Increase In Applied Stress Research Articles

In-situ electron channeling contrast imaging of local deformation behavior of lath martensite in low-carbon-steel

In-situ tensile Electron Channeling Contrast Imaging (ECCI) observations were performed on low-carbon steel lath martensite to elucidate its plastic deformation mechanisms under tensile loading. Two key deformation processes were identified through the direct observation of dislocation activity: intra-lath crystallographic slip and boundary sliding. The initial dislocation movement signifies the onset of microscopic yielding in lath martensite. As the applied stress increases, macroscopic yielding occurs when dislocations overcome internal barriers like high-dislocation-density walls within the laths. Out-of-lath-plane slip systems exhibit a strong interaction with the lath boundary, hindering their activity after initial small plastic deformation. In contrast, in-lath-plane slip systems predominantly contribute to plastic deformation. After the work hardening triggered by these slip systems, boundary sliding takes over the plastic deformation. The boundary sliding was found to accompany a rapid movement of the dislocation network adjacent to the boundary.

On pressure-driven Poiseuille flow with non-monotonic rheology.

Shear thickening fluids are liquids that stiffen as the applied stress increases. If many of these types of fluids follow a monotonic rheological curve, some experimental and numerical studies suggest that certain fluids, like cornstarch, may exhibit a non-monotonic, S-shaped rheology. Such non-monotonic behavior has however proved very difficult to observe experimentally in classical rheometer. To explain such difficulties, the possible presence of vorticity banding in the rheometer has been considered. To prevent such instabilities, we use a capillary rheometer, which is a cylindrical tube, measuring the flow rate versus the applied pressure drop. With this setup, we indeed observe a non-monotonic behavior: the flow rate increases monotonically at low pressure drops up to a maximum, after which it abruptly decreases to an almost constant flow rate regardless of further increases in pressure drop. This maximum-jump-plateau behavior occurs over a wide range of concentrations and is reproducible without hysteresis, which is in agreement with an S-shaped rheology. However, the obtained flow versus pressure difference function does not agree with the classical Wyart-Cates rheological model, which predicts an S-shaped non-monotonic function, but with neither a jump nor a plateau. To understand this jump-plateau behavior, we remark that any rheological model would establish a relationship between the flow rate and the local pressure gradient, but not the total pressure drop. We thus discuss and analyze the implications of having an S-shaped non-monotonic flow rate-pressure gradient in Poiseuille flow. In particular, we discuss the possibility of a non-uniform pressure gradient in the direction of the flow, i.e., a kind of streamwise banding. The key issue is then the selection of the gradient pressure distribution along the tube. One solution could arise from an analogy of this problem with the spinodal decomposition. It, however, leads to an increase in flow rate with up to a plateau between two values of as determined by the Maxwell construction. To account for the bump-jump behavior, we have implemented a simple dynamical stochastic version of the Wyart-Cates model, where the thickening occurs with a characteristic time. As a result, with increasing the total pressure drop, the flow rate increases monotonically up to a maximum value. Beyond this point, the flow rate drops abruptly to a lower value, forming a slowly decreasing plateau. This behavior is likely to account for the maximum-jump-plateau observed in the experiments. We also show that in such a system, the final state is quite sensitive to the initial state of the fluid, especially its homogeneity. Our results then demonstrate that the mere presence of a non-monotonic rheological curve is sufficient to predict the occurrence of stress banding in the streamwise direction and a plateau flow rate, even if the suspension remains homogeneous.

The effect of stress gradient on the formation of topologically close packed phases in a Ni-based single crystal superalloy at 1050 °C

The formation of topologically close packed (TCP) phases is detrimental to the mechanical properties of Ni-based single crystal (SX) superalloys, which are used in harsh environments with high temperature and gradient stress caused by centrifugal force during service. In order to study the precipitation behavior of TCP phases in coupling with temperature and stress gradient, a third generation Ni-based SX superalloy is selected to conduct force holding experiments at 1050 °C for 8 hours. Stress gradient can be realized under constant load by designing tensile samples with cross-sectional area continuously changed. The results indicate that the area fraction of TCP phases varies at different positions of the sample, where the effective tensile stress is different. The area fraction of TCP phases decreases at the thinner region of the sample, which means that the amount of TCP phases decreases as applied stress increases. At the thinnest position, where the stress is estimated to be 400 MPa, almost no TCP was observed. The inhibition of stress on the formation of TCP phase may be due to the reduction of Re concentration in the γ matrix, because of the segregation of Re at γ/γ′ phase interfacial dislocation cores and the dissolving of γ′ phase.

Experimental study of volumetric sand production with gas flow

The presence of sand in a production stream has been a common issue for wells drilled in high permeability sandstone formations. In the case of gas producing reservoirs, sand production can be particularly hazardous due to the high gas velocities. It can among others lead to erosion of downhole components and surface equipment and to significant production downtime. A comprehensive experimental study was conducted to investigate the effects of gas flow on sand production. This was accomplished by comparing gas flow tests to tests with oil or water flow. In these tests, hollow cylinder sandstone specimens, initially wet or at ambient humidity conditions, were isotropically compressed under simultaneous fluid injection. One-phase as well as two-phase flow experiments were carried out to simulate important processes encountered in gas fields, like water capillary flow or water-cut. The results showed that sand onset was delayed in the gas flow experiments, which agrees with experimental observations in the literature. The delayed sand onset, i.e., higher sand onset stress, was related to water evaporation due to gas flow, termed flow-through drying, which strengthens the rock. The water-sensitive strength of sandstones was also examined by completing a series of uniaxial compression tests at various water saturations. The analysis of the volumetric sand production data and the X-ray CT-scans of the tested specimens showed that the decreased capillary bonds between grains when only one fluid is present in the pores, e.g., one-phase water or gas flow, results in continuous sand production of individual grains or small-sized flakes, creating large voids in the vicinity of the hole. Under such conditions, the constantly enlarging failure zone eventually causes the total collapse of the specimen. In contrast, specimens with higher capillary cohesion due to a water-oil or water-gas interphase show post sand onset stability and lower sand rate where, due to higher grain to grain cohesion, chunks of failed rock material remain in the hole and provide additional support that prevents the rapid collapse of the specimen even as the applied stress increases.

Stress-induced influence on the evolution of γ′ phase in dendritic area during long-term stress aging at 900 °C in Ni-based superalloy K444

The evolution of γ′ phase in the Ni-based superalloy K444 alloy is investigated after long-term aging for 5000 h (LTA5,000) at 900 °C under different stresses (0 MPa, 50 MPa, 65 MPa, 80 MPa, 100 MPa and 120 MPa). The role of applied stress in the LTA5,000 process is analyzed in detail from multiple aspects, including morphology, composition partition, the formation of TCP phase, lattice misfit (δ) and dislocations configuration. The results reveal that the stress promotes the γ′ phase directional coarsening when it exceeds the critical value. For this alloy, the rafting structure is obvious when the stress is higher than 80 MPa, while γ′ phases maintain cuboid shape when the stress is below 65 MPa. It is caused by the stress gradient in the γ channels which accelerates the γ′ forming elements diffuse from horizontal γ channels to the vertical channels and only sufficient stress can activate the directional diffusion behavior. Besides, the stress induces that Al and Ti strongly assign to γ′ phases and the concentration of Cr in γ matrix increases clearly. In addition, the σ-TCP is free until the applied stress raises to 120 MPa, it is attributed to the increase in Cr concentration in the γ matrix. Moreover, the value of |δ| increases as the applied stress increases, and reducing interfacial energy becomes the driving force for directional coarsening. Finally, the calculated results based on the threshold stress demonstrate that the dislocations climb over the γ′ particles within the range of 65 MPa ∼ 120 MPa, which is in agreement with the observed phenomena.

Effect of compressive stress on corrosion protection performance of phytic acid conversion coating on carbon steel

As an environmentally friendly organic coating, phytic acid conversion coating (PACC) has been widely used for the protection of metals. In this study, the evolution of PACC on carbon steel under compressive stress is studied via multiple electrochemical and morphological approaches. Interestingly, electrochemical results show that a strengthening effect on PACC can be found when proper stress is applied, which is much distinct from the rule for oxide passive films. The proper compressive stress can effectively improve the corrosion protection performance of PACC, thus enhancing the corrosion resistance of carbon steel to chloride attack. As the applied stress increases, a detrimental effect is confirmed for compressive stress on the PACC layer. The protective PACC layer with the highest protection efficiency and coating adhesion is obtained at 100 MPa. In combination with the observations of SEM/XPS, a three-stage model is proposed to illustrate the evolution of PACC under compressive stress. In the natural stage, numerous cracks form in the PACC layer. In the strengthening stage, the PACC layer is compacted and the cracks in it are greatly healed. In the weakening stage, buckles and partial shedding emerge due to the overlarge stress.

Structure and Dynamics of Force Clusters and Networks in Shear Thickening Suspensions.

Dense suspensions can exhibit shear thickening in response to large deformation. A consensus has emerged over the past few years on the formation of force networks, that span the entire system size, that lead to increased resistance to motion. Nonetheless, the characteristics of these networks are to a large extent poorly understood. Here, force networks formed in continuous and discontinuous shear thickening dense suspensions (CST and DST, respectively) are studied. We first show the evolution of the network formation and its topological heterogeneities as the applied stress increases. Subsequently, we identify force communities and coarse grain the suspension into a cluster network, and show that cluster-level dynamics are responsible for stark differences between the CST and DST behavior. Our results suggest that the force clusters formed in the DST regime are considerably more constrained in their motion, while CST clusters are loosely connected to their surrounding clusters.

Abnormal creep stress exponents in a high-pressure die casting Mg–Al−RE alloy

Abnormal creep stress exponents (n) are frequently reported for heat-resistant magnesium alloys, but hitherto without in-depth elucidation or rationalization from the underlying creep mechanisms. In this work, tensile creep behavior of a high-pressure die casting Mg−3.76Al−3.52RE−0.23Mn (wt.%) alloy at 200 °C were studied, adopting small applied stress intervals of 1–2.5 MPa. The result indicates a roller-coaster n-value evolution of 5 → 23 → 38 → 8 → 45 → 76, as the applied stress increases from 80 MPa to 100 MPa. Microstructural analysis using transmission electron microscopy suggests that basal slip of <a> dislocations is dominant at lower stresses while dislocation climb at higher stresses and, simultaneously, twinning becomes more prominent and other sub-microstructures such as dislocation bands gradually occur as the applied stress increases such as to 87 MPa. By simultaneously introducing a threshold stress and an internal stress, the abnormal n-values are rationalized to 3.0 at relatively lower stresses and gradually to 7.7 at higher stresses, corresponding basal slip of dislocations to dislocation climb. Therefore, we suggest that alloy design aimed to improve creep resistance of magnesium alloys should pay more attention to impeding dynamic twinning during creep.

Investigating the stress level impact on the creep rupture behaviour of 2195-T84 Al-Li alloy: Experimental and constitutive modelling

In the present study, the creep rupture behaviour and microstructural evolution of 2195-T84 Al-Li alloy are investigated at different tensile stresses. It is found that as the applied stress during the creep rupture process increases, the corresponding creep strain and creep rate significantly increase. Moreover, the evolution of microstructures, including precipitates, dislocation density and creep cavities at different stages is characterized using a transmission electron microscope (TEM), X-ray diffractometer (XRD) and scanning electron microscopy (SEM). The main strengthening precipitates are identified as T1 (Al2CuLi) and θ′ (Al2Cu) phases. Obtained results show that as the applied stress increases, T1 and θ′ phases are gradually coarsened. This coarsening is more pronounced for θ′ phase. Furthermore, the creep cavities are mainly distributed at the interface between the insoluble second phase particles and the matrix, and their average sizes gradually increase as the applied stress increases. Meanwhile, the density and size of dimples on the fracture surface gradually decrease as the applied stress increases. Moreover, the main fracture mechanism changes from transgranular dimple fracture to quasi-cleavage fracture. Based on the microstructural evolution, a novel set of unified creep rupture constitutive model is proposed. The established model incorporates the evolution of microstructures, including the dislocation density, average length of T1 and θ′ precipitates and creep cavitation, and correlates microstructural variables with creep rate. The results calculated by the constitutive model are in good agreement with the experimental data, which validates the proposed model.

Turning a yield-stress calcite suspension into a shear-thickening one by tuning inter-particle friction

We show that a suspension of non-Brownian calcite particles in glycerol-water mixtures can be tuned continuously from being a yield-stress suspension to a shear-thickening suspension—without a measurable yield stress—by the addition of various surfactants. We interpret our results within a recent theoretical framework that models the rheological effects of stress-dependent constraints on inter-particle motion. Bare calcite particle suspensions are found to have finite yield stresses. In these suspensions, frictional contacts that constrain inter-particle sliding form at an infinitesimal applied stress and remain thereafter, while adhesive bonds that constrain inter-particle rotation are broken as the applied stress increases. Adding surfactants reduces the yield stress of such suspensions. We show that, contrary to the case of surfactant added to colloidal suspensions, this effect in non-Brownian suspensions is attributable to the emergence of a finite onset stress for the formation of frictional contacts. Our data suggest that the magnitude of this onset stress is set by the strength of surfactant adsorption to the particle surfaces, which therefore constitutes a new design principle for using surfactants to tune the rheology of formulations consisting of suspensions of adhesive non-Brownian particles.Graphical Impact of surfactants on the rheology of concentrated calcite suspensions. Black, schematic flow curve for bare particles. Low stress: adhesive contact, rolling and sliding between particles prevented (red). High stress: frictional contact, rolling allowed (green), constraints broken → shear thinning. Blue, with surfactant. Low stress: repulsion prevents contact, rolling and sliding allowed, removing yield stress entirely. High stress: surfactant displaced (see inset), particles bare and sliding prevented; constraints formed → shear thickening

Numerical analysis of the mechanical properties of rock materials under tiered and multi-level cyclic load regimes

In engineering practice, it is a common that rocks are subjected to cyclic loading. The mechanical properties of rock materials under tiered, single-level, and multi-level cyclic load regimes (i.e., with different frequency, minimum stress, and amplitude) were numerically investigated by using a dynamic constitutive model for rock materials. A comparison between these loads from the aspects of stress-strain curve, modulus, damage, and strength was undertaken. The results indicate that the model can effectively reflect the different mechanical properties of rock materials under different cyclic load regimes. Under the action of tiered and multi-level cyclic loading, the area and width of the hysteresis loops become larger with increasing applied stress, but the slope of stress-strain curve, area and width of the hysteresis loops, maximum strain, and the irreversible deformation vary under different loads. The loading deformation modulus decreases with the increase of applied stress and the reductions therein are different when subjected to tiered, single-level, and multi-level cyclic loading. With increasing applied stress, the damage evolution in rock specimens subjected to tiered cyclic loading present an increasing trend and the trend likes an approximately inverted “s” -shape. Under multi-level cyclic loading, the damage variables increase rapidly at the first level, then become relatively stable and increase slowly as the applied stress increases. Furthermore, the peak strength of rock differs under different load regimes, and results indicate that cyclic loading has both strengthening and damaging effects on rock strength.

Influence of micro-modulus functions on peridynamics simulation of crack propagation and branching in brittle materials

In this work, the micro-modulus function based on response function is defined. Five types of response functions are introduced to describe the spatial distribution of the intensity of nonlocal forces in prototype micro-elastic brittle (PMB) model. By using various micro-modulus functions in numerical models, the dynamic crack propagation and crack branching are reproduced, and the influence of micro-modulus function on crack propagation, single crack branching, and successive branching is analyzed. The crack propagation and branching patterns obtained by different micro-modulus functions are in close agreement with the previous experimental results that show the path length (L) before branching decreases as the applied stress increases. The times of the crack initiation and branch initiation and propagation are sensitive to the micro-modulus function in spite of the same pattern under different stress levels. The speed for crack branching and the average and maximum speeds of crack growth are influenced by the micro-modulus function. The branching speed after the first branch cracks can slightly increase or decrease or stay constant under various micro-modulus functions.

Elastocaloric cooling and heating using R-phase transformation in hot rolled Ni-Ti-Fe shape memory alloys with 2 and 4 at% Fe content

Iron-doped Ni-Ti shape memory alloys are known for their R-phase transformation. The stress-induced R-phase transformation and its reverse transformation can be utilized to create a temperature change based on elastocaloric heating and cooling. We studied the elastocaloric properties of hot rolled Ni48Ti50Fe2 and Ni46Ti50Fe4 alloys with three different approaches: 1) the calorimetry and specific heat measurements, 2) mechanical stress-strain behaviour and Maxwell relation based calculations and 3) direct measurement of adiabatic temperature change by stress application and removal. We compared the results related to different approaches and present the essential properties related to elastocaloric effect in these alloys. We show that the temperature change due to energy dissipation of the R-phase transformation is minor in comparison with the elastocaloric temperature change. The maximum adiabatic temperature change expected from the latent heat of R-B2 transformation is 12.4 K for the Ni48Ti50Fe2 alloy and 8.7 K for the Ni46Ti50Fe4 alloy. We also report an increasing tendency of the peak temperature for the elastocaloric effect when the applied stress increases.

Force-Clamp Rheometry for Characterizing Protein-based Hydrogels.

Here, we describe a force-clamp rheometry method to characterize the biomechanical properties of protein-based hydrogels. This method uses an analog proportional-integral-derivative (PID) system to apply controlled-force protocols on cylindrical protein-based hydrogel samples, which are tethered between a linear voice-coil motor and a force transducer. During operation, the PID system adjusts the extension of the hydrogel sample to follow a predefined force protocol by minimizing the difference between the measured and set-point forces. This unique approach to protein-based hydrogelsenables the tethering of extremely low-volume hydrogel samples (< 5 µL) with different protein concentrations. Under force-ramp protocols, where the applied stress increases and decreases linearly with time, the system enables the study of the elasticity and hysteresis behaviors associated with the (un)folding of proteins and the measurement of standard elastic and viscoelastic parameters. Under constant-force, where the force pulse has a step-like shape, the elastic response, due to the change in force, is decoupled from the viscoelastic response, which comes from protein domain unfolding and refolding. Due to its low-volume sample and versatility in applying various mechanical perturbations, force-clamp rheometry is optimized to investigate the mechanical response of proteins under force using a bulk approach.

Spatially Varying Small-strain Stiffness in Soils Subjected to K0 Loading

Grain-scale characteristics and formation history determine spatial variability in granular masses. We investigate the effect of spatially varying stiffness on the load-deformation response under zero-lateral strain conditions using numerical simulations of correlated random fields, where the granular medium is represented by a non-linear stress-dependent meso-scale model. Results show that stiffness heterogeneity results in higher global compressibility as compared to the homogeneous medium with the same arithmetic mean stiffness. Furthermore, the non-homogeneous stress field that develops inside the granular mass is characterized by focused load transfer along columnar regions, higher stress anisotropy and lower horizontal-to-vertical stress ratio K0 than in a granular medium of homogenous stiffness. As the applied stress increases, the inherent stress-dependent response of the granular material leads to a more homogenous stress field. While greater variance in stiffness causes lower global stiffness, a longer correlation length results in greater variance in global mechanical response among multiple realizations.

Creep deformation of a nickel-based single crystal superalloy under high stress at 1033 K

The microstructure of a nickel-based single crystal superalloy has been investigated after creep deformation at 1033 K under stress up to 850 MPa. High density of stacking faults are formed in γ′ precipitates during the creep deformation. At 700 MPa and 850 MPa, bi-directional stacking faults are observed. These stacking faults are induced by the movement of a/3<112> partial dislocations. Two formation mechanisms of a/3<112> partial dislocations, reaction and dissociation mechanisms, are simultaneously observed in this alloy after deformation. The proportion of a/3<112> dislocations formed by the dissociation mechanism increases as the applied stress increases. The primary creep strain of this alloy is 2.45% at 850 MPa, which may be attributed to the combination of these two mechanisms.

Creep rupture behavior of Hastelloy C276-BNi2 brazed joint

This paper studied the creep rupture behavior of Hastelloy C276-BNi2 brazed joint by experimental. Based on a series of creep rupture tests at 600°C under 110–250MPa, a relation between the stress and rupture time was achieved to evaluate the creep life of brazed joint. The results show that all the brazed joints were fractured in brazed seam immediately after the second creep stage. The creep model, fracture behavior and failure mechanism are dependent on the level of applied stress. The creep behavior of the brazed joint should be depicted by two-regime Norton (2RN) creep model rather than single Norton model. The creep damage tolerance factor was used to identify the cause of creep damage. From low to high stress level, the failure mode is changed from intergranular brittle to transgranular ductile fracture due to the increases of damage tolerance factor with applied stress increases.

Tension-compression asymmetry in the binding affinity of membrane-anchored receptors and ligands.

Cell adhesion plays a crucial role in many biological processes of cells, e.g., immune responses, tissue morphogenesis, and stem cell differentiation. An essential problem in the molecular mechanism of cell adhesion is to characterize the binding affinity of membrane-anchored receptors and ligands under different physiological conditions. In this paper, a theoretical model is presented to study the binding affinity between a large number of anchored receptors and ligands under both tensile and compressive stresses, and corroborated by demonstrating excellent agreement with Monte Carlo simulations. It is shown that the binding affinity becomes lower as the magnitude of the applied stress increases, and drops to zero at a critical tensile or compressive stress. Interestingly, the critical compressive stress is found to be substantially smaller than the critical tensile stress for relatively long and flexible receptor-ligand complexes. This counterintuitive finding is explained by using the Euler instability theory of slender columns under compression. The tension-compression asymmetry in the binding affinity of anchored receptors and ligands depends subtly on the competition between the breaking and instability of their complexes. This study helps in understanding the role of mechanical forces in cell adhesion mediated by specific binding molecules.

Time-Dependent and Stress-Dependent Chloride Diffusivity of Concrete Subjected to Sustained Compressive Loading

This paper studies the chloride diffusivity of concrete subjected to long-term exposure in a simulated marine environment with sustained compressive loading for up to 224 days. A series of concrete prisms were loaded under five different compressive stress levels (0, 0.2, 0.3, 0.5, and 0.7fc) and were subjected to different numbers (4, 8, 12, and 16) of wet/dry seawater exposure cycles. At the end of each exposure period, the concrete specimens were analyzed to determine their chloride penetration profile, the apparent chloride diffusion coefficient Da of the concrete, and the surface chloride content Cs. It was found that the Da and Cs values are both time and stress dependent and that these two dependencies have complex interactions. The value of Da decreases with time due to the hydration of the cement matrix, whereas the value of Cs increases over time. The effects of compressive stress on the values of Da and Cs strongly depend upon the applied stress level, which has a threshold value at approximately 30% of the ultimate compressive strength. Below this threshold, the value Cs remains constant; after this threshold, Cs increases linearly as the applied stress increases. Below the threshold, the value of Da decreases marginally as the stress level increases. However, above the threshold, Da increases rapidly with the stress level as a result of microcracking. Through regression of the chloride penetration profiles, empirical models are proposed to quantify the dependence of Da and Cs on the exposure time and stress level. The validity of these models is evaluated through comparisons with test results.

Effect of Stress on Energy Conversion and Storage Characteristics of (1-x-y)PIN-xPMN-yPT Single Crystals

Ferroelectric materials are used in many applications including, high-voltage generator, sonar, medical, electro-acoustic coupling, filters circuits and many more. In this work, the compositional dependence and stress dependence of the electromechanical properties of [011]C relaxor ferroelectric (1-x-y)Pb(In1/2Nb1/2)O3–xPb(Mg1/3Nb2/3)O3–yPbTiO3 (PIN-PMN-PT) single crystals has been determined. The specimens were subjected to mechanical loading and mechanical energy converted was found out by using Olsen-like cycle. It can be inferred that as the applied stress increases, converted energy density also increases. The maximum energy converted was found out to be 98.8 kJ/m3 for composition 0.32PIN-0.38PMN-0.30PT between 0 to 36 MPa. Also, the energy storage density for the specimens was found out. The maximum energy stored was found to be 98.8kJ/m3 for 0.24PIN-0.46PMN-0.30PT.