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Titanium and titanium alloys have been widely applied in the manufacture of aircraft engines and aircraft skins, the mechanical properties of which have a crucial influence on the safety and lifespan of aircrafts. Based on nanotwinned titanium models with different twin boundary spacings, the impacts of different loadings and twin boundary spacings on the plastic deformation of titanium were studied in this paper. It was found that due to the different contained twin boundaries, the different types of nanotwinned titanium possessed different dislocation nucleation abilities on the twin boundaries, different types of dislocation–twin interactions occurred, and significant differences were observed in the mechanical properties and plastic deformation mechanisms. For the { 10 1 - 2 } twin, basal plane dislocations were likely to nucleate on the twin boundary. The plastic deformation mechanism of the material under tensile loading was dominated by partial dislocation slip on the basal plane and face-centered cubic phase transitions, and the yield strength of the titanium increased with decreasing twin boundary spacing. However, under compression loading, the plastic deformation mechanism of the material was dominated by a combination of partial dislocation slip on the basal plane and twin boundary migration. For the { 10 1 - 1 } twin under tensile loading, the plastic deformation mechanism of the material was dominated by partial dislocation slip on the basal plane and crack nucleation and propagation, while under compression loading, the plastic deformation mechanism of the material was dominated by partial dislocation slip on the basal plane and twin boundary migration. For the {1124} twin, the interaction of its twin boundary and dislocation could produce secondary twins. Under tensile loading, the plastic deformation mechanism of the material was dominated by dislocation–twin and twin–twin interactions, while under compression loading, the plastic deformation mechanism of the material was dominated by partial dislocation slip on the basal plane, and the product of the dislocation–twin interactions was basal dislocation. All these results are of guiding value for the optimal design of microstructures in titanium, which should be helpful for achieving strong and tough metallic materials for aircraft manufacturing. [ABSTRACT FROM AUTHOR]

Microtwinning is observed as an important mechanism in γ′ precipitate-strengthened Ni-based superalloys during service at intermediate temperatures. Here, the effect of crystal orientation and stress direction on microtwinning was investigated. The result indicates that the formation of microtwins exhibits orientation dependence and tension-compression asymmetry. Based on the analysis of slip and twinning competition behavior under different conditions, it is deemed that the a/2〈110〉 matrix dislocations would dissociate into a/6〈112〉 Shockley partials in the γ matrix and then a Shockley partial sheared γ’ phase and formed microtwins.

Microtwinning is observed as an important mechanism in γ′ precipitate-strengthened Ni-based superalloys during service at intermediate temperatures. Here, the effect of crystal orientation and stress direction on microtwinning was investigated. The result indicates that the formation of microtwins exhibits orientation dependence and tension-compression asymmetry. Based on the analysis of slip and twinning competition behavior under different conditions, it is deemed that the a/2〈110〉 matrix dislocations would dissociate into a/6〈112〉 Shockley partials in the γ matrix and then a Shockley partial sheared γ' phase and formed microtwins. IMPACT STATEMENT: For the first time, this work explains why microtwinning exhibits orientation dependence and tension-compression asymmetry while stacking faults do not, although they both originate from the activity of {111} 〈112〉 slip systems. [ABSTRACT FROM AUTHOR]

In this study the mechanical anisotropy of laser powder bed fusion (LPBF) 316L stainless steel under tensile, compressive, and shear loading in different orientations with respect to the build direction is investigated. Experimental analysis revealed a moderate degree of anisotropy, which is mainly attributed to the build direction. In addition, the mechanical properties were seen to be dependent on the stress state, as evidenced by the tension-compression asymmetry. The anisotropy and asymmetry can be explained by various microstructural factors with texture orientation being one of the most significant. To incorporate such material behavior in structural analysis, a phenomenological model for anisotropic plasticity and tension-compression asymmetry was proposed and calibrated for additive 316L steel. The flexibility of the model allows it to be applied using a mechanical test set on uniaxial loading, and further enhancements may rely on data for combined stress state. The model was calibrated based on FEA of samples loading and fit well all experimental curves, though factors such as residual stresses and test imperfections could introduce some discrepancies. [ABSTRACT FROM AUTHOR]

Introduction of twins into diamond can affect the hardness of diamond, but the underlying microcosmic mechanism remains unknown. Here we have compared the mechanical properties of diamond NPs with three different models, i.e. single crystal nanopillars (SC NPs), twin crystalline nanopillars (TC NPs), and five-fold twinned nanopillars (FT NPs), with diameters from 5 to 30 nm during both tension and compression by molecular dynamics simulations. Our study reveals that the mechanical properties of diamond NPs are closely related to the models of NPs, diameters, and loading modes. The stress–strain responses present significant asymmetry during tension and compression. And the yield strength and strain for FT NPs are always higher than those of TC and SC NPs due to the effect of five-fold twin boundary. The existence of plasticity in diamond NPs is confirmed by the abundance of dislocations after yield strain. The tension–compression asymmetry is also reflected by the differences in dislocation type, dislocation evolution processes, and the fracture shape of the NPs. Moreover, the typical characteristic during tension is that stacking faults are always found following the slip of dislocations, and during compression is that dislocation networks are observed for TC and FT NPs with diameters larger than 20 nm. [ABSTRACT FROM AUTHOR]

Al-Si alloys of hypoeutectic, near-eutectic, and hypereutectic compositions, prepared from commercial pure aluminum and silicon, were investigated to study their tensile and compressive properties. The microstructures in as-cast stage were found to be refined after subsequent remelting and casting for two, five, and ten cycles. The proportion of eutectic phase and silicon particles increased with silicon content and influenced the nature of stress–strain curves. Both tensile and compressive stress–strain curves exhibited increase in flow stress with increasing strain, which could be expressed by Hollomon type relationships, but the tensile deformation occurs at lower flow stress than that in compression. While tensile specimens showed limited elongation to failure, the compression ones exhibited larger strains, up to 30% without failure. The tension–compression flow asymmetry was noted to vary as a function of the number of remelting cycles and of the silicon content in the alloy. The work hardening rate showed only stage III behavior in tension but both stage III and IV behavior in compression. The flow stress, including the yield strength, in compression was found to increase with decrease in grain size. However, in tensile deformation, instead of grain size effect, the yield strength was noted to increase with the decrease in inter-particle spacing. In both these cases, the Hall–Petch type relationships were found to be obeyed. [ABSTRACT FROM AUTHOR]

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AbstractConsidering the tension-compression asymmetry of Shape Memory Alloy (SMA), the tension and compression constitutive equations of the Functionally Graded Shape Memory Alloy (FG-SMA) beam are established based on the simplified constitutive relation of SMA. The influences of the power exponent, temperature, tension-compression asymmetry coefficient, and strain (along the length direction of the beam) on the martensite volume fraction and stress of the beam are analyzed, and the correctness of the theory is verified by comparing the results of theory with finite element analysis. The research shows that when the height of the beam is constant, the power exponent, temperature, tension-compression asymmetry coefficient are proportional (inversely proportional) to the stress (martensite volume fraction) of the cross-section, and the strain is proportional to both the stress and martensite volume fraction of the cross-section. When the other conditions are fixed, compared with compression, the martensite volume fraction (stress) under tension is larger (slightly less) at the same layer cross-section position. [ABSTRACT FROM AUTHOR]

The primary objective of this paper is to establish a dependable constitutive model for pressure-insensitive metals, with a particular emphasis on but not limited to the Cantor alloy. This model takes into account the effect of distortional hardening on deformation response of the material. It successfully establishes a quantitative relationship between the strength differential coefficient and the effective plastic strain. In order to acquire the distortional hardening parameters, the elastoplastic behavior of the Cantor alloy is determined through compressive, tensile, and shear tests. The comparison between DIC and finite element simulations confirms the accuracy of the proposed model in describing the deformation behavior of the Cantor alloy. The presented results and analyses offer a precise computational model for studying the deformation behavior of pressure-insensitive metals under various stress states. Additionally, this model can also be utilized for further research and analysis of the mechanical properties and performance of the Cantor alloy in practical applications.

Titanium and titanium alloys have been widely applied in the manufacture of aircraft engines and aircraft skins, the mechanical properties of which have a crucial influence on the safety and lifespan of aircrafts. Based on nanotwinned titanium models with different twin boundary spacings, the impacts of different loadings and twin boundary spacings on the plastic deformation of titanium were studied in this paper. It was found that due to the different contained twin boundaries, the different types of nanotwinned titanium possessed different dislocation nucleation abilities on the twin boundaries, different types of dislocation–twin interactions occurred, and significant differences were observed in the mechanical properties and plastic deformation mechanisms. For the {101-2} twin, basal plane dislocations were likely to nucleate on the twin boundary. The plastic deformation mechanism of the material under tensile loading was dominated by partial dislocation slip on the basal plane and face-centered cubic phase transitions, and the yield strength of the titanium increased with decreasing twin boundary spacing. However, under compression loading, the plastic deformation mechanism of the material was dominated by a combination of partial dislocation slip on the basal plane and twin boundary migration. For the {101-1} twin under tensile loading, the plastic deformation mechanism of the material was dominated by partial dislocation slip on the basal plane and crack nucleation and propagation, while under compression loading, the plastic deformation mechanism of the material was dominated by partial dislocation slip on the basal plane and twin boundary migration. For the {1124} twin, the interaction of its twin boundary and dislocation could produce secondary twins. Under tensile loading, the plastic deformation mechanism of the material was dominated by dislocation–twin and twin–twin interactions, while under compression loading, the plastic deformation mechanism of the material was dominated by partial dislocation slip on the basal plane, and the product of the dislocation–twin interactions was basal dislocation. All these results are of guiding value for the optimal design of microstructures in titanium, which should be helpful for achieving strong and tough metallic materials for aircraft manufacturing.

This paper provides, a closed-form analytical solution to the plane-strain problem of stress relaxation in a bended plate with tension–compression asymmetry (TCA) in viscous properties. Reversible and irreversible strains are assumed to be finite. A linear viscous model with equivalent stress that is a piecewise linear function of the principal stresses with the TCA parameter is utilized. Features of the solution specific to this model are discussed. [ABSTRACT FROM AUTHOR]

Deformation and fracture of metallic glasses are often modeled by stress-based criteria which often incorporate some sorts of pressure dependence. However, detailed mechanisms that are responsible for the shear-band formation and the entire damage initiation and evolution process are complex and the origin of such a pressure dependence is obscure. Here, we argue that the shear-band formation results from the constitutive instability, so that the shear-band angle and arrangements can be easily related to the macroscopic constitutive parameters such as internal friction and dilatancy factor. This is one reason for the observed tension-compression asymmetry in metallic glasses. The free volume coalescence leads to precipitous formation of voids or cavities inside the shear bands, and the intrinsic "ductility" is therefore governed by the growth of these cavities. Based on a generalized Stokes-Hookean analogy, we can derive the critical shear-band failure strain with respect to the applied stress triaxiality, in which the cavity evolution scenarios are sharply different between tension-controlled and shear/compression-dominated conditions. This is another possible reason for the tension-compression asymmetry. It is noted that diffusive-controlled cavity growth could also be the rate-determining process, as suggested by the recent measurements of shear-band diffusivity and viscosity that turn out to satisfy the Stokes-Einstein relationship. This constitutes the third possible reason for the tension-compression asymmetry. [ABSTRACT FROM AUTHOR]

The crack behaviors under complex stress states are very important for the safety of polymer-bonded explosives (PBXs) under accidental stimulations, but their accurate description is a challenge. Due to the advances of tracking discontinuities and multi-fields coupling, the phase-field model for complex fracture phenomena is attracting significant interest recently. Conventional phase-field fracture models are tension-compression symmetric or based on volumetric-deviatoric strain energy split, and these conventional phase-field models may lead to unrealistic fracture patterns, which hinders its further application in PBX fracture simulations. In this work, we present an extended, tension-compression asymmetric phase-field fracture model for PBXs, which distinguishes the contributions of tensile and compressive stresses to damage driving energy, and couples the mechanism of mechanical degradation and energy-driving cracking diffusion. We implemented our improved phase-field fracture model into finite element calculations and compared the simulation results with the conventional tension-compression symmetric phase-field fracture model and volumetric-deviatoric strain energy split phase-field fracture model by simulating PBX specimens under static loadings. The results show that our model not only accurately depicts the tensile and compressive cracks, but also describes compression-assisted cracking while suppressing unrealistic damage nucleation caused by small amplitudes of local compressive stresses, making it a very efficient way of describing PBX cracking under complex stress states. This new model is both mathematically and physically concise, and convenient for numerical implementation. Moreover, the novel model can be naturally extended to simulate shock-induced dynamic and/or coupled fracture of PBXs because of its feasibilities for dynamic extension and multi-field coupling. [ABSTRACT FROM AUTHOR]

The anisotropic plasticity constants of the CPB06 criterion for Ti64, previously identified with experiments performed in all three dimensions, are applied here to evaluate the effect of the direct and inverse calibration strategies of the tensile-compression asymmetry parameter k on the predictive behavior of large deformations of the hexagonal close-packed (hcp) alloy. The direct calibration strategy is based on model fitting with experimental strain hardening data up to the onset of plastic instability. The inverse calibration strategy reduces prediction errors of the load-displacement curves of both the cylindrical bar tensile and the elliptical cylinder compression tests. The results provide interesting insights into the identification of the laws modeling large deformations of hcp materials.

In the present work, an advanced plasticity model is established to character the anisotropic and asymmetric yielding and hardening behaviors of magnesium alloy. This model is comprised of the stress invariant-based Yoon2014 yield function, non-associated flow rule and yield surface-interpolation method in the description of distortional hardening. For effectively numerical implementation of the plasticity model, a semi-explicit stress integration scheme based on the return mapping method is provided. The implemented model is then applied to simulate the inhomogeneous deformations during three-point bending of an extruded magnesium alloy. Simulation results show that there are three distinct zones in the bent plate according to its deformation patterns, i.e., the compressive inner zone, the tensile outer zone, and the central zone which experiences tensile, then compressive and again tensile deformations. Compared to the counterpart Hill48 function, Yoon2014 function has better performance in simulating the inhomogeneous deformations of magnesium alloy.

ABSTRACTTexture adjusting approaches of divergence, inclination, and deviation were proposed to improve the tension-compression asymmetry in magnesium alloys. The dominant deformation mechanism during tensile and compressive yielding was quantified based on the Sachs assumption and primarily manifested as the extent of twinning inhibition and the transition between twinning and slip with the variation of texture adjustment. Its impacts on associated yield strength and asymmetry were estimated by the modified Hall-Petch relationship. Slight texture adjustments, particularly of basal texture divergence, can significantly improve the strength asymmetry while avoiding severe strength loss, illustrating the significance and potential of texture adjustment in engineering.

Ni-based single-crystal superalloys are extensively employed in turbine blades for aircraft and industrial gas engines. The blades are subjected to complex thermal and stress impacts during service, which may cause sudden low-cycle fatigue damage and subsequent flight accidents. To investigate the relationship between the microstructure and mechanical properties, fatigue-damaged microstructures are compared under cyclic deformation at 800 and 900 °C using combined electron microscope characterizations. With an increase in temperature, the deformation is more concentrated in the γ matrix channels and the cyclic deformation behaviors stabilizes, whereas cutting the γ′ phase becomes difficult with decreasing slip bands. In addition, the significance of interfacial dislocation networks in enhancing cyclic stability and hindering cyclic tension–compression asymmetry is discussed.

Biaxial tension/compression–torsion fatigue tests with varying levels of non‐proportionality were performed employing a structural adhesive designed for wind turbine rotor blades. The cycles to failure were found to be independent of the level of non‐proportionality. It is demonstrated that numerical fatigue life predictions via rainflow‐counted equivalent stress histories are not able to replicate these experimental observations and overestimate the fatigue life up to a hundredfold. The tension–compression asymmetry of the adhesive resulted in significant damage prediction differences depending on the stress space representation of the Haigh diagram. If not properly taken care of, the asymmetry will also lead to non‐conservative results. While demonstrated with a short fiber‐reinforced adhesive, the results can be transferred to other materials. Highlights: Non‐proportional fatigue tests employing a structural adhesive.The cycles to failure were found to be independent of the level of non‐proportionality.A tension–compression asymmetry significantly influences the fatigue life prediction.Rainflow‐counted equivalent stresses overestimate the non‐proportional fatigue life. [ABSTRACT FROM AUTHOR]

In order to investigate the effect of tension–compression asymmetry of propellant mechanical properties on the structural integrity of a Nitrate Ester Plasticized Polyether (NEPE) propellant grain, the unified constitutive equations under tension and compression were established, a new method for grain structural integrity assessment was proposed and the structural integrity of the NEPE propellant grain under the combined axial and transverse overloads was evaluated. The results indicate that the mechanical state of the NEPE propellant grain is in the coexistence of tension and compression under the combined axial and transverse overloads, and the tension and compression regions in the propellant grain is independent of the propellant constitutive behavior. The tension–compression asymmetry of the propellant mechanical properties has a certain impact on its mechanical response. The maximum equivalent stress and strain considering the tension–compression asymmetry falls between that obtained through the tension and compression constitutive model, and is the same as damage coefficient. The safety factor of the NEPE propellant grain considering the tension–compression asymmetry of its mechanical properties is larger than that non-considering, and the traditional method of structural integrity assessment is conservative. [ABSTRACT FROM AUTHOR]

The paper presents an analytical solution of the problem of stress relaxation in a bended viscoelastic plate. The effect of tension–compression asymmetry in the viscous properties of the material on the relaxation of the bending moment is investigated. The deformation is assumed to be finite, and the multiplicative decomposition of the deformation gradient tensor into an elastic and an inelastic part is used. Two different material models with tension–compression asymmetry are compared. [ABSTRACT FROM AUTHOR]

Organisms have evolved a strain limiting mechanism, reflected as a non-linear elastic constitutive, to prevent large deformations from threatening soft tissue integrity. Compared with linear elastic substrates, the wrinkle of films on non-linear elastic substrates has received less attention. In this article, a unique wrinkle evolution of the film-substrate system with a J-shaped non-linear stress-strain relation is reported. The result shows that a concave hexagonal array pattern is formed with the shrinkage strain of the film-substrate systems developing. As the interconnection of hexagonal arrays, a unit cell ridge network appears with properties such as chirality and helix. The subparagraph maze pattern formed with high compression is mainly composed of special single-cell ridge networks such as spiral single cores, chiral double cores, and combined multi-cores. This evolutionary model is highly consistent with the results of experiments, and it also predicts wrinkle morphology that has not yet been reported. These findings can serve as a novel explanation for the surface wrinkle of biological soft tissue, as well as provide references for the preparation of artificial biomaterials and programmable soft matter. [ABSTRACT FROM AUTHOR]

Asphalt concrete (AC) exhibits significant tension–compression (TC) asymmetry and aggregate contacts can be one of the critical contributors to this behavior. Nevertheless, the underlying mechanisms are still unclear, and there has been no study to quantify this behavior. To fill the research gap, multiscale characterization and modeling on AC were performed in this study. At the microscale level, nanoindentation tests were conducted to characterize the aggregate contact characteristics in the contact region (CR). The CR was found to have a sandwich-like structure consisting of two interfacial layers, large filler particles, and asphalt mastic. Accordingly, micromechanical models of CR were developed to predict its mechanical behavior in tension and compresison (T&C). The modeling results showed that aggregate contacts significantly increase the compressive modulus, leading to the substantial TC asymmetry of CR. The predicted viscoelastic properties of CR were further applied to the developed mesostructural model of AC. The predicted master curves in T&C showed significant asymmetry and quantitatively agreed with the experimental ones, demonstrating the effectiveness of the adopted modeling approaches. This study is the first study to quantify the asymmetric performance of AC. The outcomes can be applied to evaluate AC’s TC asymmetry effects on pavement performance.

Significant tension-compression (T–C) yield asymmetry in common wrought Mg alloys hinders their widespread applications. A double extrusion process with variable speed and partially substituted yttrium (Y) (2 wt%) for Sn was proposed to improve the T–C asymmetry of Mg–2.5Sn (wt%) alloy, and then to explore the critical grain size parameter for T–C symmetry. The result showed that the individual double extrusion or partial substitution of Sn by Y slightly refined the grains of Mg–2.5Sn alloy (from ~ 42.5 µm to 35.5 µm and 31.2 µm, respectively), along with the improved T–C asymmetry (CYS/TYS) from 0.44 to 0.51 and 0.75, respectively. When combining the above two factors, the double-extruded Mg–Sn–Y alloys obtained fine grains with ~ 8.8 µm and gained a T–C yield symmetry (CYS/TYS = 1.01). By tailoring the extrusion speed from the previous 1.2 mm/s to 2 mm/s, the grain size of double-extruded Mg–Sn–Y further deduced to ~ 5.6 µm and turned to be a reversed T–C asymmetry (CYS/TYS) of 1.37. The refined grain size combined with weakened texture led to a decreased activation or even inhibition of tension twinning and increased activation of non-basal slip, which might be the main reason for the improved or even reversed T–C asymmetry in double-extruded Mg–0.5Sn–2Y. [ABSTRACT FROM AUTHOR]

Texture adjusting approaches of divergence, inclination, and deviation were proposed to improve the tension-compression asymmetry in magnesium alloys. The dominant deformation mechanism during tensile and compressive yielding was quantified based on the Sachs assumption and primarily manifested as the extent of twinning inhibition and the transition between twinning and slip with the variation of texture adjustment. Its impacts on associated yield strength and asymmetry were estimated by the modified Hall-Petch relationship. Slight texture adjustments, particularly of basal texture divergence, can significantly improve the strength asymmetry while avoiding severe strength loss, illustrating the significance and potential of texture adjustment in engineering. Yield strength and the contribution of deformation mechanisms were predicted using the modified Hall-Petch relationship, and texture adjustment strategies were further established to improve tension-compression asymmetry. [ABSTRACT FROM AUTHOR]

The different elastic properties of tension and compression are obvious in many engineering materials, especially new materials. Materials with this characteristic, such as graphite, ceramics, and composite materials, are called bi-modulus materials. Their mechanical properties such as Young's modulus have randomness in tension and compression due to different porosity, microstructure, etc. To calibrate the mechanical properties of bi-modulus materials by bridging finite element method (FEM) simulation results and scarce experimental data, the paper presents a data-fusion computational method. The FEM simulation is implemented based on parametric variational principle (PVP), while the experimental result is obtained by digital image correlation (DIC) technology. To deal with scarce experimental data, maximum entropy principle (MEP) is employed for the uncertainty quantification (UQ) and calibration of material parameters and responses, which can retain the original probabilistic property of a priori data. The non-parametric p-box is used as a constraint for data fusion. The method presented in this paper can quantify the mechanical properties of materials with high uncertainty, which is verified by a typical example of bi-modulus graphite. It is possible to find applications in the real-time estimation of structural reliability by combining with digital twin technology in the future. [ABSTRACT FROM AUTHOR]

A homogenous microstructure of ultrafine-grained (UFG) commercially pure (CP) Ti characterized by equiaxed grains/subgrains with an average grain size of about 150 nm and strong prismatic fiber texture were obtained after 4 passes of equal channel angular pressing (ECAP). Tension–compression asymmetry in yield and work hardening behavior of UFG CP Ti were investigated by uniaxial tension and compression tests. The experimental results reveal that UFG CP Ti exhibits a relatively obvious tensioncompression asymmetry in yielding and work hardening behavior. The basal and prismatic slip are suppressed either for tension or compression, which is the easiest to activate. The tension twin system { 10 1 ¯ 2 } < 1 ¯ 011 > is easily activated in compression deformation due to the prismatic fiber texture based on the Schmidt factor, consequently resulting in a lower yield strength under compression than tension. ECAP can improve the tension-compression asymmetry of CP Ti due to grain refinement. The interaction among the dislocations, grain boundaries and deformation twins are the main work hardening mechanisms for compression deformation, while the interaction between the dislocations and grain boundaries for tension deformation. Deformation twins lead to the higher work hardening under compression than tension. [ABSTRACT FROM AUTHOR]

This work elucidates the effect of interface orientation, loading mode, and crystallography on the deformation mechanisms of Cu-Nb multilayered nanocomposites with interface structures from physical vapor deposition (PVD). Uniaxial tension and compression tests were performed using molecular dynamics simulations at room temperature. The simulations were utilized to understand the atomic-scale deformation mechanisms of tension and compression during iso-stress and iso-strain loading conditions. The Cu-Nb nanocomposites exhibit a typical tension–compression asymmetry with higher maximum compressive stress than the maximum tensile stress. In addition, the interface was shown to act as a dislocation nucleation source during iso-strain tension, and the pre-existing stacking faults were emitted during iso-strain compression, indicating different deformation mechanisms during tension and compression. Under compression normal to the interface and tension normal to the interface, unusual dislocation and twin interaction were observed, where slip transmission occurs from {111} slip planes to the unusual {100} slip-plane inside the 4-layer twin lamella. Furthermore, it was shown that the interface structures from PVD exhibit high stability against the deformation where failure was observed inside the Cu layer of the bi-metal composite. [ABSTRACT FROM AUTHOR]

Background: Advanced testing methodologies and measurement techniques to identify complex deformation and failure at high strain rates have drawn increasing attention in recent years. Objective: The objective of the current study is the development of a novel combined tension–torsion split Hopkinson bar (TTHB) conceived to generate a combination of tensile and torsional stress waves in a single loading case, and to measure material data representative of real case impact scenarios. Methods: An energy store and release mechanism was employed to generate both the longitudinal and shear waves via the rapid release of a bespoke clamp assembly. A parametric study of the material and geometry of the clamp was implemented via numerical simulations to optimise critical aspects of the wave generation. Thin-walled tube specimens made of two metallic materials were utilised to examine the capability of the developed TTHB system by comparing the experimental measurements with those obtained from conventional split Hopkinson tension and torsion bars. Results: The experimental results demonstrate that the synchronisation of the longitudinal and torsional waves was achieved within 15 microseconds. Different wave rise time were obtained via the controlled release of the clamp using fracture pins of various materials. The analysis indicates that the developed TTHB is capable of characterising the dynamic behaviour of materials under tension, torsion, as well as under a wide range of complex stress states. Conclusions: The presented apparatus, testing and analysis methods allow for the direct population of the dynamic failure stress envelopes of engineering materials and for the accurate evaluation of existing and novel constitutive models. [ABSTRACT FROM AUTHOR]

Springback of SUS430/AA1050/TA1 (SAT) laminated sheet in multi-point forming (MPF) is difficult to predict due to its complex microstructure. In this work, the polycrystalline texture and crystal structure of the three materials were measured firstly by electron backscatter diffraction (EBSD). Crystal plasticity finite element method (CPFEM) considering twinning and slip was used to predict the anisotropy and tension-compression yield asymmetry (TCYA) of the materials. The data obtained by virtual simulation are validated and used to calibrate the phenomenological yield criteria. In the springback prediction model, the three materials were modeled using the layered modeling method. The anisotropic yield criteria were implemented into finite element software ABAQUS via a user-defined material subroutine VUMAT to fully consider the anisotropy of the SAT laminated sheet. The comparison between the forming experiment and simulation results shows that the model can predict the springback accurately. Finally, the effect of plastic deformation, strain path, and TA1 layer thickness on springback were discussed. [ABSTRACT FROM AUTHOR]

The failure modes and mechanisms of single and polycrystalline aluminum under different stress states are very different. Therefore, molecular dynamics methods were used to study the tension-compression asymmetry and Bauschinger effect of single and polycrystalline aluminum under different loads. We found both single and polycrystalline aluminum exhibit work hardening under tensile and compressive loads. The difference in this hardening phenomenon is manifested in that the characteristics of single and polycrystalline aluminum are brittle fracture and ductile fracture, respectively. Furthermore, we found that the tension-compression asymmetry of polycrystalline aluminum is more obvious than that of single crystal aluminum, and it manifested as a reverse tension-compression asymmetry. The Bauschinger effect is caused by the defects introduced by preloading, and the asymmetry of the Bauschinger effect is caused by different microscopic defects. Dislocations introduced by the pre-stretching of single crystal aluminum degrade its performance under reverse loading. Polycrystalline aluminum has more twinned regions under compressive load, which hinder the movement of dislocations and grain boundaries during reverse stretching, showing the reverse Bauschinger effect. This study has important theoretical guiding significance for a comprehensive and in-depth understanding of the mechanical properties and microstructural evolution mechanism of single and polycrystalline aluminum.

We investigated the asymmetric tension—compression (T—C) behavior of ZA21 bars with bimodal and uniform structures through axial tension and compression tests. The results show that the yield strengths of bars having bimodal structure are 206.42 and 140.28 MPa under tension and compression, respectively, which are higher than those of bars having uniform structure with tensile and compressive yield strength of 183.71 and 102.86 MPa, respectively. Prismatic slip and extension twinning under tension and basal slip and extension twinning under compression dominate the yield behavior and induce the T—C asymmetry. However, due to the basal slip activated in fine grains under tension and the inhibition of extension twinning by fine grains under compression, the bimodal structure possesses a lower T—C asymmetry (0.68) compared to the uniform structure (0.56). Multiple extension twins occur during deformation, and the selection of twin variants depends on the Schmid factor of the six variants activated by parent grains. Furthermore, the strengthening effect of the bimodal structure depends on the grain size and the ratio of coarse and fine grains. [ABSTRACT FROM AUTHOR]

This study develops a new phenomenological constitutive model to capture the unique evolving cyclic elastoplastic behaviours of hexagonal close-packed (HCP) sheet metals. This new constitutive model is developed by adopting the concepts of multiple-yield surface approaches. Four phenomenological deformation modes, including Monotonic Compression (MC), Monotonic Tension (MT), Reverse Compression (RC), and Reverse Tension (RT), are considered to represent the hardening evolution of the materials, including the twining/untwining behaviours. Reference flow stress equations are introduced, and a Cazacu-Barlat 2004 (CB2004) type yield surface is employed to each deformation mode. In addition, the RT hardening parameters are defined as functions of plastic pre-strains to mitigate the interpolation error caused by parameter determination processes of existing models. For validation, the calculated stress–strain curves of AZ31B magnesium alloy are compared with experimental curves available from literature. Moreover, to show the accuracy of the proposed analytical model, the reproduced stress–strain curves are compared with those of an existing model—the modified homogeneous anisotropic hardening (HAH) model. The obtained results show that the new constitutive model can successfully reproduce experimental Tension–Compression-Tension (TCT) and Compression-Tension–Compression (CTC) stress–strain curves of HCP sheet metals with considerably less percentage errors. [ABSTRACT FROM AUTHOR]

In this study, a long-stroke SMA Restrainer (LSR) device was fabricated and its response was characterized through experimental testing. The LSR consists of a long SMA bar encased in a steel tube to prevent its buckling. First, an optimal heat treatment procedure was identified to obtain superelastic response for SMA bars. Subsequently, the LSR underwent testing using an increasing amplitude tension-compression loading protocol and its mechanical properties such as equivalent stiffness and equivalent viscous damping ratio were computed. Based on the experimental findings, a high-fidelity 3D finite element model of LSRs was developed, which then used to design a full-scale LSR and verify its performance. Nonlinear time history analyses were conducted to assess the seismic performance of the proposed restrainers when used in a four-span simply supported bridge structure. The response of a case-study bridge equipped with steel cable restrainers, SMA bar restrainers, or LSRs were comparatively evaluated using a set of 40 near-fault ground motion records. Both SMA bar restrainers and LSRs were able to effectively limit the displacement response and provide the re-centering capabilities, while LSRs producing similar moments on piers compared to steel cable restrainers. In addition, compared to tension-only SMA bar restrainers, the LSRs require about only a quarter of SMA materials used in SMA bar restrainers. • SMA bars with different heat treatment were tested for optimal superelastic response. • A full-scale Long-stroke SMA Restrainer (LSR) was tested. • The proposed LSR effectively limit displacement response of bridges, while providing self-centering capacity. • Only a quarter of SMA materials as used in SMA bar restrainers are required for the LSR. [ABSTRACT FROM AUTHOR]

This paper aims to predict defects occurring in magnesium alloy tube bending through finite element simulations. Deformation tests were conducted to identify and characterize plastic anisotropy, tension-compression asymmetry, and anisotropic hardening behaviors of extruded Mg–Al–Zn-RE alloy rectangular tubes. Limited by size, the deformation tests along the wall thickness of specimens were conducted through crystal plasticity analysis. To thoroughly investigate the material constitutive features affecting bending, two constitutive models, Yoon2014 and Hill48, were established and calibrated. The non-associative flow rule was adopted, and anisotropic hardening was depicted using the yield-surface interpolation method. In comparison, the Yoon2014 model accurately predicted the strain distribution and tube shape collapse of the bent tube with negligible deviation from the experimental data, prior to the Hill48 model. This underscores the necessity of considering comprehensive plasticity models in tube bending simulations. [Display omitted] • Plastic constitutive behavior of Mg alloy extruded tube is meticulously characterized. • Constitutive models considering asymmetrical and anisotropic plasticity are built. • Strain distribution and shape defects of bent Mg alloy tube are accurately predicted. • Asymmetrical plasticity of Mg aggravates buckling and wall thickening of bent tube. [ABSTRACT FROM AUTHOR]

The determination of Critical Resolved Shear Stress (CRSS) in titanium for basal, prismatic, and pyramidal slip-planes without empirical constants is presented by combining Density Functional Theory (DFT) and anisotropic elasticity. A new mechanism leading to tension-compression (T-C) asymmetry in the CRSS levels has been revealed for the first time. The conditions for this asymmetry are established, involving a complex interplay between the dislocation core-structure and core-advance behavior. The three conditions for T-C asymmetry that need to be simultaneously satisfied can be summarized as: (1) a medium stacking fault width, d , (3 < d / b F < 10 , where b F is the magnitude of the Burgers vector of the full dislocation), (2) an asymmetric core-structure of the extended dislocation (ξ 1 ≠ ξ 2 , where ξ 1 and ξ 2 are the core-widths of the first and second partials, respectively), and (3) intermittent motion of the partials (Δ d / b F ≠ 0 , where Δ d is the magnitude of fluctuation in stacking fault width during intermittent motion). Pyramidal-slip in titanium satisfies all three conditions, resulting in significant T-C asymmetry. The CRSS theory considers a Wigner-Seitz (W-S) cell based domain area assigned to each lattice site for the calculations of core-energies accurately capturing the slip-plane lattice. The W-S based approach is essential due to the lower symmetry of the HCP crystal circumventing potential errors associated with the one-dimensional atomic-row approximation. Dislocation core structures are obtained for all the slip-systems in titanium showing significant distortion of the disregistry profile governing the core-advance behavior. The CRSS values predicted from the theory show agreement with the experimental CRSS levels reported in the literature. [Display omitted] [ABSTRACT FROM AUTHOR]

In order to investigate the effect of tension–compression asymmetry of propellant mechanical properties on the structural integrity of a Nitrate Ester Plasticized Polyether (NEPE) propellant grain, the unified constitutive equations under tension and compression were established, a new method for grain structural integrity assessment was proposed and the structural integrity of the NEPE propellant grain under the combined axial and transverse overloads was evaluated. The results indicate that the mechanical state of the NEPE propellant grain is in the coexistence of tension and compression under the combined axial and transverse overloads, and the tension and compression regions in the propellant grain is independent of the propellant constitutive behavior. The tension–compression asymmetry of the propellant mechanical properties has a certain impact on its mechanical response. The maximum equivalent stress and strain considering the tension–compression asymmetry falls between that obtained through the tension and compression constitutive model, and is the same as damage coefficient. The safety factor of the NEPE propellant grain considering the tension–compression asymmetry of its mechanical properties is larger than that non-considering, and the traditional method of structural integrity assessment is conservative.

Quasi-static and fatigue behavior of laser powder-bed fusion Hastelloy X (LPBF-HX) specimens were studied. Tension–compression asymmetry was evaluated through a wide range of plastic strains. LPBF-HX specimens are not showing loading direction dependency during monotonic tension and compression tests at low plastic strains. LPBF-HX showed primary hardening under cyclic loading, followed by softening at high strain amplitudes in fully reversed strain-controlled fatigue tests. As the later softening masks the primary hardening, the half-life cyclic stress–strain curve coincides with the monotonic stress–strain curve. Various strain- and energy-based fatigue models were calibrated using the experimental strain-life data and were utilized to predict the fatigue life under different loading conditions with mean stresses. The Jahed–Varvani and Smith–Watson–Topper models showed good results. A fatigue design curve with 95% reliability and 90% confidence level was also constructed using the approximate Owen tolerance limit. [ABSTRACT FROM AUTHOR]

In this paper, we have investigated necking formability of anisotropic and tension-compression asymmetric metallic sheets subjected to in-plane loading paths ranging from plane strain tension to near equibiaxial tension. For that purpose, we have used three different approaches: a linear stability analysis, a nonlinear two-zone model and unit-cell finite element calculations. We have considered three materials –AZ31-Mg alloy, high purity α-titanium and OFHC copper– whose mechanical behavior is described with an elastic-plastic constitutive model with yielding defined by the CPB06 criterion (15) which includes specific features to account for the evolution of plastic orthotropy and strength differential effect with accumulated plastic deformation (48). From a methodological standpoint, the main novelty of this paper with respect to the recent work of N’souglo et al. (42) –which investigated materials with yielding described by the orthotropic criterion of Hill (24)– is the extension of both stability analysis and nonlinear two-zone model to consider anisotropic and tension-compression asymmetric materials with distortional hardening. The results obtained with the stability analysis and the nonlinear two-zone model show reasonable qualitative and quantitative agreement with forming limit diagrams calculated with the finite element simulations, for the three materials considered, and for a wide range of loading rates varying from quasi-static loading up to 40000 s− 1, which makes apparent the capacity of the theoretical models to capture the mechanisms which control necking formability of metallic materials with complex plastic behavior. Special mention deserves the nonlinear two-zone model, as it does not need prior calibration –unlike the stability analysis– and it yields accurate predictions that rarely deviate more than 10% from the results obtained with the unit-cell calculations. [ABSTRACT FROM AUTHOR]

Inspired from natural materials, functionally graded materials (FGMs) with gradual variations in their compositions and microstructures have drawn considerable attention in structural materials design to achieve superior stiffness, strength, and toughness. Taking Cu-Ni alloy as a representative model system, herein, we systematically explore the mechanical properties of Cu-Ni FGMs with varying Ni distributions and Ni contents, respectively, where the lattice constant of Cu-Ni FGMs is found to decrease with increasing Ni content. By carrying out uniaxial tension and compression, it is observed that Young's modulus of Cu-Ni FGMs is nearly independent of the atomic distribution profile but sensitive to the Ni content. Yield stress also shows the strong dependency of Ni content; under the same Ni content, however, due to the sharpness of the interface and different deformation mechanisms, the yield stress becomes sensitive to the Ni distribution. Remarkably, there exists a tension-compression (T-C) asymmetry in Cu-Ni FGMs – the T-C asymmetry of yield stress is more heavily influenced by Ni distribution and content compared to Young's modulus. Our study provides fundamental insights into the mechanical response of Cu-Ni FGMs with varying Ni distribution profiles and contents from an atomic perspective, aiding in the computational design of FGMs. [ABSTRACT FROM AUTHOR]

[Display omitted] Skeletal muscle tissue shows a clear asymmetry with regard to the passive stresses under tensile and compressive deformation, referred to as tension-compression asymmetry (TCA). The present study is the first one reporting on TCA at different length scales, associated with muscle tissue and muscle fibres, respectively. This allows for the first time the comparison of TCA between the tissue and one of its individual components, and thus to identify the length scale at which this phenomenon originates. Not only the passive stress-stretch characteristics were recorded, but also the volume changes during the axial tension and compression experiments. The study reveals clear differences in the characteristics of TCA between fibres and tissue. At tissue level TCA increases non-linearly with increasing deformation and the ratio of tensile to compressive stresses at the same magnitude of strain reaches a value of approximately 130 at 13.5% deformation. At fibre level instead it initially drops to a value of 6 and then rises again to a TCA of 14. At a deformation of 13.5%, the tensile stress is about 6 times higher. Thus, TCA is about 22 times more expressed at tissue than fibre scale. Moreover, the analysis of volume changes revealed little compressibility at tissue scale whereas at fibre level, especially under compressive stress, the volume decreases significantly. The data collected in this study suggests that the extracellular matrix has a distinct role in amplifying the TCA, and leads to more incompressible tissue behaviour. This article analyses and compares for the first time the tension-compression asymmetry (TCA) displayed by skeletal muscle at tissue and fibre scale. In addition, the volume changes of tissue and fibre specimens with application of passive tensile and compressive loads are studied. The study identifies a key role of the extracellular matrix in establishing the mechanical response of skeletal muscle tissue: It contributes significantly to the passive stress, it is responsible for the major part of tissue-scale TCA and, most probably, prevents/balances the volume changes of muscle fibres during deformation. These new results thus shed light on the origin of TCA and provide new information to be used in microstructure-based approaches to model and simulate skeletal muscle tissue. [ABSTRACT FROM AUTHOR]