Maximum Elastic Stress Research Articles

Synthesis of thermoset elastomer from microbial oil derived from the fermentation of sugarcane

AbstractIn the context of the 21st century, the integration of diverse perspectives within the circular economy framework, encompassing waste management, economic growth, and environmental sustainability, stands out as a paramount challenge. Addressing this challenge, an innovative avenue emerges through the application of microbial oil to replace traditional petroleum in the synthesis of essential commodity chemicals. This groundbreaking study takes a significant step toward this goal by introducing a pioneering polyester material boasting an exceptionally high renewable content. This material is synthesized through melt polycondensation, utilizing a novel primary feedstock derived from the oily residue extracted post‐distillation of β‐farnesene (FDR). The ingenious approach involves fermenting sugarcane syrup using a genetically engineered yeast strain of Saccharomyces cerevisiae. The outcome of this study reveals the creation of an amorphous polymer with rubber‐like attributes. These attributes include a Young's modulus of 3.9 MPa, with a maximum elastic strain and tensile stress values of 185.4% and 510 KPa, respectively, along with distinctive strain‐hardening characteristics. Remarkably, this material also exhibits indications of shape memory behavior in a temperature range spanning from −47 to 12°C, as discerned from dynamic mechanical analysis. Evidently, this novel polymer demonstrates exceptional promise in the realm of low‐temperature applications. Its intrinsic ability to uphold mechanical integrity, even when subjected to substantial deformations within its service conditions, positions it as an invaluable resource for various components requiring resilience in challenging environments.

Forced oscillations of a cylinder in the flow of viscoelastic fluids

We investigate the effects of fluid elasticity on the flow forces and the wake structure when a rigid cylinder is placed in a viscoelastic flow and is forced to oscillate sinusoidally in the transverse direction. We consider a two-dimensional, uniform, incompressible flow of viscoelastic fluid at $Re=100$ , and use the FENE-P model to represent the viscoelastic fluid. We study how the flow forces and the wake patterns change as the amplitude of oscillations, $A^*$ , the frequency of oscillations (inversely proportional to a reduced velocity, $U^*$ ), the Weissenberg number, $Wi$ , the square of maximum polymer extensibility, $L^2$ , and the viscosity ratio, $\beta$ , change individually. We calculate the lift coefficient in phase with cylinder velocity to determine the range of different system parameters where self-excited oscillations might occur if the cylinder is allowed to oscillate freely. We also study the effect of fluid elasticity on the added mass coefficient as these parameters change. The maximum elastic stress of the fluid occurs in between the vortices that are observed in the wake. We observe a new mode of shedding in the wake of the cylinder: in addition to the primary vortices that are also observed in the Newtonian flows, secondary vortices that are caused entirely by the viscoelasticity of the fluid are observed in between the primary vortices. We also show that, for a constant $Wi$ , the strength of the polymeric stresses increases with increasing reduced velocity or with decreasing amplitude of oscillations.

Insights into the intimate link between nonlinear rheological characterization and the rupture behavior of polymer gel in model fractures

A deep understanding of how fluid flows through the polymer gels has significant implications for achieving effective gel treatments in fractured reservoirs. This study aims to investigate the behavior of brine breakthrough from the gels in model fractures and relate the rupture behavior to the rheological properties of the gels. The underlying mechanisms for different gel rupture behavior are also elaborated. The effect of the injection rates, the aperture of tubes, and the gel properties on the rupture pressure of the gel were also studied. The results show that the rupture pressure of gel is highly dependent on the flow rates, and the rupture pressure is decreased with increasing the aperture sizes. It is also found that the polymer gel with a higher amount of crosslinkers has a high G’ value yet exhibits a low rupture pressure. From the visualization experiments, we found that brine is more likely to penetrate through the gel along the weak network structures, and a narrower channel would be formed at a higher injection rate. Moreover, various rheological measurements were conducted to characterize the gel properties. The results reveal that the maximum differential pressure the gel can withstand is highly dependent on the microstructure and the toughness of the gel. The defect structures, such as large cavities or cracks, would lead to a low yield stress of polymer gel, which is more susceptible to fracturing. The texture properties of the polymer gel can be interpreted by the shapes of Lissajous curves. The flexible gel shows a “banana-shaped” curve at high strain, a typical response for rubber-like materials. The toughness of the gel can significantly improve the pressure for water breakthrough. The degree of the brittleness of the polymer gels can be characterized by the reduction rate of the maximum elastic stress. The results presented in this study allow us to select an appropriate gel for water shutoff in fractured reservoirs by rheological methods, which are more robust and cost-effective.

Acyl-protein thioesterase1 alleviates senile osteoporosis by promoting osteoblast differentiation via depalmitoylation of BMPR1a

ObjectiveSenile osteoporosis (SOP) is an aging-related disease. The depalmitoylating enzyme Acyl-protein thiesterase1 (APT1) is involved in disease regulation. This study explored the mechanism of APT1 in SOP. MethodsEight-week-old SAMP6 mice were selected as SOP models and SAMR1 mice were controls, while osteoblasts were isolated from the femoral surface-soft tissues of SOP and control mice as in vitro models. Mouse femur morphological, bone mineral density (BMD), femur maximum elastic stress and maximum load, and APT1 expression were detected by HE staining, X-ray bone densitometer, material testing machine, and RT-qPCR and Western blot (WB). Osteoprotegrin (OPG)-labeled osteoblasts and APT1 localization in bone tissues were detected by immunohistochemical staining. APT1 expression was promoted in SOP mice by tail vein injection of APT1 lentivirus or promoted/silenced in osteoblasts by transfection of pcDNA3.1-APT1 overexpression or si-APT1 plasmids. SOP mouse osteoblast differentiation (OD), OD-related protein levels, osteoblast proliferation, BMPR1a palmitoylation level, and BMP/Smad pathway were detected by alizarin red staining, ALP activity detection, WB, CCK-8, and IP-ABE method. The effects of the pathway inhibitor LDN-193189 on OD were detected. ResultsAPT1 was under-expressed in osteoblasts of bone tissue in SOP mice and mainly localized in osteoblasts. SOP mice manifested increased bone marrow cavity and bone trabecular space, thinned trabecular bone, decreased BMD, maximum elastic stress, maximum load, and reduced OPG-positive osteoblasts in bone tissues, which were averted by APT1 overexpression, thus alleviating SOP. APT1 overexpression increased osteoblast calcium nodules, ALP activity, OD-related protein levels, and cell proliferation. In mechanism, APT1 overexpression inhibited BMPR1a palmitoylation in SOP mouse osteoblasts and activated the BMP/Smad pathway, thus promoting OD. ConclusionAPT1 activated the BMP/Smad pathway and promoted OD by regulating BMPR1a depalmitoylation, thus alleviating mouse SOP.

Investigation on early-age cracking resistance of basalt-polypropylene fiber reinforced concrete in restrained ring tests

The restrained shrinkage deformation of concrete structures at an early age is the main reason for cracking. Basalt-polypropylene fiber can reduce the shrinkage strain and cracking potential of concrete effectively. Investigation on cracking resistance of basalt-polypropylene fiber reinforced concrete (BPFRC) at early age by restrained ring test remain lacking. This study conducted free shrinkage test and restrained shrinkage test to analysis the early-age cracking resistance of BPFRC. The experimental results showed that incorporating basalt fiber and polypropylene fiber into concrete could decrease the free shrinkage and restrained shrinkage of concrete matrix and reduce the maximum theoretical elastic stress, actual maximum residual stress and relaxed stress, among which, the better positive effect was achieved when basalt fiber and polypropylene fiber hybridized. A numerical finite element simulation model was successfully carried out to predict the time-dependent maximum tensile stress in the concrete ring. Importantly, the damage evaluation of restrained shrinkage concrete ring was investigated based on the relations of the compressive (dc) and tensile (dt) damage factors with inelastic strains of BPFRC proposed in this paper, and a good agreement between simulation and experimental test had been observed. The findings in this study would promote the development and application of BPFRC.

A COMPARATIVE STUDY ON THE FATIGUE STRENGTH AND SERVICE LIFE OF LOWER LIMB ARTERIAL STENT AT DIFFERENT STENOSIS RATES

In order to study the influence of different stenosis rates of blood vessels on the fatigue strength and service life of lower limb arterial stent, numerical simulation was conducted for the mechanical behavior of four types of nickel-titanium alloy lower limb arterial stents (Absolute Pro, Complete SE, E-luminexx-B and Pulsar-35) under the action of radial compression, release and pulsating loads, so as to predict the fatigue life and safety of stents at different stenosis rates (0%, 30%, 50% and 70%). The study found that with increased vascular stenosis rate, both the elastic stress and strain of stent tend to increase, while the fatigue strength, service life and safety tend to decrease. When a stent is implanted in a normal blood vessel, its fatigue strength satisfies the requirement of a 10-year service life requirement, with maximum elastic stress and strain occurring on both sides of the connecting ribs at the end of stent. When the vascular stenosis rate is greater than 30%, the fatigue strength of the stent does not meet requirement of a 10-year service life, and fatigue fracture is likely to occur at the most stenotic part of the blood vessel. With increased vascular stenosis rate, the E-luminexx-B stent with the largest width of support had a significant decrease in its service life. The stent whose supporting unit is of symmetric wave peak structure has a longer service life compared with that whose supporting unit is of offset wave peak structure. The revealing of the influence of vascular stenosis rate on the mechanical properties and fatigue life of stents provides theoretical reference for the fracture failure mechanism of stents.

Fracture mechanics analysis of a closure head of a PWR reactor pressure vessel LEFM-based SIF

In this study, fracture mechanics analysis of a closure head of a PWR reactor pressure vessel has been performed under accidental conditions. For the purpose, a wide range of elliptical surface cracks starting from minute cracks to worst-case cracks was postulated at the highest stress concentration locations of the head. The analyzed accidental conditions are small break loss of coolant accident (SB-LOCA) and Rancho-Seco transient (RST). The maximum linear elastic fracture mechanics-based stress intensity factors of the worst-case crack ‘a = 0.25 th; a = 0.33 c’ under SB-LOCA and RST are 104.37 MPa√m and 137.12 MPa√m respectively, which are less than the fracture toughness value of the material i.e. 155.6 MPa√m. Hence, the accidental conditions do not threaten the structural integrity of the proposed closure head.

Lifetime optimization of solar central receivers via linear actuators

The receiver is a key component of concentrated solar power plants, whose operation can be interrupted by its early failure. In this research, a novel approach to the enhancement of solar receiver lifetime is presented, studying the use of linear actuators in the longitudinal supports of the tubes to induce forced displacements, reducing thermal stresses. Existing analytical models have been improved and joined together, in order to be implemented in a genetic algorithm to optimize the solar receiver lifetime. In a first approach, the maximum equivalent elastic stress, produced by the thermal gradients over the receiver, has been minimized. This approach reduces the maximum thermoelastic stress up to a 9.5%, and increases the lifetime of the receiver to an 8% when a forced displacement of 100 mm (1% of the tube length) is considered. When the rupture time due to creep is optimized, with the same forced displacement of 100 mm, the tube lifetime is enhanced up to a 25%. Small forced displacements (0.1% of the tube length) may lead to a lifetime improvement up to 6%. The small amount of force necessary to carry out the forced displacement of the supports makes feasible the implementation of linear actuators in future solar power plants.

Structure-activity relationship: Understanding implications of cavity design for potassium-ion storage

Structural stability of electrode material is probably the first and foremost requirement for realization of reversible potassium-ion storage in long-lasting battery applications. In this respect, careful cavity design is much needed because it not only establishes interconnected channels for ion transfer and reservoirs for electrolyte to buffer sudden changes in ion concentration, but also contributes a lot to stabilization of electrode structure. In this work, therefore, we tried to provide important insights on cavity design using multiple spherical structures as paradigms. After electrochemical characterization, a quantitative evaluation regime was proposed. This regime was mainly based on geometry and mechanics considerations. As for geometry criterion, surface expansion ratio ζ could be estimated by using r/R = [2−(1 + ζ)3/2]1/3, with r and R denoting yolk and shell radii, respectively. In terms of mechanics criterion, ζ could also be calculated by integrating εe = σe/Y with ζ = [2−(1 − εe)3]2/3 − 1, wherein εe, σe and Y denoted maximum elastic strain, maximum elastic stress and Young’s modulus, respectively. The ζ values deduced from both geometry and mechanics criteria ought to be in good consistency, so as to maintain structural integrity. Accordingly, this evaluation regime could be generalized to establish structure–activity relationship in electrode design and would provide a broad vision in understanding implications of cavity design for potassium-ion storage.

Reconsidering the strength of concrete pavements

ABSTRACT Concrete pavements develop cracks with the accumulation of fatigue damage caused by the combined effect of vehicular and environmental loadings. In Mechanistic-Empirical (ME) design, the ratio of stresses to some measure of strength is used to evaluate incremental damage, which is then related to cracking. Typically, this measure is the Modulus of Rupture (MOR) of a beam made of the same material. The flexural strength is also known to vary with the thickness of the beam. However, the variation of flexural strength due the presence of eigenstresses from temperature or moisture distributions has not received sufficient attention. A non-linear fracture mechanics model demonstrated that the thickness-adjusted flexural strength can also be used for characterising the maximum elastic stress under a linear temperature distribution. However, under a non-linear temperature distribution, the material may sustain a higher or lower maximum elastic stress than the flexural strength. To model this, a practical analytical expression for the apparent flexural strength was derived and showed excellent agreement with numerical simulations as well as with a model based on equivalent elastic deformation energy. The model was used to show that the fatigue capacity under a typical axle load varies with various non-linear temperature distributions.

Rheological behavior, electrical conductivity, and morphology of multi‐walled carbon nanotube filled poly(ethylene‐co‐vinyl acetate)/poly(methyl methacrylate) nanocomposites: Effect of nanofiller content

AbstractNanocomposites derived from poly(methyl methacrylate) (PMMA) and poly(ethylene‐co‐vinyl acetate) (EVA) with 30 wt% EVA and 0–10 wt% multi‐walled carbon nanotube (MWCNT) content were prepared. Scanning electron microscope images showed a two‐phase morphology for the unfilled blend in which the EVA droplets were dispersed within the PMMA domain. However, the size of EVA droplets was considerably smaller in the nanocomposites than that of the unfilled blend. The tensile results revealed that the maximum elastic modulus, tensile strength, yield stress, and strain at break were achieved for the nanocomposite containing 5 wt% MWCNT. The electrical conductivity sharply increased for the nanocomposite containing 2.5 wt% MWCNT, in comparison with that of the unfilled blend, beyond which the conductivity was less dependent on the MWCNT content. The rheological characterization showed that the normalized relaxation time increased up to 5 wt% nanofiller and remained reasonably constant for higher amounts of MWCNT.

The miRNA-15b/USP7/KDM6B axis engages in the initiation of osteoporosis by modulating osteoblast differentiation and autophagy.

Osteoporosis is a metabolic disease that results from oxidative stress or inflammation in renal disorders. microRNAs (miRNAs) are recently implicated to participate in osteoporosis, but the mechanism remains largely unexplored. Herein, we aimed to explore the potential role of miR‐15b in osteoblast differentiation and autophagy in osteoporosis. We established osteoporosis models through ovariectomy and determined that miR‐15b was highly expressed whereas USP7 and KDM6B were poorly expressed in tissue of osteoporosis mice. Treatment of silenced miR‐15b resulted in the elevation of decreased bone mineral density (BMD), the maximum elastic stress and the maximum load of osteoporosis mice. In osteoblasts, miR‐15 overexpression decreased proliferation but suppressed the cell differentiation and autophagy, accompanied with decreased expression of USP7. Mechanistically, miR‐15 bound and inhibited USP7 expression, while overexpression of USP7 promoted autophagy of osteoblasts. USP7, importantly, strengthened the stability of KDM6B and promoted KDM6B expression. MG132 protease inhibitor increased KDM6B and USP7 expression in osteoblasts. Silencing of KDM6B reversed the promoting effect on autophagy and proliferation induced by overexpression of USP7. Taken altogether, miR‐15b inhibits osteoblast differentiation and autophagy to aggravate osteoporosis by targeting USP7 to regulate KDM6B expression.

Estimation of creep of the turbine control valve body at increased steam temperature

The influence of the temperature of the supplied steam on the operating and strength characteristics during creep of the control valve body of the steam turbine K-325 is considered. Currently, the valves are operated with steam supply with a pressure of 24 MPa and a temperature of 540°C. The analysis of the effect of increasing the temperature of the supplied steam up to 565°C. The study was carried out in connection with a possible increase in steam temperature to increase the power of the turbine. The temperature and pressure of steam on the walls of the vessel are determined based on the numerical solution of the Navier-Stokes equations in a three-dimensional setting. It was found that an increase in the temperature of the supplied steam has little effect on the nature of the distribution of temperature and pressure on the walls of the valve body. Temperature fields and steam pressure on the walls of the valve body are used to further solve the problems of the strength of the control valve bodies of the K-325 turbine in a stationary mode of operation. The stress-strain state of the valve body of a steam turbine in a stationary mode of robots is numerically determined. The problem was solved by the finite element method in a three-dimensional setting. The results showed that the maximum elastic stresses on the outer surfaces are observed in the region of the upper transition of the inlet pipe. On the inner surfaces, the maximum values of elastic stresses are observed in the area of the branch pipes in front of the control valves. It was determined that an increase in the temperature of the supplied steam up to 565°C insignificantly affects the stress state of the valve body. The creep of the body was calculated using a model of implicit creep with hardening, taking into account the variable and constant creep components. The influence of the temperature of the supplied steam on the creep of the valve body is estimated. The results indicate that an increase in the temperature of the supplied steam by 25°C leads to an increase in creep deformation by a factor of 2.3 after 200 thousand hours of operation. Temperatures, pressures, and equivalent stresses are plotted at the body surfaces and creep curves are plotted at 540°C and 565°C.

Creep rupture life assessment of superheater tube with oxide formation in power plant

AbstractThis paper proposes a method for predicting the creep rupture life of superheater tubes, considering the continuous increasing metal temperature and stress as a result of oxide formation. The steam‐side oxide growth was assessed by the parabolic oxidation rate law, and the temperature profiles of superheater tubes was calculated combining the 3D computational fluid dynamics simulation and the 1D analytical models. Based on the estimated temperature and maximum elastic hoop stress, the parametric extrapolation technique was utilized to assess the creep rupture life. The validity of this method was confirmed by measuring the oxide thickness and the steam temperature in the field. The results found that T91 alloy in the steam outlet region and T23 alloy at the connection point was susceptible to creep rupture failure, which had the greatest cumulative damage and thickest oxide scale. The cumulative damage of superheater tubes varied significantly with tube count. Results suggested that replacement of the particular T91 alloy tubing section having the minimum creep rupture life with a better heat‐resistant steel would avoid mechanical failure by creep for the subject boiler system.

3D printing of implantable elastic PLCL copolymer scaffolds.

Poly(l-lactic acid) (PLLA) scaffolds have been used in regenerative medicine, however, they commonly suffer from low flexibility, restricting their application in the repair and reconstruction of soft tissues. In this study, poly(l-lactide-co-ε-caprolactone) (PLCL) copolymers were examined to modulate the elasticity of PLLA with the random presence of CL units in PLLA. Thermodynamic analysis revealed that the introduction of PCL could significantly decrease the melting point and glass transition temperature of PLLA, benefiting the extrusion and printing of PLCL. Diverse scaffolds with designed architectures including porous cubes with or without large holes, cambered plates with holes and round tubes could be easily constructed by 3D printing. In the process of elastic deformation, the maximum elastic stress of the copolymer scaffold was obviously increased from 19.6 to 31.5 MPa when the relative content of PCL was increased to 70%, while the elongation at break was evidently increased from 388% to about 1974%. The Young's modulus of PLCL was also significantly decreased (P < 0.05) in comparison with that of PLLA. PLCL scaffolds have good platelet and endotheliocyte adhesion ability and no obvious hemolysis was observed. In vivo subcutaneous implantation of PLCL scaffolds demonstrated superior biocompatibility. Collectively, this work highlights that copolymerization of PCL segments into PLLA is an effective approach to tune the 3D printability and the stiffness and elasticity of PLLA scaffolds. PLCL scaffolds hold great promise for the regeneration of soft tissues including but not limited to cartilage, myocardium, muscle, tendon and nervous tissues.

A closure model for the drag coefficient of a sphere translating in a viscoelastic fluid

In many large-scale industrial applications dealing with particle-laden viscoelastic fluids, the ensemble-averaged behavior of the mixture is of most interest. The first step to parametrize this behavior is to develop an accurate expression to rapidly evaluate the drag coefficient over a broad range of kinematic parameters. The drag coefficient of a spherical particle translating in a viscoelastic matrix is strongly affected by the viscoelasticity of the fluid. In this study, we aim to parametrize the effects of fluid elasticity, especially the relaxation and retardation times, as well as inertia on the drag coefficient of a sphere translating in a viscoelastic fluid described by the Oldroyd-B model. To this end, we employed three-dimensional direct numerical simulations of viscoelastic flow past a stationary sphere. The accuracy of the numerical formulation is thoroughly tested against a number of benchmark problems consisting of steady flow past a sphere in a bounded circular or square domain filled with either a Newtonian or viscoelastic fluid. Initially, the numerical computations for the drag coefficient over a wide range of geometric and flow parameters are validated by comparison with existing data and drag correction models from the literature. The drag coefficient correction is then evaluated for unconfined flow past a sphere at different Reynolds number, Re, over a wide range of Deborah number, De < 9, and polymer viscosity ratio, 0 < ζ < 1. For small Deborah number (De < 1), the drag coefficient decreases with respect to the Stokes drag coefficient, whereas, at large Deborah number (De > 1), the drag is enhanced due to the large elastic stresses that develop on both the surface and wake of the sphere. These canonical behaviors, observed in the inertia-less flow regime (Re ≤ 1) are amplified as the polymer viscosity ratio approaches unity. At higher Reynolds numbers (Re > 1), the drag coefficient correction arising from viscoelasticity is found to be always bigger than unity, but smaller than the enhancement calculated in the creeping flow limit. In this regime, the formation of an axisymmetric recirculating eddy expands the wake region behind the sphere, and shifts the maximum elastic stresses away from the rear stagnation point towards the flow separation points. Motivated by the self-similarity observed at Re < 1, we propose an approximate model for the viscoelastic drag coefficient based on rational functions, using the lowest possible degree of polynomial functions required to fit the computational data with 95% accuracy, for De ≤ 5 and 0 < ζ < 1. This expression is a key step towards formulating a momentum-exchange model for the flow of dilute suspensions of solid particles in a viscoelastic matrix.

Low-expression of lncRNA-ANCR promotes tibial fracture healing via targeting RUNX2.

To explore the role of long non-coding ribonucleic acid-anti-differentiation non-coding ribonucleic acid (lncRNA-ANCR) in tibial fracture healing in rabbits by regulating the runt-related transcription factor 2 (RUNX2) expression. A total of 60 healthy adult rabbits were evenly divided into Control group (n=20), Fracture group (n=20), and Lnc group (n=20). Then, RUNX2 transfection, Real Time Polymerase Chain Reaction (PCR) assay and relevant instruments were carried out and used to determine the differences in dry weight, bone mineral density, bone mechanical strength, and RUNX2 expression in tibiae among three groups of rabbits. Comparison of the bone mineral density in rabbit tibiae among the three groups showed that the bone mineral density was significantly lower in Fracture group than that in Control group (p<0.05), and it was slightly higher in Lnc group than in Fracture group (p<0.05). The dry weight of the full-length tibiae in Fracture group was significantly decreased compared with that in Control group (p<0.05), and Lnc group had an increased dry weight of tibiae in comparison with Fracture group (p<0.05). The maximum load, flexural strength, elastic stress, elastic strain, elastic modulus, maximum stress, and maximum strain in Fracture group were lower than those in both Control group and Lnc group (p<0.05). Compared with those in Fracture group, the amount of new collagen was overtly increased in Lnc group, and that of mature collagen was decreased (p<0.05). The relative expression level of RUNX2 in tibial bone tissues was evidently lower in Fracture group than that in Control group (p<0.05), and it was markedly higher in Lnc group than that in Fracture group (p<0.05). Down-regulating lncRNA-ANCR activates and triggers the expression of RUNX2 that facilitates the growth and metabolism of bone tissues to play an important role in the repair of bone tissues and promote the healing of the tibial fracture.

Numerical study of biomechanical characteristics of plaque rupture at stenosed carotid bifurcation: a stenosis mechanical property-specific guide for blood pressure control in daily activities

Acute stress concentration plays an important role in plaque rupture and may cause stroke or myocardial infarction. Quantitative evaluation of the relation between in vivo plaque stress and variations in blood pressure and flow rates is valuable to optimize daily monitoring of the cardiovascular system for high-risk patients as well as to set a safe physical exercise intensity for better quality of life. In this study, we constructed an in vivo stress model for a human carotid bifurcation with atherosclerotic plaque, and analyzed the effects of blood pressure, flow rates, plaque stiffness, and stenosis on the elastic stress and fluid viscous stress around the plaque. According to the maximum values of the mechanical stress, we define a risk index to predict the risk level of plaque rupture under different exercise intensities. For a carotid bifurcation where the blood flow divides, the results suggest that the stenosis ratio determines the ratio of the contributions of elastic shear stress and viscous shear stress to plaque rupture. An increase of the plaque stiffness enhances the maximum elastic shear stress in the plaque, indicating that a high-stiffness plaque is more prone to rupture for given stenosis ratio. High stress co-localization at the shoulder of plaques agrees with the region of plaque injury in clinical observations. It is demonstrated that, due to the stress-shield effect, the rupture risk of a high-stiffness plaque tends to decrease under high-stenosis conditions, suggesting the existence of a specific stenosis corresponding to the maximum risk. This study may help to complement risk stratification of vulnerable plaques in clinical practice and provides a stenosis mechanical property-specific guide for blood pressure control in cardiovascular health management.

A study on the dynamic structural behavior of Olympic sabres

Sabres used in Olympic fencing are subject to severe elastic deformations during matches and training sessions. Even though strict rules for their manufacturing are prescribed by the international fencing federation, with requirements in terms of geometrical constraints and material (steel) properties, nonetheless frequent unexpected ruptures are observed. These may cause injuries to the fencers, and involve the replacement of the blade. In this study an experimental-numerical approach is adopted to investigate the underlying failure mechanisms. To this purpose, several attacks, “bouts” in fencing, were live filmed during actual practice with digital cameras and a trajectory tracking analysis was performed on the most critical of them, taking advantage of markers fixed on the blades at different positions. The post-processed data were subsequently used as boundary conditions of a 3D finite element model of the blade. Running a non-linear transient analysis, global and local quantities such as maximum stored elastic energy, stress and strain states, strain rates and possible permanent plastic strains were evaluated. A validation of the FE model with experiments was also carried out. From the critical analysis of experimental and numerical results it was possible to speculate about the influence of materials and dynamic related effects on the structural behavior of the blade. Eventually, hypotheses on fracture mechanisms were formulated.

Elasto-plastic contact of materials containing double-layered inhomogeneities

Double-layered inhomogeneities consist of enclosed, or layered, inhomogeneities of different material properties, embedded in a matrix material. They are found in engineering materials used for components under contact and relative motion. Currently, most theoretical investigations on the double-layered inhomogeneities are limited to elastic or plane-strain problems. This work proposes a novel model, based on the numerical equivalent inclusion method, for the elasto-plastic contact of materials with double-layered inhomogeneities. The current analysis is focused on such inhomogeneities as a stiff core enclosed by a compliant outer layer. A group of in-depth parametric studies is performed to reveal the effects of this type of double-layered inhomogeneities on the contact plasticity of the matrix material. The results indicate that the plastic strain distribution in the matrix material is related to the Young's modulus and geometric eccentricity of the inner and outer inhomogeneities, as well as the location and shape of the inhomogeneities. For the cases of individual double-layered inhomogeneity embedded at different locations, the maximum equivalent plastic strain in the matrix appears in the vicinity of the double-layered inhomogeneity, wherever is the closest to the location of the theoretical maximum elastic stress from the homogeneous solution. For the cases of multiple double-layered inhomogeneities, the overlap of the plastic strain concentration regions amplifies the disturbance caused by these inhomogeneities, and the amplification effect is related to the inhomogeneity layout. If the stiff core is completely encircled in the outer inhomogeneity layer, plastic strains would initiate from the outer layer and then permeate to the matrix material.