Strain Rate Change Research Articles

Mechanical Behavior of Closed-Cell Ethylene-Vinyl Acetate Foam under Compression.

The static and dynamic compressions of closed-cell ethylene-vinyl acetate (EVA) foams with different densities were conducted under various strain rates. The stress-strain curves were processed to determine the corresponding curves of energy absorption per unit volume and energy absorption efficiency, and energy absorption diagrams were produced. The influences of density and strain rate on the elastic modulus, yield strength, energy absorption per unit volume, optimal strain, densification strain, and energy absorption diagrams were analyzed and discussed. The whole stress-strain curve can be fitted with the Rusch formula. The strain rate does not change the shape of stress-strain curve, and has little influence on the elastic modulus. There exists the optimal density of EVA foam corresponding to its maximum energy absorption efficiency. Under a fixed strain rate, the optical energy absorption per unit volume is proportional to the optical stress on the envelope line in the energy absorption diagrams of EVA foams with different densities. The change in strain rate leads to the envelope line in the energy absorption diagrams of EVA foams with a given density having the larger slope and a negative intercept where the optical energy absorption per unit volume relies linearly on the optical stress. The empirical formulas of elastic modulus, yield strength, optimal strain, and envelope lines and their slopes are derived from the tested results.

Mechanical Behaviors of the Single Crystal Two-Dimensional Silicon Carbide (SiC): A Molecular Dynamics Insight

This paper focuses on the two-dimensional silicon carbide (2D-SiC), which has an excellent opportunity to be used as an alternative to graphene in nanotechnologies such as in nanoelectronics, nanoelectromechanical systems (NEMS), nano-sensors, nano-energy harvesting devices, and nano-composites due to its unique structural, mechanical, electronic, and thermal properties. Their mechanical properties characterize the stability of the nanodevices. This study performs molecular dynamics (MD) simulation to examine the mechanical properties based on optimized Tersoff potential of the single crystal 2D-SiC at different temperatures, strain rates, point vacancies, and edge cracks. At room temperature (300 K), the obtained elastic modulus and fracture strength are 423 GPa and 68.89 GPa, respectively, along the armchair direction. As the temperature rises from 100 K to 800 K, the fracture stress falls by 21.96% and the fracture strain by 36.90%. An approximate linear reduction in fracture strength is noticed as the temperature rises from 100 K to 800 K. The elastic modulus also falls as the temperature rises but is not significant. Although the elastic modulus is unchanged, the fracture stress increases by 1.84% while the fracture strain increases by 5.84% for a change in strain rate from 0.0001 ps-1 to 0.005 ps-1. The fracture stress and strain are significantly reduced, primarily due to the edge crack, as the concentration of point vacancy grows from 0.1% to 0.6% and the edge crack size increases from 0.5 nm to 1.5 nm. Moreover, anisotropic behavior is also evaluated at 300 K temperature and 0.001 ps-1 strain rate. These findings would offer a deep understanding of the fracture mechanics of 2D-SiC and also help to address the mechanical instability issue with SiC-based nanodevices.

Abstract 17781: Spironolactone Improves Left Atrial Function and Atrioventricular Coupling in Resistant Hypertension

Purpose: To determine the blood pressure independent effects of spironolactone on left atrial (LA) size and function in patients with resistant hypertension (RHTN). Methods: Patients with RHTN (N=36, 55±7 years) were prospectively recruited. Spironolactone was initiated at 25 mg/day and increased to 50 mg/day after four weeks. Other antihypertensives were withdrawn to maintain constant blood pressure. Cardiac magnetic resonance imaging was performed at baseline and after six months of spironolactone treatment and changes in LA functional metrics were assessed (Figure 1). The statistical analysis was conducted using two-sided tests of significance. Results: LA size and function parameters improved from baseline to month-six: LA volumes indexed to body surface area (LAVI) were reduced (LAVI max 41.4±12 vs 33.2±9.7 ml/m 2 ; LAVI pre-A 32.6±9.8 vs 25.6±8.1 ml/m 2 ; LAVI min 18.5 [13.9 - 24.8] vs 14.1 [10.9 - 19.2] ml/m 2 ; all p < 0.05); left atrioventricular coupling index was reduced (28.2±11.5 vs 22.7±9.2 %, p < 0.05); LA emptying fractions (LAEF) were increased (total LAEF 52.4 [48.7 - 60.3] vs 55.9 [50.3 - 61.1] %; active LAEF 40.2±8.6 vs 43.1±7.8 %, both p < 0.05). There was a substantial increase in reservoir strain (29.1 ± 8.5 % vs 30.9 ± 5.5 %, p = 0.068) whereas active strain was significantly increased from baseline (16.3 ± 4.1 % vs 17.8 ± 4.2 %, p < 0.05). Changes in passive strain and strain rates were not statistically significant (p > 0.1). The effect of spironolactone was similar in patients with high (N = 18) and normal (N = 18) aldosterone status (defined by plasma renin activity and 24-hour urine aldosterone). Conclusion: Treatment of RHTN with spironolactone is associated with improvements in LA size and function regardless of whether aldosterone levels were normal or high. This study suggests the need for larger prospective studies examining effects of mineralocorticoid receptor antagonists on atrial function and atrioventricular coupling.

The great calving in 2017 did not have a significant impact on the Larsen C Ice Shelf in the short term

ABSTRACT The stability of the Larsen C Ice Shelf (LIS) has been widely studied due to its surface lowering, calving, and warming. In this study, we combined multisource remote-sensing data, calculated the long-term series of surface elevation above basal channels and basal melt rate, and analyzed the dynamic changes in the LIS before and after calving in 2017. We found that the basal channels were mainly concentrated in the southern LIS, and the basal melt rate was higher in the south. The Weddell Sea warm water mass entered the bottom of the LIS through the sea trough and promoted basal melting. There was an abnormal basal channel where changes in sea ice concentrations caused by wind-driven currents led to abnormal development. We did not observe significant changes in ice thickness, ice velocity and strain rate in the short term before and after the great calving in 2017. However, the basal melt rate was mainly influenced by the sea ice extent in front of the LIS from 2010 to 2020. Changes in the sea ice extent indirectly altered the proportion of High Salinity Shelf Water (HSSW) entering the ice shelf bottom water mass, which in turn promoted (or inhibited) basal melting of the LIS. In other words, we found no evidence for any dynamic changes in the LIS before and after the great calving in 2017. Since the calving area was within the Passive Shelf Ice (PSI), we have also provided additional evidence for the existence of the PSI. In addition, we speculate that the impact of calving on the LIS will not be visible in the short term, so we need to continuously observe the subsequent dynamic changes in the LIS.

Rate-dependent transition of dislocation mechanisms in a magnesium alloy

The limited slip systems of magnesium (Mg) and its alloys hamper their widespread applications in key areas. Rational design of such lightweight alloys requires fundamental knowledge of their microscopic plasticity mechanisms which, however, remain partially unresolved. Here, to obtain a better understanding of the plastic deformation mechanisms of Mg alloys, we performed tensile straining over a wide range of strain rates from 10−5 s−1 up to 2000 s−1, revealing for the first time the occurrence of a rate-dependent transition of the dislocation mechanisms. Such a shift of plasticity mechanisms is identified by two distinct activation volumes. Systematic experimental characterizations, such as transmission electron microscopy under two-beam conditions and synchrotron X-ray diffraction analysis, were employed to analyze both qualitatively and quantitatively the characteristics of dislocations at changing strain rates, revealing that the rate-dependent dislocation mechanisms are accompanied by the change of dislocation activities from easy-glide 〈a〉 dislocations to glissile 〈c + a〉 dislocations with increasing strain rates. Specifically, temporarily glissile 〈c + a〉 dislocations, enabled by a thermally activated transformation of dislocation cores from the dissociated configuration to the compact one, govern the plasticity at high strain rates. This is in stark contrast to the dominance of the easy-glide 〈a〉 dislocations at low strain rates. Meanwhile, it is found that abundant 〈c + a〉 dislocations do not necessarily lead to enhanced ductility, contrary to the common belief. We expect that these results will contribute to a further understanding of the plasticity mechanisms of Mg alloys.

Rate controlling deformation mechanisms in SS316L stainless steel manufactured using laser powder bed fusion technique

We conducted uniaxial tensile strain rate jump tests to account for the strain rate controlling mechanisms in the additively manufactured SS316L stainless steel. Special emphasis is placed on the role of high dislocation density and chemical segregation on dictating the rate sensitive behavior. Lowest strain rate sensitivity is found, despite the highest dislocation density in the as build state, which signified the role of solute segregation at cell walls in increasing the activation volume and forms the strong basis for rejecting dislocation forest as the strain rate controlling obstacles. Consistent increase in the strain rate sensitivity with annealing is found consistent with the chemical homogenisation of the solutes causing smaller inter-obstacle spacing causing decreasing activation volume. Experimental findings are supported by CALPHAD simulations and mechanical threshold stress (MTS) modelling framework. Significant difference between the instantaneous and steady state rate sensitivity is understood using the dynamic pileup dislocation density readjustment mechanism causing considerable flow transient while adjusting the mobile dislocation density on strain rate change in the presence of solute decorated dislocation cells. Present investigation helps in gaining newer insights into the deformation mechanisms of the additively manufactured alloys featuring cellular microstructures.

Constitutive description of work hardening and dynamic softening behavior under variable deformation states

By carrying out hot compression experiments for 316 L austenitic stainless steel under dynamic changes in deformation conditions, the work hardening and softening behaviors are systematically analyzed during the temperature and strain rate changing process. The response pattern of stresses to changes in temperature and strain rate during deformation is related to material properties and deformation conditions. Zener Hollomon parameters can unify the influence. The stress response pattern changes as the value of Z increases or decreases. Further a computational procedure of materials constitutive relationship suitable for the deformation process under dynamic changes in deformation conditions is proposed. The calculation procedure is based on the phenomenal model. Compared with physical-based model, the procedure shows the same accuracy for the prediction of stress and degree of recrystallization and a simple establishment process. The calculation procedure is embedded into DEFORM software. The high-temperature deformation behavior is numerically simulated under dynamic changes in deformation conditions. The dynamic changes in deformation conditions affect the center deformation resistance during the deformation process, affecting the competitive coordination of material deformation resistance and indenter-sample interfacial friction and ultimately affecting the deformation distribution. 4)The microstructure evolution during dynamic variation states is revealed by EBSD characterization. The deformation conditions variation results in the hardening and softening mechanism changing. Especially, at 1000 ℃ the strain rate changing from 0.01 s−1 to 0.001 s−1 results in the recrystallization mechanism changing from CDRX to DDRX to CDRX.

The effect of precipitate-free zone on mechanical properties in Al-Zn-Mg-Cu aluminum alloy: Strain-induced back stress strengthening

The presence of precipitate-free zone (PFZ) near grain boundaries in precipitation-strengthened Al-Zn-Mg-Cu alloys has complex impact on mechanical properties, and the mechanism remains to be elucidated in detail. In this work, two Al-Zn-Mg-Cu alloy sheets with different features of PFZ were obtained by quenching in different media after solution heat treatment, and their deformation behavior was investigated by tensile at room temperature with different strain rates. It was found that variations in the distribution of dislocations in grain boundary regions can occur at room temperature due to the change of strain rates. These variations decrease the strength and elongation in samples with narrow PFZ (about 15 nm) at a low strain rate, but increase the strength and elongation in the samples with wide PFZ (about 112 nm). When the samples with wide PFZ are deformed at a low strain rate, dislocations emitted by grain boundaries pile up at the interface between PFZ and grain interior and lead to high back stress, resulting in higher strength and ductility. Therefore, it is possible to achieve strength-ductility synergy in Al-Zn-Mg-Cu alloys by manipulating the features and deformation behavior of PFZ near grain boundaries.

"Avatar", a Modified Ex vivo Work Loop Experiments Using In vivo Strain and Activation.

Movement behaviors are emergent features of dynamic systems that result from muscle force production and work output. The interplay between neural and mechanical systems occurs at all levels of biological organization concurrently, from the tuning of leg muscle properties while running to the dynamics of the limbs interacting with the ground. Understanding the conditions under which animals shift their neural control strategies toward intrinsic muscle mechanics ('preflexes') in the control hierarchy would allow muscle models to predict in vivo muscle force and work more accurately. To understand in vivo muscle mechanics, ex vivo investigation of muscle force and work under dynamically varying strain and loading conditions similar to in vivo locomotion is required. In vivo strain trajectories typically exhibit abrupt changes (i.e., strain and velocity transients) that arise from interactions among neural activation, musculoskeletal kinematics, and loads applied by the environment. The principal goal of our "avatar" technique is to investigate how muscles function during abrupt changes in strain rate and loading when the contribution of intrinsic mechanical properties to muscle force production may be highest. In the "avatar" technique, the traditional work-loop approach is modified using measured in vivo strain trajectories and electromyographic (EMG) signals from animals during dynamic movements to drive ex vivo muscles through multiple stretch-shortening cycles. This approach is similar to the work-loop technique, except that in vivo strain trajectories are scaled appropriately and imposed on ex vivo mouse muscles attached to a servo motor. This technique allows one to: (1) emulate in vivo strain, activation, stride frequency, and work-loop patterns; (2) vary these patterns to match in vivo force responses most accurately; and (3) vary specific features of strain and/or activation in controlled combinations to test mechanistic hypotheses.

Creep of autofrettaged thick-walled FGM cylindrical vessel

In this research work, the creep of thick-walled cylindrical vessels made of FGM with autofrettaged-treated was investigated using the analytical method. At different plastic depths and a variety of internal pressures for elastic modulus and yield stress constants ( n1 and n4), changes in residual creep strain rate (radial and circumferential) and final creep strain rate are investigated. The obtained results show that the circumferential stresses in the autofrettage have different values depending on the depth of the plastic. In the equivalent stress diagram, the resulting stress at the plastic points of the vessel reaches the yield stress of the FGM, and the stress decreases along the radius. If [Formula: see text] is increased from 1 to 1.8, the maximum creep strain rate decreases (for several n4, the dimensionless plastic depth is equal to 1.8). The results show that up to a dimensionless plastic depth of 1.5, the change in residual strain rate is negligible. The slope of the graphs for the dimensionless depths above 1.6 increases sharply. A comparison of the maximum circumferential stress as a function of plastic depth for the present work and the study by Darijani shows very good agreement between the two studies.

Crystallographic rotation of Ti-6Al-4V alloy with a fiber texture component under isothermal forging through experimental and CPFEM analyses

This study examined the crystallographic rotation behavior of α phase in a Ti-6Al-4V alloy under isothermal uniaxial forging at 800 ℃ and 850 ℃ with changing strain rates. The base material of a Ti-6Al-4V alloy bar comprised a strong <101¯0> fiber texture component along the longitudinal direction (designated as the Z-direction in this study). Compression tests (until a height reduction of 50%) were conducted along the Z-direction and radius direction, which is perpendicular to the Z-direction (hereafter designated as the Y-direction). Microstructural conversions for the forging conditions with strain rates lower than 10−1 s−1 occurred under continuous dynamic recrystallization in which grain sizes became finer under deformation. However, only at the high strain rate condition of 1 s−1, grain refinement accompanied by significant crystallographic rotations of [0001] from the radius direction to the longitudinal direction occurred when compressed parallel to the Z-direction. This study investigates the origin of the significant rotation of [0001] observed during compression along the Z-direction at a high strain rate of 1 s−1 through experimental and crystal plasticity finite element analyses. The results show that the anisotropic dislocation glides among prismatic <a>, basal <a>, and pyramidal <c+a> led to the significant rotation of [0001] under isothermal forging at a high strain rate of 1 s−1. Furthermore, the possibility of the frequent occurrence of twinning is discussed, and it was determined to be lower than that of the anisotropic dislocation glide for the origin of the significant rotation of [0001] observed during forging at a high strain rate of 1 s−1. Alternatively, a sub-grain formation and its rotation occurred at strain rates below 1 s−1, corresponding to the process condition of the enhanced thermally activated process.

Effects of Strain Rate and Size on Concrete under Shear Using Push-Off Tests

The direct shear behavior of concrete sections is evaluated using push-off specimens. The experimental program consists of three strain rates, ε˙=10−7 s−1, ε˙=10−5 s−1 and ε˙=10−3 s−1, that are used to understand how changes in strain rates affect concrete sections at brittle failures. The strain rates used in the tests represent the rates of the shear strain demands on structures due to mild-to-severe earthquakes. The rate effect on a broader range is extrapolated by finite element models. A robust and realistically prescribed material damage constitutive law, Microplane Model M7, is used to perform the computational analysis. A size effect analysis is also conducted to reveal the size dependence of the monolithic concrete joints under shear. It is shown that the rate effect on shear strength of concrete joints is not negligible at and above the strain rate levels examined in this study, and the size dependence of concrete sections under direct shear complies with the Size Effect Law, Type I.

Towards understanding the role of viscoelasticity in microstructural buckling in soft particulate composites

This work investigates the interplay between viscoelasticity and instabilities in soft particulate composites undergoing finite deformation. The composite is subjected to in-plane deformation at various constant strain rates, and experiences microstructural buckling upon exceeding the critical strain level. We characterize the dependence of the critical strain and wavelength on the applied strain rate through our numerical analysis.In the simulations, we employ the single and multiple-branch visco-hyperelastic models. We find that the critical strain and wavelength – characterized by the single-branch model – show a non-monotonic dependence on the strain rate, reaching a maximum at a specific strain rate. Remarkably, different buckling patterns (with different critical wavelengths) can be activated by changing strain rates. The space of admissible buckling modes widens in composites with higher instantaneous shear modulus. In the composites characterized by the multiple-branch model, the critical strain function exhibit multiple local maxima following a superposition of the single-branch responses. Typically, the branch with a larger relaxation time has a more significant effect on the critical strain. Moreover, the local maximum (of the critical strain function) is amplified by increasing the strain–energy factor of the corresponding branch term.Finally, we perform the experiments on the 3D-printed particulate soft composite characterized by a broad spectrum of relaxation times. The comparison of the experimental and simulation results demonstrates the ability of the numerical model to predict the critical buckling characteristics.

Numerical Study on Mechanical Responses during Quench Protection in High-Temperature Superconducting Coils.

In this paper, mechanical responses and electro-thermal characteristics of a rare earth barium copper oxide (REBCO) high-temperature superconducting (HTS) insulated pancake coil during the quenching process are investigated through finite element modeling (FEM). Firstly, a two-dimensional axisymmetric electro-magneto-thermal-mechanical FEM model with real dimensions is developed. Based on the FEM model, a systematic study on the effects of the time taken to trigger the system dump, background magnetic field, material properties of constituent layers, and coil size on quench behaviors of an HTS-insulated pancake coil is implemented. The variations in the temperature, current, and stress-strain in the REBCO pancake coil are studied. The results indicate that an increase in the time taken to trigger the system dump can increase the peak temperature of the hot spot but has no influence on the dissipation velocity. An apparent slope change of the radial strain rate is observed when the quench occurs regardless of the background field. During quench protection, the radial stress and strain reach their maximum values and then decrease as the temperature decreases. The axial background magnetic field has a significant influence on the radial stress. Measures to reduce peak stress and strain are also discussed, which indicates that increasing the thermal conductivity of the insulation layer, copper thickness, and inner coil radius can effectively reduce the radial stress and strain.

Experiment on deformation and failure characteristics of sandstone at different unloading rates

The unloading effect is an important factor for the failure of surrounding rock in deep underground engineering projects, especially under high-stress conditions. To investigate the deformation and failure characteristics of sandstone at different unloading rates, the evolution law of the surrounding rock stress caused by excavation was clarified. Then, the single-side unloading test of surrounding rock at different unloading rates was conducted using a true triaxial rock mechanics test system. During the test, the acoustic emission (AE) signals were monitored using an AE monitoring system. The test results show that excavation disturbance leads to complete single-side unloading at the boundary of the surrounding rock, and partial single-side unloading occurs as the depth of the surrounding rock increases. The ultimate strength of the sandstone specimen decreases as a power function with the increasing unloading rate. The unloading rock mass is mainly subject to shear failure. However, the increasing unloading rate raises the proportion of tension cracks. The sudden high strain rate on the unloading side can be used as precursor information of rock fracture, which can effectively prevent accidents caused by loss of rock bearing capacity. The AE signal is active and releases less energy in the unloading stage. At this time, the internal fractures are in the development stage, and the energy is still mainly accumulated. The unloading and failure are not synchronized but suffer from the hysteresis effect. Additionally, the AE and the strain rate change on the unloading side are consistent. Due to the hysteresis effect of damage on unloading, anchor bolts (cables) should be installed to support the surrounding rock immediately to compensate for the stress loss caused by excavation in engineering practice.

Strain Rate Changes during Stress Echocardiography Are the Most Accurate Predictors of Significant Coronary Artery Disease in Patients with Previously Treated Acute Coronary Syndrome.

Dobutamine stress echocardiography (DSE) is a well-established non-invasive investigation for the detection of ischemic myocardial dysfunction. The aim of this study was to evaluate the accuracy of myocardial deformation parameters measured by speckle tracking echocardiography (STE) in predicting culprit coronary artery lesions in patients with prior revascularization and acute coronary syndrome (ACS). We prospectively studied 33 patients with ischemic heart disease, a history of at least one episode of ACS and prior revascularization. All patients underwent a complete stress Doppler echocardiographic examination, including the myocardial deformation parameters of peak systolic strain (PSS), peak systolic strain rate (SR) and wall motion score index (WMSI). The regional PSS and SR were analyzed for different culprit lesions. The mean age of patients was 59 ± 11 years and 72.7% were males. At peak dobutamine stress, the change in regional PSS and SR in territories supplied by the LAD showed smaller increases compared to those in patients without culprit LAD lesions (p < 0.05 for all). Likewise, the regional parameters of myocardial deformation were reduced in patients with culprit LCx lesions compared to those with non-culprit LCx lesions and in patients with culprit RCA legions compared to those with non-culprit RCA lesions (p < 0.05 for all). In the multivariate analysis, the △ regional PSS (1.134 (CI = 1.059-3.315, p = 0.02)) and the △ regional SR (1.566 (CI = 1.191-9.013, p = 0.001)) for LAD territories predicted the presence of LAD lesions. Similarly, in a multivariable analysis, the △ regional PSS and the △SR predicted LCx culprit lesions and RCA culprit lesions (p < 0.05 for all). In an ROC analysis, the PSS and SR had higher accuracies compared to the regional WMSI in predicting culprit lesions. A △ regional SR of -0.24 for the LAD territories was 88% sensitive and 76% specific (AUC = 0.75; p < 0.001), a △ regional PSS of -1.20 was 78% sensitive and 71% specific (AUC = 0.76, p < 0.001) and a △ WMSI of -0.35 was 67% sensitive and 68% specific (AUC = 0.68, p = 0.02) in predicting LAD culprit lesions. Similarly, the △ SR for LCx and RCA territories had higher accuracies in predicting LCx and RCA culprit lesions. The myocardial deformation parameters, particularly the change in regional strain rate, are the most powerful predictors of culprit lesions. These findings strengthen the role of myocardial deformation in increasing the accuracy of DSE analyses in patients with prior cardiac events and revascularization.

Metamodelling the hot deformation behaviour of titanium alloys using a mean-field approach

During the thermomechanical processing of titanium alloys in the β-domain, the β-phase undergoes restoration phenomena. This work describes them by a mean-field physical model that correlates the flow stress with the microstructural evolution. To reduce the computational time of process simulations, metamodels are developed for specific outputs of the mean-field physical model using Artificial Neural Network (ANN) and Decision Tree Regression (DTR). The performance of the obtained metamodels is evaluated in terms of the coefficient of determination (R²), the root-mean-square error (RMSE), and the mean relative error (MRE). No significant difference was observed between R2training and R2testing, meaning that all the metamodels correctly generalise the overall behaviour of the outputs for a wide range of inputs. The evolution of the metamodel outputs is compared with the model predictions in two different situations: 1) at a constant strain rate and temperature, and 2) during Finite Element (FE) simulations of the hot deformation of a hat-shaped sample, where temperature and effective strain rate vary at each element during deformation. The evolution of the outputs at constant and non-constant strain rates and temperature demonstrated the robustness of the metamodels in predicting the heterogeneous deformation within a workpiece. The computational time required by the metamodels to calculate selected outputs can be more than 100 times less than that of the model itself at a constant strain rate using MATLAB® and up to 19% less when coupled with FE simulations. The simulation results combined with microstructural analysis are used to visualise the different restoration mechanisms occurring in different regions of the hat-shaped sample as a function of the local thermomechanical history. The changes in strain rate and temperature during deformation influence the evolution of the wall dislocation density and the immobilisation rate of mobile dislocations at subgrain boundaries, leading to different kinetics of microstructure evolution.

Rheological Properties of Small-Molecular Liquids at High Shear Strain Rates.

Molecular-scale understanding of rheological properties of small-molecular liquids and polymers is critical to optimizing their performance in practical applications such as lubrication and hydraulic fracking. We combine nonequilibrium molecular dynamics simulations with two unsupervised machine learning methods: principal component analysis (PCA) and t-distributed stochastic neighbor embedding (t-SNE), to extract the correlation between the rheological properties and molecular structure of squalane sheared at high strain rates (106-1010s-1) for which substantial shear thinning is observed under pressures P∈0.1-955 MPa at 293 K. Intramolecular atom pair orientation tensors of 435×6 dimensions and the intermolecular atom pair orientation tensors of 61×6 dimensions are reduced and visualized using PCA and t-SNE to assess the changes in the orientation order during the shear thinning of squalane. Dimension reduction of intramolecular orientation tensors at low pressures P=0.1,100 MPa reveals a strong correlation between changes in strain rate and the orientation of the side-backbone atom pairs, end-backbone atom pairs, short backbone-backbone atom pairs, and long backbone-backbone atom pairs associated with a squalane molecule. At high pressures P≥400 MPa, the orientation tensors are better classified by these different pair types rather than strain rate, signaling an overall limited evolution of intramolecular orientation with changes in strain rate. Dimension reduction also finds no clear evidence of the link between shear thinning at high pressures and changes in the intermolecular orientation. The alignment of squalane molecules is found to be saturated over the entire range of rates during which squalane exhibits substantial shear thinning at high pressures.

On the formation of thrust fault-related landforms on Mercury: The key parameters controlling the mechanical structure of the lithosphere

There are numerous thrust fault-related landforms widely distributed on the surface of Mercury. Geomorphic interpretations suggest that these landforms are either thin or thick-rooted, reflecting the mechanical structure of Mercury’s lithosphere. In this work, we propose a mechanical model incorporating brittle, semi-brittle, and viscous deformation mechanisms to depict the mechanical and strength profile of Mercury’s lithosphere, where the effective elastic thickness is about 30 km. To investigate which and how parameters control the lithospheric mechanical structure, we apply the random forest algorithm to select features by predicting the deformation mechanism with given sets of randomly generated input parameters, suggesting that the lithospheric viscosity, depth, and rheological model are the most important parameters. On this basis, we discuss how the viscosity and rheological model react to the changes in temperature, strain rates, and creep laws. Our results indicate that lowering the temperature or increasing the strain rate result in an increased strength profile and expanded regions where brittle and semi-brittle deformation occurs in the lithosphere. Qualitative analyses of the importance of Peierls, dislocation, and diffusion creeps in forming the rheological structure are conducted by applying multivariate regression analysis. It shows that Peierls creep dominates and stabilizes the rheological structure in the shallow depth due to its strong stress dependence for low temperatures. Moreover, the rheological model involving Peierls creep results in a reduced strength profile and simpler mechanical structure. Therefore, we emphasize that the Peierls creep is shown to be also critical in determining the strength and mechanical structure of Mercury in addition to temperature and strain rate.

Size effect on dynamic compressive failure mode and mechanical properties of recycled concrete

AbstractIn order to study the size effect behavior of recycled concrete under dynamic compression load, a meso‐scale model of recycled concrete was established. The particle size of coarse aggregate, replacement ratio of recycled coarse aggregate, and section size were used as parameters to explore the mechanical properties of recycled concrete under uniaxial dynamic compression load, such as failure mode, compressive strength, peak strain, and residual strength. The effects of strain rate, particle size of recycled coarse aggregate, and replacement rate of recycled coarse aggregate on the size effect of dynamic compression of recycled concrete were further analyzed. The results showed that under uniaxial dynamic compression load, the damage degree of recycled concrete deep and along with the change of the strain rate change, recycled concrete dynamic compression strength and strain rate were positively correlated. There is the critical strain rate is 1/s, which weaken the influence of the size effect, when the strain rate reaches 10/s, the dynamic compressive strength of recycled concrete increases with the increase of section size. At the same time, when the strain rate is high, the particle size of recycled coarse aggregate and the replacement rate of recycled coarse aggregate have little effect on the size effect behavior of recycled concrete.