bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonplanar Crack Growth Using the Surface Integral Method
Authors: Forth S. C. , Keat W. D.
Published: February 2008

A surface integral formulation, based on representing a crack as a distribution of force dipoles, has been developed for modeling the propagation of a three-dimensional nonplanar fracture. The minimum strain energy density and maximum circumferential stress theories were used to determine the direction of crack growth. The extension of the fracture surface was based on the Paris law for fatigue. Remeshing of the fracture during growth was accomplished by adding a ring of elements to the existing mesh at the conclusion of each increment of crack growth. This promoted the efficiency of the algorithm by eliminating the need to recalculate the entire coefficient matrix. Use of the surface integral method, coupled with growth criteria, has yielded an accurate model for three-dimensional nonplanar crack growth under mixed mode loading conditions. The study of several penny-shaped precracks under mixed-mode loading conditions produced the expected growth trajectory, and compared favorably to existing two-dimensional, three-dimensional, and experimental results found in the literature.
Simulation and Experimental Analysis of Diesel Fuel-Injection Systems With a Double-Stage Injector
Authors: Catania A. E. , Dongiovanni C. , Mittica A. , Negri C. , Spessa E.
Published: February 2008

A double-spring, sacless-nozzle injector was fitted to the distributor-pump fuel-injection system of an automotive diesel engine in order to study its effect on the system performance for two different configurations of the pump delivery valve assembly with a constant-pressure valve and with a reflux-hole valve, respectively. Injection-rate shapes and local pressure time histories were both numerically and experimentally investigated. The NAIS simulation program was used for theoretical analysis based on a novel implicit numerical algorithm with a second-order accuracy and a high degree of efficiency. The injector model was set up and stored in a library containing a variety of system component models, which gave a modular structure to the computational code. The program was also capable of simulating possible cavitation propagation phenomena and of taking the fluid property dependence on pressure and temperature, as well as flow shear and minor losses into account. The experimental investigation was performed on a test bench under real operating conditions. Pressures were measured in the pumping chamber at two different pipe locations and in the injector nozzle upstream of the needle-seat opening passage. This last measurement was carried out in order to determine the nozzle-hole discharge flow coefficient under nonstationary flow conditions, which was achieved for the first time in a sacless-nozzle two-stage injector over a wide pump-speed range. The numerical and experimental results were compared and discussed.
Qualitative Characterization of Anti-Swirl Gas Dampers
Authors: Zirkelback N. L.
Published: February 2008

Anti-swirl gas dampers have applications in high-temperature turbomachinery. Nozzles comprising the circumference of the damper inject air against the direction of shaft rotation, providing a force tangential to the rotor surface that acts to damp rotor vibration. The present work involves prediction of experiments by using direct damping and cross-coupled stiffness coefficients given in Vance and Handy (1997) that characterize the rotordynamic performance of an anti-swirl damper. Direct stiffness added at the damper location is vital to representing the changes in critical speeds and the onset and cease of backward whirl given in the experiments. This direct stiffness arises due to the release of air axially across the annulus of the damper. With the addition of this significant direct stiffness, experimental results compare well with the present rotordynamic model. Predictions using the experimentally obtained damping coefficients adequately reproduce the reduction in vibration amplitudes at the critical speeds. However, applying the cross-coupled stiffness coefficients in predictions fails to show increases in the speed at which backward whirl begins and does not reproduce the wrecking instability experienced in the tests. A further study investigates the magnitude of the cross-coupled stiffness coefficient necessary to cause instability in the rotor-damper system.
Experimental Rotordynamic Characterization of Annular Seals: Facility and Methodology
Authors: Darden J. M. , Earhart E. M. , Flowers G. T.
Published: February 2008

Annular seals are known to enhance rotordynamic stability margins and minimize vibration response levels in high-speed rotating machinery. Theoretical predictions for the rotordynamic characteristics of annular seals exist but additional experimental data is needed to properly anchor these results. NASA’s Marshall Space Flight Center (MSFC) has developed an annular seal test rig and facility to experimentally characterize axially fed annular seals. The objective of MSFC’s annular seal test rig is to obtain the rotordynamic coefficients (direct and cross-coupled stiffness, damping, and added mass) for a variety of high Reynolds number annular seals. The MSFC test rig supports centered-seal testing with inlet pressures up to 138 bars (2000 psi) and flow rates of over 946 liters per minute (250 gpm). The rig’s shaft is powered by a 186 kilowatt (250 horsepower) steam turbine capable of rotational speeds of over 20,000 revolutions per minute (rpm). A description of the identification process used to obtain rotordynamic coefficients is given as well as procedures for ensuring quality data. Experimental results for a smooth annular seal with an L/D =0.5 is presented. Excellent agreement between experimental and theoretical results is obtained.
Experimental and Three-Dimensional CFD Investigation in a Gas Turbine Exhaust System
Authors: Sultanian B. K. , Nagao S. , Sakamoto T.
Published: February 2008

Both experimental and three-dimensional CFD investigations are carried out in a scale model of an industrial gas turbine exhaust system to better understand its complex flow field and to validate CFD prediction capabilities for improved design applications. The model consists of an annular diffuser passage with struts, followed by turning vanes and a rectangular plenum with side exhaust. Precise measurements of total/static pressure and flow velocity distributions at the model inlet, strut outlet and model outlet are made using aerodynamic probes and locally a Laser Doppler Velocimeter (LDV). Numerical analyses of the model internal flow field are performed utilizing a three-dimensional Navier-Stokes (N-S) calculation method with the industry standard k-ε turbulence model. Both the experiments and computations are carried out for three load conditions: full speed no load (FSNL), full speed mid load (FSML, 57 percent load), and full speed full load (FSFL). Based on the overall comparison between the measurements and CFD predictions, this study concludes that the applied N-S method is capable of predicting complicated gas turbine exhaust system flows for design applications.
High-Speed Rotor Losses in a Radial Eight-Pole Magnetic Bearing: Part 2—Analytical/Empirical Models and Calculations
Authors: Kasarda M. E. F. , Allaire P. E. , Maslen E. H. , Brown G. R. , Gillies G. T.
Published: February 2008

The continual increase in the use of magnetic bearings in various capacities, including high-speed aerospace applications such as jet engine prototypes, dictates the need to quantify power losses in this type of bearing. The goal of the present study is to develop and experimentally verify general power loss equations for the high-speed operation of magnetic bearings. Experimental data from a large-scale test rotor have been presented in Part 1 of this study. Analytical/empirical predictions are presented here for the test bearings, a pair of eight-pole planar radial bearings, for comparison to the experimental results from Part 1. Expressions for the four loss components, eddy current, alternating hysteresis, rotating hysteresis, and windage, are also presented. Analytical/empirical predictions for the test bearings at three different bias flux levels demonstrate good correlation with corresponding experimental data. Throughout most of the speed range the dominant loss mechanism appears to be eddy currents.
NOx-Abatement Potential of Lean-Premixed GT Combustors
Authors: Sattelmayer T. , Polifke W. , Winkler D. , Do¨bbeling K.
Published: February 2008

The influence of the structure of perfectly premixed flames on NOx formation is investigated theoretically. Since a network of reaction kinetics modules and model flames is used for this purpose, the results obtained are independent of specific burner geometries. Calculations are presented for a mixture temperature of 630 K, an adiabatic flame temperature of 1840 K, and 1 and 15 bars combustor pressure. In particular, the following effects are studied separately from each other: • molecular diffusion of temperature and species; • flame strain; • local quench in highly strained flames and subsequent reignition; • turbulent diffusion (no preferential diffusion); • small scale mixing (stirring) in the flame front. Either no relevant influence or an increase in NOx production over that of the one-dimensional laminar flame is found. As a consequence, besides the improvement of mixing quality, a future target for the development of low-NOx burners is to avoid excessive turbulent stirring in the flame front. Turbulent flames that exhibit locally and instantaneously near laminar structures (“flamelets”) appear to be optimal. Using the same methodology, the scope of the investigation is extended to lean-lean staging, since a higher NOx-abatement potential can be expected in principle. As long as the chemical reactions of the second stage take place in the boundary between the fresh mixture of the second stage and the combustion products from upstream, no advantage can be expected from lean-lean staging. Only if the primary burner exhibits much poorer mixing than the second stage can lean-lean staging be beneficial. In contrast, if full mixing between the two stages prior to afterburning can be achieved (lean-mix-lean technique), the combustor outlet temperature can in principle be increased somewhat without NO penalty. However, the complexity of such a system with a larger flame tube area to be cooled will increase the reaction zone temperatures, so that the full advantage cannot be realized in an engine. Of greater technical relevance is the potential of a lean-mixlean combustion system within an improved thermodynamic cycle. A reheat process with sequential combustion is perfectly suited for this purpose, since, first, the required low inlet temperature of the second stage is automatically generated after partial expansion in the high pressure turbine, second, the efficiency of the thermodynamic cycle has its maximum and, third, high exhaust temperatures are generated, which can drive a powerful Rankine cycle. The higher thermodynamic efficiency of this technique leads to an additional drop in NOx emissions per power produced.
Prediction of the Three-Dimensional Reacting Two-Phase Flow Within a Jet-Stabilized Combustor
Authors: Kurreck M. , Willmann M. , Wittig S.
Published: February 2008

Numerical calculations of the two-phase flow in an experimentally well-investigated research combustor are presented. The comparison between measurements and calculations demonstrates the capabilities of the state-of-the-art Euler/Lagrange method for calculating two-phase flows, when applied to a complex reacting liquid-fueled combustor. The governing equations for gaseous and liquid phase are presented, with special emphasis on the control of the coupling process between the two phases. The relaxation method employed, together with a convergence history, shows a suitable way to achieve a fast and accurate solution for the strongly coupled two-phase flow under investigation. Furthermore, methods are presented to simulate the stochastic behavior of the atomization process caused by an air-blast atomizer. In addition to the numerical methods, experimental techniques are presented that deliver detailed information about droplet starting conditions.
The Development of the Whittle Turbojet
Authors: Meher-Homji C. B.
Published: February 2008

Sir Frank Whittle passed away on August 8, 1996, at the age of 89, in Maryland. His work in developing the turbojet can truly be said to represent one of the greatest mechanical engineering achievements in the last 70 years. The development of the turbojet demanded that Whittle face almost insurmountable technical and institutional challenges. The technical challenges included developing centrifugal compressor pressure ratios of 4:1 from the prevailing technology level of 2.5:1, increasing compressor efficiencies from 65 to 80 percent while designing for combustion intensities that were 10 times the prevailing state of the art in boiler technology. He was also responsible for utilizing a vortex turbine design approach. The institutional challenges that he faced included changing a paradigm on aircraft propulsion technology and nurturing Power Jets Ltd. to produce excellent engine designs with minimal resources in terms of money, technical manpower, and governmental support. It is the object of this paper to document the epic long-drawn-out struggle fought by Sir Frank against entrenched technical opinion, which ultimately resulted in the turbojet revolution. The technical aspects of his pioneering work with emphasis on the problems he encountered will also be discussed.
Sensor-Based Analyzer for Continuous Emission Monitoring in Gas Pipeline Applications
Authors: Schubert P. F. , Sheridan D. R. , Cooper M. D. , Banchieri A. J.
Published: February 2008

Continuous emissions monitoring of gas turbine engines in pipeline service have typically been monitored using either laboratory derived instruments (CEMS), or predicted using data from low cost sensors on the engines and algorithms generated by mapping engine performance (PEMS). A new cost-effective system developed under a program sponsored by the Gas Research Institute (Chicago) combines the advantages of both systems to monitor engine emissions in gas transmission service. This hybrid system is a sensor-based analyzer that uses a sensor array, including a newly developed NOx sensor, to directly monitor NOx, CO, and O2 emissions at the stack. The gases are measured hot and wet, The new systems were installed and tested on a gas-fired Rolls Royce Spey turbine engine and on Ingersoll-Rand KVG-410 and Cooper GMVH-10 reciprocating engines in gas transmission service. These systems passed the Relative Accuracy Test (Part B) required under U.S. EPA regulations (40 CFR 60).
Direct Prediction of the Effects of Mistuning on the Forced Response of Bladed Disks
Authors: Mignolet M. P. , Hu W.
Published: February 2008

In this paper, a novel approach to determine reliable estimates of the moments of the steady-state resonant response of a randomly mistuned bladed disk is presented, and the use of these moments to accurately predict the corresponding distribution of the amplitude of blade vibration is described. The estimation of the moments of the response is accomplished first by relying on a “joint cumulant closure” strategy that expresses higher order moments in terms of lower order ones. A simple modeling of the error terms of these approximations is also suggested that allows the determination of an improved, or accelerated, estimate of the required moments. The evaluation of the distribution of the amplitude of blade response is then accomplished by matching the moments computed by the cumulant closure with those derived from a three-parameter model recently derived. A first order approximation of the moments obtained for a simple structural model of a bladed disk yields a new parameter that can be used as a measure of the localization of the forced response. Then, numerical results demonstrate that the method provides extremely accurate estimates of the moments for all levels of structural coupling which in turn lead to a description of the amplitude of blade response that closely matches simulation results. Finally, a comparison with existing perturbation techniques clearly shows the increased accuracy obtained with the proposed joint cumulant closure formulation.
Diesel Engine Smoke Reduction by Controlling Early Thermal Cracking Process and Activation Later Stage Combustion
Authors: Murayama T. , Chikahlsa T. , Fujiwara Y.
Published: February 2008

In this investigation, extensive soot reduction was attempted with two parallel approaches: blending of oxygenated fuel and generation of strong turbulence during the combustion process. In the early stage of usual diesel combustion, the droplets in the spray are thermally cracked to low boiling point components as C2 to C5 due to the shortage of oxygen, and these components result in the formation of soot. To control the thermal cracking process, the addition of oxygenated additives to the fuel was attempted. As a result, remarkable soot reduction was obtained by adding small amounts of oxygen to the fuel. To generate strong turbulence, a small chamber connected to the main combustion chamber was added and small amount of fuel were injected into the chamber, resulting in significant soot reduction. Additionally, a clearly apparent Nox reduction was obtained by the mixing control.
A Plastic Fracture Mechanics Analysis of Small Case B Fatigue Cracks Under Multiaxial Loading Conditions
Authors: Wang Y. , Pan J.
Published: February 2008

The near-tip fields of small Case B cracks in power law, hardening materials are investigated under generalized plane-strain and general yielding conditions by finite element analyses. The results for two different crack orientations are examined and compared. The results indicate that the plastic deformation patterns near the tips of the cracks of two different orientations are remarkably similar in terms of the global coordinates. The results of the J-integral from the finite element analyses are used to correlate to a fatigue crack growth criterion for Case B cracks. The trends of constant ΔJ-contours on the Γ-plane for two cracks of different orientations are virtually the same. Further, the trends are compared reasonably well with those of the experimental results of constant fatigue life and constant fatigue crack growth rate.
A Metal Matrix Composite Damage and Life Prediction Model
Authors: Ahmad J. , Santhosh U. , Hoff S.
Published: February 2008

A simple analytical model is derived for the prediction of time-dependent deformation and damage response of metal matrix composites under fiber direction loading. The model can be used in conjunction with a number of viscoplastic constitutive models to describe the matrix material behavior. Damage in the form of progressive fiber fractures is incorporated using a mechanistic approach. The criterion for fiber fractures can be based on statistical information on fiber strength. When used in conjunction with a prescribed failure condition for a composite, the model provides a means for predicting composite life under general thermomechanical load conditions. Based on comparison of results with detailed finite element analyses and with laboratory test data, it appears that the simple model provides reasonably accurate predictions.
Modeling Piston-Ring Dynamics, Blowby, and Ring-Twist Effects
Authors: Tian T. , Noordzij L. B. , Wong V. W. , Heywood J. B.
Published: February 2008

A ring-dynamics and gas-flow model has been developed to study ring/groove contact, blowby, and the influence of ring static twist, keystone ring/groove configurations, and other piston and ring parameters. The model is developed for a ring pack with three rings. The dynamics of the top two rings and the gas pressures in the regions above the oil control ring are simulated. Distributions of oil film thickness and surface roughness on the groove and ring surfaces are assumed in the model to calculate the forces generated by the ring/groove contact. Ring static and dynamic twists are considered, as well as different keystone ring/groove configurations. Ring dynamics and gas flows are coupled in the formulation and an implicit scheme is implemented, enabling the model to resolve detailed events such as ring flutter. Studies on a spark ignition engine found that static twist or, more generally speaking, the relative angle between rings and their grooves, has great influence on ring/groove contact characteristics, ring stability, and blowby. Ring flutter is found to occur for the second ring with a negative static twist under normal operating conditions and for the top ring with a negative static twist under high-speed/low-load operating conditions. Studies on a diesel engine show that different keystone ring/groove configurations result in different twist behaviors of the ring that may affect the wear pattern of the keystone ring running surfaces.
Analysis of Failure Modes Resulting in Stress Corrosion Cracking of 304N Tubing in a High Pressure Heater Desuperheater
Authors: Karg D. C. , Svensen L. M. E. , Ford A. W. , Catapano M. C.
Published: February 2008

Santee Cooper (South Carolina Public Service Authority) experienced twenty-three tube failures in a high pressure feedwater heater that was in service less than three years. The tube failures were located at baffles adjacent to both exits of the dual flow desuperheater. Metallurgical analysis of the failed tubes indicated that stress corrosion cracking of the 304N stainless steel was the primary failure mode (Rudin, 1994; Shifter, 1994). The investigation to determine the factors leading to the onset of stress corrosion cracking included analysis of heater acceptance tests, the heater manufacturer’s proposal and manufacturing procedures, operational data, eddy current reports, metallurgical reports, and a heater design review for vibration and wet wall potential (formation of condensation on the outside diameter (OD) of the tube prior to the desuperheater exit).
Testing and Modeling Gas Turbines Using Multisine Signals and Frequency-Domain Techniques
Authors: Evans C. , Borrell A. , Rees D.
Published: February 2008

The frequency-domain identification of gas turbine dynamics is discussed. Models are directly estimated from engine data and used to validate linearized thermodynamic models derived from the engine physics. This work is motivated by the problems previously encountered when using time-domain methods. A brief overview of frequency-domain techniques is presented and the design of appropriate multisine test signals is discussed. Practical results are presented for the modelling of the fuel feed to shaft speed dynamics of a twin-spool engine. The gathered data are analyzed and the frequency response functions of the engine are estimated. The identification of parametric s-domain models is discussed in detail and a comparison made between the identified models and the linearized thermodynamic models. The influence of engine nonlinearities on the linear models is also examined.
Diffusion Bonding of CMSX-4 to UDIMET 720 Using PVD-Coated Interfaces and HIP
Authors: Larker R. , Ockborn J. , Selling B.
Published: February 2008

There is an increasing interest in development of manufacturing methods for Dual Property BLISKs, consisting of creep resistant airfoils and fatigue resistant disks bonded together by a durable joint. Optimum heat treatments are, however, very different for creep resistant single crystal CMSX-4 and fatigue resistant polycrystalline Udimet 720 selected in this study, but fortunately the first aging treatment for CMSX-4 (1140°C, 2–6 h, AC) is similar to the partial solution treatment of U 720 HS2 (115°C, 4h, OQ). Based on this, diffusion bonding was performed by HIP at 1120°C and 200 MPa argon pressure for 4 h, followed by cooling to 400°C. Subsequently, a shortened Udimet 720 HS2 two-step aging treatment was adopted by heating to 650°C for 6 h followed by cooling to 400°C, heating to 760°C for 2 h, and finally cooling to R.T. under remaining HIP pressure. Plasma etching followed by thin (80 nm) PVD coating with either nickel or titanium were used to clean and protect the polished surfaces before joining. The selection of coatings was governed by the possibility to reduce oxidized nickel by flushing with hydrogen at 330°C during evacuation of the HIP capsules, and by the large solubility of oxygen in titanium. Hot tensile testing was performed at 750°C on both joined and reference materials subjected to the modified heat treatment. Initially solution treated Udimet 720 and CMSX-4 comprised the reference materials. The testing showed that joints with Ti-PVD coatings were almost as strong as Udimet 720 (although with very limited elongation), while the joints with Ti-PVD coatings were weaker.
Performance of a Low Heat Rejection Diesel Engine With Air Gap Insulated Piston
Authors: Rama Mohan K. , Vara Parasad C. M. , Murali Krishna M. V. S.
Published: February 2008

A threaded air gap insulated piston provided effective insulation without causing sealing problems. The performance of the diesel engine with the air gap insulated piston was obtained with different piston crown materials, at differing magnitudes of air gap with varying injection timings. The engine using Nimonic for the piston crown with an air gap of 3 mm at an injection timing of 29.5° bTDC reduced the BSFC by 12 percent at part loads and 4 percent at full load. The performance in terms of P-θ and T-θ was predicted employing a zero dimensional multizone combustion model, and the model results have been validated with measured pressures and the exhaust gas temperatures. More appropriate piston surface temperatures were employed in Annand’s equation to improve the computer predictions using finite element modeling of the piston. The measured temperatures of air in the air gap using an L-link mechanism provided excellent validation for the finite element prediction of isotherms in the piston.
Evaluating Alternative Internal Combustion Engines: 1950–1975
Authors: Amann C. A.
Published: February 2008

GMR (General Motors Research Laboratories, now GM R&D Center) has long sought ways to improve automotive combustion engines. Following World War II, considerable effort was also devoted to evaluating new powerplants that embodied different operating cycles from those of the established spark-ignition and diesel engines. Two internal combustion variants receiving attention during the third quarter of the 20th century were the free-piston diesel and the gas turbine. Research on those two alternatives is reviewed. Because their shortcomings were judged to outweigh their advantages, neither has found commercial application in highway vehicles.
The Creep Damage Behavior of the Plasma-Sprayed Thermal Barrier Coating System NiCr22Co12Mo9-NiCoCrAlY-ZrO2/7%Y2O3
Authors: Schmidt U. T. , Vo¨hringer O. , Lo¨he D.
Published: February 2008

During creep loading metallic substrates impose deformation on deposited ceramic thermal barrier coatings (TBC). Strain accomodation of the TBC is not attained by plastic deformation, but by means of crack initiation, crack opening, crack propagation or sliding of adjacent crack faces. In technical applications a distinction is made between tolerated or desired cracks perpendicular to the surface, and detrimental cracks parallel to the substrate-coating interface. Thus, TBC can respond to creep deformation by segmentation or spallation, the latter being referred to as failure. The parameters influencing the probability of either segmentation or spallation are temperature, creep rate, magnitude of creep deformation, layer thickness, and microstructure of the TBC. It can be stated that spallation failure probability increases with increasing creep rate, creep deformation, and layer thickness. The presence of pores between single spraying layers also strongly augments the likelyhood of spallation. No significant influence of temperature on spallation failure probability can be found in the range from 850°C to 1050°C. Light microscopy and scanning electron microscopy investigations show that the microstructure of the ceramic TBC changes during creep, and that the density of cracks detected on micrographs with low magnification (×50) increases with increasing creep deformation. On the other hand, the density of microcracks visible with high magnification (×500) is constant, or even decreases with increasing creep deformation. These findings are explained by sintering processes enabled by stress relaxation due to formation of macroscopic cracks perpendicular to the surface as a response to creep deformation. A relationship between microstructural changes and the emission of acoustic signals recorded during creep is presented.
On Nonuniqueness and Stability in Barber’s Model of Thermoelastic Contact
Authors: Olesiak Z. S. , Pyryev Yu. A.
Published: February 2008

In the recent our paper we have derived an algorithm which lets us find numerical solutions of special cases of a nonstationary problem of thermoelasticity with imperfect boundary conditions of Barber’s model. In this paper we show the results of computational simulation for a case of the thermal resistance function. We have obtained a number of solutions for different situations and have discussed the unique, nonunique, stable, and unstable solutions. We have found cases when the unique and nonunique solutions alternate. The results have been presented in the form of diagrams.
The Wrinkling of Thin, Flat, Rectangular Webs
Authors: Lin C. C. , Mote C. D.
Published: February 2008

A web is termed wrinkled when one of the in-plane principal stresses is tensile and the other is sufficiently compressive. A criterion is derived that predicts wrinkling of isotropic, compressible rectangular webs under uniform in-plane principal stresses. The compressive stress at impending wrinkling depends on the flexural stiffness, and it equals zero in the case of a membrane. A criterion of wrinkling is also derived using isotropic, incompressible membrane theory. This criterion predicts an infinite number of wrinkle waves in a wrinkled region. With small flexural stiffness, the number of wrinkle waves becomes finite at wrinkling and it is predictable along with the shape and the size of the wrinkled region. The number of the wrinkle waves increases as the aspect ratio of the rectangular web increases, as the in-plane principal tension increases, and as the flexural stiffness decreases. Analyses of wrinkling of a rectangular web under simple shear and under uniform longitudinal stretching illustrate the above predictions.
Intergranular Crack Nucleation in Bicrystalline Materials Under Fatigue
Authors: Shodja H. M. , Hirose Y. , Mura T.
Published: February 2008

During cyclic deformation of polycrystalline materials, as substantiated by many experimental observations, due to existence of high stress concentration at the interfaces the preferential site of crack nucleation is intercrystalline. Accordingly, it is assumed that the highly localized cyclic deformation persistent slip band (PSB) occurs along the grain boundary (GB) which results in intergranular crack initiation. In the present work the irreversible accumulation of dislocations are used to characterize PSB by means of double pile-up which are composed of vacancy and interstitial dipoles. We shall give the mechanism and a quantitative remedy of ratcheting of plastic deformation peculiar to fatigue deformation. In a manner conceptually analogous to Griffith theory (1921), the critical number of cycles to failure and hence the S-N curves for crack initiation is obtained by considering the free energy of the system. The Gibbs free energy change ΔG increases with the fatigue cycle number due to cyclic increment of elastic strain energy which in turn stems from cyclic pile-up of dislocations along the slip planes. The Gibbs free energy change attains its maximum value at a critical cycle number beyond which the state of dislocation dipole accumulation becomes energetically unstable. In our theory we postulate that this critical state is the onset of crack initiation. We shall give a quantitative value for the fatigue limit and analyze the dependence of the S-N curve on several important physical parameters such as grain size; surface energy; yield strength; width of the PSB; and the ratio of the shear modulus of the bicrystalline material.
Intracellular Calcium Dynamics During Photolysis
Authors: Busch N. A. , Reiken S. R. , Toner M. , Yarmush M. L.
Published: February 2008

The objective of this investigation was to gain a deeper understanding of the intracellular events that precede photolysis of cells. A model system, consisting of malignant melanoma cells pretreated with the calcium sensitive fluorescent dye, Fluo-3, was used to examine the intracellular calcium dynamics in single-cell photolysis experiments. Exposure of the cells to 632 nm laser light in the presence of photosensitizer, tin chlorin e6, resulted in a rise in intracellular calcium. The increase in intracellular calcium was blocked using a variety of calcium channel blocking agents, including verapamil, nifedipine, and nickel. Treatment with the channel blockers was also effective in either decreasing or eliminating cell death despite the presence of lethal doses of photosensitizer and irradiation. These results show that intracellular calcium rises prior to plasma membrane lysis, and that this early rise in intracellular calcium is necessary for membrane rupture.
Effect of Intraluminal Thrombus Thickness and Bulge Diameter on the Oxygen Diffusion in Abdominal Aortic Aneurysm
Authors: Vorp D. A. , Wang D. H. J. , Webster M. W. , Federspiel W. J.
Published: February 2008

The intraluminal thrombus (ILT) commonly found within abdominal aortic aneurysm (AAA) may serve as a barrier to oxygen diffusion from the lumen to the inner layers of the aortic wall. The purpose of this work was to address this hypothesis and to assess the effects of AAA bulge diameter (dAAA) and ILT thickness (δ) on the oxygen flow. A hypothetical, three-dimensional, axisymmetric model of AAA containing ILT was created for computational analysis. Commercial software was utilized to estimate the volume flow of O2 per cell, which resulted in zero oxygen tension at the AAA wall. Solutions were generated by holding one of the two parameters fixed while varying the other. The supply of O2 to the AAA wall increases slightly and linearly with dAAA for a fixed δ. This slight increase is due to the enlarged area through which diffusion of O2 may take place. The supply of O2 was found to decrease quickly with increasing δ for a fixed dAAA due to the increased resistance to O2 transport by the ILT layer. The presence of even a thin, 3 mm ILT layer causes a diminished O2 supply (less than 4 × 10−10 μmol/min/cell). Normally functioning smooth muscle cells require a supply of 21 × 10−10 μmol/min/cell. Thus, our analysis serves to support our hypothesis that the presence of ILT alters the normal pattern of O2 supply to the AAA wall. This may lead to hypoxic cell dysfunction in the AAA wall, which may further lead to wall weakening and increased potential for rupture.
Experimental Investigation of Oscillatory Flow Through a Symmetrically Bifurcating Tube
Authors: Peattie R. A. , Schwarz W.
Published: February 2008

To provide a quantitative description of the convection field of gas transport through the lung under both low and high-frequency ventilation conditions, volume-cycled, purely oscillatory flow has been investigated in a symmetrically bifurcating model bronchial bifurcation. Significant differences in the flow properties that developed as the Reynolds number varied from 750 to 950 and the dimensionless frequency varied from 3 to 12 are described. At low frequency, the axial velocity field was found to approximate closely that of a steady flow through a bifurcation. However, even at α = 3, secondary velocity fields were confined to within a few diameters of the bifurcation, with less than 10 percent of the magnitude of the axial velocity. At high frequency they were still slower and more limited. These secondary velocity observations are discussed in terms of a physical mechanism balancing inviscid centripetal acceleration with viscous retardation. As the dimensionless frequency increased but the flow amplitude decreased, the magnitude of the axial drift velocity field was found to decrease. In addition, a burst of high-frequency velocity fluctuations was detected in both the axial and secondary velocity measurements in the parent tube, in low-frequency flow, during the deceleration phase of expiration. The position and timing of this burst suggest that it derives from the free shear layer in the parent tube. Stability criteria for the flow were therefore evaluated.
Creep Contributes to the Fatigue Behavior of Bovine Trabecular Bone
Authors: Bowman S. M. , Guo X. E. , Cheng D. W. , Keaveny T. M. , Gibson L. J. , Hayes W. C. , McMahon T. A.
Published: February 2008

Repetitive, low-intensity loading from normal daily activities can generate fatigue damage in trabecular bone, a potential cause of spontaneous fractures of the hip and spine. Finite element models of trabecular bone (Guo et al., 1994) suggest that both creep and slow crack growth contribute to fatigue failure. In an effort to characterize these damage mechanisms experimentally, we conducted fatigue and creep tests on 85 waisted specimens of trabecular bone obtained from 76 bovine proximal tibiae. All applied stresses were normalized by the previously measured specimen modulus. Fatigue tests were conducted at room temperature; creep tests were conducted at 4, 15, 25, 37, 45, and 53°C in a custom-designed apparatus. The fatigue behavior was characterized by decreasing modulus and increasing hysteresis prior to failure. The hysteresis loops progressively displaced along the strain axis, indicating that creep was also involved in the fatigue process. The creep behavior was characterized by the three classical stages of decreasing, constant, and increasing creep rates. Strong and highly significant power-law relationships were found between cycles-to-failure, time-to-failure, steady-state creep rate, and the applied loads. Creep analyses of the fatigue hysteresis loops also generated strong and highly significant power law relationships for time-to-failure and steady-state creep rate. Lastly, the products of creep rate and time-to-failure were constant for both the fatigue and creep tests and were equal to the measured failure strains, suggesting that creep plays a fundamental role in the fatigue behavior of trabecular bone. Additional analysis of the fatigue strain data suggests that creep and slow crack growth are not separate processes that dominate at high and low loads, respectively, but are present throughout all stages of fatigue.
Simulation of the Superelastic Response of SMA Orthodontic Wires
Authors: Raboud D.
Published: February 2008

Shape-memory alloys have properties that make them well suited to a variety of applications. One application for which their unique combination of properties (large elastic range, low modulus of elasticity, ability to deliver nearly constant forces over a wide range of deformations) seems ideally suited is for orthodontic retraction appliances where these properties are very desirable. The mechanical response of shape-memory alloys is modeled by a simple constitutive model that captures the essential superelastic behavior of the shape-memory wires. An initial value approach that iteratively converges to the appropriate boundary conditions is utilized to deliver numerical solutions. Qualitative agreement is shown with previous experimental works. The possible benefits of using such wires in an orthodontic retraction appliance are then investigated.
Increasing Quadriceps Loads Affect the Lengths of the Ligaments and the Kinematics of the Knee
Authors: Hsieh Y.-F. , Draganich L. F.
Published: February 2008

The relationships between the lengths of the ligaments and kinematics of the knee and quadriceps load, for low to physiologic levels of quadriceps loads, have not previously been studied. We investigated the effects of increasing levels of quadriceps force, necessary to balance increasing levels of externally applied flexion moments, on the kinematics of the tibiofemoral joint and on the separation distances between insertions of selected fibers of the major ligaments of the knee in twelve cadavera. Static measurements were made using a six-degree-of-freedom digitizer for flexion angles ranging from 0 to 120 deg in 15 deg increments. Quadriceps generated extension of the knee was performed by applying loads to the quadriceps tendon to equilibrate each of four magnitudes of external flexion moments equivalent to 8.33, 16.67, 25.00, and 33.33 percent of values previously reported for maximum isometric extension moments. The magnitude of quadriceps force increased linearly (p < 0.0001) as external flexion moment increased throughout the entire range of flexion. Anterior translation, internal rotation, and abduction of the tibia increased linearly (p < 0.0001, p < 0.001, p < 0.001) as external flexion moment and, hence, quadriceps load increased. For the fibers studied, the anterior cruciate ligament (p < 0.0076), posterior cruciate ligament (p < 0.0001), and medial collateral ligament (p < 0.0383) lengthened linearly while the lateral collateral ligament (p < 0.0124) shortened linearly as quadriceps load increased. Based on these results for low to physiologic levels of quadriceps loads, it is reasonable to assume that the ligament lengths or knee kinematics expected with higher quadriceps loads can be extrapolated.
Material Characterization of Human Medial Collateral Ligament
Authors: Quapp K. M. , Weiss J. A.
Published: February 2008

The objectives of this study were to determine the longitudinal and transverse material properties of the human medial collateral ligament (MCL) and to evaluate the ability of three existing constitutive models to describe the material behavior of MCL. Uniaxial test specimens were punched from ten human cadaveric MCLs and tensile tested along and transverse to the collagen fiber direction. Using load and optical strain analysis information, the tangent modulus, tensile strength and ultimate strain were determined. The material coefficients for each constitutive model were determined using nonlinear regression. All specimens failed within the substance of the tissue. Specimens tested along the collagen fiber direction exhibited the typical nonlinear behavior reported for ligaments. This behavior was absent from the stress–strain curves of the transverse specimens. The average tensile strength, ultimate strain, and tangent modulus for the longitudinal specimens was 38.6 ± 4.8 MPa, 17.1 ± 1.5 percent, and 332.2 ± 58.3 MPa, respectively. The average tensile strength, ultimate strain, and tangent modulus for the transverse specimens was 1.7 ± 0.5 MPa, 11.7 ± 0.9 percent, and 11.0 ± 3.6 MPa, respectively. All three constitutive models described the longitudinal behavior of the ligament equally well. However, the ability of the models to describe the transverse behavior of the ligament varied.
Stress Intensity Factors for a Vertical Surface Crack in Polyethylene Subject to Rolling and Sliding Contact
Authors: Eberhardt A. W. , Kim B. S.
Published: February 2008

Pitting wear is a dominant form of polyethylene surface damage in total knee replacements, and may originate from surface cracks that propagate under repeated tribological contact. In the present study, stress intensity factors, KI, and KII, were calculated for a surface crack in a polyethylene-CoCr-bone system in the presence of rolling or sliding contact pressures. Variations in crack length and load location were studied to determine probable crack propagation mechanisms and modes. The crack tip experienced a wide range of mixed-mode conditions that varied as a function of crack length, load location, and sliding friction. Positive KI values were observed for shorter cracks in rolling contact and for all crack lengths when the sliding load moved away from the crack. KII was greatest when the load was directly adjacent to the crack (g/a = ±1), where coincidental Mode I stresses were predominantly compressive. Sliding friction substantially increased both KImax and KIImax. The effective Mode I stress intensity factors, Keff, were greatest at g/a = ±1, illustrating the significance of high shear stresses generated by loads adjacent to surface cracks. Keff trends suggest mechanisms for surface pitting by which surface cracks propagate along their original plane under repeated reciprocating rolling or sliding, and turn in the direction of sliding under unidirectional sliding contact.
Analysis of Indentation: Implications for Measuring Mechanical Properties With Atomic Force Microscopy
Authors: Costa K. D. , Yin F. C. P.
Published: February 2008

Indentation using the atomic force microscope (AFM) has potential to measure detailed micromechanical properties of soft biological samples. However, interpretation of the results is complicated by the tapered shape of the AFM probe tip, and its small size relative to the depth of indentation. Finite element models (FEMs) were used to examine effects of indentation depth, tip geometry, and material nonlinearity and heterogeneity on the finite indentation response. Widely applied infinitesimal strain models agreed with FEM results for linear elastic materials, but yielded substantial errors in the estimated properties for nonlinear elastic materials. By accounting for the indenter geometry to compute an apparent elastic modulus as a function of indentation depth, nonlinearity and heterogeneity of material properties may be identified. Furthermore, combined finite indentation and biaxial stretch may reveal the specific functional form of the constitutive law—a requirement for quantitative estimates of material constants to be extracted from AFM indentation data.
Significance of Nonsagittal Power Terms in Analysis of a Dynamic Elastic Response Prosthetic Foot
Authors: Geil M. D. , Parnianpour M. , Berme N.
Published: February 2008

Dynamic elastic response prosthetic feet generally utilize a solid ankle, limiting dominant motion to the sagittal plane. However, researchers often use total rotational ankle joint power in the analysis of these feet. This investigation measured joint power terms in each plane for the Carbon Copy High Performance prosthetic foot. The significance of the frontal and transverse plane terms was assessed. Addition of these terms to the dominant sagittal power term revealed only slight differences, indicating that the sagittal power term is likely sufficient.