Shear Stress History Research Articles

Evaluation of the accuracy and efficiency of the modified maximum variance method for multiaxial fatigue analysis under constant amplitude loading

An optimized version of the Maximum Variance Method (MVM) is employed to determine maximum shear stress amplitudes in high-cycle multiaxial fatigue analysis using the critical plane approach, increasing accuracy and reducing computational burden. This refined approach posits that the MVM assumes that the critical plane is the one subjected to the maximum variance of the shear stress history. Comprehensive statistical analyses, including Analysis of Variance (ANOVA) and multiple comparison techniques such as Tukey’s HSD and Bonferroni correction, were conducted to systematically investigate the impact of critical factors on fatigue resistance, such as mean stress, phase, synchrony, failure criteria, and the strategy for calculating maximum shear stress amplitude. These analyses highlighted significant effects of mean stress and phase on the accuracy of fatigue strength predictions, underscoring the effectiveness of the enhanced MVM in managing complex loading scenarios. The results demonstrate that the proposed methodology performs equal to or better than conventional methods, with significantly lower computational costs.

A numerical approach for simulation of stress-controlled cyclic shear loading in a soil column considering partial drainage

Liquefaction strength curves (LSCs) are commonly generated based on undrained stress-controlled cyclic testing to define the liquefaction potential of sands. However, these LSCs do not consider the effect of partial drainage on liquefaction potential. This is mainly because there are very limited experimental techniques to run stress-controlled tests for partially drained sands. This manuscript proposes a procedure to numerically simulate a saturated sand column with a free drainage boundary at the top (or top and bottom), accelerated horizontally at the bottom with constant acceleration cycles of duration designed to generate constant cyclic shear stress histories in the column. This is done for columns subjected to low and high overburden pressures. The paper starts with numerical simulations of a typical undrained cyclic direct simple shear, CDSS tests by applying a stepped velocity wave to a single soil element, an already established approach. This approach only works for a single element in undrained condition which is fixed at the base. Then, the proposed procedure is implemented by applying stepped acceleration time histories at the base of soil columns having one or two drainage boundaries. In these stepped acceleration runs, the durations of the acceleration cycles are controlled to achieve a partially drained stress-controlled CDSS kind-of-loading at specific elevations within the soil columns. The technique is used to show the effect of partial drainage on sand liquefaction behavior at both low and high overburden pressures. The results show that the effect of drainage is much more significant at higher overburden. The existence of both top and bottom drainage boundaries resulted in less liquefaction vulnerability, as compared to having only one drainage boundary at the top of the sand column. The results show that liquefiable sand layers in the field may be less prone to liquefaction under high overburden pressure than predicted by the current state-of-practice, which relies mainly on undrained small-scale cyclic tests.

P84: The impact of temporal synchronization on platelet activation in LVAD patients: CFD and PIV study for the HeartMate3 pulsatility mode.

Study: LVAD patients continue to experience high rates of stroke. Stasis and high shear stresses contribute to platelet activation in the left ventricle, that can lead to thrombus formation. A pulsatility mode was incorporated into the HeartMate3 to improve in-pump washout. The pump speed modulation occurs every two seconds, independently of the cardiac cycle even in patients with preserved native contractility. This study examines intraventricular washout under different LVAD pulsatility/native cardiac cycle synchronization. Methods: A patient-specific 3D time-resolved computational fluid dynamics (CFD) simulation captures the hemodynamics in the left ventricle implanted with an LVAD. Lagrangian particle tracking models platelet motion from the mitral valve, across the left ventricle and out through the outflow cannula. The residence time and shear stress history for each platelet is tracked to determine its risk of activation. Four different temporal synchronizations between the LVAD pulsatility and the native contractility are studied: contraction, peak contraction, relaxation, and peak relaxation. The flow rate through the cannula is prescribed from flow meter data in the experimental campaign. Particle image velocimetry (PIV) experiments measured the in-plane and through plane velocity fields at two orthogonal planes in the ventricle. This PIV data is compared to the CFD to benchmark the simulations. Results: The different pulsatility mode synchronizations result in significantly different time-dependent flow rates through the cannula. The difference between these flow rates results in different hemodynamics patterns in the ventricle. The velocity fields from the CFD models agree closely with the results from the PIV experiments, and take these in a new direction with the use of Lagrangian platelet tracking. Recirculation zones develop underneath the aortic valve and around the cannula/apex pocket, resulting in areas of stasis for the platelets. By identifying platelets that experience both high shear stress history and long residence time, the thrombogenicity of the different synchronizations is evaluated.

Modeling the Mechanical Microenvironment of Coiled Cerebral Aneurysms.

Successful occlusion of cerebral aneurysms using coil embolization is contingent upon stable thrombus formation, and the quality of the thrombus depends upon the biomechanical environment. The goal of this study was to investigate how coil embolization alters the mechanical micro-environment within the aneurysm dome. Inertialess particles were injected in three-dimensional, computational simulations of flow inside patient aneurysms using patient-specific boundary conditions. Coil embolization was simulated as a homogenous porous medium of known permeability and inertial constant. Lagrangian particle tracking was used to calculate the residence time and shear stress history for particles in the flow before and after treatment. The percentage of particles entering the aneurysm dome correlated with the neck surface area before and after treatment (pretreatment: R2 = 0.831, P < 0.001; post-treatment: R2 = 0.638, P < 0.001). There was an inverse relationship between the change in particles entering the dome and coil packing density (R2 = 0.600, P < 0.001). Following treatment, the particles with the longest residence times tended to remain within the dome even longer while accumulating lower shear stress. A significant correlation was observed between the treatment effect on residence time and the ratio of the neck surface area to porosity (R2 = 0.390, P = 0.007). The results of this study suggest that coil embolization triggers clot formation within the aneurysm dome via a low shear stress-mediated pathway. This hypothesis links independently observed findings from several benchtop and clinical studies, furthering our understanding of this treatment strategy.

Study on fractional-order elastic-plastic constitutive model of river silt based on critical state theory

The conventional constitutive models for mairne silts are usually built on different loading surfaces and plastic potential surfaces. In this paper, based on the concept of critical state, combined with the modified Cam-clay model and fractional-order calculus algorithm, the fractional-order elastic-plasticity constitutive model of the marine silt is studied and proposed. The basic characteristics of the model, such as the shear shrinkage, friction and stress history dependence of the marine silt, are comprehensively considered, and the parameters are easy to obtain. The experimental results and predicted results of different initial void ratios under undrained conditions are compared. The results show that the constitutive model can well simulate the strain hardening of the marine silt with different initial void ratios under undrained conditions. The predicted results of the model under different initial confining pressures are not consistent with the experimental results under fixed λ value. By changing λ, the stress-strain relationship of the marine silt can be effectively simulated. At the same time, the maximum deviator stress increases with the increase of the initial confining pressure, and the phenomenon that increases with the decrease of the initial void ratio can be predicted by this model.

Geotechnical characterization of the estuarine deltaic deposits in the Guayaquil city through in situ and laboratory tests

According to previously available research and seismic microzonation studies a large area of the Guayaquil (Ecuador, South America) sits on estuarine deltaic deposits which consist of weak and highly compressible clays with diatoms. The nature of these fine-grained deposits may determine difficulties in a proper estimation of the soil properties. In this respect, the paper provides a detailed geotechnical and geophysical characterization of these soft clays, carried out in the estuarine complex of the Ecuadorean city. Borehole logs, standard penetration tests (SPT), piezocone tests (CPTu), a seismic dilatometer test (SDMT), a non-invasive geophysical survey, and laboratory tests were performed and then compared to analyze the static and dynamic geotechnical parameters of these deposits. The interpretation of the results highlighted the higher reliability of CPTu and SDMT rather than SPT and characterization lab testing to estimate soil shear strength, compressibility and stress history due to the soft nature of these clays, underlining also a certain sensitivity to the presence of the diatoms.

Parameters of Sand-Tyre Chips Mixture for Hardening Soil Small Constitutive Model

Hardening Soil model with the small strain extension (HSS) is lately one of the most popular constitutive models to describe soil behaviour. It is versatile – includes the phenomena of shear strength, stress history, dilatancy, volumetric and shear hardening, hyperbolic stress-strain relationship in axial compression, stiffness dependency on stress and its degradation with strain, as well as the regain of the high stiffness after sharp loading reversals. Even though the model is advanced and complex, accordingly to its authors, it is relatively easy to calibrate based on results of standard tests and empirical formulas. In this paper an attempt was undertaken to estimate the parameters of untypical anthropogenic soils – mixtures of sand and scrap tyre rubber in order to build a database for future numerical analyses. A literature review was conducted and, eventually, the material parameters were determined based on results of a series of laboratory tests (cyclic and monotonic triaxial with bender elements, direct shear) published by researchers of Wollongong University of Australia.

Effects of the atrium on intraventricular flow patterns during mechanical circulatory support.

Simulations of the ventricular flow patterns during left ventricular assist device (LVAD) support are mainly performed with idealized cylindrical inflow, neglecting the influence of the atrial vortex. In this study, the influence of the left atrium (LA) on the intra-ventricular flow was investigated via Computational Fluid Dynamics (CFD) simulations. Ventricular flow was simulated by a combined Eulerian (carrier flow)/Lagrangian (particles) approach taking into account either the LA or a cylindrical inflow section to mimic a fully support condition. The flow deviation at the mitral valve, the blood low-velocity volume as well as the residence time and shear stress history of the particles were calculated. Inclusion of the LA deflects the flow at the mitral valve by 25°, resulting in an asymmetric flow jet entering the left ventricle. This reduced the ventricular low-velocity volume by 40% (from 6.4 to 3.9 cm3), increased (40%) the shear stress experienced by particles and correspondingly increased (27%) their residence time. Under the studied conditions, the atrial geometry plays a major role in the development of intraventricular flow patterns. A reliable prediction of blood flow dynamics and consequently thrombosis risk analysis within the ventricle requires the consideration of the LA in computational simulations.

Platelet adhesion potential estimation in a normal and diseased coronary artery model: effects of shear stress magnitude versus shear stress history

Platelets play a salient role in the pathogenesis of coronary diseases; primarily through their adhesion to other platelets, endothelial cells and plasma proteins. It is necessary for platelets to activate in order for them to adhere to these different substrates. One of the key regulatory mechanical factors in platelet activation is shear stress, which has been shown to alter multiple platelet functions through the activation of mechanoreceptors. Our goal was to investigate how different numerical shear stress tracking techniques affect platelet adhesion estimates within physiologically relevant computational models. Previously, we developed a physiological coronary artery computational fluid dynamics model. Shear stress waveforms, obtained from these models, were used to monitor in vitro platelet and endothelial cell adhesion marker expression. In this work, the adhesion marker expression data was regressed to obtain numerical functions for receptor expression predictions. These functions were input into a customized adhesion model utilizing different shear stress tracking techniques. For the normal vascular conditions and minimal pathological disease models, shear stress tracking did not significantly affect the adhesion estimates. However, for the severe pathological model, the two shear stress tracking methods had vastly different estimates. Therefore, shear stress tracking methods must be chosen accurately to predict platelet adhesion potentials for accurate modeling techniques.

Lagrangian Trajectory Simulation of Platelets and Synchrotron Microtomography Augment Hemodynamic Analysis of Intracranial Aneurysms Treated With Embolic Coils.

As frequency of endovascular treatments for intracranial aneurysms increases, there is a growing need to understand the mechanisms for coil embolization failure. Computational fluid dynamics (CFD) modeling often simplifies modeling the endovascular coils as a homogeneous porous medium (PM), and focuses on the vascular wall endothelium, not considering the biomechanical environment of platelets. These assumptions limit the accuracy of computations for treatment predictions. We present a rigorous analysis using X-ray microtomographic imaging of the coils and a combination of Lagrangian (platelet) and Eulerian (endothelium) metrics. Four patient-specific, anatomically accurate in vitro flow phantoms of aneurysms are treated with the same patient-specific endovascular coils. Synchrotron tomography scans of the coil mass morphology are obtained. Aneurysmal hemodynamics are computationally simulated before and after coiling, using patient-specific velocity/pressure measurements. For each patient, we analyze the trajectories of thousands of platelets during several cardiac cycles, and calculate residence times (RTs) and shear exposure, relevant to thrombus formation. We quantify the inconsistencies of the PM approach, comparing them with coil-resolved (CR) simulations, showing the under- or overestimation of key hemodynamic metrics used to predict treatment outcomes. We fully characterize aneurysmal hemodynamics with converged statistics of platelet RT and shear stress history (SH), to augment the traditional wall shear stress (WSS) on the vascular endothelium. Incorporating microtomographic scans of coil morphology into hemodynamic analysis of coiled intracranial aneurysms, and augmenting traditional analysis with Lagrangian platelet metrics improves CFD predictions, and raises the potential for understanding and clinical translation of computational hemodynamics for intracranial aneurysm treatment outcomes.

The effects of unloading on undrained deformation of a kaolin clay under cyclic loading

Excavation induced unloading may induce additional shear stress in soils. Such additional shear stress or unloading history may affect the long-term deformation of soft clay under subsequent cyclic loadings. A series of undrained cyclic triaxial tests were performed on a kaolin clay to study unloading effect on cyclic behaviour of the soil. The effect of unloading sequence on deformation of the clay under cyclic loading was simulated by reducing cell pressure in between loading cycles. The dependence of axial strain accumulation on the amplitude of cell pressure reduction and unloading schemes was investigated under different initial confining pressures and cyclic loading frequencies. The results indicate that reducing confining pressure accelerates the strain accumulation during subsequent loading cycles and the greater the reduction in cell pressure, the greater the accumulated strain and excess pore water pressure. Such acceleration of axial strain accumulation after unloading would be attributed to the variation of stress state and the soil's ability in memorizing the unloading history. The direction of strain accumulation is controlled by the shear stress ratio rather than the amplitude of unloading. The final accumulated strain is not much affected by unloading schemes if the samples are the same in the initial confining pressure and the amplitude of unloading. An empirical model was developed to fit the axial strain accumulation both before and after unloading. The effects of cell pressure reduction on the parameters of the empirical model were also analysed.

Fluid dynamics characterization and thrombogenicity assessment of a levitating centrifugal pump with different impeller designs

Technical guidelines nowadays recommend and regulate the use Computational Fluid Dynamics (CFD) to assess the performance of medical devices. CFD coupled to blood damage models has emerged as a powerful tool to evaluate the hemocompatibility of blood recirculating devices. The present study is aimed at evaluating the hydrodynamic performance and the thrombogenic potential of two prototypes of magnetically levitating centrifugal pumps. The two devices differ in the impeller configuration - 6-blades vs. 12-blades – and have been designed to be used in Cardiopulmonary Bypass (CPB) circuits during open heart surgery and in Extracorporeal Membrane Oxygenation (ECMO) to support patients with severe cardiac or respiratory failure. The pumps have been modelled using Direct Numerical Simulation coupled to Lagrangian analysis to predict platelet activation due to abnormal shear stress histories. Numerical results have been compared with experimental data in terms of head generation for different working points. Results show that the 6-blades pump has i) smaller stagnation areas, ii) lower stress levels and iii) higher strain rate, resulting in a lower thrombogenic potential, whereas the 12-blade impeller guarantees a more stable performance at high flow rates, suggesting its preferential use for more demanding applications, such as CPB.

Medium amplitude parallel superposition (MAPS) rheology. Part 1: Mathematical framework and theoretical examples

A new mathematical representation for nonlinear viscoelasticity is presented based on the application of the Volterra series expansion to the general nonlinear relationship between shear stress and shear strain history. This theoretical and experimental framework, which we call medium amplitude parallel superposition (MAPS) rheology, reveals a new material property, the third-order complex modulus, which describes completely the weakly nonlinear response of a viscoelastic material in an arbitrary simple shear flow. In this first part, we discuss several theoretical aspects of this mathematical formulation and new material property. For example, we show how MAPS measurements can be performed in strain- or stress-controlled contexts and provide relationships between the weakly nonlinear response functions measured in each case. We show that the MAPS response function is a superset of the response functions that have been previously reported in medium amplitude oscillatory shear and parallel superposition rheology experiments. We also show how to exploit inherent symmetries of the MAPS response function to reduce it to a minimal domain for straightforward measurement and visualization. We compute this material property for a few constitutive models to illustrate the potential richness of the datasets generated by MAPS experiments. Finally, we discuss the MAPS framework in the context of some other nonlinear, time-dependent rheological probes and explain how the MAPS methodology has a distinct advantage over these others because it generates data embedded in a very high-dimensional space without driving fluid mechanical instabilities, and is agnostic to the flow protocol.

Simulation of thrombosis in a stenotic microchannel: The effects of vWF-enhanced shear activation of platelets

This study was undertaken to develop a numerical/computational simulation of von Willebrand Factor (vWF) - mediated platelet shear activation and deposition in an idealized stenosis. Blood is treated as a multi-constituent mixture comprised of a linear fluid component and a porous solid component (thrombus). Chemical and biological species involved in coagulation are modeled using a system of coupled convection-reaction-diffusion (CRD) equations. This study considers the cumulative effect of shear stress (history) on platelet activation. The vWF activity is modeled as an enhancement function for the shear stress accumulation and is related to the experimentally-observed unfolding rate of vWF. A series of simulations were performed in an idealized stenosis in which the predicted platelets deposition agreed well with previous experimental observations spatially and temporally, including the reduction of platelet deposition with decreasing expansion angle. Further simulation indicated a direct relationship between vWF-mediated platelet deposition and degree of stenosis. Based on the success with these benchmark simulations, it is hoped that the model presented here may provide additional insight into vWF-mediated thrombosis and prove useful for the development of more hemo-compatible blood-wetted devices in the future.

Torsional Fatigue Failure of Additively Manufactured Stainless Steel of Reduced Specimen Size

The torsional fatigue response of as-built direct metal laser sintered (DMLS) stainless steel (SS) GP1 of reduced specimen size and horizontal build orientation subjected to cyclic loading at a highly plastic shear strain has been investigated. The average shear modulus, G, was found to be 55.36 GPa, with shear stress history revealing cyclic hardening to stabilization and softening to fracture. Findings suggest that stainless steel GP1 of reduced specimen size experiences fatigue crack initiation at voids near the surface, resulting in a star-spline brittle fracture response, and premature torsional fatigue failure within the low cycle fatigue regime. A noninteraction deformation constitutive model through finite element simulations has been used to successfully simulate the deformation response of the gage section under torsional fatigue conditions.

Rate effects on the interface shear behaviour of normally and overconsolidated clay

This paper presents a study on the effects of shearing velocity, surface roughness and overconsolidation ratio on the strength and deformation behaviour of fine-grained soil–structure interfaces. The results of shear box direct interface shear tests, augmented with soil deformation measurements from particle image velocimetry analyses, indicate that shearing velocity (i.e. velocity at which the continuum surface is displaced) has a controlling effect on the drainage conditions mobilised during shearing. As shearing velocity was increased, undrained conditions were progressively mobilised, characterised by smaller interface strength and volumetric changes for tests on normally consolidated specimens. Increases in shearing velocity resulted in increases in interface strength for overconsolidated specimens. The results presented herein indicate that, as the magnitude of surface roughness was increased, drained shearing conditions were favoured for a given shearing velocity. Soil deformation measurements indicate that the shear zone where localisation occurs becomes thicker as the surface roughness increases, and the magnitude of soil deformations decreases as shearing velocity is increased. The findings presented herein are relevant for prediction and interpretation of soil–structure interface behaviour, as well as for development of constitutive models that capture the combined effect of shear rate, surface roughness and stress history.

Left Ventricular Assist Device Inflow Cannula Insertion Depth Influences Thrombosis Risk

Left ventricular assist device (LVAD) use has continued to grow. Despite recent advances in technology, LVAD patients continue to suffer from devastating complications, including stroke and device thrombosis. Among several variables affecting thrombogenicity, we hypothesize that insertion depth of the inflow cannula into the left ventricle (LV) influences hemodynamics and thrombosis risk. Blood flow patterns were studied in a patient-derived computational model of the LV, mitral valve (MV), and LVAD inflow cannula using unsteady computational fluid dynamics (CFD). Hundreds of thousands of platelets were tracked individually, for two inflow cannula insertion depth configurations (12 mm-reduced and 27 mm-conventional) using platelet-level (Lagrangian) metrics to quantify thrombogenicity. Particularly in patients with small LV dimensions, the deeper inflow cannula insertion resulted in much higher platelet shear stress histories (SH), consistent with markedly abnormal intraventricular hemodynamics. A larger proportion of platelets in this deeper insertion configuration was found to linger in the domain for long residence times (RT) and also accumulated much higher SH. The reduced inflow depth configuration promoted LV washout and reduced platelet SH. The increase of both SH and RT in the LV demonstrates the impact of inflow cannula depth on platelet activation and increased stroke risk in these patients. Inflow cannula depth of insertion should be considered as an opportunity to optimize surgical planning of LVAD therapy.

Small Left Ventricular Size Is an Independent Risk Factor for Ventricular Assist Device Thrombosis.

The prevalence of ventricular assist device (VAD) therapy has continued to increase due to a stagnant donor supply and growing advanced heart failure (HF) population. We hypothesize that left ventricular (LV) size strongly influences biocompatibility and risk of thrombosis. Unsteady computational fluid dynamics (CFD) was used in conjunction with patient-derived computational modeling and virtual surgery with a standard, apically implanted inflow cannula. A dual-focus approach of evaluating thrombogenicity was employed: platelet-based metrics to characterize the platelet environment and flow-based metrics to investigate hemodynamics. Left ventricular end-diastolic dimensions (LVEDds) ranging from 4.5 to 6.5 cm were studied and ranked according to relative thrombogenic potential. Over 150,000 platelets were individually tracked in each LV model over 15 cardiac cycles. As LV size decreased, platelets experienced markedly increased shear stress histories (SHs), whereas platelet residence time (RT) in the LV increased with size. The complex interplay between increased SH and longer RT has profound implications on thrombogenicity, with a significantly higher proportion of platelets in small LVs having long RT times and being subjected to high SH, contributing to thrombus formation. Our data suggest that small LV size, rather than decreased VAD speed, is the primary pathologic mechanism responsible for the increased incidence of thrombosis observed in VAD patients with small LVs.

Multiaxial cycle counting method for non-proportional multiaxial variable loading signals based on modified maximal shear stress

One of the most challenging task in field of multiaxial fatigue is fatigue lifetime estimation of components loaded with multiaxial non-proportional variable amplitude loading. While this task consists of multiple smaller problems, one of the most crucial ones is loading cycles identification (and extraction) for future use with multiaxial damage criterions. By now, several cycle counting methods have been proposed for multiaxial loading conditions. The most wildly accepted methods are Bannantine-Socie’s method and Wang-Brown’s method (which has been later modified by Meggiolaro and Castro). The aim of this paper is the comparison of newly developed method with Bannantine-Socie’s method and Wang-Brown’s method. The new cycle counting method is based on cycle identification in relative maximum shear stress histories (calculated from multiaxial loading histories). The extracted data than composes part of each loading channel of multiaxial loading histories corresponding to identified loading cycle. The comparison of chosen methods has been done by using data sets created by authors as well as using real measured data from real operation.

Post-cyclic undrained shear behavior of marine silty clay under various loading conditions

The post-cyclic undrained shear characteristics of marine silty clay was investigated by conducting a series of cyclic triaxial tests on undisturbed specimens. The effects of confining pressure, initial undrained shear stress ratio (qs/p0), cyclic stress ratio (qcy/p0) and number of cycles (N) on the post-cyclic undrained shear behavior were investigated. Results show that the post-cyclic shear strength decreases with the increase in qs/p0, qcy/p0 and N. The confining pressure has no apparent effect on the normalized post-cyclic monotonic undrained shear strength for the silty clay. During the post-cyclic shear, the specimens with small qs/p0 and qcy/p0 contract, while the cases with large qs/p0 and qcy/p0 dilate. Also, the terminal value of normalized post-cyclic shear-induced excess pore pressure decreases with qs/p0 or qcy/p0 increasing. The Equivalent Hvorslev Line (EHL) was proposed to form a failure envelop for the post-cyclic silty clay and the slope of EHL increases with the increase in qcy/p0 or qs/p0, regardless of the number of cycles. Moreover, a post-cyclic strength model, which could consider the influence of initial shear stress and previous cyclic loading history was proposed on the basis of Yasuhara model. This model demonstrates a good agreement with the test results.