Obvious Softening Research Articles

Dynamic Softening and Hardening Behavior and the Micro-Mechanism of a TC31 High Temperature Titanium Alloy Sheet within Hot Deformation.

TC31 is a new type of α+β dual phase high temperature titanium alloy, which has a high specific strength and creep resistance at temperatures from 650 °C to 700 °C. It has become one of the competitive candidates for the skin and air inlet components of hypersonic aircraft. However, it is very difficult to obtain the best forming windows for TC31 and to form the corresponding complex thin-walled components. In this paper, high temperature tensile tests were carried out at temperatures ranging from 850 °C to 1000 °C and strain rates ranging from 0.001 s−1 to 0.1 s−1, and the microstructures before and after deformation were characterized by an optical microscope, scanning electron microscope, and electron back-scatter diffraction. The dynamic softening and hardening behaviors and the corresponding micro-mechanisms of a TC31 titanium alloy sheet within hot deformation were systematically studied. The effects of deformation temperature, strain rate, and strain on microstructure evolution were revealed. The results show that the dynamic softening and hardening of the material depended on the deformation temperature and strain rate, and changed dynamically with the strain. Obvious softening occurred during hot tensile deformation at a temperature of 850 °C and a strain rate of 0.001 s−1~0.1 s−1, which was mainly caused by void damage, deformation heat, and dynamic recrystallization. Quasi-steady flowing was observed when it was deformed at a temperature of 950 °C~1000 °C and a strain rate of 0.001 s−1~0.01 s−1 due to the relative balance between the dynamic softening and hardening. Dynamic hardening occurred slightly with a strain rate of 0.001 s−1. Mechanisms of dynamic recrystallization transformed from continuous dynamic recrystallization to discontinuous dynamic recrystallization with the increase in strain when it was deformed at a temperature of 950 °C and a strain rate of 0.01 s−1. The grain size also decreased gradually due to the dynamic recrystallization, which provided an optimal forming condition for manufacturing thin-walled components with the desired microstructure and an excellent performance.

Triaxial Mechanical Characteristics and Microscopic Mechanism of Graphene-Modified Cement Stabilized Expansive Soil

To investigate the effect of graphene (G) on the triaxial mechanical characteristics of cement stabilized soil, a triaxial unconsolidated and undrained (UU) test was conducted on cement stabilized expansive soil (CES) and graphene-modified cement stabilized expansive soil (GCES) with 0.05%, 0.1%, 0.15%, and 0.2%G content, respectively. The experimental results showed that compared with CES, the deviatoric stress-strain curve of GCES showed a more obvious softening characteristic. The addition of G had a good modification effect on the triaxial mechanical characteristics of CES, and with the increase of the content of G, the compressive strength and shear strength of CES increased first and then decreased, and reached the best value when the G content was 0.1%. The failure angle predicted by the Mohr-Coulomb failure angle formula had a good correlation with the measured failure angle of soil specimens, so it could provide help for the prediction of soil failure surface in practical engineering. Besides, the scanning electron microscope (SEM) analysis indicated that the internal structure of CES was improved due to G addition.

Rapid in situ test and determination of dam foundation weak mudstone bearing capacity

In a hydropower station project in the middle reaches of the Minjiang River, the dam foundation rock mass is the upper cretaceous Guankou formation (K2g) brick red weak mudstone. In previous investigations, the single-pipe core drilling technology was employed, which greatly disturbed the cores. Most of the cores were broken, and the mudstone cores quickly lost water, cracked, and disintegrated after being taken out, making it difficult to obtain several columnar cores. Indoor uniaxial compressive strength tests were conducted using only a few short columnar mudstone cores, but the obtained mudstone uniaxial compressive strength was extremely low, and the converted bearing capacity of the dam foundation mudstone was only 0.25 Mpa, which does not meet the dam foundation bearing capacity standard. Large-area pile foundation treatment is required for the dam foundation mudstone at a high cost, and the project construction period is delayed. To obtain the accurate bearing capacity of the dam foundation mudstone, we used a self-developed “self-anchored dam foundation rock mass bearing capacity rapid test device” and excavated the dam foundation ship lock section to the elevation of the foundation surface. In situ tests were conducted on 12 dam foundation mudstone rock masses in 15 days, and the longitudinal wave velocities of the rock masses were tested. Reliable bearing capacity parameters of the weak mudstone of the dam foundation were obtained, and the relationship between the bearing capacity of the dam foundation mudstone and the longitudinal wave velocity was established. In situ tests showed the minimum and maximum bearing capacities of 0.986 and 1.972 Mpa, respectively, for the dam foundation weak mudstone. After excavating the dam foundation mudstone, it showed high bearing capacity without obvious softening and relaxation. The in-situ-measured minimum bearing capacity (0.986 Mpa) is consistent with the recommended bearing capacity of dam foundation mudstone, which is much greater than the maximum load of the dam foundation (0.5 Mpa). The value meets the bearing capacity requirements for dam foundation rock masses without engineering treatment. This study obtained the real bearing capacity of mudstone for dam foundations, thereby overcoming the problem of the low bearing capacity of weak mudstone obtained through laboratory experiments. The method not only saves cost but also lays the foundation for scheduled pouring of dams.

Study on microstructure evolution of Ti–6Al–3Nb–2Zr–1Mo alloy with bimodal structure during hot compression in the near β region

Hot compression tests were performed different reductions on Ti–6Al–3Nb–2Zr–1Mo alloy with bimodal structure in the near β phase region to investigate the microstructure evolution. The stress–strain curves showed obvious flow softening. With the increase of deformation strain, the alloy gradually undergoes dynamic recrystallization (DRX). Simultaneously, the thickness of lamellar αs grains significantly decreases. The high-density dislocation area provides a suitable place for β grains nucleation. Subsequently, β grains grow in lamellar α grains and become approximately 60° with the lamellar α grains under the driving force of the dislocation. The calculated DRX critical true strain of the alloy deformed at 0.01 s−1 and 980 °C is 0.095. Discontinuous dynamic recrystallization (DDRX) is the main DRX mechanism, while continuous dynamic recrystallization (CDRX) is the assistant DRX mechanism. In addition, the twin boundaries provides opportune locations for the nucleation of DRX grains, thereby promoting the DRX of the alloy. Moreover, the initial texture disappears, and different new texture components appear during hot deformation. The texture density first increases with increased deformation strain. With the further increase of deformation strain, the texture density gradually decreases, due to the combined action of sufficient DRX and dynamic phase transformation.

Mechanical Performance of Vegetation-Growing Concrete Reinforced by Palm Fibers under Triaxial Compression

Fibers have been widely applied to improve the mechanical properties of natural soil and traditional cemented soils, but rarely investigated in substrates for slope eco-protection. Vegetation-growing concrete (VC) substrate is a kind of ecological cemented soil and has very wide popularization and application prospects. In this study, unconsolidated-undrained (UU) triaxial compression tests were conducted on the control specimens of VC substrate containing three cement contents and the palm fiber reinforced specimens manufactured by adding four fiber contents. The control specimens were shown to possess increasing yield strength, ultimate strength, yield strain and shear modulus with cement content, which resulted from the cemented texture improved by the hydrate products as recognized previously. Instead of inextensible reinforcement provided by cement, the palm fibers were found to provide extensible reinforcement, through which the obvious softening behavior of VC substrate could transfer to elasto-plasticity. Attributed to the unique ductility, the fibers added could bring with considerable increase in ductility of reinforced substrate, except for additional increases in yield strength, ultimate strength and yield strain.

Nonlinear vibration analysis of a cantilever beam with multiple breathing edge cracks

The nonlinear vibration analysis of cracked structures could be a suitable indicator for diagnosing defects. When breathing cracks appear on the beam, its behavior can be modeled as a bilinear oscillator. In this paper, the nonlinear vibration behavior of a cantilever beam with multiple breathing cracks is investigated. For this purpose, the multi-cracked beam’s stiffness is calculated by introducing the effective length for the local stiffness reduction in the vicinity of cracks. Furthermore, owing to the importance of the damping variation in a cracked beam, its exact value is evaluated through theoretical expressions. A continuous polynomial function with the Weierstrass approximation is exploited to simulate the bilinear behavior of breathing cracks. By performing nonlinear analysis using the perturbation technique, the cracked beam’s dynamic behavior is studied. Moreover, the sensitivity of the response to the crack parameters in the primary resonance of the structure is analyzed. Finally, the sensitivity of the beam’s nonlinear response to the different number of breathing cracks and various crack depths is investigated. It is shown that the beam with a higher number of cracks or deeper ones has obvious softening behavior, and hence a more significant jumping can appear in the response.

Single‐Crystal Elasticity of High‐Pressure Ice up to 98 GPa by Brillouin Scattering

AbstractKnowledge of the elasticity of high‐pressure ice is key to constructing the internal structure of various planetary bodies. Here, we present direct experimental constraints on the single‐crystal elasticity of ice with 0.5 mol/L (0.9 mol.%) NaCl up to 98 GPa at 300 K. The change from ice‐VII to the dynamically disordered ice‐VII is characterized by an obvious softening of the longitudinal and bulk moduli between 42.4 and 54 GPa. Further transition to ice X at ∼64 GPa results in an apparent change in the pressure dependence for the elasticity. Using the obtained elasticity, our modeling reveals that the boundary between the ice‐VII layer and the silicate mantle or core in moderate‐size icy planetary bodies would be associated with a sudden velocity jump. In large planetary bodies, the change from ice X to the silicate layer will be characterized by an abrupt reduction in velocity.

A Novel Thermostable Keratinase from Deinococcus geothermalis with Potential Application in Feather Degradation

Keratinase can specifically attack disulfide bridges in keratin to convert them from complex to simplified forms. Keratinase thermal stability has drawn attention to various biotechnological industries. In this study, a keratinase DgeKer was identified from a slightly thermophilic species, D. geothermalis. The in silico analysis showed that DgeKer is composed of signal peptide, N-terminal propeptide, mature domain, and C-terminal extension. DgeKer and its C-terminal extension-truncated enzyme (DgeKer-C) were cloned and expressed in E. coli. The purified DgeKer and DgeKer-C showed maximum activity at 70 °C and pH 9–The thermal stability assay (60 °C) showed that the half-life value of DgeKer and DgeKer-C were 103.45 min and 169.10 min, respectively. DgeKer and DgeKer-C were stable at the range of pH from 9 to 11 and showed good tolerance to some metal ions, surfactants and organic solvent. Furthermore, DgeKer could degrade feathers at 70 °C for 60 min. However, the medium became turbid with obvious softening of barbules after being treated with DgeKer-C, which might be due to C-terminal extension. In summary, a thermostable keratinase DgeKer with high efficiency degradation of feathers may have great potential in industry.

High strain rate impact effect and failure behavior of 3D six-directional braided composites

The high strain rate impact effect and failure behavior of three-dimensional six-directional (3D6d) braided composites with different braiding angles are reported in this paper. The results show that 3D6d braided composites exhibit typical strain rate sensitivity. Under out-of-plane dynamic compression, the stress–strain curves rise linearly and present obvious softening characteristics. With the increase of strain rate, both the strength and modulus of composites increase, while the failure strain decreases. Moreover, the composites with larger braiding angle exhibit higher dynamic curves and better out-of-plane high strain rate properties. The fracture morphology shows that composites exhibit different dynamic damage and failure modes at different strain rates and braiding angles. As the strain rate increases, the composites show clearer shear fracture feature and more severe brittle failure. The damage is mainly manifested as shear of the sixth and braiding fibers, tear of the fifth fibers on the shear plane, interfacial debonding and matrix cracking. As the braiding angle increases, the damage and fracture of composites are reduced obviously and the composites become less brittle failure at high stain rates.

Spin-phonon coupling and dielectric spectroscopy in nano-crystalline Pr[formula omitted]CoMnO[formula omitted] double perovskite manganite

In this paper, we present the spin-phonon coupling and dielectric response of nano-crystalline Pr2CoMnO6 employing Raman and dielectric spectroscopic study. Pr2CoMnO6 is a manganite compound , it undergoes a paramagnetic to ferromagnetic (PM–FM) phase transition around Tc∼172 K. Temperature-dependent Raman scattering experiment is carried out across Tc to study the spin-phonon behavior in this material. The results from Raman study reveal an obvious softening of the phonon mode involving stretching vibrations of the (Co/Mn)O6 octahedra in ferromagnetic temperature regions, indicating a close correlation between magnetism and lattice in Pr2CoMnO6 and conform the spin phonon coupling. Further, we have carried out detailed study on dielectric response, impedance spectroscopy, electric modulus and AC conductivity of Pr2CoMnO6 ceramics in the temperature range of 20 K - 300 K and frequency range of 1kHz - 5.5MHz. We found Pr2CoMnO6 shows strong frequency dependence with large dispersion and large dielectric constant. It is found that thermally activated relaxation mechanism is involve and material is deviated from Debye’s model which is confirmed by Nyquist plot and complex modules behavior.

Uniaxial Dynamic Compressive Behaviors of Hydraulic Asphalt Concrete under the Coupling Effect between Temperature and Strain Rate.

To investigate the compressive dynamic properties of hydraulic asphalt concrete under various temperatures, four temperatures and four strain rates have been set to perform the uniaxial compression experiments using hydraulic servo machine in this paper. The influence of temperature and strain rate on the failure modes, stress-strain curves and mechanical characteristic parameters of hydraulic asphalt concrete is analyzed and the results reveal that the failure modes and stress-strain curves have significant temperature effect. When the temperature is between −20 °C and 0 °C, the failure mode is dominated by brittle failure of asphalt binder, and hydraulic asphalt concrete shows obvious strain softening. With the addition of temperature, the failure modes of specimens are transferred from brittle failure to ductile failure since the asphalt changes from elastic-brittleness to viscoelasticity. Influenced by temperature effect, the compressive stress-strain curves of hydraulic asphalt concrete show strain hardening while the peak stress of hydraulic asphalt concrete is obviously decreased, and the variation coefficient of peak stress has a power relation with temperature. With successive increases in strain rate, the stress-strain curves of hydraulic asphalt concrete gradually are transferred from strain hardening to strain softening. The peak stress and stiffness modulus of specimens under compression gradually increase, and the dynamic increase factor of peak stress is linearly related with the logarithm value of strain rate after dimensionless treatment. In terms of the quantitative analysis of the experimental data, two relationship models of the coupling effect between temperature and strain rate are proposed. The proposed models have good applicability to the quantitative analysis of the experimental results in the manuscript. This paper offers important insights into the application and development of hydraulic asphalt concrete in hydraulic engineering.

Effects of ageing on low-cycle fatigue (LCF) properties of AZ80 magnesium alloy wheel hub

A hollow billet Expanding-Reducing Extrusion (ERE) wheel hub forming process was carried out with AZ80 magnesium alloy. The effects of ageing on Low-cycle fatigue (LCF) properties were investigated, and the microstructure was characterized with scanning electronic microscopy (SEM). The LCF testing results exhibited that the stress amplitude increased with increasing ageing time, while the plastic strain decreased with increasing ageing time. Cyclic softening followed by hardening was observed for 0 h and 16 h aged samples; however, there was no obvious cyclic softening or hardening for 36 h aged sample. The tension-compression asymmetry were not obvious due to the low strain, precipitation and the non typical basal texture. Fracture observations indicated that the fatigue cracks were initiated on the surface, and the crack propagation area decreased with increasing ageing time. Thus, it can be concluded that ageing has strong effects on the fatigue life for the studied magnesium alloy.

Softening Behaviors of Severely Deformed Zn Alloy Studied by the Nanoindentation

Zamak 3 alloy treatment by sliding-friction treatment (SFT) was investigated by nanoindentation to explore the influence of microstructure and strain rate on nanoscale deformation at room temperature. The results show that obvious material softening occurs in the ultrafine-grained (UFG) Zn alloy and strain-hardening happens in the twinning-deformed layer, respectively. It can be concluded that almost constant values of V in the UFG Zn alloy contribute to the dislocations moving along the grain boundary (GB) not cross the grain interior. In the twinning-deformed layer, the highly frequent dislocation–twinning boundary (TB) interactions are responsible for subsequent inverse Cottrell–Stokes at lower stress, which is quite different from dislocation–dislocation reaction inside the grain in their coarse-grained (CG) counterpart.

Phonon softening and higher-order anharmonic effect in the superconducting topological insulator SrxBi2Se3

We report the anharmonic effect of the Raman-scattering spectrum in the low-carrier density superconductor SrxBi2Se3, which is dominated by the quartic term. Compared to the parent Bi2Se3, the superconducting SrxBi2Se3 crystals show obvious phonon softening in the three optical phonon modes , , and . Based on the simulations by the Fano function, we present compelling evidence of the enhanced electron–phonon coupling in SrxBi2Se3 for its smaller asymmetric parameter q. Moreover, an anomalous broadening and intensity antiresonance in the characteristic Raman peaks were observed as the temperature decreased to around 160 K. Thus, the superconducting topological material SrxBi2Se3 represents a counterintuitive example where the electron–phonon interaction is strengthened by the accumulation of electron carriers.

Cyclic behavior and constitutive model of high strength low alloy steel plate

Cyclic loading tests with different protocols were conducted for a high strength low alloy steel. Based on the test results, the low cycle fatigue and extremely low cycle fatigue performance of the high strength steel was discussed and an appropriate constitutive model was established. The experimental results implied that the low and extremely cycle fatigue failure showed different fracture features. For low cycle fatigue, there was a strong negative linear correlation between the fatigue life and the strain amplitude in the log–log scale. However, they did not conform the linear relation when transiting to the low cycle fatigue. The steel represented obvious cyclic softening and good energy dissipation capacity. With the increase of strain amplitude, the softening degree decreased and equivalent damping ratio increased. The cyclic stress–strain curve could be descried by the Ramberg-Osgood model, and the strength coefficient and strain hardening exponent of this steel were larger than those of steel with lower strength. A combined hardening model was calibrated to describe the cyclic plasticity of the steel, and it was verified by the simulation results.

Microstructure and Mechanical Properties of GMAW Welded Joints Er-Containing Aluminum Alloy

This present work explored the welding process of gas metal arc welding for 4mm 5E61 Er-containing aluminum alloy, and then analyzed the microstructure and mechanical properties of the welded joint. The results demonstrated that when the welding current was 160A-220A, the welded joint penetration depth range was 5.75mm to 6.72mm, the melting width ranging from 9.68mm to 11.61mm. When the arc voltage increased from 17.5V to 22.5V, the penetration depth of the welded joint reduced from 6.95mm to 5.57mm, and the melting width ranged from 6.64mm to 11.86mm. When the welding current was 170A, the arc voltage was 17.5V, and the welding speed was 10mm/s. In the third case, a fully penetrated welded joint can be obtained and the joint strength was the highest value. The yield strength reached 192 MPa, the tensile strength can be 301 MPa, and the fracture location occurred in the HAZ. The weld zone of the welded joint mainly consist of the equiaxed dendrites size of 50 μm. The micro-hardness of the weld zone was lower than that of the base metal, and there was no obvious softening phenomenon in the heat affected zone.

Experimental and numerical study on the compressive behavior of micro-expansive ultra-high-performance concrete-filled steel tube columns

In concrete-filled steel tube (CFST) columns, conventional ultra-high-performance concrete (UHPC) has a large shrinkage and could easily be deboned from the steel tube, resulting in a failure of the effective lateral restraint of the steel tube to the core concrete. To solve this problem, a new type of micro-expansive and self-compacting UHPC was adopted to short CFST columns, which were further subjected to push-out and axial compressive tests. Test results indicated that the UHPC with the expansive agent could achieve an effective bonding between the core concrete and the steel tube. Compared with the compressive behavior of conventional UHP-CFST columns with a low confinement factor, the load-shortening curves of micro-expansive UHP-CFST columns do not show any obvious softening section. Furthermore, a finite element model of the micro-expansive UHP-CFST column was developed to predict the relationship of load and displacement by ABAQUS. Finally, the axial capacities of the short micro-expansive UHP-CFST columns obtained by the finite element analyses (FEA) were compared with that calculated by different design codes. It was concluded that the axial capacity of the composite members calculated by current codes is less than the FEA results and the formulas provided by the codes need to be further validated or modified.

Physical and Mechanical Properties of Gypsum‐Like Rock Materials

In the process of tunnel construction, gypsum rock is often encountered, and the volume of gypsum rock expands when encountering water, which is easy to cause the occurrence of rock fall, collapse, and other disasters, bringing serious challenges to the safe construction of the tunnel. Therefore, in this paper, four groups of samples under different moisture content are tested by ultrasonography, uniaxial compression, conventional triaxial compression, Brazilian splitting, X‐ray diffraction, and SEM, and then the physical and mechanical properties of gypsum rock are studied, and the conclusion is as follows: the density of the water saturated sample, and the longitudinal wave velocity of the natural sample are the highest. Both the water saturation and dehydration conditions have a weakening effect on the remolded sample of high‐strength gypsum powder. The peak intensity of the sample gradually increases with the increase of confining pressure, and the relationship between the peak intensity and confining pressure of the sample conforms to the Coulomb strength criterion. After high‐temperature dehydration, the sample showed obvious plastic softening characteristics. The cohesion and internal friction angle of the sample are closely related to the water content. The cohesion is the largest in the 45°C dehydrated sample, the internal friction angle is the smallest in the saturated sample, whereas the cohesion is the smallest and internal friction is the largest in the high‐temperature dehydrated sample. The characteristics of failure for the natural and 45°C dehydrated samples are almost the same and most samples show shear or shear‐tensile failure. The shear plane begins at the edge of the end face of the sample and exhibits a typical diagonal shear failure. The high‐temperature dehydrated samples are completely broken under uniaxial and triaxial compression conditions. After high‐strength gypsum powder was used to make the remolded sample, the calcium sulfate disappeared, the water content increased, and the main mineral components of the natural and saturated samples were the same. After dehydration at 45°C, the sample began to release structural water and generate SiO2. After high‐temperature dehydration, the hemihydrate gypsum continued to dehydrate and become soluble anhydrous gypsum.

Effect of multi-pass hot deformation on flow stress and microstructure of Ti-6Al-4V titanium alloy prepared by hot isostatic pressing

The multi-pass hot compressive tests of Ti-6Al-4V alloy prepared by hot isostatic pressing were carried out. The results show that the flow stress with the strain rates of 1s−1 and 5s−1 both in first-pass at 1050°C and second-pass at 1000°C deformation, also with the strain rate of 5s−1 in third-pass at 950°C deformation, exhibit obvious dynamic softening behavior. For first-pass deformation with the strain rate ranging from 0.001s-1 to 0.1s-1, a few equiaxed β grains can be observed and the larger strain rate resulted in the smaller dynamic recrystallization (DRX) grains. For second-pass deformation, a number of finer DRX β grains and a few DRX α grains can be observed under 0.01s−1 strain rate. The grain boundary is indistinguishable and a small number of DRX grains adhere to the grain boundary under 0.1s−1 and 1s-1 strain rates; there are plenty of lamellar α structure and a few basketweave structures. For third-pass deformation, there are equiaxed β grains and equiaxed primary α grains under 0.1s-1 strain rate. Moreover, a small number of globularization α grains can be observed. As the strain rate increasing to 1s-1, a great many of interlaced fine lamellar α phases precipitated inside the β grain.

Thermal effect on the evolution of hydraulic fracture conductivity: An experimental study of enhanced geothermal system

This paper aimed to investigate the thermal effect on the evolution of hydraulic fracture conductivity during heat extraction. Meanwhile, the degradation mechanisms of fracture conductivity were analyzed in terms of the mechanical damage of fracture surfaces. Experimental results of micro-indentation tests on the studies granite show the existence of a threshold value of temperature. The Young's modulus and hardness of fracture surface significantly decreased when the thermal treatment levels were higher than 300 °C. Similarly, the degradation of fracture conductivity was limited (<10%) before 300 °C and then significantly deteriorated at 400 °C and 500 °C. Moreover, cyclic thermal treatment tended to aggregate the mechanical damages of fracture surfaces further. At the thermal treatment level of 300 °C, obvious thermal softening of fracture surfaces and degradation of fracture conductivity were observed only when the number of cycles was larger than 10, while both of these two phenomena became more severe with the increasing of the number of cycles at the thermal treatment level of 400 °C. The evolution of fracture conductivity can be accounted for by the temperature-related integrity of fracture surfaces and proppant pack. It was indicated that the thermal softening of fracture surface tended to enhance the proppant embedding, which might be beneficial for the alleviation of proppant crushing.