Increasing Strain Rate Research Articles

Dynamic Shear Mechanical Properties and Microscopic Deformation Characteristics of Laser Cladding IN625 Alloy

To reveal the dynamic shear mechanical properties of laser cladding Inconel 625 (IN625) alloy, dynamic shear mechanical properties experiments are conducted on forged specimens and laser cladding specimens by split Hopkinson press bar experimental device. Optical microscopy, scanning electron microscopy, and electron backscattered diffraction (EBSD) are used to characterize the microscopic morphology, crystal orientation, and grain size of laser cladding specimens before and after dynamic shear deformation. In the results, it is shown that the yield strength and shear strength of both forged and laser clad specimens increase with the increase of shear strain rate, reflecting a remarkable strain‐rate‐strengthening effect, and the shear strength of the laser cladding specimen is higher than that of the forged specimen. After dynamic shear loading, the grains in the shear region of the laser cladding specimen are severely deformed, deformation bands appear, and some dendrites are transformed into equiaxed crystal. After shear deformation, the preferred orientation strength of the crystal increases, the small‐size grain and low‐angle grain boundary increase, and a large number of dislocations are formed in the shear region, which dominates the dynamic shear deformation behavior of the material. Dislocation is an important dominant mechanism of dynamic shear deformation.

Progressive damage and fracture behavior of brittle rock under multi-axial prestress constraint and cyclic impact load coupling

To explore the progressive damage and fracture mechanics characteristics of brittle rock materials under combined dynamic-static loading. Taking account of the coupling effect of the constraint states of uniaxial stress (σ1 ≥ σ2 = σ3 = 0), biaxial stress (σ1 ≥ σ2 > σ3 = 0) and true triaxial stress (σ1 ≥ σ2 ≥ σ3 ≠ 0) and impact load, the strain rate effect and prestress constraint effect of dynamic mechanical characteristics of sandstone are studied. The progressive damage evolution law of sandstone under the coupling of true triaxial stress constraint and cyclic impact load is discussed. The results show that with the increase of axial stress σ1, the dynamic compressive strength and peak strain gradually decrease, and the strain rate gradually increases, resulting in crushing failure under high strain rate. When the axial stress is fixed, the lateral stress constraint reduces the damage degree of sandstone and improves the dynamic compressive strength. With the increase of strain rate, the sample changes from slight splitting failure to inclined shear failure mode. Under the true triaxial stress constraint, the intermediate principal stress σ2 obviously enhances the dynamic compressive strength of sandstone. Under the constraints of triaxial stress, biaxial stress and uniaxial stress, the enhancement effect of dynamic compressive strength and the deformation resistance of sandstone are weakened in turn. Under the coupling of true triaxial stress constraint and high strain rate, sandstone samples show obvious progressive damage evolution effect under repeated impacts, and eventually inclined shear failure occurs, resulting in complete loss of bearing capacity.

Dynamic enhancement induced by interface for additively manufactured continuous carbon fiber reinforced composites

Additive manufacturing (AM) has become one advanced manufacturing method for continuous carbon fiber reinforced polymer (CCFRP) composites, which is promising to work as aerospace materials. To promote their efficient design and extensive application, series of tensile and fiber-pullout tests were implemented to investigate the rate-dependent mechanical behavior and fracture mechanism. Both tensile strength and fracture strain significantly increase with the increase of strain rate in the studied range of 0.0001–1600 s−1. Fiber-pullout test also confirms that the interfacial shear strength of composites increases with the increasing loading rate. Dynamic enhancement is analyzed quantitatively, and it is mainly contributed by the rate-dependent behavior of fiber-matrix interface. Fracture analysis indicates the stronger interfacial bonding under dynamic loading, which is attributed to the impeded interfacial crack propagation by high strain rate. A constitutive model is built to predict the rate-dependent strength of CCFRP composites, and it fits well with the experimental results.

Rate sensitive plasticity-based damage model for concrete under dynamic loading

In this manuscript, a rate-sensitive plasticity-based damage model for concrete subjected to dynamic loads is presented. The developed rate-sensitive damage model incorporates the key experimental evidence related to strain rate and damage rate. With increasing strain rates, the model is able to predict a decrease in the rate of damage evolution due to artificial stiffening effects together with a final higher level of damage. The major contribution of the work is to include the effect of damage rate, strain rate and also to consider all the physical mechanisms of damage. This is achieved by using a power law model to relate the rate of damage to the equivalent plastic strain rate. The damage parameter considers hydrostatic, tension and compression damage. Such a damage definition helps in the prediction of pulverized damage due to a loss in cohesive strength at increased hydrostatic stress, shear-induced compressive damage and tensile microcracking. Strong volumetric deformation of the material that includes hydrostatic and compaction damage is also considered in the model. Because hydrostatic damage accounts for the reduction in stiffness and compaction damage accounts for the increase in stiffness under pure compressive loading. A strain rate-dependent failure surface is considered and with increasing strain rate there is an increase in the size of the failure surface which is capped at an upper limiting value. The incremental effective stress-strain relationship is defined in terms of rate of damage, accumulated damage and viscosity parameters reflecting the inherent inertial, thermal and viscous mechanisms respectively. The stress-strain relationship in the model also accounts for stress reversals that occur due to interference of an incident and reflected wave, by including a Heaviside function. The developed model is implemented in LS-DYNA using vectorized UMAT. Verification, validation and parameterization of the developed model have been made through several numerical analyses.

Damage Model of Carbon-Fiber-Reinforced Concrete Based on Energy Conversion Principle

In order to enhance the practical application of carbon-fiber-reinforced concrete (CFRC) in engineering, it is necessary to study the damage mechanism of CFRC. Experimental research on the mechanical properties of CFRC under multiple strain rates was conducted. Five different fiber contents were analyzed to study the compressive strength and tensile strength of CFRC, and the damage characteristics of CFRC under multiple strain rates were analyzed based on failure modes and energy changes. An energy-based damage constitutive model was established. The results showed the following: (1) When the carbon fiber content was 0.4%, CFRC had the best comprehensive performance, with a 15.02% increase in compressive strength and a 51.12% increase in tensile strength. With the increase in strain rate, the compressive strength of the concrete increased. (2) Under high strain rates, carbon fiber significantly enhanced the compressive strength of the concrete, and the input energy, elastic strain energy, and dissipated energy increased. The peak value of the elastic strain energy conversion rate increased, and the minimum value of the dissipated energy conversion rate decreased. (3) Under the same strain rate, the CFRC had a larger inflection point of dissipated energy corresponding to the strain compared to the reference group of concrete during the loading process. A constitutive model for CFRC was established based on damage mechanics and probability statistics. The research results will provide theoretical references for the application of carbon-fiber-reinforced concrete.

Hot Deformation Behavior of a Marine Engineering Steel for Novel 980 MPa Grade Extra‐Thick Plate

The hot deformation behavior of a marine engineering steel for a novel 980 MPa grade extra‐thick plate is investigated through hot compression tests at temperatures of 850–1200 °C and strain rates of 0.01–10 s−1. The flow stress curves are corrected to eliminate the effects of friction on the flow stress, and a modified constitutive model is established to accurately quantify the flow behaviors of the steel. Hot working maps of the steel are derived by using a dynamic materials model and the Prasad instability criterion. The effects of the deformation parameters on the dynamic recrystallization (DRX) nucleation mechanisms are evaluated using scanning electron microscopy and electron backscatter diffraction. The flow stress and peak strain increased with decreasing deformation temperature and increasing strain rate, respectively. The activation energy for hot deformation after friction correction is 380.49 kJ mol−1. The DRX mechanism involves the migration of subgrains. The deformation parameters are optimized by combining the degree of DRX and the size of recrystallized grain. The optimum hot working window is determined as 1100–1200 °C and 0.1–10 s−1.

Analysis of the Dynamic Mechanical Properties and Energy Dissipation of Water-Saturated Fissured Sandstone Specimens

To investigate the dynamic mechanical properties of water-saturated fissure rock at different strain rates, prefabricated sandstone specimens with a 45° dip angle were treated with water saturation and the impact compression test was performed with a Split Hopkinson Pressure Bar (SHPB) test device at different impact pressures. The results show that the clusters of dynamic stress–strain curves of water-saturated and natural sandstone specimens with a 45° dip angle of prefabricated fissures are basically similar under different impact air pressures. A distinct strain rate effect was observed for dynamic strain and dynamic compressive strength, both of which increased with increasing strain rate. From the failure pattern of the specimen, it can be seen that cracks appeared from the tip of the prefabricated fissure under axial stress, spreading to both ends and forming wing cracks and anti-wing cracks associated with shear cracks. As the strain rate increased, the energy dissipation density of the specimen gradually increased, and the macroscopic cracks cross-expanded with each other. The fracture form of the specimen showed a small block distribution, and the average particle size of the specimen gradually decreased. The specimen crushing energy dissipation density was negatively correlated with fracture size, reflecting a certain rate correlation. The sandstone fragments’ fractal dimension increases with the increase in crushing energy dissipation density, and the fractal dimension may be applied as a quantitative index to characterize sandstone crushing.

Numerical Investigation of Partially Premixed Flames Under Transcritical Conditions

Understanding the characteristics of partially premixed flames (PPFs) under transcritical conditions is of critical importance for the development of fuel-rich staged rocket engines. While substantial progress has been achieved for transcritical non-premixed flames (NPFs), comparatively little effort has been made to investigate transcritical PPFs. To this end, a series of transcritical counterflow gaseous hydrogen/liquid oxygen (GH2/LOX) PPFs is simulated to investigate the thermodynamic structure of PPF and to examine the effects of molecular diffusion modeling, strain rate, and the equivalence ratio at the fuel-rich side on PPFs in physical space and mixture fraction space, as well as reduced temperature/reduced pressure space. The comparisons between the NPF and PPF demonstrate that the PPF exhibits a bimodal structure in physical space: a premixed reaction zone at the fuel inlet side and a non-premixed reaction zone at the oxidizer side. In mixture fraction space, the C-shaped structure of PPF is observed owing to the differential diffusion of species. It is found that the choice of molecular diffusion model has a significant impact on PPF structure. The presence of a loop at subcritical pressures in a reduced temperature/reduced pressure space is caused by the differential diffusion of species and the formation of [Formula: see text] in the non-premixed reaction zone. Furthermore, the results indicate that the premixed reaction zones in the PPFs are very sensitive to the change in strain rate and/or equivalence ratio of the premixed mixture at the fuel inlet side. For a given equivalence ratio, increasing strain rate can suppress the differential diffusion effect and the C-shaped structure, while it has a negligible impact on the non-premixed reaction and hence on loop formation.

Investigation on Creep Deformation and Age Strengthening Behavior of 304 Stainless Steel under High Stress Levels.

The creep deformation behavior and age strengthening behavior of 304 stainless steel under high stress levels were systematically studied by uniaxial creep test, tensile test, XRD diffraction test and transmission electron microscopy. The results show that the total creep strain and the initial creep strain rate increase with the increase in stress level, and the creep strain in the whole aging process is mainly produced in the initial creep stage. The calculated stress exponent shows that the main mechanism of creep deformation of 304 stainless steel at 453 K is dislocation slip. The strength and plasticity of 304 stainless steel after creep aging are improved simultaneously. Microstructural observations indicate an increase in dislocation density and martensite content, as well as austenite and twins, leading to an improvement in strength and plasticity, respectively. In addition, considering the influence of dislocation density on creep behavior, the relative dislocation density increase is introduced into the hyperbolic sine creep model, and a simple mechanism-based creep aging constitutive model is established. The creep strain predicted by the model is in good agreement with the experimental data of 304 stainless steel. The findings can provide theoretical support for the application of creep age forming in 304 stainless steel parts.

Study on the Constitutive Modeling of (2.5 vol%TiB + 2.5 vol%TiC)/TC4 Composites under Hot Compression Conditions.

The hot deformation behavior of titanium matrix composites plays a crucial role in determining the performance of the formed components. Therefore, it is significant to establish an accurate constitutive relationship between material deformation parameters and flow stress. In this study, hot compression experiments were conducted on a (2.5 vol%TiB + 2.5 vol%TiC)/TC4. The experiments were performed under temperatures ranging from 1013.15 to 1133.15 K and strain rates ranging from 0.001 to 0.1 s-1. Based on the stress-strain data obtained from the experiment, the constitutive models were established by using the Arrhenius model and the BP neural network algorithm, respectively. Considering the relationship between strain rate, hot working temperature, and flow stress, a comparative analysis was conducted to evaluate the prediction accuracy of two different constitutive models. The research results indicate that the flow stress of (2.5 vol%TiB + 2.5 vol%TiC)/TC4 increases with decreasing temperature and increasing strain rate, and the stress-strain curve shows obvious work hardening and softening behaviors. Both the Arrhenius model and the BP neural network algorithm are effective in predicting the hot compression flow stress of (2.5 vol%TiB + 2.5 vol%TiC)/TC4, but the average relative error and root mean square error of the BP neural network algorithm are smaller and the correlation coefficient is higher, thus possessing higher accuracy and reliability.

Research on the Tensile Mechanical Properties and Fracture Characteristics of GFRP Mortar Tubes under Impact Loads

To investigate the protective effect of glass fiber-reinforced plastic (GFRP) pipes on cement mortar, dynamic splitting tests were carried out on cement mortar and GFRP-mortar pipe specimens by using a 74 mm diameter Hopkinson pressure bar (SHPB) test device combined with a high-speed video camera device to obtain the crack expansion process and rupture and fragmentation morphology of the specimens. The results show that the tensile strength of cement mortar rings is lower and that the stress?strain curve is single-peaked. Compared with cement mortar rings, the tensile strength of GFRP-mortar ring specimens increases significantly, and the stress-strain curve is bimodal. For 0.5 MPa air pressure and wall thicknesses of 0, 2, 3, and 4 mm, the hollow rate of the 0.187 specimen peak stress is 3.21, 1.02, 1.05, and 1.26 times that of the 0.292 specimen. For 0.5 MPa air pressure and a hollow rate of 0.187, the wall thickness of the 2, 3, and 4 mm specimen peak stress is 1.06, 1.31, and 1.69 times that of the 0 mm specimen. The increase in the wall thickness, the decrease in the hollow rate and the increase in the strain rate make the dynamic tensile strength of the specimens increase. The dynamic modulus of elasticity of the GFRP-mortar pipe specimens was lower than that of the mortar specimens, and the increase in strain rate and the thickness of the GFRP pipe wall resulted in an increase in the dynamic modulus of elasticity of the specimens. The cement mortar specimen under impact load was split and tensile damage occurred; the GFRP-mortar specimen suffered tensile damage at the two ends of the applied force, and the upper and lower ends were crushed. With the increase in the strain rate, the GFRP-mortar specimens transitioned from cracks to the formation of “V” area damage, and the specimen crushing degree increased, but the existence of GFRP pipe can have a protective effect on the mortar, reducing the degree of specimen rupture and crushing. The increase in the hollow ratio reduces the ability of the specimen to resist the external load, and the degree of specimen rupture and fragmentation increases.

Tensile impact behaviors and damage mechanism of woven carbon fiber-reinforced-polymer laminates considering adiabatic shear band

In this study, the tensile impact behavior and damage mechanism of a two-dimensional woven carbon-fiber-reinforced-polymer (CFRP) laminates under different high strain rates are investigated. The damage morphologies are illustrated and studied using meso-macroscale finite element analysis (FEA) and Split Hopkinson bar (SHB) test, FEA results are compared and validated with the SHB test results, and proved to be correct with enough accuracy. The matrix crack first appeared, the tensile stress wave propagated along the interface, which causes shear failure at the interface, which dominated the failure of the specimen and formed adiabatic shear bands inside each ply, the adiabatic shear bands begin to develop in areas with interfacial damage, they are formed after structural damage and propagate in the direction of the interface, accompanied by damage evolution. High strain rate 1400, 1600, and 2400 s−1 impacts tend to lead to direct failure of composite laminates compared to low strain rate 600, and 800 s−1 impacts, the Young’s modulus with high strain rate 1400, 1600, and 2400 s−1 are more than twice as much compared to low strain rate 600, and 800 s−1. The plain-woven CFRP laminates is characterized with strain-rate-hardeningeffect and strain rate sensitivity, with increase of high strain rate from 600 to 2400 s−1, the Young’s modulus increases from 8.91 to 35.7 GPa, whereas the yield strength increases from 397 to 1298 MPa. The delamination occurs between the plies after crack generation inside the epoxy resin matrix, and begins at a certain strain value after the elastic phase, the micro-cracks propagated along the fiber bundle-matrix interface, thus causing interfacial debonding due to shear stress, these factors result in a strain-rate hardening effect.

Study on Deformation Behavior and Mechanical Properties of 42CrMo High-Strength Steel with Multi-Station Warm Upsetting

In order to find out the deformation behavior and mechanical properties of 42CrMo steel under warm upsetting conditions, the Gleeble-3500 thermal simulation testing machine was used to carry out a warm upsetting physical simulation experiment on 42CrMo steel. By controlling deformation temperature, strain rate, and constant temperature deformation pass, the microstructure evolution rule under different warm upsetting conditions was analyzed, and its hardness value was measured. Then, the simulation experiment is carried out based on the Deform-3D finite element platform. The results show that, with the increase in deformation temperature, 42CrMo steel has a temperature rise softening effect, which significantly reduces the peak value of rheological stress. At 650 °C, the maximum peak value of rheological stress is only 45.3% of that of cold upsetting deformation at room temperature, and the stress-strain curve tends to be gentle at the plastic deformation stage, which is the most suitable temperature for warm upsetting deformation. The maximum peak flow stress of 42CrMo steel increases with the increase in strain rate, but the number of deformation channels has little influence on the stress-strain curve. The warm, upsetting deformation can refine the internal grain structure significantly, and the grain refinement mechanism is mechanical crushing. When the temperature is slightly higher, the broken grain will recover, and the grain size will grow. During the process of warm upsetting, the strain rate has a great influence on the microhardness of the sample. The deformation pass has little influence on the hardness, and the hardness increases slightly with the increase in the deformation pass. Through the Deform-3D simulation, the correlation coefficient R and the average absolute relative error (AARE) between the simulation value and the experimental value were calculated, and the correlation coefficient R-value was 0.9948, and the average absolute relative error (AARE) was 2.05%, indicating that the simulation can accurately reflect the relationship between displacement and applied load.

Hybrid fibre reinforced recycled aggregate concrete: dynamic mechanical properties and durability

Research into recycled aggregate concrete (RAC) has been on the rise as there is a need to preserve natural resources and promote sustainable infrastructure practices. In our previous study, a new hybrid-fibre reinforced recycled aggregate concrete (FRRAC) with micro basalt fibre (BF) and recycled macro polypropylene fibre (PF) was developed and its physical and mechanical properties were investigated. As a continuous work, this study investigated the dynamic mechanical properties and durability of the developed FRRAC. Dynamic compressive and splitting tensile strength tests were undertaken using a Ø 100 mm split Hopkinson pressure bar (SHPB) system. The failure processes and failure modes of RAC and FRRAC specimens under different strain rates were recorded using a high-speed camera. For dynamic compressive strength, the stress-strain curves, dynamic increase factor for compressive strength (DIFf’c) and energy absorption capacities of RAC and FRRAC were evaluated and compared in the strain rate range from around 27.9 s-1 to 138.0 s-1. The stress-time histories, crack-opening displacement values and dynamic increase factor for splitting tensile strength (DIFfst) at the strain rates of around 3.03 s-1 to 8.17 s-1 of the two materials were obtained and compared. The test results indicated that RAC specimens were slightly more strain rate sensitive than natural aggregate concrete (NAC) under dynamic compression and splitting tension. On the other hand, FRRAC specimens showed similar strain rate sensitivity as their counterparts with no fibres under dynamic compression but showed lower DIFfst values with increasing strain rate under splitting tension. Empirical formulae for DIFf’c, DIFfst and energy absorption capacities against strain rates were proposed. In addition, the durability performance was assessed by examining the water absorption, volume of permeable voids (VPV), water sorptivity and chloride ion penetration resistance. Durability test results indicated that RAC and FRRAC in this study are suitable for structural applications.

High-temperature compression deformation strengthening mechanism of Mg–Gd–Y–Zn–Zr alloy formed by cold metal transfer (CMT) arc additive forming

The Mg–Gd–Y–Zn–Zr alloy structural parts were formed by CMT arc additive forming. The high-temperature compression deformation strengthening mechanism of the alloy at 623–723 K, 0.01–1s−1, and 50 % deformation was studied. Upon a decrease in the temperature or an increase in the strain rate, the effect of fine-grain strengthening was obvious, which led to the improvement of the mechanical properties of the alloy after deformation. The grain refinement was mainly attributed to the DDRX and CDRX processes during hot deformation. Because the compression specimen was loaded along the building direction, the twins after deformation were compression twins {10–11}, and the content increased with an increase in the strain rate or a decrease in the temperature. As the strain rate increased, the texture distribution on the {10-10} plane became more concentrated and the texture intensity decreased from 6.79 to 2.24. This was attributed to the fact that the alloy was mainly dominated by a non-basal slip during high-temperature deformation, which increased the sensitivity of the strain rate to the texture.

Atomistic simulation of structural transition and grain refinement in Fe nanowires driven by high strain rate compression

The compression-induced structural transition (ST) and grain refinement of BCC Fe nanowires have been investigated based on atomistic simulations. It is found that high strain rate compression can cause nanowires to reach higher pressures and meet ST conditions, and the threshold stress of ST is found to be much lower than that of bulk materials. Thus, the compression process of nanowires at high strain rates includes elastic deformation, ST and its reverse process, grain refinement, and buckling instability. The occurrence of grain refinement is due to the fact that after undergoing ST and its reverse process, the (001) cross section of the nanowire can be transformed into different (111) planes. Furthermore, the dependence of stress threshold and nucleation structure on strain rate is revealed. When the strain rate increases to a certain threshold, HCP nucleation is found to occur on the side surface of the nanowire, but its reverse process will quickly occur due to the pressure release on the side. With the strain rate increase, the stress threshold of ST no longer satisfies a constant power-law change, and the power-law index will increase. When the strain rate exceeds 5 × 1010 s−1, the elastic deformation prior to ST also exhibits strong nonequilibrium characteristics, causing a sharp increase in the number of HCP nuclei. Especially, the nanowires will ultimately be in a disordered state, rather than a nanocrystalline structure. Also, the cylindrical and prismatic nanowires are both considered to understand the influence of boundary morphology, and the differences in nucleation and the similarity in deformation are explained. Note that, as the strain rate increases, the cylindrical nanowires undergo HCP nucleation directly, while the prismatic nanowires undergo significant twinning deformation first.

Cryogenic deformation behavior of Al–Zn–Mg–Cu alloy and its sensitivity to temperature and strain rate

The flow behavior and deformation mechanism of solution quenched Al 7075 alloys at different cryogenic temperatures and strain rates are systematically investigated in the present work. Uniaxial tensile experiments were conducted at five different temperatures (room temperature (RT), −80 °C, −120 °C, −160 °C, −190 °C) and three different strain rates (0.00025 s−1, 0.001 s−1, 0.01 s−1). Deformation characteristics and microstructure of the sample were studied by optical microscope (OM), scanning electron microscopy (SEM), electron back scattered diffraction (EBSD) and transmission electron microscopy (TEM). The results showed that the deformation behavior of solid solution quenched 7075 aluminum alloy is synergistically affected by the deformation temperature and strain rate. The strength and plasticity of the sample are simultaneously elevated at −190 °C and −160 °C compared to that at RT with strain rates of 0.00025 s−1 and 0.001 s−1, respectively. The critical temperatures for plastic enhancement are −160 °C, −120 °C and −80 °C with strain rates of 0.00025s−1, 0.001s−1 and 0.01s−1, respectively. The proportion of increase in tensile strength and plasticity at −190 °C is more significant with the increase of strain rate. Different strain rates can lead to different Portevin-Le Chatelier (PLC) types of stress-strain curves. The PLC effect exhibits serrated fluctuations of type C, type B and type A at RT with strain rates of 0.00025s−1, 0.001s−1 and 0.01s−1, respectively. When the strain rates are 0.001s−1 and 0.01s−1, the PLC effect is completely suppressed at cryogenic temperatures. Interestingly, the PLC effect reappears at −160 °C and −190 °C with a strain rate of 0.00025s−1. The pinning effect of clusters formed by solute atom diffusion on dislocations as well as the grid-like entanglement formed by dislocations can result in the unstable plastic flow of aluminum alloy. In summary, the tensile properties and the PLC type of the 7075 aluminum alloys are determined by the combination of deformation temperature and strain rate.

Hot deformation behavior and hot working window of a solid solution strengthened Ni–Cr–Fe-based superalloy

The hot deformation behavior of Ni–Cr–Fe-based ChS57 alloy was investigated by isothermal compression tests in the temperature range of 1000–1200 °C with strain rates covering 0.001–10 s−1. Based upon the hyperbolic sine function and dynamic material model, the constitutive model and processing maps of the alloy were established, the microstructure evolution was systematically analyzed and the optimization of the hot working window was completed. The results indicate that the flow stress increases remarkably with increasing strain rate and decreasing deformation temperature, while the amount of strain required to reach peak stress also gradually increases. The hot deformation process of this alloy is the result of the competition between dynamic softening and work hardening, with a deformation activation energy of 451.34 kJ/mol; dynamic recrystallization (DRX) is the main softening mechanism, while both continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) exist; the low temperatures and medium strain rates (1000–1025 °C, 0.045–0.37s−1) are the instability zone parameters of the alloy when flow localization and deformation bands are the microstructural manifestations of unstable hot working. The optimum hot working window for ChS57 alloy is the medium-low temperatures and low strain rates（1050–1100 °C, 0.001–0.003s−1）and the medium-high temperatures and high rates（1125–1175 °C, 1-10s−1 or 1100 °C/10s−1）, where the microstructure appears as randomly oriented and uniformly distributed full DRX grains.

Experimental study on mechanical properties of sprayed ECC under quasi-static and cyclic loading

The tunnel lining will be subjected to the cyclic action of high ground stress during service. However, there is a lack of research on the damage evolution mechanism of sprayed engineered cementitious composites (ECC) under cyclic loading. Herein, we studied the strain rate effect and fatigue damage evolution mechanism of sprayed ECC by conducting compression and bending tests at different quasi-static strain rates and high-stress cyclic loading-unloading compression and bending tests. The results show that with the increase of strain rate, the compressive peak stress and bending strength of sprayed ECC show an upward trend, as well as the increase of the change rate. At 8.3 × 10−3 s−1, the compressive SGF is 1.52, with an energy storage limit density of 0.501MJ/m3. At 2.1 × 10−2 s−1, the bending SGF is 1.52, with a peak fracture energy of 49.29 J. In addition, under the compression test, the tensile shear failure of specimens develops from finer double shear planes to coarser multiple shear planes. Under the bending test, the position of the main crack of specimen is gradually transferred from the mid-span to both sides. Under high stress cycle test, the input energy and dissipation energy of the two tests have the similar trend, showing “U” type. The variation of elastic energy and dissipation energy conversion rate exhibits a “U” type with different directions. At the last loading, the dissipation energy conversion rate of compression test is 46.8%, while that of bending test is 70.4%. The damage variable of specimen increases significantly with the increase of the number of cycles, accompanied by an increase in the degree of breakage. In addition, the damage variables based on deformation characteristics are higher than those based on energy characteristics.

Quantitative study on dynamic instantaneous dissolution of precipitated phases in 2195-T6 Al-Li alloy based on characterizations with SANS and TEM

The kinetics of the dynamic instantaneous dissolution of the precipitated phases in 2195-T6 Al-Li alloy under high strain rate loading was firstly studied quantitatively by means of the small angle neutron scattering (SANS) and transmission electron microscopy (TEM) techniques, and the thermodynamics and transformation mechanism were also illustrated in this paper. The high strain rate loadings were performed by the split Hopkinson pressure bar. The maximum flow stresses increased but the pulse durations decreased with increasing strain rates. TEM observations indicated that the size and volume fraction of the T1 phases decreased. The SANS fitting results showed that the radius and volume fraction of the T1 phase decreased after loading. The dissolution activation energy (Qd) was calculated to be 28.4 kJ/mol by combining the Johnsone-Mehl-Avrami-Kolmogorov (JMAK) equation and the Arrhenius equation, indicating that the instantaneous dissolution of T1 phase was kinetically feasible. The T1 phase generated a larger distortion energy, and the surface energy as well as the internal pressure of T1 phase increased in dissolving during dynamic loading to increase the free energy difference (driving force for dissolving) between T1 phase and matrix, making the instantaneous dissolution of the T1 phases be thermodynamically feasible. The diffusion rates of Cu and Li atoms were significantly accelerated to promote the instantaneous dissolution of T1 phase particles. The results of the Vickers hardness values showed that the hardness increased after loadings, due to the high density of dislocations and only partial dissolution of the T1 phase particles.