Dynamic Mechanical Deformation Research Articles

NUMERICAL SIMULATION OF DYNAMIC MECHANICAL PROPERTIES OF GRAPHENE-REINFORCED ALUMINUM MATRIX COMPOSITES

The effects of the strain rate of dynamic loading, volume fraction, shapes and orientation of graphene on the dynamic mechanical properties, deformation and damage of graphene/7075Al composites were simulated using the finite element method, and the damage mechanism of the composites under dynamic load was revealed. The flow strain-stress curves were obtained. The effect of strain rates on the flow stress of the composites was analyzed. The results show that graphene/7075Al composites exhibit a significant strain rate effect. With an increase in the volume fraction of graphene, the yield strength of the composites increases and the composites have significant stress-softening characteristics at a strain rate of 10000 s–1, leading to a reduced strain-hardening effect. The shape and orientation of graphene have an important influence on the dynamic mechanical properties and the extent of damage made to the composites. The flow stress of the composites increases with varying the graphene shape and orientation in the following order, respectively: prism < cylinder < platelet < disc and 0°< 3D < 90° < 45°. The extent of the damage to the composites increases in the following order: disc < platelet < cylinder < prism and 0° < 3D < 90° < 45°. The dynamic failure mechanism of graphene/7075Al composites is interface damage.

A rate-dependent damage model to characterize dynamic fracture behavior of Ti6Al4V under high strain rate loading

Material damage and fracture behavior are affected by loading rates. This paper aims to investigate dynamic deformation and fracture mechanisms of Ti6Al4V over a wide strain rate range from quasi-static to 104 /s. A high strain rate test apparatus is developed and a new rate-dependent material damage law is proposed. The damage energy of Ti6Al4V keeps nearly constant at the strain rate lower than 10 /s, while it declines dramatically under the strain rate high than 103 /s. Ductile to brittle transition of Ti6Al4V occurs at the strain rate of 16,000 /s, which leads to the transition of material fracture mechanism from adiabatic shear to embrittlement fracture. The research proves that developing appropriate damage evolution law is important to improve prediction accuracy of material deformation and guarantee structural design safety especially for dynamic loading components.

Stress wave propagation and incompatible deformation mechanisms in rock discontinuity interfaces in deep‐buried tunnels

AbstractComplex weak structural planes and fault zones induce significant heterogeneity, discontinuity, and nonlinear characteristics of a rock mass. When an earthquake occurs, these characteristics lead to extremely complex seismic wave propagation and vibrational behaviors and thus pose a huge threat to the safety and stability of deep buried tunnels. To investigate the wave propagation in a rock mass with different structural planes and fault zones, this study first introduced the theory of elastic wave propagation and elastodynamic principles and used the Zoeppritz equation to describe wave field decomposition and develop a seismic wave response model accordingly. Then, a physical wave propagation model was constructed to investigate seismic waves passing through a fault, and dynamic damage was analyzed by using shaking table tests. Finally, stress wave attenuation and dynamic incompatible deformation mechanisms in a rock mass with fault zones were explored. The results indicate that under the action of weak structural planes, stress waves appear as a complex wave field decomposition phenomenon. When a stress wave spreads to a weak structural plane, its scattering may transform into a tensile wave, generating tensile stress and destabilizing the rock mass; wave dynamic energy is absorbed by a low‐strength rock through wave scattering, which significantly weakens the seismic load. Wave propagation accelerates the initiation and expansion of internal defects in the rock mass and leads to a dynamic incompatible deformation. This is one of the main causes for large deformation and even instability within rock masses. These findings provide an important reference and guide with respect to stability analysis of rock mass with weak structural planes and fault zones.

Deciphering the microstructural evolution and adiabatic shearing behavior of the titanium alloy with stress-induced ω phase transformation during dynamic compression

Metastable β titanium alloys are attracting great interest due to their intricate plastic deformation mechanisms. In this work, a novel metastable β titanium alloy Ti-6Mo-3.5Cr-1Zr is developed by controlling the phase stability. The alloy exhibits a compressive strength of ∼ 1152 MPa with large malleability (∼34 %) during dynamic compressions. The microstructural evolution and adiabatic shearing behavior of the alloy are investigated by applying stop rings to control the compressive strains. Microstructural characterizations and differential scanning calorimetry show that the content of stress-induced ω (SIω) phase increases with the accumulation of strain, resulting in strain hardening of the material. The formation of adiabatic shear band (ASB) is observed in samples with 30% strain, whilst the formation and expansion of ASB are hindered by stress-induced structures. The microstructural evolution along outer edge → transition zone → center of the ASB region corresponds to (β + ω)→(β + α)→(β), which is caused by the temperature increasement in ASB. In addition, the average grain sizes in ASB region are gradually refined under the action of dynamic recrystallization. These results provide insights into the dynamic deformation mechanism and adiabatic shearing behavior of metastable β titanium alloys.

Formation of ultrafine grains in magnesium alloy during dynamic extrusion at room temperature

Magnesium alloy is a class of lightweight structural materials with promise for many applications. Here, the authors report a significant grain refinement, high deformation twin density and obvious nanograined structures in magnesium alloy through a novel high strain rate plastic deformation method—dynamic extrusion. By using finite element simulation and transmission electron microscopy (TEM), the authors find that the stain rate during plastic deformation affects the microstructure evolution process, which results in the dynamic deformation mechanisms transition. The results are of interests not only for understanding the deformation mechanisms under dynamic plastic deformation but also for nanostructures formation of the magnesium alloy.

Superior dynamic shear properties and deformation mechanisms in a high entropy alloy with dual heterogeneous structures

Both heterogeneous grain structure and dual nanoprecipitates were designed in a Al0.5 Cr0.9FeNi2.5V0.2 high entropy alloy (HEA), and the dynamic shear responses were investigated by hat-shaped specimens in split Hopkinson pressure bar tests. The present HEA with heterogeneous structure displays an unprecedented synergy of dynamic shear strength and ductility, as compared to the literature data for other metals and alloys. The excellent dynamic shear properties in the unaged samples could be due to the dynamical grain refinement, the dislocations hardening, and the precipitation hardening. The aged samples with a higher volume fraction of coherent L12 nanoprecipitates display even better dynamic shear properties, as compared to the unaged samples, which can be attributed to the triggered planar dislocation slip, the stored higher density of dislocations, the formation of dislocation substructure and the more pronounced precipitation hardening for postponing the occurrence of the adiabatic shear band (ASB). The high strain rate, high strain/stress magnitude, high adiabatic temperature rise, and fast-cooling process within ASB were observed to induce the dynamic recrystallization and the phase transformation from FCC phase to B2 phase, and this newly observed phase transformation phenomenon was not observed before under quasi-static deformation conditions.

Phase Composition Effects on Dynamic Behavior and Strain Rate Sensitivity in Metastable β-Ti Alloys.

In this study, high strain rate tension tests are conducted to determine and compare the dynamic mechanical behaviors and deformation mechanisms of different phase composition α-β metastable β-Ti alloys using a split Hopkinson tension bar. Two typical bimodal equiaxed αp + β and lamellar αs + β Ti-45551 alloy microstructures are formed through different hot working and thermal processing for investigating the effect of phase composition or microstructure on mechanical properties and strain rate sensitivity. It is demonstrated that dislocation nucleation and motion in the α/β phase and dislocation tangle or pile up at the α/β interface are typical deformation modes in both of the typical dual-phase Ti alloys at quasi-static loading conditions. Under dynamic loading, both the strength and ductility show a clearly positive strain rate dependence, which is directly related to dislocation activation in the α + β Ti-45551 alloy. Based on microstructure characterizations, it is shown that deformation twinning starts to become a major deformation mechanism in equiaxed αp + β microstructures under dynamic loading conditions. However, deformation twins are not favored in the lamellar αs + β Ti-45551 alloy due to its nano phase size. Finally, the mechanical behaviors and strain rate sensitivity are strongly dependent on the phase composition of metastable β-Ti alloys.

Abnormal work hardening in a TRIP-assisted metastable β titanium alloy under high strain rate loading

In this study, the dynamic tensile deformation behavior and microstructural evolution of a metastable β titanium alloy (Ti–10 V–2Fe–3Al, Ti-1023) with a single β phase structure were investigated at ambient temperature and high strain rates ranging from 1000 s−1 to 4000 s−1 on a split Hopkinson tension bar (SHTB) system. The dynamic tensile properties and work hardening ability exhibited a significant strain rate dependence. The calculated strain rate sensitivity (0.026–0.090) showed a strong strain and strain rate hardening ability. Microstructural analyses indicated that the dynamic deformation mechanisms were dislocation slip and deformation-induced α" martensite at all strain rates, while deformation-induced ω phase was activated beyond a strain rate of 2000 s−1. Deformation-induced phase transformations during dynamic tensile deformation led to a significant transformation-induced plasticity (TRIP) effect and a double-yielding phenomenon. As strain rate increased, deformation-induced martensitic transformation and the corresponding TRIP effect were suppressed, while work hardening rate first increased and then rapidly decreased. This abnormal TRIP effect on work hardening behavior was mainly due to the deformation-induced ω phase, promoting the dislocation plasticity in the β matrix and contributing to extra work hardening ability.

Unveiling the local deformation behavior of typical microstructures of nickel-based single crystals under nanoindentation

In the present study, consider the typical microstructures (γ, γ’ and interface phases) in nickel-based single crystal, three ideal simplified atomic models of Ni, Ni3Al and Ni/Ni3Al interface were established. And nanoindentation molecular dynamics (MD) simulations were performed on the above models. Based on the dynamic deformation behavior and mechanism of nickel-based single crystal microstructure under the local loading, the nanoindentation plastic deformation is divided into two stages Pi and Pii. In the initial Pi stage, dislocations only move and react on the contact surface between the indenter and the substrate. Subsequently, in stage Pii, edge dislocations climb away from the contact surface, screw dislocations interact with each other and finally form dislocation shear loops. The local plastic deformation is dominated by the nucleation and emission of dislocation loops, affected by its resistance and dislocation velocity. It is noted that in the interface model, dislocation shear loops inevitably interact with interface dislocations, which will lead to further plastic deformation. Combined with the dislocation theory and the above research, a model based on the movement of dislocations was established better describe the local plastic behavior of the typical microstructure in nickel-based single crystal under indentation.

Study of solute–solvent interaction in the solutions of PVA with PEG at 338 K by ultrasonic technique

The implementation of low Intensity ultrasound in polymer system acts as a dynamic mechanical deformation. It can be widely used to observe the variations in chemical and physical phenomenon related to change in the visco-elastic behavior. The velocity of ultrasound is related to the density and polymer storage modulus whereas the absorption of ultrasonic wave is connected with energy dissipation in the polymers. Frequently the technique is used in association with rheological methods as a medium of proffer a better understanding about the visco-elastic behavior of polymer materials. The aim of this paper is to study of solute–solvent interaction in the solution of polymer polyvinyl alcohol(PVA) andpolyethylene glycol(PEG) mixed with double distilled water have been done. The mixtures of samples such as Poly Vinyl Alcohol (PVA), Poly ethylene glycol (PEG) and DD water have been prepared at different concentrations. The solution has been magnetically stirred to find homogeneous solution. The Ultrasonic velocities, densities and viscosities are measured at 338 K by using the ultrasonic Interferometer, specific gravity bottle and Oswald viscometer respectively. The ultrasonic waves of two different frequencies such as 2 MHz and 5 MHz have been used. Depending upon the various data obtained, the different acoustical parameters such as adiabatic compressibility, intermolecular free length, acoustic impedance, relaxation time, ultrasonic attenuation coefficient (α/f2) have been evaluated. The obtained results have been discussed in detail and it has been observed that these results are very much helpful in describing the complete molecular interaction in the present sample. The observed variations in acoustical parameters show that all these parameters increase by increasing of the polymer concentration except of adiabatic compressibility and intermolecular free length.

Study on Dynamic Tensile Mechanical Behavior and Deformation Mechanisms of CrCoNi Medium Entropy Alloy at Room and Cryogenic Temperature

Study on Dynamic Tensile Mechanical Behavior and Deformation Mechanisms of CrCoNi Medium Entropy Alloy at Room and Cryogenic Temperature

Dynamic mechanical response and deformation behavior of a novel hierarchical cellular structure material – The thousands of eyes Bodhi

Natural cellular structures encourage researchers to seek new design inspirations for impact protection engineering materials. The thousands of eyes Bodhi (TEB) is a novel hierarchical cellular structure material, which includes the first order filled-cells, the second order novel-closed-cells, and the third order open-cells. In this study, dynamic compressive tests at different high strain rates in the longitudinal and transverse directions were conducted by a split Hopkinson pressure bar (SHPB), respectively. A high-speed camera was employed to capture the macroscopic deformation mechanisms and a scanning electron microscope (SEM) was employed to study the microscopic failure mechanisms. The dynamic mechanical response and strain rate effects of TEB samples in the transverse and longitudinal directions were investigated. The anisotropic behavior of TEB samples is observed under dynamic compression tests. The tests results show that the dynamic mechanical properties of TEB samples and dynamic deformation behavior of each order cellular structure exist a significant strain rate dependence. The TEB material dissipates the impact energy through the dynamic compression mechanisms of each order cellular structure which improves the energy absorption and impact resistance. Thus, it provides a bio-inspired template for the design of energy absorption and crashworthiness engineering materials.

Quasi-Static, Dynamic Compressive Properties and Deformation Mechanisms of Ti-6Al-4V Alloy with Gradient Structure

Gradient structure metals have good comprehensive properties of strength and toughness and are expected to improve the dynamic mechanical properties of materials. However, there are few studies on the dynamic mechanical properties of gradient structured materials, especially titanium alloys. Therefore, in this study, ultrasonic surface rolling is used to prepare a gradient structure layer on the surface of Ti-6Al-4V, and the quasi-static and dynamic compressive properties of coarse-grained Ti-6Al-4V (CG Ti64) and gradient-structured Ti-6Al-4V (GS Ti64) are investigated. The results show that a GS with a thickness of 293 µm is formed. The quasi-static compressive strength of GS Ti64 is higher than that of CG Ti64. Both CG Ti64 and GS Ti64 exhibit weak strain hardening effects and strain rate insensitivity during dynamic compression, and the compressive strength is not significantly improved. The lateral expansion of CG Ti64 is more obvious, while the lateral side of GS Ti64 is relatively straight, indicating that uniform deformation occurs in GS Ti64. The α phase in the GS produces dislocation cells and local deformation bands, and the lamellar structure is transformed into ultrafine crystals after dynamic compression. Both of them produce an adiabatic shear band under 2700 s−1, a large crack forms in CG Ti64, while GS Ti64 forms a small crack, indicating that GS Ti64 has better resistance to damage. The synergistic deformation of GS and CG promotes Ti-6Al-4V to obtain good dynamic mechanical properties.

Dynamic compression behaviour and microstructure of ZK60 alloy under different strain rates

In this study, the split Hopkinson pressure bar was used to investigate the effect of ageing treatment on dynamic compression properties and microstructure of extruded ZK60 alloy under different strain rates. The experimental results demonstrated that the amount of the precipitated MgZn and MgZn2 phases increases after ageing treatment. The peak flow stress increases with increasing strain rate. Due to ageing precipitation strengthening, the dynamic compressive strength of the ZK60 alloy is improved. EBSD and TEM results indicated that the dynamic compression deformation mechanism of the ZK60 alloy is the interaction of twins, dislocations, and dynamic recrystallisation. Furthermore, the extension twinning boundaries and the precipitated phases hinder the slip of dislocations during deformation, which significantly affects dislocation strengthening.

Investigation of Dynamic Mechanical Properties of Coal after Freeze-Thaw Cyclic Conditions

Due to the extensive excavation of the mine pit, a special frozen rock slope is formed, which transforms the permafrost (coal rock) of certain thickness in the frozen state to the melting state. To evaluate the dynamic mechanical properties and deformation characteristics of coal under cyclic freeze-thaw conditions, freeze-thaw experiments with different cycle times were conducted. And the mechanical properties of coal under quasistatic and dynamic conditions were investigated by using GCTS multifunctional rock mechanic experimental apparatus and SHPB dynamic loading apparatus, respectively. The results show that with the increase of freeze-thawing times, mass of both water-saturated and dried coal samples gradually decreased, the postpeak becomes gentler, and the specimens show ductile damage characteristics. The damage of the coal samples changed more after 30 freeze-thaw cycles, when deterioration of the coal samples was highest. The elastic modulus of the coal sample after freeze-thawing decreases continuously with the increase of the number of freeze-thaw cycles, and its trend decreases approximately linearly. Dynamic compressive strength of the coal samples decreases after freeze-thaw cycles, and this trend is consistent with the quasistatic loading conditions.

Dynamic mechanical behaviors of nacre-inspired graphene-polymer nanocomposites depending on internal nanostructures

Nacre, a natural nanocomposite with a brick-and-mortar structure existing in the inner layer of mollusk shells, has been shown to optimize strength and toughness along the laminae (in-plane) direction. However, such natural materials more often experience impact load in the direction perpendicular to the layers (i.e., out-of-plane direction) from predators. The dynamic responses and deformation mechanisms of layered structures under impact load in the out-of-plane direction have been much less analyzed. This study investigates the dynamic mechanical behaviors of nacre-inspired layered nanocomposite films using a model system that comprises alternating multi-layer graphene (MLG) and polymethyl methacrylate (PMMA) phases. With a validated coarse-grained molecular dynamics simulation approach, we systematically study the mechanical properties and impact resistance of the MLG-PMMA nanocomposite films with different internal nanostructures, which are characterized by the layer thickness and number of repetitions while keeping the total volume constant. We find that as the layer thickness decreases, the effective modulus of the polymer phase confined by the adjacent MLG phases increases. Using ballistic impact simulations to explore the dynamic responses of nanocomposite films in the out-of-plane direction, we find that the impact resistance and dynamic failure mechanisms of the films depend on the internal nanostructures. Specifically, when each layer is relatively thick, the nanocomposite is more prone to spalling-like failure induced by compressive stress waves from the projectile impact. Whereas, when there are more repetitions, and each layer becomes relatively thin, a high-velocity projectile sequentially penetrates the nanocomposite film. In the low projectile velocity regime, the film develops crazing-like deformation zones in PMMA phases. We also show that the position of the soft PMMA phase relative to the stiff graphene sheets plays an important role in the ballistic impact performance of the investigated films. Our study provides insights into the effect of nanostructures on the dynamic mechanical behaviors of layered nanocomposites, which can lead to effective design strategies for impact-resistant films.

Dynamic impact behavior and deformation mechanisms of Cr26Mn20Fe20Co20Ni14 high-entropy alloy

In this work, the dynamic impact behavior and linked deformation mechanisms of a Cr26Mn20Fe20Co20Ni14 high-entropy alloy (HEA) with face-centered cubic structure were systematically explored. The HEA displays uniform plastic deformation without any adiabatic shear bands at a strain rate range from 1000 to 3000 s−1. Moreover, the yield strength exhibits a pronounced strain rate dependence, increasing by 28% from 282 MPa at 1000 s−1 to 360 MPa at 3000 s−1. The strain hardening exponent, strain rate sensitivity and temperature rise were calculated to be 0.899–0.95, 0.076 and 64.7 K, respectively, indicative of high strain and strain-rate hardening capabilities and strong resistance to thermal softening induced adiabatic shear localization. Detailed microstructural analyses decipher a transition of deformation mechanism from dislocation slip at 1000 s−1 to stacking faults (SFs) at 2000 s−1 and further to twinning at 3000 s−1, respectively. The strengthening mechanisms are delineated as manifold interactions between glide dislocations and SFs, SFs and SFs (i.e. the Lomer-Cottrell locks), and SFs and twins. Our work provides a comprehensive understanding on the dynamic impact behavior, deformation and strengthening mechanisms of Cr26Mn20Fe20Co20Ni14 high-entropy alloy.

Numerical Investigation on Dynamic Response Characteristics and Deformation Mechanism of a Bedded Rock Mass Slope Subject to Earthquake Excitation

In order to systematically reveal the dynamic response characteristics of rock mass slopes subject to seismic excitation, time-domain and frequency-domain analyses are used to investigate the dynamic response of a bedded rock slope from multiple perspectives, using the two-dimensional numerical dynamic analyses. Based on the numerical simulation results, the influence of the weak bedded structural planes on the propagation characteristics of seismic waves in the slope is analyzed. The time-domain analysis suggests that the topographic and geological conditions have an influence on the dynamic response of the slope. The effects of ground motion direction on the dynamic response characteristics of the slope are identified. In addition, according to the frequency-domain analysis, the impacts of slope surface, elevation, and structural plane on the seismic response characteristics of the slope are also clarified. The intrinsic characteristics of the slope are investigated by using Fourier spectral analysis and modal analysis, and the deformation response characteristics of the slope are clarified. The relationship between different natural frequencies of the slope, the predominant frequency of the seismic wave, and the dynamic response characteristics of the slope is discussed. Moreover, the dynamic failure mechanism of the slope is analyzed. This work provides a reference for the seismic analysis of this type of slope.

Relaxation behavior of high-entropy bulk metallic glass: Influences of physical aging and cyclic loading

<p indent="0mm">In this study, the correlation among the relaxation behavior, deformation mechanism, and structural heterogeneity of metallic glasses is examined using La₃₀Ce₃₀Ni₁₀Al₂₀Co₁₀ high-entropy bulk metallic glass as the model glass. Dynamic mechanical analysis (DMA) was used to investigate the influences of physical aging and cyclic loading on the dynamic relaxation and deformation mechanism of the model glass. The correlation between the deformation units and the structural heterogeneity was analyzed on the basis of the activation energy spectrum (AES). The results demonstrated that La₃₀Ce₃₀Ni₁₀Al₂₀Co₁₀ high-entropy bulk metallic glass displays an apparent β relaxation. A more stable state can be achieved during physical aging below the glass transition temperature and in cyclic loading because the atomic rearrangements are suppressed during these processes. This structural relaxation during the physical aging process enables the transition from liquid-like regions to an elastic matrix. Further, more time is required to activate the deformation units that are frozen during the transition. The applied stress results in a decrease in the number of liquid-like regions, which suppresses the activation of the deformation units during the cyclic loading process. As a result, the intensity of the β relaxation in the metallic glass decreases. Therefore, the origin of the β relaxation in La₃₀Ce₃₀Ni₁₀Al₂₀Co₁₀ high-entropy bulk metallic glass is ascribed to the liquid-like regions.

Effect of α phase fraction on the dynamic mechanical behavior of a dual-phase metastable β titanium alloy Ti–10V–2Fe–3Al

Dynamic mechanical behavior of a metastable β titanium alloy Ti–10V–2Fe–3Al (Ti-1023) with α+β dual-phase structure is investigated by a split Hopkinson tension bar (SHTB) system at two high strain rates of 2000 s −1 and 4000 s −1 . α phase fraction in Ti-1023 alloy is tuned via different heat treatments. Dynamic tensile stress-strain curves, deformation mechanisms and fracture are discussed in detail. As α phase fraction decreases from 60.5% to 4.5%, there is a transition from one-stage to three-stage work hardening, as well as a transition from single-to double-yielding phenomenon, which is caused by stress-induced martensitic transformation (SIMT). Positive strain rate effects are found on yield strength, ultimate tensile strength and triggering stress, while negative strain rate effect is found on uniform elongation of dual-phase Ti-1023 alloy. The connections between deformation mechanisms (including SIMT, hierarchical α" martensite structures, kinking and dislocation slip) and dynamic mechanical properties are discussed in detail. No adiabatic shear bands are found in the solution-treated alloys under SHTB loading.