Small Stress Research Articles

Stress Orientations in the Nankai Trough Constrained Using Seismic and Aseismic Slip

AbstractThe strength of subduction thrust faults is key to understanding seismogenesis at the provenance of Earth's largest earthquakes. Earthquake focal mechanisms are routinely inverted to constrain the stress state at seismogenic depths. However, on some megathrusts, deformation is accommodated by both earthquakes and types of slow fault slip. We employ focal mechanisms of short‐term slow slip events (SSEs), a type of slow fault slip, and earthquakes in a regional stress inversion to investigate the stress state of the Nankai Trough megathrust and interpret the results in the context of regional tectonics Previous studies using earthquake‐only stress inversions found principal stress orientations in this region that are incompatible with thrust faulting on the megathrust. When both SSEs and earthquakes are considered, the stress state of the central and eastern Nankai Trough megathrust is well oriented for thrust faulting. Our results suggest that slow fault slip source regions may appear to have misoriented stress fields if slow fault slip constitutes a substantial proportion of fault slip and the stress field is not well constrained by earthquakes. In the SSE region, we find that faults are well oriented for failure, suggesting they have strengths similar to their surroundings. Combined with low Vp/Vs ratios and sensitivity to small stress changes, our results imply that the megathrust and surroundings operate at low deviatoric stresses in the SSE source region. Further, we show that the coefficient of friction for areas hosting SSEs is low (μ=0.19–0.50), implying frictionally weak materials in the SSE source region.

Yielding, thixotropy, and strain stiffening of aqueous carbon black suspensions

We study experimentally the rheological behavior of carbon black (CB) suspensions in water at different ionic strengths and concentrations. We show by means of standard rheometry completed by local magnetic resonance imaging-rheometry that these suspensions first appear to be thixotropic yield stress fluids: they exhibit a yield stress increasing with the time of rest, their apparent viscosity decreases under shear, and a viscosity bifurcation occurs around the yield stress, the fluid evolving either towards stoppage or to steady flow at a large shear rate for a small stress change. Then, an original effect appears when we follow the mechanical state of the material in the solid regime by measuring its apparent elastic modulus at small deformation during a creep test under various stresses. In contrast with various other yield stress fluids for which the elastic modulus under small deformation appears to be constant for any deformation in the solid regime, for CB suspensions, this modulus widely increases while deformation increases up to yielding. We suggest that this strain stiffening effect finds its origin in the specificities of the (van der Waals) interactions and of the (rough) structure (aggregates) of the particles: the slight relative rotation of particles in contact due to deformation would, on average, tend to increase the net area of contact between particles, which stiffens the whole material structure. This is supported by the observation that the relative increase of elastic modulus is approximately proportional to sample deformation, whatever the material characteristics (ionic strength, concentration) and whatever the deformation history.

Effects of Nanoscale Roughness on the Lubricious Behavior of an Ionic Liquid

AbstractWhile many mechanistic studies have focused on the lubricious properties of ionic liquids (ILs) on ideally smooth surfaces, little is known about the mechanisms by which ILs lubricate contacts with nanoscale roughness. Here, substrates with controlled density of nanoparticles are prepared to examine the influence of nanoscale roughness on the lubrication by 1‐hexyl‐3‐methyl imidazolium bis(trifluoromethylsulfonyl)imide. Atomic force microscopy is employed to investigate adhesion, hydrodynamic slip, and friction at the lubricated contact as a function of surface topography for the first time. This study reveals that nanoscale roughness has a significant influence on the slip along the surface and leads to a maximum slip length on the substrates with intermediate nanoparticle density. This coincides with the minimum friction coefficient at sufficiently small contact stresses, likely due to the lower resistance of the IL film to shear. However, at the higher pressures applied with a sharp tip, friction increases with nanoparticle density, indicating that the IL is not able to alleviate the increased dissipation due to roughness. The results of this work point toward a complex influence of the surface topology on friction. This study can help design ILs and nanopatterned substrates for tribological applications and nano‐ and microfluidics.

Experimental study of micro-deformation of structural steel under simulation of operating conditions

Diagnostics of the load bearing capacity of machines and structures, service properties of structural materials are carried out by destructive and non-destructive methods. In the system of non-destructive methods of diagnosing and monitoring of the service properties of materials and products, radiation technologies are of particular importance, since they have instrumental capabilities for diagnosing and controlling changes in the soundness of a solid body at different structural levels. The X-ray diffractometry method provided determination of the stability of the stress-strain state of structural steel samples subjected to prolonged (over 5 years) static loads not exceeding the yield strength of the material. The purpose of the study is to determine experimentally the effect of continuous (since 2013) elastic stress and climatic factors on the change in the half-width of the diffraction line profile. It is shown that the straight-line dependence of the half-width of the diffraction line profile is maintained in the range of elastic stresses not exceeding 0.5σt. The results of changes in the microstructural state (micro-strains) identified by the characteristics of the diffraction lines profile are presented and discussed. The results of the experimental study of the effect of small steady elastic stresses reveal that periodic annual fluctuation of temperatures (2013 – 2018) does not cause a significant change in the properties of the diffraction line profile of the 08ps structural steel samples. On the contrary, low climatic temperatures contributed to the elimination of individual instrumental errors attributed to design conditions of the experiment. A sharp change in the true half-width of the diffraction line profile at the stress values σ > 0.5σt, probably indicates the minimum margin of safety of 08ps structural steel when setting the permissible stress value (σ). The revealed changes in microplastic deformation observed in structural steels in the range of elastic stresses corresponding to real operational loads require further study and analysis.

Elastocaloric effects in polycrystalline Ni-Fe-Ga foams with hierarchical pore architecture

Ni-Fe-Ga foam with hierarchical pore architecture was created by replication casting. The annealed foam had an open porosity 53% and reversible superelasticity with recoverable strain 4.9%. An adiabatic temperature change of 4.1 K with low hysteresis and coefficient of performance 22 were obtained under an external stress 70 MPa. The foam showed improved cyclic stability ($\ensuremath{\sim}2.8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ in 194 cycles) because of low hysteresis loss under small stress and high crack initiation/propagation resistance in small size nodes and struts, which demonstrated a feasible strategy to enhance the mechanical and functional stabilities of ferromagnetic shape memory alloys via tailoring the material architectures. Nevertheless, the achieved elastocaloric cooling stability still cannot satisfy (${10}^{7}$ cycles) the commercial application, therefore, it should be further explored to become a promising candidate for elastocaloric material.

Thermal Guanidine Metathesis for Covalent Adaptable Networks.

We demonstrate that a dynamic chemical reaction that we term thermal guanidine metathesis (TGM) can serve as the basis for covalent adaptable network (CAN) materials. CANs are a class of cross-linked polymers that transition from thermoset to thermoplastic-like rheological behavior upon significant activation of reversible exchange reactions within the network and thus can be reprocessed. Small molecule studies indicate the TGM reaction proceeds by a dissociative mechanism, and guanidine-cross-linked network polymers can be reprocessed at elevated temperature. These TGM-based CANs exhibit dynamic behavior, such as dissolution in the presence of monofunctional exchange partners and stress relaxation above Tg. Additionally, differences in the activation energies obtained by small molecule kinetic studies and stress relaxation analysis are consistent with key predictions of the Semenov-Rubinstein model of thermoreversible gelation of highly cross-linked networks.

Stability and dynamics of the fractured zygomaticomaxillary complex fixed using non-resorbable and resorbable 2- and 3-point miniplates, under physiological and maximal occlusal loads: a finite element analysis

Background: Given that zygoma fractures are the second most common facial trauma, knowledge of their treatments is of clinical value. Among these are fixation methods, which despite their importance are neglected in many aspects. Objectives: Therefore, for the first time in the literature, the present finite element analysis evaluated displacements and dynamics of the zygoma fixed using four 2- and 3-point resorbable and four non-resorbable plates under normal and maximal mastication loads. Methods: A maxillofacial CT scan of a man with linear fractures without severe displacements was used to model the zygoma and its adjacent bones. Seven combinations of resorbable and seven combinations of non-resorbable mini-plates 2mm thick were fixed on the zygoma (orbital rim, zygomaticomaxillary buttress [ZMB], and frontozygomatic [FZ]) using 6mm miniscrews. ZMB was fixed using an L-shaped 4-hole plate. The infraorbital rim was fixed with a curved 5-hole miniplate. The FZ suture area was fixed with a 4-hole miniplate. The model underwent 150N and 750N loads. Minimum and maximum displacements, rotational displacements, stresses, and strains of the zygoma models were calculated. Results: Non-resorbable fixation methods can yield much smaller stresses, strains, and displacements compared to resorbable fixations. Also the parameters were much smaller under the 150N load compared to the 750N load. The worst results belonged to the fixation of Rim and ZMB and the best results belonged to the fixation of ZMB-Rim, and especially FZ-ZMB, Rim-FZ, and FZ-ZMB-Rim. Conclusions: In patients with heavy masticatory forces, it is not recommended to use resorbable platesZygomatic fractures are the second most common facial injury, knowledge of their management is important. However, fixations methods, which are of importance are variable.

Plasticity through De-Twinning in Twinned BCC Nanowires

The deformation behaviour of twinned FCC nanowires has been extensively investigated in recent years. However, the same is not true for their BCC counterparts. Very few studies exist concerning the deformation behaviour of twinned BCC nanowires. In view of this, molecular dynamics (MD) simulations have been performed to understand the deformation mechanisms in twinned BCC Fe nanowires. The twin boundaries (TBs) were oriented parallel to the loading direction [110] and the number of TBs is varied from one to three. MD simulation results indicate that deformation under the compressive loading of twinned BCC Fe nanowires is dominated by a unique de-twinning mechanism involving the migration of a special twin–twin junction. This de-twinning mechanism results in the complete annihilation of pre-existing TBs along with reorientation of the nanowire. Further, it has been observed that the annihilation of pre-existing TBs has occurred through two different mechanisms, one without any resolved shear stress and other with finite and small resolved shear stress. The present study enhances our understanding of de-twinning in BCC nanowires.

Low-temperature superelasticity and elastocaloric effect in textured Ni–Mn–Ga–Cu shape memory alloys

The superelastic behavior and elastocaloric effect of a [001]-textured Ni50Mn23Ga25Cu2 polycrystal have been investigated below room temperature (256–294 K). The adiabatic temperature change caused by rapid unloading of uniaxial stress reached −4.3 K at 259 K. A small stress hysteresis of 14 MPa was observed. Due to the low hysteresis energy loss (0.37 MJ m−3) stemming from the texture structure, the coefficient of performance (COPmat) of our alloy was calculated to be 18, indicating that it is a promising elastocaloric candidate for low-temperature refrigeration.

An Elasto-plastic Contact Solving Method for Two Spheres

The traditional Hertz contact theory has been widely used in solving contact problems. However, it is only applicable to the elastic contact, and cannot truly reflect the contact stress distribution and contact radius in the elasto-plastic contact. In this work, based on the Hertz contact theory, a fast solving method is proposed to calculate the contact stress distribution and contact radius in the elasto-plastic contact between two spheres. It is assumed that the elastic contact only occurs at the outer edge of contact patch and its contact stress distribution satisfies the Hertz contact theory, and the contact stress distribution at the inner edge of contact patch can be superimposed by a constant contact stress and several small ellipsoidal contact stress distributions. Moreover, based on the equivalent relation between the resultant force of contact stress and the normal external load, the contact radius in the elasto-plastic contact can be solved. Finally, an elasto-plastic contact example of two spheres is given based on the power-law hardening material model, and the influences of material parameters, contact radii and normal external loads on the accuracy of the proposed method are discussed by comparing the differences between the numerical results by finite element method and the predicted ones by the proposed method. It is shown that the proposed method can accurately calculate the maximum contact stress and contact radius in the elasto-plastic contact, and the relative errors of both maximum contact stress and contact radius are within $$\pm \,5{\%}$$. To sum up, the proposed fast solving method can be applied to perform the elasto-plastic contact analysis in engineering practice.

Assessment of Internal Stresses Using Dislocation Dipole Heights in Cyclically Deformed [001] Copper Single Crystals

There have been a number of studies on dipole separations in cyclically deformed FCC single crystals in single slip while there are no such studies in multiple slip. The dipole heights provide insight into the presence of long-range internal stresses (LRIS). In this study, we investigated how LRIS compare with the single slip studies through measuring the dislocation of dipole heights. [001] oriented copper single crystals were cyclically deformed in strain-control to saturation at ambient temperature. Transmission electron microscopy (TEM) confirms a labyrinth dislocation microstructure with high dislocation density walls and low dislocation density channels. The maximum dipole heights under the saturation stress were approximately independent of location, being nearly equal in the walls and within the channels. This, by itself, supports a uniform stress across the microstructure and low long-range internal stresses. The maximum value for dipole heights suggests dipole strengths (local stresses) that are about a factor of 2.4 higher than the applied stress based on the usual athermal equations. Considering the small “extra” stress that may be provided by tripoles or small dislocation pile-ups, a nearly homogenous stress distribution with only small internal stresses may be present, which is consistent with the observation of uniform dipole height across the heterogeneous dislocation microstructure. This observation that the stress state appears to be homogenous and higher than the applied stress has also been reported in the case of cyclically deformed metals in single slip.

Electromagnetic–Mechanical Coupling Optimization of an IPM Synchronous Machine with Multi Flux Barriers

In this paper, an interior permanent magnet (IPM) synchronous machine with multiflux barriers is proposed to meet the wide speed regulation application requirements of electric vehicles. Based on the flux barrier characteristic, an electromagnetic–mechanical coupling optimization strategy is employed for the machine design. In order to facilitate the optimization design, the rotor barriers are divided into two optimization zones, the maximum stress zone and the maximum deformation zone. The electromagnetic–mechanical coupling optimization strategy is divided into two stages accordingly. In the first stage, the machine is regarded as a synchronous reluctance machine by ignoring permanent magnets, where the dimensions of the arc-shaped barriers are optimized to achieve a large reluctance torque and small stress. In the second stage, the dimensions of the arc-shaped PMs and the elliptical barrier are optimized with three objectives of minimum torque ripple, minimum flux linkage, and minimum deformation. After machine optimization, the comparison investigations are carried out on the basis of finite-element analysis by considering both the electromagnetic performances and mechanical performances.

Fatigue Properties of Cruciform and Flange-gusset Joints Using Normal and High Strength Steels

The objective of this paper is to investigate and compare the fatigue behavior of Normal Strength Steel (NSS) and High Strength Steel (HSS) welded specimens (cruciform and flange gusset joints). Numerous specimens were tested in the framework of the research project. The experimental S-N curves of the test specimens are determined and analyzed by mathematical statistics to consider the uncertainties and scatter of test results. Statistical analysis of the S-N curves showed that the fatigue behavior of as-welded flange gusset joints is identical in the cases of HSS and NSS specimens, which means that there is no difference in the fatigue lifetime depending on the steel grade. However, the as-welded HSS cruciform joints can be more favorable and their fatigue lifetime is longer than the specimens made from NSS, if the stress range is below 100 MPa. The current study shows that the fatigue behavior of as-welded HSS details depends on the geometry of the investigated specimens and its fatigue lifetime can be larger in the small stress range region than for NSS specimens.

Decreasing Density of Grown-in Dislocations in AlN/c-Sapphire Templates Grown by Plasma-Activated Molecular Beam Epitaxy

AlN/c-sapphire templates grown by plasma-activated molecular beam epitaxy have been studied using multicrystal X-ray diffractometry and a multi-beam optical stress meter system. Studies of the seed and buffer layers grown at different ratios of Al and N* growth flows and substrate temperatures have shown that templates with small tensile elastic stresses (<0.5 GPa) and densities of screw and edge grown-in dislocations 4 × 108 and 8 × 109 cm−2, respectively, can be produced.

Numerical Simulation and Measurement of Welding Residual Stresses in Orthotropic Steel Decks Stiffened with U-Shaped Ribs

This study aims to investigate the magnitude and distribution of welding residual stresses in orthotropic steel decks stiffened with U-shaped ribs. Numerical simulation of the welding residual stresses is performed using the thermal elastic–plastic finite element method. The residual stresses are subsequently measured using the hole-drilling strain-gage method. Based on the results of this study, it can be determined that: (1) The longitudinal welding residual stresses (σz) in the weld zones are tensile stresses, where the peak value exceeds the minimum yield strength (fyl); however, relatively low compressive stresses exist in the areas located far away from the welds; (2) The stress gradient in the weld zone is steep, and the widths of the areas experiencing tensile stress are narrow; (3) In the case of the upper surface of the deck, the weld that is first deposited has the highest peak stress, while similar stresses are determined for the remaining welds. In the case of the lower surface of the deck, the peak stress in the weld zones is greater than that on the upper surface; (4) In the case of the weld zone of the U-shaped ribs, the tensile stresses reach up to 1.15fyl; however, at locations a short distance away from the welds, the σz varies from tensile to compressive, and finally transforms into a small tensile stress at the base of the U-shaped ribs. In addition, the simulated and measured σz are compared, and the deviation between them is analyzed.

Is There a Relationship Between Stress in Walls of Abdominal Aortic Aneurysm and Symptoms?

BackgroundAbdominal aortic aneurysm (AAA) is a permanent and irreversible dilation of the lower region of the aorta. It is typically an asymptomatic condition that if left untreated can expand to the point of rupture. In simple mechanical terms, rupture of an artery occurs when the local wall stress exceeds the local wall strength. It is therefore understandable that numerous studies have attempted to estimate the AAA wall stress and investigate the relationship between the AAA wall stress and AAA symptoms. Materials and methodsWe conducted computational biomechanics analysis for 19 patients with AAA (a proportion of these patients were classified as symptomatic) to investigate whether the AAA wall stress fields (both the patterns and magnitude) correlate with the clinical definition of symptomatic and asymptomatic AAAs. For computation of AAA wall stress, we used a very efficient method recently presented by the Intelligent Systems for Medicine Laboratory. The Intelligent Systems for Medicine Laboratory's method uses geometry from computed tomography images and mean arterial pressure as the applied load. The method is embedded in the software platform BioPARR—Biomechanics based Prediction of Aneurysm Rupture Risk, freely available from http://bioparr.mech.uwa.edu.au/. The uniqueness of our stress computation approach is three-fold: i) the results are insensitive to unknown patient-specific mechanical properties of arterial wall tissue; ii) the residual stress is accounted for, according to Y.C. Fung's Uniform Stress Hypothesis; and iii) the analysis is automated and quick, making our approach compatible with clinical workflows. ResultsSymptomatic patients could not be identified from the plots (pattern) of AAA wall stress and stress magnitude. Although the largest stress was predicted for a patient who suffered from AAA symptoms, the three patients with the smallest stress were also symptomatic. ConclusionsThe results demonstrate, contrary to the common view, that neither the wall stress magnitude nor the stress distribution appears to be associated with the presence of clinical symptoms.

Small Sample Stress: Probing Oxygen-Deprived Ammonia-Oxidizing Bacteria with Raman Spectroscopy In Vivo.

The stress response of ammonia-oxidizing bacteria (AOB) to oxygen deprivation limits AOB growth and leads to different nitrification pathways that cause the release of greenhouse gases. Measuring the stress response of AOB has proven to be a challenge due to the low growth rates of stressed AOB, making the sample volumes required to monitor the internal stress response of AOB prohibitive to repeated analysis. In a proof-of-concept study, confocal Raman microscopy with excitation resonant to the heme c moiety of cytochrome c was used to compare the cytochrome c content and activity of stressed and unstressed Nitrosomonas europaea (Nm 50), Nitrosomonas eutropha (Nm 57), Nitrosospira briensis (Nsp 10), and Nitrosospira sp. (Nsp 02) in vivo. Each analysis required no more than 1000 individual cells per sampling; thus, the monitoring of cultures with low cell concentrations was possible. The identified spectral marker delivered reproducible results within the signal-to-noise ratio of the underlying Raman spectra. Cytochrome c content was found to be elevated in oxygen-deprived and previously oxygen-deprived samples. In addition, cells with predominantly ferrous cytochrome c content were found in deprived Nitrosomonas eutropha and Nitrosospira samples, which may be indicative of ongoing electron storage at the time of measurement.

Prospective of development of large-scale tidal current turbine array: An example numerical investigation of Zhejiang, China

Despite the rapid development of tidal current energy, understanding of its potential environmental impacts is still far from complete, especially for the region with considerable input of freshwater and sediment. As the large-scale tidal energy stations were deployed in Canada, the UK, China, Netherlands, France, and the US, concerns about its harm to the environment are growing. To address such issues, the Zhejiang area, one of the top tidal sites in the world, is taken as an example and a three-dimensional two-way-nested model was constructed. The embedded array of 5 turbines, 50 turbines, and 200 turbines are estimated to produce an average power of 0.9 MW, 7.8 MW, and 22.1 MW, respectively. The results show that currents are decelerated significantly downstream of the array, while accelerations are observed in the neighboring channel. Moreover, significant increases in tidal elevation are found in the small basins characterized by both deep water and great velocity deficits. Modifications to sediment transport are predicted through changes in the bed shear stress. Notably, disturbance from the turbines unlikely changes the locations of sediment erosion and deposition. However, considerable reductions in bed shear stress extending over 10 km downstream could potentially slow down the sediment transport rates. The presence of the turbine array also induces a more noticeable effect in the areas with more benign hydrodynamic conditions. Especially in the coastal waters, the relatively small bed shear stresses decrease greatly in percentage, reaching up to 40%.

Shear thickening of suspensions of dimeric particles

In this article, I study the shear thickening of suspensions of frictional dimers by the mean of numerical simulations. I report the evolution of the main parameters of shear thickening, such as the jamming volume fractions in the unthickened and thickened branches of the flow curves, as a function of the aspect ratio of the dimers. The explored aspect ratios range from $1$ (spheres) to $2$ (dimers made of two kissing spheres). I find a rheology qualitatively similar than the one for suspensions of spheres, except for the first normal stress difference $N_1$, which I systematically find negative for small asphericities. I also investigate the orientational order of the particles under flow. Overall, I find that dense suspensions of dimeric particles share many features with dry granular systems of elongated particles under shear, especially for the frictional state at large applied stresses. For the frictionless state at small stresses, I find that suspensions jam at lower volume fraction than dry systems, and that this difference increases with increasing aspect ratio. Moreover, in this state I find a thus far unobserved alignment of the dimers along the vorticity direction, as opposed to the commonly observed alignment with a direction close to the flow direction.

Effects of hydrogen influence on strained steel 1020

It is shown that the method of small stresses σ ∼0.1σy action on the metal created by the authors allows studying plasticity changes caused by the hydrogen action with high accuracy. The effects of elastic hysteresis amplitude ωh reduction under the action of several (n) hydrogenation cycles on non-strained samples and a significant (2.5 times) increase of ωh value after several hydrogenation cycles on strained samples have been detected. The effect of ωh(n) → const was also detected and on the basis of which it was concluded that the hydrogen harmful action was best observed during the first cycles of hydrogen action. It is shown that a giant increase of ωh over several months is a sign of unpredictable destruction of low carbon steel after hydrogen action.