Increasing Strain Rate Research Articles

THE INFLUENCE OF THE STRAIN RATE ON THE VALUE OF FRICTION COEFFICIENT

The Male and Cockroft ring compression test stands as one of the methods utilized to determine the coefficient of friction in forming processes. This approach eliminates the need for force measurement. This study presents the outcomes of the Male and Cockroft ring compression test conducted on Hardox 450 material at varying strain rates, conducted without lubricant. The experiments were carried out using a ZD 40 hydraulic press, CFA-80 pneumatic die hammer and split Hopkinson pressure bar test at the Faculty of Mechanical Engineering of Brno University of Technology. The test results were documented on a calibration diagram, revealing a significant influence of strain rate on the coefficient of friction. Specifically, the findings indicate that as the strain rate increases, the coefficient of friction decreases.

Cardiac Magnetic Resonance Imaging with Myocardial Strain Assessment Correlates with Cardiopulmonary Exercise Testing in Patients with Pectus Excavatum

Objectives: To evaluate correlations between cardiac magnetic resonance imaging (cMRI) at rest including strain imaging and variables derived from quantitative cardiopulmonary exercise testing using a treadmill in patients with pectus excavatum. Methods: We retrospectively correlated the results of cMRI and cardiopulmonary exercise testing in 17 patients with pectus excavatum, in whom both examinations were performed during their pre-operative clinical evaluation. In addition to cardiac volumetry, we assessed the strain rates of both ventricles using a feature-tracking algorithm of a piece of commercially available post-processing software. Results: Right ventricular (RV) ejection fraction correlated negatively with heart rate at anaerobic threshold (rho = −0.543, p = 0.024). A positive correlation between radial strain rate at the RV base and percentage of predicted maximum heart rate (rho = 0.72, p = 0.001) was shown, with equivalent results for circumferential strain rate (rho = −0.64, p = 0.005). Radial strain rate at the RV base correlated in a strongly negative way with maximum oxygen uptake (rho = −0.8, p < 0.001), with a correspondingly positive correlation for circumferential strain rate (rho = 0.73, p = 0.001). Conclusions: Quantitative parameters derived from cMRI at rest, especially those acquired at the most severely compressed RV base, correlated with cardiopulmonary exercise testing variables. The compression of the RV base by the sternum might be partially compensated by an increased strain rate to induce higher heart frequencies during exercise. However, high strain rates were associated with a higher disease severity and a lower maximum oxygen uptake, indicating a limitation of this compensation mechanism.

High‐Temperature Deformation Behavior and Microstructural Evolution of N06625 Nickel‐Based Alloy

Herein, the hot deformation behavior and microstructure evolution of N06625 superalloy are studied. Hot deformation behavior at 950–1200 °C and strain rate 0.1–10 s−1 are carried out by Gleeble‐3800 thermal simulator machine. The activation energy (Q) of the alloy is calculated by the change of stress–strain curve. The microstructure of the experimental material is characterized by electron backscatter diffraction, and it is found that the recrystallization fraction rate increases with the increase of temperature, but decreases first and then increases with the increase of strain rate. Discontinuous dynamic recrystallization is the main nucleation mechanism of N06625 superalloy. Continuous dynamic recrystallization (CDRX) is the auxiliary nucleation mechanism; CDRX gradually decreases with the increase of temperature. In addition, it is found that the evolution of special boundaries is closely related to the strain rate. The longer deformation time is beneficial to the interaction of grain boundaries, which leads to the special boundary fraction, and the increase of special boundary fraction can effectively weaken the preferred orientation caused by recrystallization growth.

Effect of process parameters on the static recrystallisation and precipitation behaviours of V–Ti–N microalloyed steel

Hot rolling process often consisting of several successive deformation stages, static recrystallisation and precipitation may occur in microalloyed steels during the inter-pass periods between two stages, which have a great impact on the mechanical properties of the final products. To obtain the products with high performances, the process parameters must be controlled. In this study, static recrystallisation and precipitation in a vanadium titanium microalloyed steel was investigated by double compression method. Especially, the effect of process parameters, such as, temperature, pass time interval, strain and strain rate on the static recrystallisation and precipitation behaviours was discussed in details. The results showed that the static recrystallisation and precipitation behaviours of the tested steel could be affected by the process parameters significantly. At a higher temperature of 950 °C or 900 °C, static recrystallisation fraction increased with increasing time interval, while plateaus could be observed on softening curves due to the vanadium titanium carbonitride precipitated in the tested steel at a lower temperature of 850 °C or 800 °C. In addition, an increase in strain rate or an increase in strain could promote static recrystallisation. Especially, plateaus could not be observed on the softening curves obtained from the tested steel deformed at the strain rate of 1 s−1 and at the temperature of 850 °C or 800 °C. The prohibition on static recrystallisation was not recognisable as a larger strain of 35% was applied at 850 °C but it could still be observed at 800 °C.

Dynamic splitting tensile properties of high-strength ultrahigh-toughness cementitious composites (HS-UHTCCs)

In this study, the splitting tests were conducted to examine the effect of strain rate on the splitting tensile performance of high-strength ultrahigh-toughness cementitious composites (HS-UHTCCs). Hydraulic machine and split Hopkinson pressure bar (SHPB) were used to investigate the tensile properties of HS-UHTCCs. The strain rates ranged from 10-6 to 10-3 s−1 for hydraulic machine experiments and from 5 to 15 s−1 for SHPB tests, respectively. The splitting tensile strength, tensile stress-strain curves, dynamic increase factor (DIF) and energy absorption ability of HS-UHTCCs were analyzed. The new DIF models proposed for HS-UHTCCs fit well with the experimental data. In addition, the failure modes of specimens and fibers were investigated. The splitting tensile strain was obtained from digital image correlation (DIC) tests. The results indicate that the strain rate has a significant effect on the splitting tensile strength and energy absorption ability of HS-UHTCCs. On the contrary, the average splitting tensile strain showed a decreasing trend with the increase of strain rate. Besides, the DIF model of HS-UHTCCs was proposed with experimental data. The failure modes of specimens changed at high strain rates, transitioning from remaining intact to completely splitting into half. Two failure patterns of polyethylene (PE) fibers included pull-out failure and rupture were observed, while steel fibers experienced pull-out failure.

Investigating the Dynamic Behavior and Tensile Properties of Coal Gangue Shotcrete Under Impact Loading Conditions SHPB Device

Shotcrete is the primary support material for coal mine tunnel linings and is often subjected to dynamic loads such as blasting and impact. To promote the resource utilization of coal gangue (CG), this study incorporated it into shotcrete and investigated the dynamic mechanical properties of coal gangue shotcrete (CGSC) at different coal gangue aggregate (CGA) replacement ratios. Impact tensile splitting tests were conducted using a split Hopkinson pressure bar (SHPB) device, and the specimen failure process was recorded with a high-speed camera. The results showed that the dynamic tensile splitting strength (DSTS) and energy absorption of CGSC exhibit a significant strain rate effect, with the strain rate sensitivity of DSTS decreasing as the CGA replacement ratio increases. As the strain rate increases, the elastic limit stress of the specimens rises, and the failure mode tends to become more fragmented. Based on energy theory, this paper establishes a dynamic tensile splitting strength model for CGSC, providing theoretical support for its application in engineering.

A visco-hyperelastic constitutive model of hydrogel considering the coupling effect between segment motion and interchain slippage

Segment motion induces interchain slippage, leading to a complex coupling between hyperelastic and viscoelastic behaviors in hydrogels. Traditional models, which treat these behaviors separately and introduce a coupling free energy, struggle to capture this visco-hyperelastic coupling mechanism accurately. In this work, we develop a visco-hyperelastic constitutive model incorporating viscoelastic contributions into the general hyperelastic free energy to capture the coupling mechanism. The model introduces both the end-to-end distance and the envelope radius to describe the three-dimensional chain conformation. A joint probability distribution of these two parameters is used to capture the relationship between the chain conformation and the chain distribution. We also propose a two-level macro-micro transition framework to link the evolution of end-to-end distance and envelope radius to macroscopic deformations. In the first level of such macro-micro transition, the elongation of individual chains in various directions within the network is mapped to the overall chain elongation. In the second level, both interchain slippage and total chain elongation are incorporated into the overall deformation of the polymer network. A comparison between our predicted results with our experimental data demonstrates the predictability of the new model. Finally, the modeling results show that the interchain slippage always involves two sequential stages, i.e., orientation and relative slippage; and the increasing strain rate enhances the asynchronous responses between segment motion and relative slippage due to multichain interactions, making the viscoelastic responses of hydrogel more obvious.

Dynamic Mechanical Properties and Damage Morphology Analysis of Concrete with Different Aggregates Based on FDM-DEM Coupling

To study the dynamic compressive mechanical properties of concrete with different aggregates (limonite and lead-zinc ore), a dynamic mechanical experiment was carried out by the Φ 100 mm SHPB equipment. Based on the coupling of the finite difference method (FDM) and the discrete element method (DEM), a three-dimensional numerical model was constructed. The effects of various strain rates and aggregate types on the dynamic mechanical properties of concrete, the dynamic increase factor (DIF), and the dynamic impact damage process were analyzed and discussed. The results show that both types of concrete have a significant strain rate strengthening effect. The dynamic compressive strength, peak strain, and DIF of the two types of concrete gradually increase with the increasing strain rate. The DIF and dynamic compressive strength growth of lead-zinc ore concrete was greater than that of limonite concrete, and the strain rate sensitivity of lead-zinc ore concrete was stronger than that of limonite concrete. The constructed three-dimensional coupling model can better simulate the experimental process, and the stress-strain curves and damage patterns show good agreement with the experimental results. The relative errors between the calibration results of the microscopic parameters and the experiment values are all within 1%.

Effects of aggregate crushing and strain rate on fracture in compressive concrete with a DEM-based breakage model

This study looked at how breakable aggregates affected the mesoscopic dynamic behavior of concrete in the uniaxial compression condition. In-depth dynamic two-dimensional (2D) studies were conducted to examine the impact of aggregate crushing and strain rate on concrete’s dynamic strength and fracture patterns. Using a DEM-based breakage model, concrete was simulated as a four-phase material consisting of aggregate, mortar, ITZs, and macropores. The concrete mesostructure was obtained from laboratory micro-CT tests. Collections of spherical particles were used to imitate aggregate breakage of different sizes and shapes by enabling intra-granular fracturing between them. The mortar was described in terms of unbreakable spheres with different diameters. Compared to the mortar, the aggregate strength was always stronger. A qualitative consistency was achieved between the DEM results and the available experimental data. Concrete’s dynamic compressive strength rose significantly with strain rate and just somewhat with aggregate strength. The fracture process was impacted considerably by aggregate crushing and strain rate. The number of broken contacts grew with an increase in strain rate and a decrease in aggregate strength.

Damage characteristics and failure mechanism analysis of NEPE propellant at high strain rates

The study of mechanical properties and failure mechanism of propellants under impact loads is crucial for analyzing structural integrity of propellant charges. An experimental investigation was conducted on NEPE propellant using a separated Hopkinson pressure bar to conduct high strain rate uniaxial impact tests. Deformation and failure processes of the propellant under impact conditions were recorded with a high-speed camera. The microscopic failure forms of the propellants were observed using a scanning electron microscope and an optical microscope. Stress-strain curves and high-speed images revealed that the damage behavior and failure mechanism of NEPE propellants are significantly influenced by strain rate. As the strain rate increases, there is a notable increase in the deformation degree of the propellant specimens, with a more pronounced shear effect. This leads to an earlier occurrence of failure and a more severe degree of failure. The predominant failure forms observed in NEPE propellants include transgranular failure, matrix tearing, and cavity merging. A nonlinear visco-hyperelastic constitutive model with damage at high strain rates was established to provide a precise account of the mechanical response of NEPE propellant under high strain rates.

Effect of strain rates on stress corrosion sensitivity of 7085-T7452 thick-plate friction stir welding joint

The stress corrosion behavior of 7085-T7452 high-strength aluminum alloy base material (BM) and its friction stir welding (FSW) joints under varying strain rates during slow strain rate tensile (SSRT) was investigated. The results show that the stress corrosion sensitivity (ISSRT) of both the BM and FSW joints decreases with the increase of strain rate from 10−7 s−1 to 10−5 s−1. Moreover, the BM demonstrates lower ISSRT than the joints. The cracks in the joints exhibit dendritic paths along grain boundaries, with evidence of crack arrest marking (CAM) at localized areas in 3.5 wt% NaCl solution, suggesting both anodic dissolution and hydrogen-induced cracking simultaneously govern the propagation of stress corrosion cracks. There is the forced more test time in the slower strain rate, and the accumulation of stress corrosion coupling damage accelerates with the time, eventually leading to the greater degradation in the stress corrosion resistance and the higher ISSRT.

On revealing the mechanisms involved in brittle-to-ductile transition of fracture behaviors for γ-TiAl alloy under dynamic conditions

To understand the high-temperature behaviors of γ-TiAl is essential for its applications in aerospace industries. In this study, the high-temperature mechanical tests were carried out with strain rates ranging from 0.001 to 0.1 s−1 for the forged Ti-46Al-5Nb-1.8Cr-0.2Ta-0.1B alloy. The dynamic behaviors, fracture modes and microstructure evolution at elevated temperatures were analyzed to reveal the mechanisms involved in damage evolution. The results show that the brittleness of γ-TiAl is enhanced by strain rate hardening, and the brittle-to-ductile transition temperature (BDTT) increases with the increase of strain rates. The fracture morphology shows that the fracture characteristics transfer from the brittle fracture with mixed mode of intergranular and transgranular below and within the BDTT to the ductile fracture above BDTT. In addition, micro-cracks appear in fracture zones at the transitory stage, and cavities appear at the ductile stage, which is related to dynamic recrystallization (DRX). In this case, it is deduced that the brittle-to-ductile transition is induced by the competition between plastic deformation and crack propagation, and the results can be used to optimize the processing parameters towards high surface integrity when manufacturing γ-TiAl components.

The nanoscale tensile behavior of polyaspartate polyurea: A coarse-grained molecular dynamics simulation study

Polyaspartate polyurea can be applied in concrete enhancement, where its mechanical properties in different tension states are key to achieving the desirable enhancing effect. In this study, the coarse-grained (CG) model of amorphous polyaspartate polyurea and equivalent interactions were developed, and its nanoscaled mechanical behaviors under tensile, succssive cyclic tensile, and stress relaxation were simulated. The CG model, with significantly reduced freedom degrees, has almost the same glass transition temperature and density as the full-atomic one. The unixial tension mechanical properties of the CG model varies with the strain rate. With the increasing strain rate, the yield strength increases, whereas the yield strain declines. The deformation of the model is primarily caused by slippage of beads and bending of bond angles. Under successive loading-reloading cyclic tension, the model performs with stress memory behavior, and the viscous strain happens when the strain of unloading is equal to or exceeds 0.5. Under stress relaxation, the axial stress declines with the increasing time, and its evolution is primarily caused by the varied van der Waals interactions.

Impact resistance performance of 3D woven TZ800H plates with different textile architecture

Two typical methods commonly used to improve the mechanical properties and impact resistance properties of 3D woven composites are studied, namely weave pattern and layered architectures. The mechanical property and impact resistance performance were studied by utilising the quasi-static compressive test, split Hopkinson pressure bar (SHPB) test and ballistic impact test. The compressive responses in warp and weft directions with different strain rates 0.001, 500 and 1300 s-1 were presented and analysed, providing strain rate influence on the material strength of different 3D woven composites. The impact resistance performance including damage mode, ballistic limit and specific energy absorption of three structures were discussed through impact tests. The results reveal that as the strain rate increases, the compressive strength and Young's modulus in both directions of 3D woven composites exhibit a significant increase. The compressive strength and modulus in the warp direction of the composites can be enhanced by using shallow interlocking of the warp tow or layered architectures. However, the two methods degrade the failure strain and weaken the strain rate strengthening effect of compressive strength in the weft direction, resulting in a significant decrease in the average strain energy density. For the ballistic impact case, the crimp of warp tows would decrease its load-bearing capacity, while resisting matrix crack growth under the ballistic impact. The significant reduction in the average strain energy density in the weft direction leads to a decrease in ballistic limit and specific energy absorption capacity under ballistic impact.

Tensile properties and constitutive modeling of Kevlar29 fibers: From filaments to bundles

This paper experimentally investigates the tensile properties of Kevlar29 filaments and fiber bundles, revealing the size and strain rate effects on the mechanical properties of fiber bundles. The fracture modes and mechanisms of the fibers were analyzed, and viscoelastic constitutive models at the fiber bundle scale and constitutive models for filaments-bundles based on the Weibull distribution were established. The results show that Kevlar29 fiber bundles exhibit significant strain rate effects: as the strain rate increases, tensile strength and modulus increase, while fracture strain and toughness decrease. Under quasi-static loading, the fracture modes of the fibers are mainly fibrillation or shear flow, but as the strain rate increases, the failure modes shift towards brittle fracture. Both constitutive models can accurately predict the tensile properties of fiber bundles at different strain rates. The accuracy and applicability of the filament-to-bundle constitutive model were verified through numerical simulations, demonstrating that the model better describes the size effect of fibers and quantifies the damage to the fiber bundles.

On the dynamic failure mechanism of Vit-1 bulk metallic glass: Coupling effects of pre-made damage and strain rate

To clarify the two distinct effects and the underlying physical mechanisms of pre-made damage on the failure strength of bulk metallic glass (BMG), i.e. strengthening or weakening, the influence of axial pre-compression on the dynamic failure of Vit-1 BMG are investigated experimentally. Quasi-static and dynamic compression tests were conducted on pre-compressed BMG specimens for a wide range of nominal strain rates between 0.001 s−1 to 6000 s−1. The focus is on the coupled effects of pre-compression and strain rate on the dynamic failure stress. Two contrasting mechanisms were identified that influence how the pre-made shear bands affect the propagation of cracks under different strain rates. It will be shown that the dynamic failure stress exhibits a transition from strengthening to knockdown with increasing strain rate; and, that the extent of these strengthening and knockdown effects depends on the pre-compression strain level. Finally, a failure model that captures the effects of both strain rate and pre-made damage is proposed.

Study on the uniaxial tensile mechanical behavior of two-dimensional single-crystal aluminum nitride

Abstract To investigate the tensile behavior and mechanical properties of single-crystal aluminum nitride (AlN) at the microscopic level, molecular dynamics simulations were used to study the effects of crystal orientation, strain rate, environmental temperature, and hole defect size on fracture strength, fracture mechanism, and potential energy during uniaxial tensile. The results show that the tensile strength of AlN in the [100] crystal direction is stronger. The anisotropic behavior characteristics of Al-N bonds fracture mechanism, crack growth rate, and cracking degree are significant when stretched along the [100], [010], and [110] crystal directions. Under high temperature condition, the lattice structure undergoes changes, causing grain boundaries to move and slip. This facilitates the breaking of bonds, leading to a decrease in tensile strength and a reduction in stored potential energy. Hole defects cause more lattice damage, reducing the energy required for Al-N bonds breakage and facilitating the propagation of microcracks. Additionally, it was found that the strain rate affects the stress–strain behavior of the model. An increase in strain rate leads to an increase in breaking stress, and the rapid deformation of AlN results in more energy being stored in the lattice in the form of potential energy. Therefore, the tensile strength and potential energy are improved.

Strain Rate Sensitivity of Low‐Temperature Superplastic Heterogeneous Medium‐Mn Steel Fabricated by a Novel High‐Ratio Differential Speed Rolling

In the present article, insights into the high‐temperature deformation behavior of medium‐Mn steel (MMS), which is prepared via high‐ratio differential speed rolling (HRDSR), are provided. Moreover, through innovative bidirectional jump experiments, variations in the strain rate sensitivity index m under various conditions are obtained. In the research findings, it is indicated that an increase in strain rate (SR) leads to a hardening of the alloy. During high‐temperature deformation, the value of m decreases with the increase in SR, but the rate of decrease gradually slows down. Furthermore, the higher the temperature (T), the greater the impact of changes in SR on m. In addition, the change in deformation mechanism during deformation leads to microstructural changes, and under the main deformation mechanism of grain‐boundary sliding, m generally increases with strain. Interestingly, at a T of 760 °C, the material exhibits a strong texture with high orientation, resulting in larger m values and superior superplasticity (≈691%). This study not only enriches the research content of HRDSR but also has significant implications for the superplasticity research of MMS. Moreover, a reference is provided for the research of other superplastic materials.

A thermodynamic based constitutive model considering the mutual influence of multiple physical fields

In multiple physical fields, the mutual influence among these fields can significantly impact material elastoplasticity. This paper proposes a thermodynamic-based constitutive model that incorporates the mutual influence of multiple physical fields. Rather than treating physical field characteristics as adjustable “parameters” affecting material coefficients, the proposed model employs a thermodynamic dissipation potential derived from the Onsager reciprocity relations, accounting for thermodynamic forces coupling. This dissipation potential ensures that the thermodynamic flow in the stress field is influenced by both stress field and other physical fields thermodynamic forces, which describes the plastic flow under multiple physical fields, while preserving thermodynamic duality. The paper begins with the formulation of a generalized thermodynamic model applicable to diverse materials and types of coupled fields, which is then degraded to a specific model for AA5182-O AlMg alloy under the influence of temperature and strain rate fields coupling. Given the universal applicability of the generalized model, such degradation provides a structured approach framework for developing thermodynamics-based constitutive models. For different materials encountered in practical engineering, new thermodynamic forces can be introduced to describe their unique mechanical properties while preserving the overarching thermodynamics-based model framework, thereby facilitating model scalability. The paper concludes with a validation example, showing that within the Portevin-Le Chatelie (PLC) regime, the plastic flow stress of AA5182-O AlMg alloy decreases with increasing strain rate at low temperatures but increases at high temperatures. The accurate simulation of these distinct strain rate effects crucially relies on integrating the mutual influence of temperature field and strain rate field.

Influence of Grain Size Evolution on Mantle Plume and LLSVP Dynamics

AbstractRecent seismic tomography models suggest large‐radius primary plumes originating from the core‐mantle boundary, with grain size variations potentially explaining these observations. Additionally, grain size variations are thought to enhance the long‐term stability of Large Low Shear Velocity Provinces (LLSVPs), identified as thermochemical piles near the core‐mantle boundary. Nevertheless, geodynamic models investigating these hypotheses remain limited. To address this gap, we constructed a series of geodynamic numerical models incorporating grain size evolution, plate tectonics, and the spontaneous generation of deep mantle plumes above LLSVPs. Our results reveal that grain size evolution does not significantly affect the plume width, primarily because the increased strain rate in the mantle plume suppresses both its grain size and viscosity. The region adjacent to the plumes, characterized by the accumulation of mantle materials with larger grain size and low‐temperature remnants of subducted slabs, displays a higher viscosity compared to the area near the subducted slabs. Furthermore, grain size evolution plays a crucial role in enhancing the stability of LLSVPs by increasing the viscosity ratio between LLSVPs and the ambient mantle. These findings underscore the need for incorporating grain size evolution in geodynamic models to gain a better understanding of the dynamics of plumes and lower mantle.