Oscillatory Stress Research Articles

Cohesive Living Bacterial Films with Tunable Mechanical Properties from Cell Surface Protein Display.

Engineered living materials (ELMs) constitute a novel class of functional materials that contain living organisms. The mechanical properties of many such systems are dominated by the polymeric matrices used to encapsulate the cellular components of the material, making it hard to tune the mechanical behavior through genetic manipulation. To address this issue, we have developed living materials in which mechanical properties are controlled by the cell-surface display of engineered proteins. Here, we show that engineered Esherichia coli cells outfitted with surface-displayed elastin-like proteins (ELPs, designated E6) grow into soft, cohesive bacterial films with biaxial moduli around 14 kPa. When subjected to bulge-testing, such films yielded at strains of approximately 10%. Introduction of a single cysteine residue near the exposed N-terminus of the ELP (to afford a protein designated CE6) increases the film modulus 3-fold to 44 kPa and eliminates the yielding behavior. When subjected to oscillatory stress, films prepared from E. coli strains bearing CE6 exhibit modest hysteresis and full strain recovery; in E6 films much more significant hysteresis and substantial plastic deformation are observed. CE6 films heal autonomously after damage, with the biaxial modulus fully restored after a few hours. This work establishes an approach to living materials with genetically programmable mechanical properties and a capacity for self-healing. Such materials may find application in biomanufacturing, biosensing, and bioremediation.

Acceleration effect on stress singularity and edge strength in piezoelectric/Piezomagnetic heterostructures

The acceleration effect on edge stress singularity and edge strength in piezoelectric/piezomagnetic heterostructures is investigated in this work. For the stress singularity analysis, we propose a stress function based iterative approach based on the Lekhnitskii stress functions with body forces and harmonic assumption of initial stress fields. The acceleration effect is introduced by adding body force terms to the equilibrium equations. The governing equations are obtained by applying variational principle in each process and solved by general eigenvalue problems to obtain homogeneous solutions, as well as to obtain particular solutions based on the forms of load and acceleration conditions. During the iterations, the stress oscillations can be gradually eliminated and the stress concentration can be predicted exactly located at the interfaces. Finally, an example of symmetrically layered heterostructure is presented under both in-plane acceleration and out-of-plane acceleration. It is found that both accelerations have significant effect on the edge normal and shear stresses which may further cause failure. The edge strength is also evaluated by calculating the edge average stress. This work may help to understand the acceleration effect on edge stresses and edge strength for heterostructures, as well as the trend of stress magnitude change caused by external accelerations.

Frictional Properties of Simulated Fault Gouges Subject to Normal Stress Oscillation and Implications for Induced Seismicity

AbstractUnder critical conditions where experimental fault slip exhibits self‐sustained oscillation, effects of normal stress oscillation (NSO) on fault strength and stability remain poorly understood, as do potential effects of NSO on natural and induced seismicity. In this study, we employed double direct‐shear testing to investigate the frictional behavior of a synthetic, slightly velocity‐weakening (SVW) fault gouge (characterized by self‐sustained oscillation under quasi‐static shear loading), when subjected to NSO at different amplitudes (5%–20% of 5 MPa) and frequencies (0.001–1 Hz). During the experiment, fault displacement and gouge layer thickness were measured. Transmitted ultrasonic waves were also employed to probe grain contact states within the gouge layer. Our results show that fault weakening and unstable slip can be triggered at NSO frequencies ranging from 0.03 to 0.1 Hz and amplitudes exceeding 5%. Interestingly, an amplified shear stress drop and weakening effect were observed when the NSO frequency fell in 0.05–0.1 Hz. Analysis of transmitted ultrasonic waves in tests on the SVW gouge revealed fault dilation, accompanied by unstable slip and weakening. By extending an existing microphysical model (the “Chen‐Niemeijer‐Spiers [CNS]” model), to account for elastic effects of NSO on gouge microstructure and grain contact state, the mechanical and wave data obtained in our experiments on the SVW gouge was reproduced, suggesting an approach for modeling fault instability under upper crustal (SVW) conditions where normal stress is perturbed by subsurface operations, such as periodic gas storage stimulation of reservoir formations.

Mechanical properties and cage transformations in CO2-CH4 heterohydrates: a molecular dynamics and machine learning study

Abstract The substitution of natural gas hydrates with CO2 offers a compelling dual advantage by enabling the extracting of CH4 while simultaneously sequestering CO2. This process, however, is intricately tied to the mechanical stability of CO2-CH4 heterohydrates. In this study, we report the mechanical properties and cage transformations in CO2-CH4 heterohydrates subjected to uniaxial straining via molecular dynamics (MD) simulations and machine learning (ML). Results indicate that guest molecule occupancy, the ratio of CO2 to CH4 and their spatial arrangements within heterohydrate structure greatly dictate the mechanical properties of CO2–CH4 heterohydrates including Young’s modulus, tensile strength, and critical strain. Notable, the introduction of CO2 within clathrate cages, particularly within 512 small cages, weakens the stability of CO2–CH4 heterohydrates in terms of mechanical properties. Upon critical strains, unconventional clathrate cages form, contributing to loading stress oscillation before fracture of heterohydrates. Intriguingly, predominant cage transformations, such as 51262–4151063 or 425864 and 512–425861 cages, are identified, in which 4151062 appears as primary intermediate cage that is able to transform into 4151063, 425862, 425863, 512 and 51262 cages, unveiling the dynamic nature of heterohydrate structures under straining. Additionally, ML models developed using MD data well predict the mechanical properties of heterohydrates, and underscore the critical influence of the spatial arrangement of guest molecules on the mechanical properties. These newly-developed ML models serve as valuable tools for accurately predicting the mechanical properties of heterohydrates. This study provides fresh insights into the mechanical properties and cage transformations in heterohydrates in response to strain, holding significant implications for environmentally sustainable utilization of CO2–CH4 heterohydrates.

Numerical simulation of non-Newtonian blood flow in a human arterial bifurcation model

Hemodynamic factors play a role in atherogenesis and the localization of atherosclerotic plaques. The human aorta and coronary arteries are susceptible to arterial disease, and there have been many studies of flows in models of these vessels. Atherosclerosis is a systemic disease occurring in specific sections of the cardiovascular tree such as the carotid and the coronary arteries. This paper aims to simulate the human arterial bifurcation and investigates some hemodynamic parameters as blood propagates from the aortic artery toward the carotid artery. We analyze the velocity, static pressure, shear stress. The simulation results showed disturbed flow patterns, such as flow separation and stagnation, as well as abnormal hemodynamic parameters (HPs) distributions including the low and high wall shear stress (WSS) and oscillation of wall shear stress. Based on our simulating characteristic results, the CFD tools can not only monitor the hemodynamic parameters obviously, but also help to analyze the diagnostic for the treatment of vascular diseases. CFD can be an effective technology for the examination and treatment of patients with failure of blood supply of the head and brain.

Rapid Normal Stress Oscillations Cause Weakening and Anelastic Dilation in Gouge‐Bearing Faults

AbstractFault normal stress (σn) changes dynamically during earthquakes. However, the impact of these changes on fault strength is poorly understood. We explore the effects of rapidly varying σn by conducting rotary‐shear experiments on simulated fault gouges at 1 μm/s, under well‐drained, hydrothermal conditions. Our results show both elastic and anelastic (time‐dependent but recoverable) changes in gouge layer thickness in response to step changes and sinusoidal oscillations in σn. In particular, we observe dilation associated with marked weakening during ongoing σn‐oscillations at frequencies >0.1 Hz. Moreover, recovery of shear stress after such oscillations is accompanied by transient (anelastic) compaction. We propose a microphysically based friction model that explains most of the observations made, including the effects of temperature and step versus sinusoidal perturbation modes. Our results highlight that σn‐oscillations above a specific frequency threshold, controlled by the loading regime and frictional properties of the fault, may enhance seismic hazards.

Unsteady flow past an impulsively started infinite vertical plate in presence of thermal stratification and chemical reaction

AbstractThe purpose of this study is to analyze how thermal stratification affects fluid movement past an impulsively initiated infinite upright plate when first‐order chemical reactions are present. Laplace's transform method is applied to achieve a closed‐form solution for the nondimensional governing equations when . The formula , where t is the time and s is a parameter, can be used to obtain the Laplace transform of an exponentially ordered piece‐wise continuous function . The unstable flow past an infinite vertical plate, starting abruptly in the presence of heat stratification and chemical reactions, has never been studied before. The research focuses on the combined impact of thermal stratification and chemical processes on the flow of an incompressible viscous fluid over an indefinitely tall plate. In this study, the significant findings that resulted from heat stratification are compared to the condition in which heat stratification is absent. The impacts of a number of parameters, such as are investigated and visually displayed with respect to the following variables: concentration, velocity, shear stress, rate of heat transfer, temperature, and rate of mass transfer. It is demonstrated that applying stratification increases the frequency of oscillation in shear stress and heat transfer rate. The results obtained can be useful in the design of heat exchangers and other engineering applications.

Rheology and Extrusion Printing of κ-Carrageenan/Olive Oil Emulsion Gel Tablets with Varying Surface Area to Volume Ratios for Release of Vitamin C and Curcumin.

In this work, κ-carrageenan and olive oil at different oil to κ-carrageenan ratios (OCR) are homogenized to create emulsion gels. Interestingly, confocal imaging shows that the oil droplets are stabilized in the κ-carrageenan-structured gel matrix without using any surfactants. Rheological studies show that the oil droplets enhanced the oscillatory yield stress and the maximum printable height of the emulsion gels. The creation of the emulsion gels with an OCR of 1:9-3:7 led to an improvement in the structural integrity of extrusion printed structures. The emulsion gel with an OCR of 3:7 efficiently encapsulates vitamin C in the aqueous phase and curcumin in the hydrophobic oil phase, enabling the extrusion 3D printing of tablets with varying surface area to volume (SA/V) ratios. The release of vitamin C and curcumin is influenced by the preparation method of printing versus casting and the SA/V ratio of the tablets. The hollow cylinder with the highest SA/V ratio was observed to have the highest vitamin C release, whereas for curcumin, the printed tablets had a higher release compared to the cast tablet. Additionally, through rheo-dissolution experiments, we observe a lower modulus and higher vitamin C release from the 3D-printed disc versus the higher modulus and lower vitamin C release from the cast disc tablet.

An integrated three-dimensional aeromechanical analysis for the prediction of stresses on modern coaxial rotors

This paper presents the first application of an Integrated Three-Dimensional aeromechanical analysis—defined as the coupled solution of three-dimensional finite element-based structural dynamics with a three-dimensional Reynolds-Averaged Navier-Stokes-based fluid dynamics—to predict the stresses on a modern coaxial rotor. The coupling was carried out with the University of Maryland’s structural dynamic solver X3D and the U.S. Army’s CREATETM–AV Helios suite of fluid dynamic solvers. A modern four-bladed hingeless coaxial rotor model—inspired by the gross dimensions of the Sikorsky S-97 Raider but generic and open source otherwise—is developed as a demonstration test case. The new structural solver is driven by parallel and scalable solvers and advanced high performance computing. It is enabled by high-order three-dimensional brick finite elements unified with multibody dynamics, integrated aerodynamics, and a special 3D-to-1D fluid-structure interfaces refines the power of delta-coupling procedure while retaining the advantages of existing CFD mesh motion schemes. The analysis predicts the three-dimensional stresses on the rotor blades and hub, together with the deformations, airloads, and wake, in an integrated manner. Two flight conditions are studied: a low-speed flight at 37 knots and a high-speed flight at 150 knots. Interesting three-dimensional unsteady stress patterns are revealed all across the blade but particularly inboard of 50% rotor radius—patterns that change from flight to flight and have remained invisible until now—since they could neither be predicted nor measured in flight. The maximum axial stresses exhibited 3/rev variation at low speed, and 2/rev variation at high speed flight. The lower rotor carried higher oscillatory stress burden at low speed, whereas both the rotors shared the same stress burden at high speed flight. The key conclusion is that such analysis is now indeed possible, and the stress patterns they reveal provide deeper insights into the dynamics of advanced rotors, and these might provide a path toward mitigating them in the future.

Plastic zones for ductile layer sandwiched between two different substrates

This study is an extension of previously developed models of plasticity occurring in the vicinity of ​​a longitudinal crack in a thin elastic-plastic layer sandwiched between two identical, ideally elastic substrates: Varias–Suo–Shih [1], Rogowski-Molski [2], and Rogowski [3]. The solution presented in the current study describes the plastic zone size problem in a thin constrained layer bonded to two different substrates. A well-known two-term K-T Williams [4] stress solution for interfacial cracks is used to obtain the plastic zone size. A new equation with coefficients dependent on the layer and substrate material properties was obtained, allowing for the calculation of the plastic zone size in the thin ductile layer based on the values of the complex stress intensity factor K and T-stresses. The proposed model is tested for several arbitrarily selected interfacial crack geometries, load conditions, bonding layers, and substrate material properties. Because of the complicated oscillatory description of the local stress field, the final solution of the mathematical equation is obtained numerically, ignoring a small zone of intense stress oscillations near the crack tip. Good agreement is obtained between the results from the proposed model and the FEM results in terms of quantity and quality. A novel approach may be utilized in the future as a method for estimating the parameter of fracture criterion for bonded joints.

Dynamic stress prediction for a Pump-Turbine in Low-Load Conditions: Experimental validation and phenomenological analysis

To meet the increasing demand for higher flexibility in hydropower, an operating range extension in an existing hydroelectric powerplant is considered. To this end, the dynamic flow and structural behavior of the low-head pump-turbine is investigated in turbine mode in partial load and deep partial load conditions, to evaluate the feasibility of this flexibilization due to the increased high cycle fatigue damage. In a first step, different numerical simulation approaches that combine fluid dynamics and structural dynamics are applied and compared to in-situ dynamic strain and pressure measurements on the impeller. As a result, the modelling requirements for a numerical workflow that leads to unmatched accuracy in predicting the broad-band dynamic stresses of deep partial load are identified. In a second step, the operating-point-dependent fatigue crack initiation spots are identified based on dynamic stresses. Herein, the emergence of an additional dominant critical spot near the blade leading edge is observed, which is unexpected for low-head pump turbines. Thanks to the detailed numerical flow and structural analyses, the root cause for the dynamic stress concentration is studied and the emergence of the new critical spot is understood. It turns out in deep partial load, that the main source of pressure oscillations on the impeller blade surfaces is the vortex-dominated flow detachment zone around the blade leading edge, which, in combination with the geometry of the investigated impeller, translates into the main stress oscillations. The results of this work were later used to extend the operating range of the hydroelectric powerplant from initially 25% to 100% power output to now 0% to 100%.

EXPERIMENTAL STUDY OF NON-EQUILIBRIUM RHEOLOGICAL EFFECTS DURING THE FLOW OF PARAFFINIC OILS OF TIMAN-PECHORA BASIN. THIXOTROPY, SUPER ANOMALOUS VISCOSITY, OSCILLATIONS OF SHEAR STRESS

Laboratory experiments were carried out to study the non-stationary and non-equilibrium flow regimes of some paraffinic and high paraffinic crude oils of the Timan-Pechora Basin in a rotational viscometer. The presence of thixotropic properties (shear thinnig and hy steresis loops on the flow curves) is shown. The threshold character of the dependence of the area of the hysteresis loop on the oil tempera ture is demonstrated. Experiments have shown that under certain conditions on the flow curves of the studied oils, there are areas of a decrease in shear stress with an increase in shear rate, similar to areas of a «super anomalous» viscosity (SAV) in plastic lubricants. The shape and size of these areas, i.e. the intensity of the «superanomality» manifestation strongly depends on the experiment conditions and are poorly reproducible from experiment to experiment. In experiments, the SAV phenomenon at temperatures below the gelation temperature was always observed when the sample of paraffinic oil was at rest for a sufficiently long time before deformation. If, however, it was in the gap between the viscometer cylinders after the end of the next measurement cycle for a short time (no more than a few minutes), then at the next cycle of recording the flow curve, the SAV phenomenon can be practically imperceptible, although the hysteresis remains. Experiments show that if a sample of high paraffinic oil is deformed at a constant low shear rate, i.e. constant angular velocity of rotation of the device cylinder, and a temperature below the temperature of gelation, then on the falling curve of the dependence of shear stress on time («shear thinning»), oscillations in shear stress with a period equal to the period of rotation of the viscometer cylinder can be observed.

Multilayer heterostructure inhomogeneous model for the functionally graded spherical shell with rotation effect for arbitrarily varying material properties

With the development of functionally graded materials (FGMs) composite technology, structural safety analysis related to FGMs has been widely concerned. An innovative multilayer heterostructure power-law inhomogeneous (MHPI) model is taken to solve the rotating FGMs hollow spherical shell with arbitrarily varying material properties (elastic modulus and density) along the radial direction. The basic idea of MHPI model is that the rotating FGMs spherical shell is divided into multiple sublayers, and two power functions approximate the elastic modulus and density of each sublayer respectively. For each single sublayer, the analytical solutions include two undetermined constants, which can be obtained from the boundary and continuity conditions. Many numerical examples of the rotating FGMs spherical shell are discussed, involving seven property profiles gradient laws assumptions, three boundary conditions, and nine volume fraction gradient models. Finally, this study further discusses the rotating FGMs spherical shell optimization design. The greatest advantage of this study is that the elastic response of the rotating FGMs spherical shell under various gradient assumptions is completely discussed. The numerical results show that the MHPI model can solve the problem of circumferential stress oscillation well and have good convergence and high accuracy. For many numerical examples, the error of the MHPI model results is about 5‰ when sublayer number reaches 15.

Discrete modeling of elastic heterogeneous media

Discrete models provide advantages in simulating fracture in quasi-brittle materials due, in part, to their simplicity in representing cracking and other forms of displacement discontinuity. However, the stress analyses that form the basis for fracture simulation are complicated by difficulties in modeling the Poisson effect and other aspects of elastic behavior. The capabilities of Voronoi-cell lattice models, which are a form of particle-based lattice model, for elastic stress analysis are evaluated. It is found that the conventional means for representing the Poisson effect in particle-based lattice models result in spatially correlated stress oscillations that, at first glance, mimic the effects of material heterogeneity. The correlation length is dependent on discretization size. Alternatively, material heterogeneity can be introduced into elastically uniform lattice models via random assignments of material properties, independent of mesh size and geometry.

Importance of sign conventions on analytical solutions to the wave-induced cyclic response of a poro-elastic seabed

This paper discusses the influence of different sign conventions for strains and stresses, i.e. the solid mechanics sign convention and the soil mechanics sign convention, on the form of governing partial differential equations (the static equilibrium equations and the continuity equation) used to describe the wave-induced cyclic response of a poro-elastic seabed due to propagation of a sinusoidal surface water-wave. Some selected analytical solutions, obtained by different authors and published in specialist literature in the form of complex functions describing the wave-induced pore-fluid pressure, effective normal stress and shear stress oscillations in the seabed, have been analysed and compared with each other mainly with respect to different sign conventions for stains and stresses and also with regard to different orientations of the positive vertical axis of the two-dimensional coordinate system and different directions of surface water-wave propagation. The performed analyses of the analytical solutions has indicated many inaccuracies, or even evident errors and exemplary mistakes of wrong-signed values of basic wave-induced response parameters (the shear stress in particular), thereby disqualifying these solutions and their final equations from practical engineering applications. Most of the mistakes found in the literature must be linked to authors’ lack of understanding and consistency in an uniform application of a certain sign convention for strains and stresses in the soil matrix at both stages of mathematical formulation of the governing problem and correct interpretation of equations of the final analytical solution. The present paper, based mostly on a thorough literature review, ought to draw attention and arouse interest among coastal scientists and engineers in proper identification and use of the existing analytical solutions to the wave-induced cyclic seabed response – solutions which differ very often in the applied sign convention for stresses in the soil matrix.

Coupling strength comparison in optical fiber with multiple sector shape stress elements

AbstractThe maximum stress oscillation amplitude and maximum coupling strength of the optical fiber with multiple sector stress elements are studied. Optimized fiber structure with craft feasible is designed and the related structural parameters are given. Comparing the research results, it is found that the optical fiber with triple sector stress elements can achieve the strongest coupling strength to realize the single circular polarization state conversion by the shortest coupling distance. In addition, though the maximum coupling strength is decreased as the number of stress elements increases, it remains the same order of magnitude as the triple sector's. So from the craft perspective, whether it is an optical fiber with triple sector stress elements or the other optical fiber with more sector stress elements, the optical fiber with the property of single circular polarization transmission can be achieved if the spinning rate satisfies the phase matching condition.

Fretting fatigue crack nucleation analysis under a variable coefficient of friction

Fretting fatigue is a phenomenon that occurs when two surfaces come into contact and experience oscillatory stresses and displacements of small amplitude. This can lead to a significant reduction in mechanical parts’ lifetime. The complexity of this type of problems relies on the many variables that can affect the results. One of its most important variables is the Coefficient of Friction (CoF), which has been observed to vary throughout a single fretting test. However, this variation is rarely considered in fretting fatigue finite element analyses. In order to analyse the effects of a Variable Coefficient of Friction (VCoF) on nucleation lifetime, a cylinder-on-plane finite element model is developed. To predict nucleation lifetime, two critical plane damage parameters, Fatemi-Socie (FS) and Findley parameter (FP), are used along with an evolving CoF during the first stress cycles. These parameters are evaluated by calculating initiation lifetime where the maximum value of the damage parameter occurs (hotspot method) and by the quadrant averaging method, because of its capability of predicting lifetime with relative accuracy and negative crack angles. To simulate the evolution of the CoF, a Python script that changes the value of CoF before each simulation is developed. This coefficient varies between 0.3633 and 0.65, following arbitrary points on the coefficient’s evolution curve. Once the simulation is concluded, the script evaluates the damage parameters and their equivalent lifetime throughout the contact surface and saves those values in an accumulation parameter that follows Miner’s rule. This process is repeated until CoF reaches its maximum value. The lifetime to nucleation is statistically analysed for all methodologies. The results suggest that, regarding the hotspot method, FP values remain within ± 3 N deviations from the experimental results, while FS results are nearer, within ± 2 N deviations. In addition, the quadrant method results remain at ± 3 N deviations, but providing more accurate results for the FP parameter. The application of a VCoF to these methods leads to less conservative and less deviated results, providing more accuracy for conservative approaches and more consistency for predictions in general. It is concluded that VCoF analysis is an interesting tool to determine more accurately nucleation lifetime for conservative approaches. Additionally, it reduces the predictions variation for differently loaded fretting setups regardless the desired approach.

Characterization of hot workability of a Cu containing high strength low alloy steel using processing map

Hot workability of an experimental Cu containing martensitic high strength low alloy (HSLA) steel was characterized by conducting isothermal hot compression tests at different temperatures (900–1100 °C) and strain rates (0.0003–1 s−1). Flow curves exhibits predominantly single peak behaviour at temperatures for a strain rate > 0.01 s−1. At temperature > 950 °C and for low strain rates of 0.0003 and/or 0.001 s−1, multiple peaks characterised by oscillations in flow stress was observed. The strain rate sensitivity (m), temperature sensitivity (S) of the material was calculated and the processing map was developed to identify optimum condition for thermo-mechanical processing. The material exhibits three suitable processing windows in temperature – strain rate space viz., (1) 1050–1100 °C; 0.1–0.01 s−1 (2) 980–1020 °C; 0.1–1 s−1 and (3) 980–1020 °C; 0.001–0.0003 s−1. The microstructure of the material deformed under all the above mentioned processing conditions exhibited defect free equiaxed dynamically recrystallized grains with an average grain size of 30–40 μm. Any or combination of these processing conditions can be used by the industry for designing suitable forging process (roughing, finishing etc.) based on the infrastructure available. Further, using Avrami approach, the effect of strain, strain rate and temperature on the kinetics of dynamic recrystallization (DRX) was also evaluated. The results obtained are presented and discussed.

Calcium hydroxide-loaded nanoparticles dispersed in thermosensitive gel as a novel intracanal medicament.

Design, produce and assess the viability of a novel nanotechnological antibacterial thermo-sensible intracanal medicament This involves encapsulating calcium hydroxide (Ca(OH)2) within polylactic-co-glycolic acid (PLGA) nanoparticles (NPs) and dispersing them in a thermosensitive gel (Ca(OH)2-NPs-gel). In addition, perform in vitro and exvivo assessments to evaluate tissue irritation and penetration capacity into dentinal tubules in comparison to free Ca(OH)2. Reproducibility of Ca(OH)₂-NPs was confirmed by obtaining the average size of the NPs, their polydispersity index, zeta potential and entrapment efficiency. Moreover, rheological studies of Ca(OH)2-NPs-gel were carried out with a rheometer, studying the oscillatory stress sweep, the mean viscosity value, frequency and temperature sweeps. Tolerance was assessed using the membrane of an embryonated chicken egg. In vitro Ca(OH)2 release was studied by direct dialysis in an aqueous media monitoring the amount of Ca(OH)2 released. Six extracted human teeth were used to study the depth of penetration of fluorescently labelled Ca(OH)2-NPs-gel into the dentinal tubules and significant differences against free Ca(OH)2 were calculated using one-way anova. Ca(OH)2-NPs-gel demonstrated to be highly reproducible with an average size below 200 nm, a homogeneous NPs population, negative surface charge and high entrapment efficiency. The analysis of the thermosensitive gel allowed us to determine its rheological characteristics, showing that at 10°C gels owned a fluid-like behaviour meanwhile at 37°C they owned an elastic-like behaviour. Ca(OH)2-NPs-gel showed a prolonged drug release and the depth of penetration inside the dentinal tubules increased in the most apical areas. In addition, it was found that this drug did not produce irritation when applied to tissues such as eggs' chorialantoidonic membrane. Calcium hydroxide-loaded PLGA NPs dispersed in a thermosensitive gel may constitute a suitable alternative as an intracanal antibacterial medicament.

The Effect of Normal Stress Oscillations on Fault Slip Behavior Near the Stability Transition From Stable to Unstable Motion

AbstractTectonic fault zones are subject to normal stress variations with a wide range of spatio‐temporal scales. Stress perturbations cover a wide range of frequencies and amplitudes from high frequency seismic waves generated by earthquakes to low frequency transients associated with solid Earth tides. These perturbations can reactivate critically stressed faults and trigger earthquakes. Here, we describe lab experiments to illuminate the physics of such changes in friction and the mechanics of earthquake triggering and fault reactivation. Friction tests were done in a double direct shear configuration for conditions near the stability transition from stable to unstable motion. We studied simulated fault gouge composed of quartz powder and conducted experiments at reference normal stress from 10 to 13.5 MPa. After shearing to steady state sliding, we applied sinusoidal normal stress oscillations of amplitude 0.5–2 MPa, and period of 0.5–50 s. We performed numerical simulations using measured values of rate and state friction (RSF) parameters to assess our data. Our results show that low frequency stress oscillations cause a Coulomb‐like response of shear strength that transitions from stable slip to slow lab earthquakes as frequency increases. At the critical frequency predicted by RSF we observe periodic stick‐slip behavior. Perturbations of high amplitude and short period weaken the fault, while lower amplitudes strengthen the fault. We find that a modified RSF formulation is able to accurately match our laboratory data. Our findings highlight the complex effects of stress perturbations on fault strength and the mode of fault slip.