motion from velocity measurements of a single slice can lead to underestimations of the trajectory if there is a through-plane component of the motion that is not truly represented by the measured velocities. This underestimation can be overcome if volumetric cine phase-contrast velocity data are acquired and full three-dimensional analysis is performed.     

The large strain deformation of some aluminum alloys

The stress-strain behavior of both non-heat-treatable and heat-treatable aluminum alloys has been examined over a large strain range by a variety of deformation modes. In superpurity aluminum deformed in torsion, the work hardening rate approaches zero at strains of four to five, while a definite saturation in the flow stress is observed at much lower strains in the precipitation hardened alloys. In the non-heat-treatable alloys, a saturation in the flow stress is not approached at even very large strains. Nevertheless, the stress-strain behavior of all the alloys can be reasonably represented by a saturation type stress-strain equation. The deformation behavior of the alloys can be qualitatively understood in terms of the dislocation accumulation processes and slip morphology in the different alloys. Finally, it is shown that alloys deformed on a commercial rolling mill exhibit equivalent stress-strain behavior to that found in these laboratory deformation studies.     

Texture evolution during plane strain deformation of aluminum

Plane strain forming in commercial processes commonly includes elevated temperature multi-step reductions. An understanding of microstructural development during this process is critical because it dictates the properties of the material subsequent to hot forming. Earing behavior, formability, and mechanical response of the final product are all dependent upon the processing parameters which dictate microstructural evolution. The present work focuses upon the development of hot deformation textures in aluminum during this type of processing. Commercial purity aluminum specimens exhibiting two different starting textures were deformed in channel die compression experiments to simulate the plane strain deformation conditions imposed by the rolling mill. Initial structures consisted of a randomly textured material and a preferentiallycube oriented texture to investigate the effects of starting texture on hot deformation processing and the resulting microstructures. Rate and temperature dependence of texture evolution was experimentally and theoretically investigated in conjunction with this. The relative stability of cube orientations within polycrystals deformed in plane strain is demonstrated for certain deformation conditions. Finally, experimental observations of the evolution of orientation flow from the alpha to the beta fiber are discussed.     

The phenomenon of strain avalanches in cyclic deformation

By studying cyclic deformation of both mono- and polycrystalline copper with a fast response recorder, we have discovered an interesting substructure in hysteresis loops. This effect is caused by strain avalanches, which occur in loop patches, persistent slip bands and cellular dislocation structures. The behavior of the strain avalanches changes as life is expended and offers a means of measuring the current state of fatigue damage. They are especially pronounced when cracks are propagating. We believe that strain avalanches provide a useful tool for understanding cyclic deformation and fracture and for a novel method of nondestructive testing.     

The deformation of silicon at high temperatures and strain rates

Polycrystalline and 〈100〉 single crystalline semiconductor grade silicon samples have been subjected to uniaxial compression at strain rates from 10⁻⁵ to 12 s⁻¹ at temperatures ranging from 1100 to 1380 °C. Both intrinsic and p-type polycrystalline material and p-type single crystalline material were tested. Except at the highest temperature and lowest strain rate, no steady state deformation was observed for the polycrystalline material. In all other cases strain hardening was observed which increased with increasing strain rates. The polycrystalline material could be compressed by as much as 50 pct at 1380 °C and a strain rate of 7 s⁻¹ without cracking. An axial stress of approximately 170 MPa produces a strain rate of 5 s⁻¹ at 1380 °C. The stress necessary to produce a given strain rate increases rapidly with decreasing temperature while the ductility rapidly decreases. A preliminary forming limit diagram has also been determined for the polycrystalline material at 1380 °C. The deformation rate-controlling process in the polycrystalline material at high stresses could be the production of vacancies on jogged dislocations. Formerly with the Department of Materials Science and Engineering, University of Pennsylvania     

Effects of strain rate and deformation heating in tensile testing

The effects of strain, strain rate and deformation heating in tensile testing of sheet steel are numerically analyzed. An experimentally determined strain-hardening formula is used as a basis of the calculation. The effects of heat conduction and free or forced con-vection during the test are taken into account by using the approximate solution method originally given by Bishop for metal extrusion. The calculated values agree well with the experimental data. It is shown that deformation heating considerably affects the uni-form strain. Consequently the stretchability of sheet metals can be improved by using very low forming speeds or more economically by using an effective cooling system.     

Plastic deformation of delta-ferritic iron at intermediate strain rates

The plastic deformation of delta-ferritic iron, represented by an electrolytic iron and Fe-0.028 C, Fe-0.044 C and Fe-3.0 Si alloys, has been measured for the temperature range 1200 to 1525‡C and the strain rate range 2.8 x 10^-5 to 2.3 x 10^-2 s^-1. For the bamboo-like tension specimens the plastic flow behavior is approximately nonlinear viscous. Delta-ferrite is more than four times weaker than austenite, and does not exhibit dynamic recrystallization. At the melting point of iron the extrapolated steady-state flow stress increases from 0.31 to 1.86 MN/m² (45 to 370 psi) over the range of strain rate examined.