Stress Waveform Research Articles

Experimental measurement of dynamic fluid shear stress on the aortic surface of the aortic valve leaflet

Aortic valve (AV) calcification is a highly prevalent disease with serious impact on mortality and morbidity. Although exact causes and mechanisms of AV calcification are unclear, previous studies suggest that mechanical forces play a role. Since calcium deposits occur almost exclusively on the aortic surfaces of AV leaflets, it has been hypothesized that adverse patterns of fluid shear stress on the aortic surface of AV leaflets promote calcification. The current study characterizes AV leaflet aortic surface fluid shear stresses using Laser Doppler velocimetry and an in vitro pulsatile flow loop. The valve model used was a native porcine valve mounted on a suturing ring and preserved using 0.15% glutaraldehyde solution. This valve model was inserted in a mounting chamber with sinus geometries, which is made of clear acrylic to provide optical access for measurements. To understand the effects of hemodynamics on fluid shear stress, shear stress was measured across a range of conditions: varying stroke volumes at the same heart rate and varying heart rates at the same stroke volume. Systolic shear stress magnitude was found to be much higher than diastolic shear stress magnitude due to the stronger flow in the sinuses during systole, reaching up to 20 dyn/cm(2) at mid-systole. Upon increasing stroke volume, fluid shear stresses increased due to stronger sinus fluid motion. Upon increasing heart rate, fluid shear stresses decreased due to reduced systolic duration that restricted the formation of strong sinus flow. Significant changes in the shear stress waveform were observed at 90 beats/min, most likely due to altered leaflet dynamics at this higher heart rate. Overall, this study represents the most well-resolved shear stress measurements to date across a range of conditions on the aortic side of the AV. The data presented can be used for further investigation to understand AV biological response to shear stresses.

Strain stiffening of non-colloidal hard sphere suspensions dispersed in Newtonian fluid near liquid-and-crystal coexistence region

Concentrated hard sphere suspensions often show an interesting nonlinear behavior, called strain stiffening, in which the viscosity or modulus starts to increase at critical strain amplitude. Sudden increase of rheological properties is similar to shear thickening; however, the particle dynamics in the strain stiffening under oscillatory shear flow does not necessarily coincide with the mechanism of shear thickening under step shear flow. In this study, we have systematically investigated the nonlinear rheology of non-colloidal (>1 μm) hard sphere suspensions dispersed in Newtonian fluid near liquid-and-crystal coexistence region in order to better understand the strain stiffening behavior. The suspensions near liquid-and-crystal coexistence region are known to locally form the closed packing structure. The critical strain amplitude which is the onset of strain stiffening was different for the storage and loss modulus. But they converged to each other as the suspension forms a more crystalline structure. The critical strain amplitude was independent of medium viscosity, imposed angular frequency, and particle size, but was strongly dependent upon particle volume fraction. The onset of strain stiffening was explained in terms of shear-induced collision due to particle motion in the closed packing structure. Nonlinear stress wave-forms, which reflect the micro-structural change, were observed with the onset of strain stiffening. During the strain stiffening, enhanced elastic stress before and after flow reversal was observed which originates from changes in the suspension microstructure. Nonlinearity of the shear stress in terms of Fourier intensity was extremely increased up to 0.55. Beyond the strain stiffening, the suspension responded liquid-like and the nonlinearity decreased but the elastic shear stress was still indicating the microstructure rearrangement within a cycle.

Application of stochastic system identification to the study of the compliance of electroactive polymers

Electroactive polymers have shown promising applications as transducers that can mimic biological muscle. The modulus or the compliance of many of these devices can change significantly as they are actuated making these materials attractive for applications that require tunable stiffness. We have developed a dynamic mechanical analyzer that is capable of making in situ measurements of the dynamic compliance transfer function of conducting polymers as a function of an electrochemical stimulus. We do this by simultaneously applying a stochastic stress waveform over a potential waveform and calculating the compliance as it changes over the course of electrochemical excitation. Using these signals we can calculate the compliance transfer function between 0.1 and 100 Hz and the impulse response function with up to 3% variation in its parameters. These functions are then computed as charge is injected into the polymer and it is shown that the low frequency gain of the transfer function can change by 30%-40% in the electrochemical system tested.

OSO204 鉛フリーはんだにおけるクリープ疲労き裂進展のEBSDによる研究(放射光およびEBSD法による材料評価,OS-2 回折法による材料評価)

Crack propagation tests of lead-free solder were conducted using center-notched plates under cyclic tension-compression of three stress waveforms: pp waveform having fast loading and unloading, cch waveform with tension and compression holds, cph waveform having tension hold. Fatigue and creep components of the J integral were evaluated from the load displacements relations using simple estimates. For fatigue loading of pp waveform, the crack propagation rate was expressed as a power function of the fatigue J integral. The creep component due to the hold time greatly accelerated the crack propagation rate. The creep crack propagation rate was expressed as a power function of the creep J integral for each case of cph and cch waveforms. The crack propagation rate for cph waveform was higher than that for cch waveform. SEM observation after the removal of the surface layer showed many tiny cracks in the vicinity of the main crack for cph waveform, suggesting crack propagation acceleration. The grain fragmentation was observed near the crack by EBSD and its amount increased with increasing crack propagation rate.

G030063 水素封入試験片の疲労条件下におけるき裂成長挙動の負荷波形特性([G03006]材料力学部門一般セッション(6):高温・環境)

G030063 水素封入試験片の疲労条件下におけるき裂成長挙動の負荷波形特性([G03006]材料力学部門一般セッション(6):高温・環境)

Identification of acoustic emission wave modes for accurate source location in plate-like structures

Acoustic emission (AE) technique is a popular tool used for structural health monitoring of civil, mechanical and aerospace structures. It is a non-destructive method based on rapid release of energy within a material by crack initiation or growth in the form of stress waves. Recording of these waves by means of sensors and subsequent analysis of the recorded signals convey information about the nature of the source. Ability to locate the source of stress waves is an important advantage of AE technique; but as AE waves travel in various modes and may undergo mode conversions, understanding of the modes (‘modal analysis’) is often necessary in order to determine source location accurately. This paper presents results of experiments aimed at finding locations of artificial AE sources on a thin plate and identifying wave modes in the recorded signal waveforms. Different source locating techniques will be investigated and importance of wave mode identification will be explored. Copyright © 2010 John Wiley & Sons, Ltd.

Alternative method for electrostatic discharge stress testing of individual system components

Electrostatic discharge (ESD) testing of integrated circuits (ICs) is necessary to ensure that the products can withstand several types of ESD threats, including those encountered in the factory and in the field. This study discusses the testing of components with electrostatic stress waveforms that were originally designed for predicting system failures in the field. IC manufacturers are struggling to obtain reliable data when applying system tests to their components due to interface and repeatability problems found with this form of evaluation. To alleviate these problems and some of the confusion, a new test methodology has been designed and implemented. It is used for testing individual components that have pins that are to be connected to the external ports on a completed design. This study reports on a new method that solves these IC testing problems and compares it to the current approach most often employed.

Study of acoustic wave propagation through the cross section of green wood

An experimental approach was used to model stress wave propagation in green wood (Douglas fir). Based on the assumption that wood is an orthotropic material, the stress wave velocity through the cross section was calculated using plane strain motion equations. The experiments were carried out in two steps under axial restraint, while the wave propagation time was measured on discs and bars sliced from the discs. Mechanical and physical properties were determined in free vibration. The results showed a significant difference in propagation velocity between waves propagating throughout the whole disc volume and guided waves in bars. The acoustic anisotropy of green wood is discussed and the stress wave form simulation is presented. Good agreement between the simulation and experimental results was obtained.

Hemodynamic Environments from Opposing Sides of Human Aortic Valve Leaflets Evoke Distinct Endothelial Phenotypes In Vitro

The regulation of valvular endothelial phenotypes by the hemodynamic environments of the human aortic valve is poorly understood. The nodular lesions of calcific aortic stenosis (CAS) develop predominantly beneath the aortic surface of the valve leaflets in the valvular fibrosa layer. However, the mechanisms of this regional localization remain poorly characterized. In this study, we combine numerical simulation with in vitro experimentation to investigate the hypothesis that the previously documented differences between valve endothelial phenotypes are linked to distinct hemodynamic environments characteristic of these individual anatomical locations. A finite-element model of the aortic valve was created, describing the dynamic motion of the valve cusps and blood in the valve throughout the cardiac cycle. A fluid mesh with high resolution on the fluid boundary was used to allow accurate computation of the wall shear stresses. This model was used to compute two distinct shear stress waveforms, one for the ventricular surface and one for the aortic surface. These waveforms were then applied experimentally to cultured human endothelial cells and the expression of several pathophysiological relevant genes was assessed. Compared to endothelial cells subjected to shear stress waveforms representative of the aortic face, the endothelial cells subjected to the ventricular waveform showed significantly increased expression of the “atheroprotective” transcription factor Kruppel-like factor 2 (KLF2) and the matricellular protein Nephroblastoma overexpressed (NOV), and suppressed expression of chemokine Monocyte-chemotactic protein-1 (MCP-1). Our observations suggest that the difference in shear stress waveforms between the two sides of the aortic valve leaflet may contribute to the documented differential side-specific gene expression, and may be relevant for the development and progression of CAS and the potential role of endothelial mechanotransduction in this disease.

Defining the regulation of KLF4 expression and its downstream transcriptional targets in vascular endothelial cells

The Kruppel-like factor 2 (KLF2) and Kruppel-like factor 4 (KLF4) transcription factors have recently been shown to act as critical regulators of endothelial homeostasis. While several insights have been made into the signaling mechanisms orchestrating endothelial KLF2 expression, those governing the expression of KLF4 in the vascular endothelium remain largely unknown. Here, we show that diverse vasoprotective stimuli including an atheroprotective shear stress waveform, simvastatin, and resveratrol induce the expression of KLF4 in cultured human endothelial cells. We further demonstrate that the induction of KLF4 by resveratrol and atheroprotective shear stress occurs via a MEK5/MEF2-dependent signaling pathway. Since MEK5 activation is also critical for the expression of KLF2, we assessed the individual contribution of KLF4 and KLF2 to the global transcriptional activity triggered by MEK5 activation. Genome-wide transcriptional profiling of endothelial cells overexpressing KLF4, KLF2, or constitutively active MEK5 revealed that 59.2% of the genes regulated by the activation of MEK5 were similarly controlled by either KLF2 or KLF4. Collectively, our data identify a significant degree of mechanistic and functional conservation between KLF2 and KLF4, and importantly, provide further insights into the complex regulatory networks governing endothelial vasoprotection.

Quantifying the effects of shear stress and shear exposure duration regulation on flow induced platelet activation and aggregation

A heightened flow shear stress magnitude or increased exposure duration at any shear stress magnitude enhances platelet activation. It has been suggested that increased circulating activated platelets are an indication of enhanced cardiovascular risk and that activated platelets can perpetuate cardiovascular diseases. However, the combined effects that shear stress and shear exposure duration have on platelet activation and aggregation are not clear. Our objective was to identify a new parameter ("shear stress-exposure time") to predict platelet functional changes better than the absolute magnitude of shear stress. Platelets were exposed to shear stress waveforms, with different combinations of shear stress magnitude and exposure duration, in a circulation flow loop. Timed samples were removed from the flow loop to quantify thrombogenicity (modified prothrombinase assay), aggregation potential (optical platelet aggregometry), and cell surface marker expression (flow cytometry). Flow induced platelet activation and aggregation was enhanced with increased shear stress-exposure time. Platelets that were exposed to waveforms with the same shear stress-exposure time had the same thrombogenicity and aggregation potential. This was true even for waveforms that exposed platelets to shear stress magnitudes varying from 4 to 40 dynes/cm(2). These results indicate that shear stress-exposure time is a good predictor of platelet activation and aggregation levels and it may be able to predict the likelihood of cardiovascular disease onset.

Channel hot-carrier degradation under AC stress in short channel nMOS devices with high-k gate stacks

The channel hot-carrier (CHC) degradation in nMOS transistors with high-k dielectric subjected to different AC stress configurations was studied. The influence of the gate AC stress waveform parameters on the CHC degradation was analysed, when a constant drain voltage was applied. For short channel devices, CHC degradation has been found to be dependent on duty-cycle and rise/fall times and independent of frequency. These results have been observed independently of the gate dielectric material (SiO 2 or high-k). Moreover, the CHC damage in a nMOS transistor subjected to stress conditions which simulate its operation within a CMOS inverter has been also analysed. Lower degradation is observed in this case.

Assessment of low-temperature embrittlement of asphalt binders using an acoustic emission approach

Thin films of paving grade asphalt binders moulded on granite substrates were stressed under rapid cooling and thermal restraint conditions at temperatures ranging from 15°C to -50°C. This temperature range encompasses typical pavement temperatures in colder climates. Differential thermal contraction between the granite substrates and asphalt binders induces progressively higher thermal stress levels in the binders as the temperature drops resulting in thermal crack formation, and the corresponding release of elastic energy in the form of transient stress waves. Using piezoelectric sensors (Digital Wave, Model B-1025), a four-channel acoustic emission (AE) system was used to record the acoustic emission activity during the binder/granite cooling process. It was observed that AE events of high energy clustered around a narrow temperature range. Assuming the cracking temperature (T CT ) to be the temperature at which the AE signal energy exceeds a pre-selected threshold energy level, this AE testing approach was found to be sensitive and repeatable for predicting cracking temperatures (T CT ) in four paving grade asphalt binders processed to two distinct ageing levels. The effect of binder type, ageing level and binder film thickness on cracking temperature is discussed. These AE-based T CT predictions showed strong correlations with predictions based on either the AASHTO TP1 or the MPIA protocols.

Biomechanical Regulation of Endothelium-dependent Events Critical for Adaptive Remodeling

Alterations in hemodynamic shear stress acting on the vascular endothelium are critical for adaptive arterial remodeling. The molecular mechanisms regulating this process, however, remain largely uncharacterized. Here, we sought to define the responses evoked in endothelial cells exposed to shear stress waveforms characteristic of coronary collateral vessels and the subsequent paracrine effects on smooth muscle cells. A lumped parameter model of the human coronary collateral circulation was used to simulate normal and adaptive remodeling coronary collateral shear stress waveforms. These waveforms were then applied to cultured human endothelial cells (EC), and the resulting differences in EC gene expression were assessed by genome-wide transcriptional profiling to identify genes distinctly regulated by collateral flow. Analysis of these transcriptional programs identified several genes to be differentially regulated by collateral flow, including genes important for endothelium-smooth muscle interactions. In particular, the transcription factor KLF2 was up-regulated by the adaptive remodeling coronary collateral waveform, and several of its downstream targets displayed the expected modulation, including the down-regulation of connective tissue growth factor. To assess the effect of endothelial KLF2 expression on smooth muscle cell migration, a three-dimensional microfluidic assay was developed. Using this three-dimensional system, we showed that KLF2-expressing EC co-cultured with SMC significantly reduce SMC migration compared with control EC and that this reduction can be rescued by the addition of exogenous connective tissue growth factor. Collectively, these results demonstrate that collateral flow evokes distinct EC gene expression profiles and functional phenotypes that subsequently influence vascular events important for adaptive remodeling.

Dynamic Stress Analysis of the Arterial Wall Utilizing Physiological Pressure Waveforms

Arterial diseases, promoted by alterations in arterial wall properties, are among the main causes of mortality. Mechanical stresses of the arterial wall caused by pulsatile luminal pressure define arterial function in normal and pathological conditions. This study aims to determine dynamic stress distribution in the arterial wall subjected to physiological pressure waveforms. Finite element models of a typical artery are developed to evaluate Von Misses stress in the arterial wall due to physiological pressure waveforms and with differing mechanical properties. Mechanical parameters include Young's modulus of elasticity, non-linear stress-strain relationship and visco-elastic parameter. The appropriate boundary conditions are allocated to allow radial expansion. Application of physiological pulsatile pressure results in stress waves with the values and waveforms markedly influenced by mechanical properties of the arterial wall and blood pressure pulse. Elevated elastic modulus of the arterial wall results in significant increase in maximum stress. Viscoelastic property leads to reduction of the peak stress and smoothening of the stress waveform. The pressure waveform is also a major parameter affecting the stress pattern in arterial wall. Hypertensive arteries result in higher and sharper stress waves not only because of a higher systolic value but also because of the sharper waveform. The combination of these parameters produces the resultant stress pattern in the arterial wall.

Corrosion Fatigue Crack Growth Rate for Petroleum Refining Pressure Vessel Materials (2.25Cr-1Mo Steel)

Abstract The 2.25Cr-1Mo steel is used for pressure vessel components and has sometimes suffered from hydrogen embrittlement. Therefore, it is important to investigate the effect of hydrogen on the embrittlement for 2.25Cr-1Mo steel. Under corrosion fatigue conditions, the effect of hydrogen embrittlement (HE) on corrosion fatigue crack growth rate (CFCGR) appears in different ways depending on the applied stress wave forms. This behavior has been clarified by previous theoretical and experimental analyses. In this paper, based on the cited theoretical analyses, the effect of stress waveform on CFCGR for 2.25Cr-1Mo steel was investigated and the sensitivity to hydrogen embrittlement is examined for this material.

GRP78 Upregulation by Atheroprone Shear Stress Via p38-, α2β1-Dependent Mechanism in Endothelial Cells

The initiation of atherosclerosis is in part dependent on the hemodynamic shear stress environment promoting a proinflammatory phenotype of the endothelium. Previous studies demonstrated increased expression of ER stress protein and unfolded protein response (UPR) regulator, GRP78, within all vascular cells in atherosclerotic lesions and its regulation in the endothelium by several atherosclerotic stressors; however, regulation of GRP78 by shear stress directly has not been established. Using an in vitro model to simulate human arterial shear stress waveforms, atheroprone or atheroprotective flow was applied to human endothelial cells. GRP78 was found to be significantly upregulated (3-fold) in a sustained manner under atheroprone, but not atheroprotective flow up to 24 hours. This response was dependent on both sustained activation of p38, as well integrin alpha2beta1. Increased GRP78 correlated with the activation of the ER stress sensing element (ERSE1) promoter by atheroprone flow as a marker of the UPR. Shear stress regulated GRP78 through increased protein stability when compared to other flow regulated proteins, such as connexin-43 and vascular cell adhesion molecule (VCAM)-1. Increased endothelial expression of GRP78 was also observed in atheroprone versus atheroprotective regions of C57BL6 mice. This study supports a role of the hemodynamic environment in preferentially inducing GRP78 and the UPR in atheroprone regions, before lesion development, and suggests a potential atheroprotective (ie, prosurvival), compensatory effect in response to ER stress within atherosclerotic lesions.

Active-Clamping Single-Stage AC-DC Converter with the Input Current Shaper Charged in the Duty-Off Time

SummarySingle-stage a.c.-d.c. converters employing boost-type input current shapers (ICS) usually suffer from relatively high switch current stress and line current waveform distortion caused by the voltage-follower control. To remedy these drawbacks, this paper presents a novel single-stage soft-switching a.c.-d.c. converter. Unlike the conventional single-stage designs, the proposed ICS scheme is intentionally arranged to be charged in the duty-off time. With this design, the switch current stress in the duty-on time is greatly reduced. Moreover, this design produces a.c. modulation effect on the charging time of the ICS so that the waveforms of the average charging current and discharging current of the boost inductor can compensate each other automatically. Consequently, the input i-v curve has nearly linear relationship. In the proposed topology, an auxiliary transformer winding is employed to achieve the above functions. By programming the turns ratio between the auxiliary and secondary windings, the proposed converter can be guaranteed to comply with the IEC 1000-3-2 Class D specifications at as high efficiency as possible. Experimental results, obtained from a prototype circuit with universal line input and 48 V/100 W output, are given to confirm the predicted performance.

Design of an Ex Vivo Culture System to Investigate the Effects of Shear Stress on Cardiovascular Tissue

Mechanical forces are known to affect the biomechanical properties of native and engineered cardiovascular tissue. In particular, shear stress that results from the relative motion of heart valve leaflets with respect to the blood flow is one important component of their mechanical environment in vivo. Although different types of bioreactors have been designed to subject cells to shear stress, devices to expose biological tissue are few. In an effort to address this issue, the aim of this study was to design an ex vivo tissue culture system to characterize the biological response of heart valve leaflets subjected to a well-defined steady or time-varying shear stress environment. The novel apparatus was designed based on a cone-and-plate viscometer. The device characteristics were defined to limit the secondary flow effects inherent to this particular geometry. The determination of the operating conditions producing the desired shear stress profile was streamlined using a computational fluid dynamic (CFD) model validated with laser Doppler velocimetry. The novel ex vivo tissue culture system was validated in terms of its capability to reproduce a desired cone rotation and to maintain sterile conditions. The CFD results demonstrated that a cone angle of 0.5 deg, a cone radius of 40 mm, and a gap of 0.2 mm between the cone apex and the plate could limit radial secondary flow effects. The novel cone-and-plate permits to expose nine tissue specimens to an identical shear stress waveform. The whole setup is capable of accommodating four cone-and-plate systems, thus concomitantly subjecting 36 tissue samples to desired shear stress condition. The innovative design enables the tissue specimens to be flush mounted in the plate in order to limit flow perturbations caused by the tissue thickness. The device is capable of producing shear stress rates of up to 650 dyn cm(-2) s(-1) (i.e., maximum shear stress rate experienced by the ventricular surface of an aortic valve leaflet) and was shown to maintain tissue under sterile conditions for 120 h. The novel ex vivo tissue culture system constitutes a valuable tool toward elucidating heart valve mechanobiology. Ultimately, this knowledge will permit the production of functional tissue engineered heart valves, and a better understanding of heart valve biology and disease progression.

Platelet Activation Due to Hemodynamic Shear Stresses: Damage Accumulation Model and Comparison to In Vitro Measurements

The need to optimize the thrombogenic performance of blood recirculating cardiovascular devices, e.g., prosthetic heart valves (PHV) and ventricular assist devices (VAD), is accentuated by the fact that most of them require lifelong anticoagulation therapy that does not eliminate the risk of thromboembolic complications. The formation of thromboemboli in the flow field of these devices is potentiated by contact with foreign surfaces and regional flow phenomena that stimulate blood clotting, especially platelets. With the lack of appropriate methodology, device manufacturers do not specifically optimize for thrombogenic performance. Such optimization can be facilitated by formulating a robust numerical methodology with predictive capabilities of flow-induced platelet activation. In this study, a phenomenological model for platelet cumulative damage, identified by means of genetic algorithms (GAs), was correlated with in vitro experiments conducted in a Hemodynamic Shearing Device (HSD). Platelets were uniformly exposed to flow shear representing the lower end of the stress levels encountered in devices, and platelet activity state (PAS) was measured in response to six dynamic shear stress waveforms representing repeated passages through a device, and correlated to the predictions of the damage accumulation model. Experimental results demonstrated an increase in PAS with a decrease in "relaxation" time between pulses. The model predictions were in very good agreement with the experimental results.