bluuebunny/crossref_metadata_2025 huggingface dataset:
Gas Turbine Parameter Corrections
Authors: Volponi A. J.
Published: February 2008

The various parameters appearing along an engine’s gas path, such as flows, pressures, temperatures, speeds, etc., not only vary with power condition but also with the ambient conditions at the engine’s inlet. Since a change in inlet temperature and/or pressure will contribute to an attendant change in a gas path parameter’s value, it would be difficult to characterize the aero-thermodynamic relationships between gas turbine engine parameters, (even at a constant engine operating point) unless the ambient conditions are somehow accounted for. This is usually accomplished through the use of corrected engine parameters. Although most of these corrections are well known by practitioners in the industry, knowledge of their origin does not appear to be as commonplace. The purpose of this paper is to fill that gap and furnish a summary of the commonly used corrections for the “major” gas path parameters that are used in performance analysis, diagnostics, and control design, and to offer a derivation of these corrections. We will suggest both an analytic approach as well as an empirical approach. The latter can be used to establish the correction for parameters not directly addressed in this paper, as well as to fine tune the correction factors when actual engine data is available.
Characteristics of Discharge Coefficient in a Rotating Disk System
Authors: Maeng D. J. , Lee J. S. , Jakoby R. , Kim S. , Wittig S.
Published: February 2008

The discharge coefficient of a long orifice in a rotating system is measured to examine the rotational effect on discharge behavior. The rotating system is comprised of a rotating disk and two stators on both sides of the rotating disk. Test rig is constructed to simulate the real turbine operating conditions. Pressure ratios between upstream and downstream cavities of the orifice range from 1.05 to 1.8, and rotational speed of the rotor disk is varied up to 10,000 rpm. The orifice hole bored through the rotor disk has length-to-diameter ratio of 10. For a better interpretation of discharge behavior, three-dimensional velocity field in the downstream and upstream cavities of the rotor is measured using a Laser Doppler Velocimetry. A new definition of the rotational discharge coefficient is introduced to consider the momentum transfer from the rotor to the orifice flow. Additional loss in the discharge coefficient due to pressure loss in the orifice hole at the inlet and exit regions is quantitatively presented in terms of the Rotation number and the compressibility factor. The effect of corner radiusing at the orifice inlet is also investigated at various rotational conditions.
Rotor Power Losses in Planar Radial Magnetic Bearings—Effects of Number of Stator Poles, Air Gap Thickness, and Magnetic Flux Density
Authors: Allaire P. E. , Kasarda M. E. F. , Fujita L. K.
Published: February 2008

Rotor Power losses in magnetic bearings cannot be accurately calculated at this time because of the complexity of the magnetic field distribution and several other effects. The losses are due to eddy currents, hysteresis, and windage. This paper presents measured results in radial magnetic bearing configurations with eight pole and 16 pole stators and two laminated rotors. Two different air gaps were tested. The rotor power losses were determined by measuring the rundown speed of the rotor after the rotor was spun up to speeds of approximately 30,000 rpm, DN = 2,670,000 mm-rpm, in atmospheric air. The kinetic energy of the rotor is converted to heat by magnetic and air drag power loss mechanisms during the run down. Given past publications and the opinions of researchers in the field, the results were quite unexpected. The measured power losses were found to be nearly independent of the number of poles in the bearing. Also, the overall measured rotor power loss increased significantly as the magnetic flux density increased and also increased significantly as the air gap thickness decreased. A method of separating the hysteresis, eddy current and windage losses is presented. Eddy current effects were found to be the most important loss mechanism in the data analysis, for large clearance bearings. Hysteresis and windage effects did not change much from one configuration to the other.
Experimentally Determined Rotor Power Losses in Homopolar and Heteropolar Magnetic Bearings
Authors: Kasarda M. E. F. , Allaire P. E. , Norris P. M. , Mastrangelo C. , Maslen E. H.
Published: February 2008

The identification of parameters that dictate the magnitude of rotor power losses in radial magnetic bearings is very important for many applications. Low loss performance of magnetic bearings in aerospace equipment such as jet engines and flywheel energy storage systems is especially critical. Two basic magnetic bearing designs are employed in industrial practice today: the homopolar design, where the flux paths are of a mixed radial/axial orientation, and the heteropolar design, where the flux paths are primarily radial in nature. The stator geometry and flux path of a specific bearing can have a significant effect on the rotor losses. This paper describes the detailed measurement of rotor losses for experimentally comparable homopolar and heteropolar designs. The two test bearing configurations are identical except for geometric features that determine the direction of the flux path. Both test bearing designs have the same air gap length, tip clearance ratio, surface area under the poles, and bias flux levels. An experimental test apparatus was used where run down tests were performed on a test rotor with both bearing designs to measure power losses. Numerous test runs where made for each bearing configuration by running multiple levels of flux density. The components of the overall measured power loss, due to hysteresis, eddy currents, and windage, were determined based on theoretical expressions for power loss. It was found that the homopolar bearing had significantly lower power losses than the heteropolar bearing.
Statistics of Response of a Mistuned Bladed Disk Assembly Subjected to White Noise and Narrow Band Excitation
Authors: Cha D. , Sinha A.
Published: February 2008

This paper deals with the statistics of the response of a mistuned bladed disk assembly subjected to random excitation. Analytical techniques are developed to compute this statistics for two types of random excitation: white noise and narrow band. The validity of the analytical methods has been established by comparison with the results from numerical simulations. The sensitivities of the response to mistuning have been examined as a function of the width of the frequency band of the random excitation, the dominant frequency of the random excitation and the structural coupling between adjacent blades.
Computational Prediction of Tube Erosion in Coal Fired Power Utility Boilers
Authors: Lee B. E. , Fletcher C. A. J. , Behnia M.
Published: February 2008

Erosion of boiler tubes causes serious operational problems in many pulverized coal-fired utility boilers. A new erosion model has been developed in the present study for the prediction of boiler tube erosion. The Lagrangian approach is employed to predict the behavior of the particulate phase. The results of computational prediction of boiler tube erosion and the various parameters causing erosion are discussed in this paper. Comparison of the numerical predictions for a single tube erosion with experimental data shows very good agreement.
Rotation of Clamped Parts due to Reciprocating Motion
Authors: Edelstein E. S. , Blech J. J.
Published: February 2008

Cylindrical and spherical parts, clamped in mechanical systems, rotate when the system is subjected to a dynamic reciprocating environmental condition. This rotation is a combination of the clamped part high “receptance” magnification near its natural frequencies, with frictional stick-slip at the clamping contact areas, when the system is excited to a reciprocating motion not collinear with the preloading direction. Such rotation may result in wear and fatigue failures in those systems, due to the rotation and the slip motion involved. Three cases are analyzed: cylindrical pins in hinges, balls in ball bearings, and spherical vessels clamped between spherical cavities.
An Exact Three-Dimensional Solution for Simply Supported Rectangular Piezoelectric Plates
Authors: Bisegna P. , Maceri F.
Published: February 2008

An exact three-dimensional solution for the problem of a simply supported rectangular homogeneous piezoelectric plate is obtained, in the framework of the linear theory of piezoelectricity. The plate is made of a transversely isotropic material, is earthed on the lateral boundary, and is subjected to prescribed surface charge and tractions on the end faces. The limit of this solution as the plate thickness aspect ratio approaches zero is explicitly carried out. The analytical results obtained may constitute a reference case when developing or applying two-dimensional plate theories for the analysis of more complex piezoelectric problems. A numerical investigation in the case of a square uniformly loaded plate is also performed, in order to evaluate the influence of the thickness-to-side ratio on the three-dimensional solution of the plate problem.
Consistent Infinitesimal Finite Element Cell Method: Three-Dimensional Scalar Wave Equation
Authors: Wolf J. P. , Song Chongmin
Published: February 2008

To calculate the unit-impulse response matrix of an unbounded medium governed by the scalar wave equation for use in a time-domain analysis of unbounded medium-structure interaction, the consistent infinitesimal finite element cell method is developed for the three-dimensional case. Its derivation is based on the finite element formulation and on similarity. The discretization is only performed on the structure-medium interface, yielding a reduction of the spatial dimension by 1. The procedure is exact in the radial direction and converges to the exact solution in the finite element sense in the circumferential directions.
Anisotropic Beam Theories With Shear Deformation
Authors: Murakami H. , Reissner E. , Yamakawa J.
Published: February 2008

We investigate the effect of constitutive coupling of stretching, bending, and transverse shearing deformation on the deflection of an anisotropic cantilever beam with narrow rectangular cross-section. To this end, we have developed a hierarchy of beam models by applying a variational principle for displacements and transverse stresses to the associated plane stress problem.
Electrorheological Dampers, Part I: Analysis and Design
Authors: Gavin H. P. , Hanson R. D. , Filisko F. E.
Published: February 2008

Electrorheological (ER) materials are suspensions of specialized, micron-sized particles in nonconducting oils. When electric fields are applied to ER materials, they exhibit dramatic changes (within milli-seconds) in material properties. Pre-yield, yielding, and post-yield mechanisms are all influenced by the electric field. Namely, an applied electric field dramatically increases the stiffness and energy dissipation properties of these materials. A previously known cubic equation which describes the flow of fluids with a yield stress through a rectangular duct can be applied to annular flow, provided that certain conditions on the material properties are satisfied. An analytic solution and a uniform approximation to the solution, for the rectangular duct Poiseuille flow case is presented. A numerical method is required to solve the flow in annular geometries. The approximation for rectangular ducts is extended to deal with the annular duct case.
Application of Objective Rates in Mechanical Modeling of Solids
Authors: Reinhardt W. D. , Dubey R. N.
Published: February 2008

A unified formulation is developed for deformation-related spins, and for objective rates based on them. The approach generalizes the underlying concepts, and allows new rates to be constructed. Mathematical and thermodynamical restrictions on these are shown. As a result, it can be demonstrated that the Eulerian strain rate is an objective rate of logarithmic strain, based on a spin easily derivable from the general form. Interrelations between other known spins and objective rates emerge very clearly. Consequences of the proposed formalism are explored in hypoelastic and in rigid-plastic constitutive relations, the latter involving purely isotropic and purely kinematic hardening. The application of the resulting models to the simple shear deformation is shown.
Modeling of Cyclic Ratchetting Plasticity, Part I: Development of Constitutive Relations
Authors: Jiang Y. , Sehitoglu H.
Published: February 2008

The existing plasticity models recognize that ratchetting direction strongly depends on the loading path, the stress amplitude, and the mean stresses, but their predictions deviate from experiments for a number of materials. We propose an Armstrong-Frederick type hardening rule utilizing the concept of a limiting surface for the backstresses. The model predicts long-term ratchetting rate decay as well as constant ratchetting rate for both proportional and nonproportional loadings. To represent the transient behavior, the model encompasses a memory surface in the deviatoric stress space which recalls the maximum stress level of the prior loading history. The coefficients in the hardening rule, varying as a function of the accumulated plastic strain, serve to represent the cyclic hardening or softening. The stress level effect on ratchetting and non-Masing behavior are realized with the size of the introduced memory surface. Simulations with the model checked favorably with nonproportional multiaxial experiments which are outlined in Part 2 of this paper.
Modeling of Cyclic Ratchetting Plasticity, Part II: Comparison of Model Simulations With Experiments
Authors: Jiang Y. , Sehitoglu H.
Published: February 2008

The material constants of the new plasticity model proposed in the first part of the paper can be divided into two independent groups. The first group, c(i) and r(i) (i = 1, 2, ..., M), describes balanced loading and the second group, χ(i) (i = 1,2, . . ., M), characterizes unbalanced loading. We define balanced loading as the case when a virgin material initially isotropic will undergo no ratchetting and/or mean stress relaxation, and unbalanced loading as the loading under which a virgin material initially isotropic will produce strain ratchetting and/or mean stress relaxation. The independence of the two groups of material constants and the interpretation of the model with a limiting surface concept facilitated the determination of material constants. We describe in detail a computational procedure to determine the material constants in the models from simple uniaxial experiments. The theoretical predictions obtained by using the new plasticity model are compared with a number of multiple step ratchetting experiments under both uniaxial and biaxial tension-torsion loading. In multiple step experiments, the mean stress and stress amplitude are varied in a stepwise fashion during the test. Very close agreements are achieved between the experimental results and the model simulations including cases of nonproportional loading. Specifically, the new model predicted long-term ratchetting rate decay more accurately than the previous models.
Magnetomechanical Instabilities in Elastic-Plastic Cylinders, Part I: Elastic Response
Authors: Littlefield D. L.
Published: February 2008

The effects of electromagnetic fields on instabilities in metal cylinders are considered in this analysis. The cylinder is assumed to be infinitely long and perfectly conducting. An axial electric current is introduced in the cylinder, giving rise to an azimuthal magnetic field in the surrounding vacuum, causing mechanical distortion in the cylinder. The current is assumed to be small so that the deformation remains elastic; in an accompanying paper (Littlefield, 1996) larger currents are considered where plastic flow becomes important. After solutions to the idealized motion of the cylinder under uniaxial strain conditions are developed, small perturbations to the motion are considered. The equations governing the motion of these disturbances are derived using linear perturbation theory. Solutions to the equations indicate that electromagnetic fields can have a substantial effect on the stability spectrum in the cylinder. In general, the frequency of oscillating perturbations is suppressed by the azimuthal magnetic field, and distending instabilities are possible if the magnetic field is above a threshold value. The underlying physical mechanisms contributing to these deviations are proposed.
Magnetomechanical Instabilities in Elastic-Plastic Cylinders, Part II: Plastic Response
Authors: Littlefield D. L.
Published: February 2008

The analysis of elastic instabilities in metal cylinders when subjected to electromagnetic fields (Littlefield, 1996a) is extended in this work to include elastic-plastic flow. The cylinder is assumed to be infinitely long and perfectly conducting. The Prandtl-Reuss elastic-plastic material model is the assumed constitutive law, with the von Mises yield criterion employed to limit the effective stress. An axial electric current, assumed to be conducting along the surface of the cylinder, generates a confining pressure, causing plastic flow that is initially assumed to be uniform throughout the cross section. The propagation of small axisymmetric disturbances to this uniform motion is studied by applying linear perturbation theory. Solutions to these equations exhibit a wide range of instability modes, as was the case for the purely elastic results, and the frequency of the oscillating disturbances appears to be suppressed by electromagnetic effects. However, in contrast to the elastic result, no threshold magnetic field exists, and distending instabilities are possible for all levels of electric current. Physical mechanisms resulting in these distinctions are suggested.
The Sliding Velocity Flow of Rough Collisions in Multibody Systems
Authors: Batlle J. A.
Published: February 2008

Single-point rough collisions in multibody systems with perfect constraints, under the assumptions of Coulomb’s friction and infinite tangential stiffness at the collision point, require usually an integration over the normal impulse. The evolution of the sliding velocity, which is needed in the integration, is determined by an autonomous nonlinear flow. The phase-space geometry of this flow depends upon five parameters associated with the system collision configuration and the friction coefficient μ, and gives a global picture of the system behavior in collisions with the configuration considered and arbitrary initial velocities. This geometry is studied using μ, as a control parameter, and a set of threshold values of μ, associated with changes in qualitative behavior are determined.
Riser Response Analysis by Modal Phase Reconstruction
Authors: Lucor Didier , Triantafyllou M. S.
Published: February 2008

A methodology is presented to identify the vortex-induced vibrational modes of a riser, based on data from computational fluid dynamics coupled to a long beam under tension and placed in cross flow; two inflow velocity profiles are tested: (a) uniform and (b) exponential shear. The modes we estimate are, in fact, nonlinear equilibria between the flow-induced excitation forces and the structural dynamics and are characterized by varying amplitude and phase along the span; these are complex modes, mixtures of traveling and standing waves.
Empirical Wind Turbine Load Distributions Using Field Data
Authors: Agarwal Puneet , Manuel Lance
Published: February 2008

In the design of land-based or offshore wind turbines for ultimate limit states, long-term loads associated with return periods on the order of the service life (20years, usually) must be estimated. This requires statistical extrapolation from turbine load data that may be obtained by simulation or by field tests. The present study illustrates such extrapolation that uses field data from the Blyth offshore wind farm in the United Kingdom, where a 2MW wind turbine was instrumented, and environment and load data were recorded. From this measurement campaign, the load data available are in two different formats: as 10min statistics (referred to as “summary” data) or as full time series (referred to as “campaign” data). The characteristics of the site and environment and, hence, that of the turbine response are strikingly different for winds from the sea and winds from the shore. The load data (here, only the mudline bending moment is studied) at the Blyth site are hence separated depending on wind regime. By integrating load distributions conditional on the environment with the relative likelihood of the different environmental conditions, long-term loads associated with specified return periods can be derived. This is achieved here using the peak-over-threshold method based on campaign data but long-term loads are compared with similar estimates based on the summary data. Winds from the shore are seen to govern the long-term loads at the site. Though the influence of wave heights on turbine long-term loads is smaller than that of wind speed, there is possible resonance of tower dynamics induced by the waves; still, to first order, it is largely the wind speed and turbulence intensity that control design loads. Predicted design loads based on the campaign data are close to those based on the summary data discussed in a separate study.
An Investigation of the Fatigue Performance of Riser Girth Welds
Authors: Maddox Stephen J. , Speck Julian B. , Razmjoo G. Reza
Published: February 2008

Increasing deep-water oil and gas recovery has highlighted the need for high integrity, high fatigue performance girth welds in steel catenary riser systems. Such systems include girth welds made from one side. However, the widely used fatigue design classification, UK Class F2, for such welds is not well founded, but probably overconservative for pipeline welds. In an attempt to justify upgrading current fatigue design classifications and providing a better basis for design, fatigue tests were performed on a range of girth-welded pipes produced by pipeline welding contractors. This paper presents the results of those tests and their evaluation in terms of the factors that influence the fatigue performance of girth welds, including welding process, welding position, backing system, joint alignment, weld quality, specimen type, and fatigue loading conditions. Conclusions are drawn regarding the scope for adopting higher design classifications and the conditions that must be met to justify them.
Biomechanics of Human Thoracic Ribs
Authors: Yoganandan N. , Pintar F. A.
Published: February 2008

Considerable advances have been made to determine the failure biomechanical properties of the human thoracic spinal column and its components. Except for a few fundamental studies, there is a paucity of such data for the costovertebral elements. The present study was designed to determine the biomechanics of the human thoracic spine ribs from a large population. Seventh and eighth ribs bilaterally were tested from 30 human cadavers using the principles of three-point bending techniques to failure. Biomechanical test parameters included the cross-sectional area (core, marrow, and total), moment of inertia, failure load, deflection, and the Young’s elastic modulus. The strength-related results indicated no specific bias with respect to anatomical level and hemisphere (right or left), although the geometry-related variables demonstrated statistically significant differences (p < 0.05) between the seventh and the eighth ribs. This study offers basic biomechanical information on the ultimate failure and geometric characteristics of the human thoracic spine ribs.
Biomechanical Properties of Human Cadaveric Ankle-Subtalar Joints in Quasi-Static Loading
Authors: Parenteau C. S. , Viano D. C. , Petit P. Y.
Published: February 2008

The biomechanical properties of human ankle-subtalar joints have been determined in a quasi-static loading condition. The moving center of rotation was determined and approximated by a fixed point. The moment-angle characteristics of the ankle-subtalar joints about the fixed center of rotation have been measured under four basic movements: dorsiflexion, plantarflexion, inversion, and eversion. The method linearly increases rotation of the calcaneus until failure, and measures the moments, forces, and linear and rotational displacements. Failure was identified as the initial drop of moment on plot showing the moment representing gross injury or microfilament damage. In this study, 32 human ankle-subtalar joints have been tested to failure. The center of rotation of the ankle-subtalar joints was determined for a pure dorsiflexion (9 specimens), plantarflexion (7 specimens), inversion (8 specimens), and eversion (8 specimens), Failure in the joints occurred at an average moment of −33.1 ± 16.5 Nm in dorsiflexion, 40.1 ± 9.2 Nm in plantarflexion, −34.1 ± 14.5 Nm in inversion, and 48.1 ± 12.2 Nm in eversion. The failure angle was also determined in all four motions. Failure was best predicted by an angle of −44.0 ± 10.9 deg in dorsiflexion, 71.6 ± 5.7 deg in plantarflexion, −34.3 ± 7.5 deg in inversion, and 32.4 ± 7.3 deg in eversion. Injury was identified in every preparation tested in inversion and eversion, while it resulted in five of the nine preparations in dorsiflexion, and in three of the seven in plantarflexion. Injury occurred at −47.0 ± 5.3 deg and −36.2 ± 14.8 Nm in dorsiflexion, and at 68.7 ± 5.9 deg and 36.7 ± 2.5 Nm in plantarflexion. The results obtained in this study provide basic information of the ankle-subtalar joint kinematics, biomechanics, and injury. The data will be used to form a basis for corridors of the ankle-subtalar joint responses.
The Influence of Strain Rate on the Passive and Stimulated Engineering Stress–Large Strain Behavior of the Rabbit Tibialis Anterior Muscle
Authors: Myers B. S. , Woolley C. T. , Slotter T. L. , Garrett W. E. , Best T. M.
Published: February 2008

The passive and stimulated engineering stress–large strain mechanical properties of skeletal muscle were measured at the midbelly of the rabbit tibialis anterior. The purpose of these experiments was to provide previously unavailable constitutive information based on the true geometry of the muscle and to determine the effect of strain rate on these responses. An apparatus including an ultrasound imager, high-speed digital imager, and a servohydraulic linear actuator was used to apply constant velocity deformations to the tibialis anterior of an anesthetized neurovascularly intact rabbit. The average isometric tetanic stress prior to elongation was 0.44 ± 0.15 MPa. During elongation the average stimulated modulus was 0.97 ± 0.34 MPa and was insensitive to rate of loading. The passive stress–strain responses showed a nonlinear stiffening response typical of biologic soft tissue. Both the passive and stimulated stress–strain responses were sensitive to strain rate over the range of strain rates (1 to 25 s−1). Smaller changes in average strain rate (1 to 10, and 10 to 25 s−1) did not produce statistically significant changes in these responses, particularly in the stimulated responses, which were less sensitive to average strain rate than the passive responses. This relative insensitivity to strain rate suggests that pseudoelastic functions generated from an appropriate strain rate test may be suitable for the characterization of the responses of muscle over a narrow range of strain rates, particularly in stimulated muscle.
The Vertical Displacement of the Center of Mass During Walking: A Comparison of Four Measurement Methods
Authors: Saini M. , Kerrigan D. C. , Thirunarayan M. A. , Duff-Raffaele M.
Published: February 2008

Measuring the vertical displacement of the center of mass (COM) of the body during walking may provide useful information about the energy required to walk. Four methods of varying complexity to estimate the vertical displacement of the COM were compared in 25 able-bodied, female subjects. The first method, the sacral marker method, utilized an external marker on the sacrum as representative of the COM of the body. The second method, the reconstructed pelvis method, which also utilized a marker over the sacrum, theoretically controlled for pelvic tilt motion. The third method, the segmental analysis method, involved measuring motion of the trunk and limb segments. The fourth method, the forceplate method, involved estimating the COM displacement from ground reaction force measurements. A two-tailed paired t-test within an ANOVA showed no statistically significant difference between the sacral marker and the reconstructed pelvis methods (p = 0.839). There was also no statistically significant difference between the sacral marker and the segmental analysis method (p = 0.119) or between the reconstructed pelvis and the segmental analysis method (p = 0.174). It follows that the first method, which is the most simple, can provide essentially the same estimate of the vertical displacement of the COM as the more complicated second and third measures. The forceplate method produced data with a lower range and a different distribution than the other three methods. There was a statistically significant difference between the forceplate method and the other methods (p < 0.01 for each of the three comparisons). The forceplate method provides information that is statistically significantly different from the results of the kinematic methods. The magnitude of the difference is large enough to be physiologically significant and further studies to define the sources of the differences and the relative validity of the two approaches are warranted.
A Least-Squares Estimation Approach to Improving the Precision of Inverse Dynamics Computations
Authors: Kuo A. D.
Published: February 2008

A least-squares approach to computing inverse dynamics is proposed. The method utilizes equations of motion for a multi-segment body, incorporating terms for ground reaction forces and torques. The resulting system is overdetermined at each point in time, because kinematic and force measurements outnumber unknown torques, and may be solved using weighted least squares to yield estimates of the joint torques and joint angular accelerations that best match measured data. An error analysis makes it possible to predict error magnitudes for both conventional and least-squares methods. A modification of the method also makes it possible to reject constant biases such as those arising from misalignment of force plate and kinematic measurement reference frames. A benchmark case is presented, which demonstrates reductions in joint torque errors on the order of 30 percent compared to the conventional Newton–Euler method, for a wide range of noise levels on measured data. The advantages over the Newton–Euler method include making best use of all available measurements, ability to function when less than a full complement of ground reaction forces is measured, suppression of residual torques acting on the top-most body segment, and the rejection of constant biases in data.
Effects of Disturbed Flow On Endothelial Cells
Authors: Chiu J.-J. , Wang D. L. , Chien S. , Skalak R. , Usami S.
Published: February 2008

Atherosclerotic lesions tend to localize at curvatures and branches of the arterial system, where the local flow is often disturbed and irregular (e.g., flow separation, recirculation, complex flow patterns, and nonuniform shear stress distributions). The effects of such flow conditions on cultured human umbilical vein endothelial cells (HUVECs) were studied in vitro by using a vertical-step flow channel (VSF). Detailed shear stress distributions and flow structures have been computed by using the finite volume method in a general curvilinear coordinate system. HUVECs in the reattachment areas with low shear stresses were generally rounded in shape. In contrast, the cells under higher shear stresses were significantly elongated and aligned with the flow direction, even for those in the area with reversed flow. When HUVECs were subjected to shearing in VSF, their actin stress fibers reorganized in association with the morphological changes. The rate of DNA synthesis in the vicinity of the flow reattachment area was higher than that in the laminar flow area. These in vitro experiments have provided data for the understanding of the in vivo responses of endothelial cells under complex flow environments found in regions of prevalence of atherosclerotic lesions.
A Constitutive Law for Mitral Valve Tissue
Authors: May-Newman K. , Yin F. C. P.
Published: February 2008

Biaxial mechanical testing and theoretical continuum mechanics analysis are employed to formulate a constitutive law for cardiac mitral valve anterior and posterior leaflets. A strain energy description is formulated based on the fibrous architecture of the tissue, accurately describing the large deformation, highly nonlinear transversely isotropic material behavior. The results show that a simple three-coefficient exponential constitutive law provides an accurate prediction of stress–stretch behavior over a wide range of deformations. Regional heterogeneity may be accommodated by spatially varying a single coefficient and incorporating collagen fiber angle. The application of this quantitative information to mechanical models and bioprosthetic development could provide substantial improvement in the evaluation and treatment of valvular disease, surgery, and replacement.
Finite Element Implementation of Anisotropic Quasi-Linear Viscoelasticity Using a Discrete Spectrum Approximation
Authors: Puso M. A. , Weiss J. A.
Published: February 2008

The objective of this work was to develop a theoretical and computational framework to apply the finite element method to anisotropic, viscoelastic soft tissues. The quasi-linear viscoelastic (QLV) theory provided the basis for the development. To allow efficient and easy computational implementation, a discrete spectrum approximation was developed for the QLV relaxation function. This approximation provided a graphic means to fit experimental data with an exponential series. A transversely isotropic hyperelastic material model developed for ligaments and tendons was used for the elastic response. The viscoelastic material model was implemented in a general-purpose, nonlinear finite element program. Test problems were analyzed to assess the performance of the discrete spectrum approximation and the accuracy of the finite element implementation. Results indicated that the formulation can reproduce the anisotropy and time-dependent material behavior observed in soft tissues. Application of the formulation to the analysis of the human femur-medial collateral ligament–tibia complex demonstrated the ability of the formulation to analyze large three-dimensional problems in the mechanics of biological joints.
The Acoustic and Structural Properties of the Human Femur
Authors: Weiss S. , Zimmerman M. C. , Harten R. D. , Alberta F. G. , Meunier A.
Published: February 2008

This study evaluates the variations in the acoustic properties of the human femur at ten evenly spaced locations along its length, as well as differences that exist within given transverse sections. Six pairs of human femora, three male and three female, were sectioned, ground, and polished, and scanned with a microprocessor-driven scanning acoustic microscope. Images with a resolution of approximately 140 μm were used to calculate the average acoustic impedances for each transverse cross section and each quadrant within a cross section. The mean acoustic impedance for all the cross sections was 7.69 ± 0.18 Mrayls. Variations were observed among the cross sections, and the central sections (4–7) had values that were statistically greater than the other more distal and proximal sections. Within the cross sections, the posterior quadrant had a lower average acoustic impedance compared to the other quadrants and this was statistically significant (Tukey’s multiple comparison test). The cross sections were further analyzed to determine several geometric parameters including the principal moments of inertia, polar moment of inertia, and the biomechanical shape index. The product of the acoustic impedance and the maximum moment of inertia provided a result that attempted to account for the acoustic property variation and the change in shape at the different section locations.
Articular Joint Mechanics With Biphasic Cartilage Layers Under Dynamic Loading
Authors: Wu J. Z. , Herzog W. , Epstein M.
Published: February 2008

The composition and amount of extracellular matrix produced by chondrocytes are thought to be influenced by the stress and strain states in the vicinity of the chondrocytes. During daily activities, such as walking and running, articular joints are loaded dynamically. In the present study, a solution is proposed to simulate the responses of a joint under dynamic loading. In order to show the characteristics of the proposed solution, numerical simulations were carried out, in which the contact radius, the relative approach displacement between the centers of the contacting bodies, or the contact force were controlled. As a result of the history-dependent material properties of the articular cartilage, the predicted parameters changed nonperiodically, when the controlled parameters varied periodically. For a constant load, the contact radius and the relative displacement between the contacting bodies were predicted to increase at decreasing rates. When the contact force was varied dynamically, the predicted mean values of the contact radius, the relative displacement between the contacting bodies, and the contact pressure at the center of the contact area depended on the amplitude and the duration of the loading. When the relative displacement between the contacting bodies was controlled, the amplitudes and the cycling frequency must be limited to avoid a loss of contact between the articular joint surfaces. The proposed solution is valid for a long but limited time period, the exact extent of which is yet to be determined. It can be used to simulate the effects associated with cartilage degeneration in diseases such as osteoarthritis.
Total Shoulder Arthroplasty Biomechanics: A Study of the Forces and Strains at the Glenoid Component
Authors: Karduna A. R. , Williams G. R. , Iannotti J. P. , Williams J. L.
Published: February 2008

The objective of this study was to examine how changes in glenohumeral joint conformity and loading patterns affected the forces and strains developed at the glenoid. After removal of soft tissue (muscles, ligaments, and labrum), force-displacement data were collected for both natural and prosthetically reconstructed joints. Joints were shown to develop higher forces for a given translation as joint conformity increased. A rigid body model of joint contact forces was used to determined the so-called effective radial mismatch of each joint. For the purposes of this study, the effective radial mismatch is defined as the mismatch required for a rigid body joint to have the same force-displacement relationship as the joint in question. This parameter is an indication of the deformation at the articular surface. The effective radial mismatch dramatically increased with increasing medial loads, indicating that under physiological loads, the effective radial mismatch of a joint is much greater than its measured mismatch at no load. This increase in effective mismatch as medial loads were increased was found to be threefold greater in cartilaginous joints than in reconstructed joints. Rosette strain gages positioned at the midlevel of the glenoid keel in the reconstructed joints revealed that anterior/posterior component loading leads to fully reversible cyclic keel strains. The highest compressive strains occurred with the head centered in the glenoid, and were larger for nonconforming joints (ε = 0.23 percent). These strains became tensile just before rim loading and were greater for conforming joints (ε = 0.15 percent). Although recorded peak strains are below the yield point for polyethylene, the fully reversed cyclic loading of the component in this fashion may ultimately lead to component toggling and implant failure.
Load Sharing Between Solid and Fluid Phases in Articular Cartilage: I — Experimental Determination of in Situ Mechanical Conditions in a Porcine Knee
Authors: Mukherjee N. , Wayne J. S.
Published: February 2008

The in situ mechanical conditions of cartilage in the articulated knee were quantified during joint loading. Six porcine knees were subjected to a 445 N compressive load while cartilage deformations and contact pressures were measured. From roentgenograms, cartilage thickness before and during loading allowed the calculation of tissue deformation on the lateral femoral condyle at different times during the loading process. Contact pressures on the articular surface were measured with miniature fiber-optic pressure transducers. Results showed that the medial side of the lateral femoral condyle had higher contact pressures, as well as deformations. To begin to correlate the pressures and resulting deformations, the intrinsic material properties of the cartilage on the lateral condyle were obtained from indentation tests. Data from four normal control specimens indicated that the aggregate modulus of the medial side was significantly higher than in other areas of the condyle. These experimental measures of the in situ mechanical conditions of articular cartilage can be combined with theoretical modeling to obtain valuable information about the relative contributions of the solid and fluid phases to supporting the applied load on the cartilage surface (see Part II).
In Vivo Determination of the Direction of Rotation and Moment-Angle Relationship of Individual Elbow Muscles
Authors: Zhang L. , Butler J. , Nishida T. , Nuber G. , Huang H. , Rymer W. Z.
Published: February 2008

The direction of rotation (DOR) of individual elbow muscles, defined as the direction in which a muscle rotates the forearm relative to the upper arm in three-dimensional space, was studied in vivo as a function of elbow flexion and forearm rotation. Electrical stimulation was used to activate an individual muscle selectively, and the resultant flexion-extension, supination-pronation, and varus-valgus moments were used to determine the DOR. Furthermore, multi-axis moment-angle relationships of individual muscles were determined by stimulating the muscle at a constant submaximal level across different joint positions, which was assumed to result in a constant level of muscle activation. The muscles generate significant moments about axes other than flexion-extension, which is potentially important for actively controlling joint movement and maintaining stability about all axes. Both the muscle DOR and the multi axis moments vary with the joint position systematically. Variations of the DOR and moment-angle relationship across muscle twitches of different amplitudes in a subject were small, while there were considerable variations between subjects.
Muscular Resistance to Varus and Valgus Loads at the Elbow
Authors: Buchanan T. S. , Delp S. L. , Solbeck J. A.
Published: February 2008

Although the contributions of passive structures to stability of the elbow have been well documented, the role of active muscular resistance of varus and valgus loads at the elbow remains unclear. We hypothesized that muscles: (1) can produce substantial varus and valgus moments about the elbow, and (2) are activated in response to sustained varus and valgus loading of the elbow. To test the first hypothesis, we developed a detailed musculoskeletal model to estimate the varus and valgus moment-generating capacity of the muscles about the elbow. To test the second hypothesis, we measured EMGs from 11 muscles in four subjects during a series of isometric tasks that included flexion, extension, varus, and valgus moments about the elbow. The EMG recordings were used as inputs to the elbow model to estimate the contributions of individual muscles to flexion-extension and varus-valgus moments. Analysis of the model revealed that nearly all of the muscles that cross the elbow are capable of producing varus or valgus moments; the capacity of the muscles to produce varus moment (34 Nm) and valgus moment (35 Nm) is roughly half of the maximum flexion moment (70 Nm). Analysis of the measured EMGs showed that the anconeus was the most significant contributor to valgus moments and the pronator teres was the largest contributor to varus moments. Although our results show that muscles were activated in response to static varus and valgus loads, their activations were modest and were not sufficient to balance the applied load.
Engineered Alignment in Media Equivalents: Magnetic Prealignment and Mandrel Compaction
Authors: Barocas V. H. , Girton T. S. , Tranquillo R. T.
Published: February 2008

We predicted and measured the evolution of smooth muscle cell (SMC) orientation in media-equivalents (MEs) for four fabrication conditions (F−, M−, F+, M+) under Free or Mandrel compaction (F/M) with and without magnetic prealignment of the collagen fibrils in the circumferential direction (±). Mandrel compaction refers to SMC-induced compaction of the ME that is constrained by having a nonadhesive mandrel placed in the ME lumen. Predictions were made using our anisotropic biphasic theory (ABT) for tissue-equivalent mechanics. Successful prediction of trends of the SMC orientation data for all four fabrication cases was obtained: maintenance of the initial isotropic state for F−, loss of initial circumferential alignment for F+, development of circumferential alignment for M−, and enhancement of initial circumferential alignment for M+. These results suggest two mechanisms by which the presence of the mandrel leads to much greater mechanical stiffness in the circumferential direction reported for mandrel compacted MEs relative to free compacted MEs: (1) by inducing an increasing circumferential alignment of the SMC and collagen, and (2) by inducing a large stress on the SMC, resulting in secretion and accumulation of stiffening components.
Residual Strain in Ischemic Ventricular Myocardium
Authors: Summerour S. R. , Emery J. L. , Fazeli B. , Omens J. H. , McCulloch A. D.
Published: February 2008

Structural remodeling during acute myocardial infarction affects ventricular wall stress and strain. To see whether acute myocardial infarction alters residual stress and strain in the left ventricle (LV), we measured opening angles in rat hearts after 30 minutes of left coronary artery occlusion. The mean opening angle in 18 ischemic hearts (51 ± 20 deg) was significantly greater than in five sham-operated controls (29 ± 11 deg, P < 0.05). To determine whether these alterations in residual strain may be associated with strain softening caused by systolic overstretch of the noncontracting ischemic tissue, we also measured opening angles in isolated hearts that had been passively inflated to high LV pressures (120 mmHg). The mean opening angle of the strain-softened hearts was not significantly different from the sham-operated hearts (34 ± 27 deg, P = 0.74). Mean collagen area fractions in the myocardium were not significantly different between ischemic hearts (0.027 ± 0.014) and the nonischemic group (0.022 ± 0.011). Although there were significant differences in opening angles measured with ischemia, they do not appear to be a result of altered extracellular collagen content or softening associated with overstretch. Thus, there is a significant change in residual strain associated with acute ischemia that may be related to changes in collagen fiber structure, myocyte structure, or metabolic state.