Tensile-compressive Asymmetry Research Articles

Mechanical Behavior of Additive Manufacturing (AM) and Wrought Ti6Al4V with a Martensitic Microstructure

Processing and microstructure are fundamental in shaping material behavior and failure characteristics. Additively manufactured materials, due to the rapid heating and solidification process, exhibit unique microstructures compared to their as-cast counterparts, resulting in distinct material properties. In this work, the response of the titanium alloy Ti6Al4V has been investigated for different processing conditions through quasi-static testing. AM Ti6Al4V was fabricated by employing Selective Laser Sintering (SLS) and Selective Laser Melting (SLM) techniques. Both materials present a similar microstructure consisting of an acicular martensitic α′-phase. Commercial Ti6Al4V-grade 5 (supplied as bars) was also examined after heat treatment to achieve a microstructure akin to the AM material. The heat treatment involved rapid heating above the β-phase region and water quenching to obtain a full martensite microstructure. A similar constitutive behavior and tensile–compressive asymmetry in strength were noted for the investigated materials. However, AM alloys exhibited a significantly higher deformation at failure, reaching nearly 40%, compared to only 6.1% for the wrought martensitic material, which can be attributed to the dissimilar distribution of both α′ laths and prior-β grain boundaries in the investigated materials. The results indicate that AM can be implemented for the fabrication of martensitic microstructures with mechanical properties superior to those obtained with conventional water-quenching.

Failures of geometrically asymmetric steel-UHPC composite beams

Geometrically asymmetric steel-ultra-high performance concrete (UHPC) composite structures (SUCSs) suffer several typical failure modes during applications in structural engineering. But the competing mechanism of these failure modes remains a challenge to be urgently overcome. In the present paper, the failure modes of the geometrically asymmetric SUCSs under lateral loads are theoretically predicted and numerically studied. Considering both the geometric asymmetry of the structure and the tensile-compressive asymmetry of UHPC, a theoretical model is proposed to predict the critical failure load for each failure mode (e.g., bending and core shear), and then a failure mechanism map is constructed by comparing each critical failure load. The theoretical predictions are in line with the present numerical results and the previous experimental measurements. The results demonstrate that geometric asymmetry, together with some other geometric parameters and material properties, have remarkable influences on the failure modes of structures. This work provides an effective approach for predicting and designing the failure modes of geometrically asymmetric SUCSs in structural engineering applications, such as undersea tunnels, offshore platforms, and nuclear shielding walls.

Effect of extrusion ratio on microstructures, mechanical properties, and high cycle fatigue behavior of Mg–5Zn–1Mn alloy

The effects of extrusion ratio (ER) on the microstructures, static mechanical properties, and tension-compression high cycle fatigue (HCF) behavior of the as-extruded Mg–5Zn–1Mn (ZM51) alloy was investigated systematically, and the HCF mechanism was discussed. The results show that texture weakening and grain refinement caused by increasing the ER can effectively enhance the static mechanical properties and alleviate the tensile-compressive asymmetry of the alloy. The fatigue strength at 107 cycles is 124 ± 5, 138 ± 4, and 142 ± 5 MPa for ER8, ER11.5, and ER23, respectively, and the corresponding fatigue ratio is approximately 0.45, 0.49, and 0.50, which means that the as-extruded ZM51 alloy possesses outstanding fatigue resistance. The failure analysis and microstructure evolution of post-fatigued samples demonstrated that the stress amplitudes and microstructure characteristics strongly influence the HCF mechanism of the alloy. At high-stress load, close to the tensile yield strength 160–180 MPa, abundant {101‾2} residual twin bands were observed near the fracture surface. Twinning/detwinning deformation is the dominant fatigue mechanism due to the plastic deformation incompatibility between the matrix and the residual twins. At low-stress load, close to the fatigue strength 125–145 MPa, the fatigue crack initiation mechanism transits from twinning/detwinning to dislocation slip. Grain refinement and texture weakening inhibit the activation of {101‾2} twins, alleviate the twin-dislocation interaction, and promote dislocation slip to dominate the fatigue deformation, thereby enhancing the fatigue life of the alloy. This work provides new insight into the design and development for enhancing the fatigue resistance of wrought Mg alloy to ensure the long-term service safety of structural materials.

A constitutive model of nanoporous NiTi shape memory alloys considering tensile-compressive asymmetry, grain size and porosity

A constitutive model of the macroscopic behaviors of nanoporous Shape Memory Alloys (SMAs) is developed, and the effects of tensile-compressive asymmetry, grain size, and porosity are all considered because the influences of these factors cannot be ignored. A finite-volume, three-phase micromechanical model that contains the grain boundary phase, grain-core phase, and uniformly distributed pores is first given to represent the grains of nanoporous NiTi SMAs. The secant equivalent bulk modulus and shear modulus of the nanoporous NiTi SMAs were calculated by using the extended Mori-Tanaka method. Due to the high plastic yield limit of the grain boundary phase, the grain boundary phase is regarded as the region where no phase change occurs. And the overall phase transformation can be caused by the grain-core phase. The phase transformation is established according to the J2, J3 theory and the von Mises equivalence effect. The martensite volume fraction is represented in this article as a linear interpolation function of equal effectiveness. Based on the strain-hardening behavior, the equivalent stress of nanoporous NiTi SMAs is represented by a power function consisting of transformation onset stress, hardening modulus, equivalent phase change strain, and hardening index. Then, the constitutive model describing the superelastic behavior can be obtained. By comparing the numerical simulation results with the experimental results, it is verified that the constitutive model proposed in this paper can better describe the superelastic behavior of the nanoporous NiTi SMAs. Finally, through numerical simulations, it was found that tensile-compressive asymmetry, grain size, and porosity have significant effects on the superelastic behavior of nanoporous NiTi SMAs. This study will provide a theoretical basis for the application of nanoporous NiTi SMAs.

The discrepancy in basal slip hardening rates of cast and extruded Mg-Y-Zr alloys

The differences in the deformation mechanisms during uniaxial tension of cast and extruded Mg–2Y-0.5Zr (wt%) alloys have been investigated by using EBSD-based slip trace analysis and VPSC simulation methods, and a novel method for counting the activity of slip/twinning using grain area has been proposed. Slip trace analysis reveals that enhanced ductility and reduced tensile-compressive asymmetry observed in the Mg–2Y-0.5Zr (wt%) alloy can be attributed to the addition of the Y element, which modified the texture, and promoted pyramidal <c + a> slip and suppressed prismatic <a> slip. The extruded alloy exhibits a distinctive pseudo-fiber bimodal microstructure, which contributes to its superior ductility. The VPSC results indicate that the CRSSnon-basal/CRSSbasal ratio of the extruded alloys decreases, whereas the CRSStwinning/CRSSbasal ratio increases, which is attributed to the different effects of the grain refinement on the different deformation modes. In addition, both the VPSC results and the slip trace analysis results indicate that the extruded alloy exhibits reduced non-basal slip activity compared to the cast alloy at similar strain levels. The work-hardening rate curves and VPSC parameters give a reasonable explanation that the key to the increase in the ductility of Mg–Y–Zr alloys after extrusion is the decrease in the basal hardening rate rather than the non-basal slip activity. The refinement of grain size leads to a significant decrease in basal slip hardening rate, primarily due to the decrease in the additional hardening effect caused by tensile twinning and the generation of an additional GBS mechanism in the pseudo fiber microstructure.

Characterization approaches affect asymmetric load predictions of hexagonal close-packed alloy

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.

Combining Machine Learning and Molecular Dynamics to Predict Mechanical Properties and Microstructural Evolution of FeNiCrCoCu High-Entropy Alloys.

Compared with traditional alloys, high-entropy alloys have better mechanical properties and corrosion resistance. However, their mechanical properties and microstructural evolution behavior are unclear due to their complex composition. Machine learning has powerful data processing and analysis capabilities, that provides technical advantages for in-depth study of the mechanical properties of high-entropy alloys. Thus, we combined machine learning and molecular dynamics to predict the mechanical properties of FeNiCrCoCu high-entropy alloys. The optimal multiple linear regression machine learning algorithm predicts that the optimal composition is Fe33Ni32Cr11Co11Cu13 high-entropy alloy, with a tensile strength of 28.25 GPa. Furthermore, molecular dynamics is used to verify the predicted mechanical properties of high-entropy alloys, and it is found that the error between the tensile strength predicted by machine learning and the tensile strength obtained by molecular dynamics simulation is within 0.5%. Moreover, the tensile-compression asymmetry of Fe33Ni32Cr11Co11Cu13 high-entropy alloy increased with the increase of temperature and Cu content and the decrease of Fe content. This is due to the increase in stress caused by twinning during compression and the decrease in stress due to dislocation slip during stretching. Interestingly, high-entropy alloy coatings reduce the tensile-compression asymmetry of nickel; this is attributed to the reduced influence of dislocations and twinning at the interface between the high-entropy alloy and the nickel matrix.

Constitutive model for nanocrystalline NiTi shape memory alloys considering grain size effects and tensile–compressive asymmetry

Mechanical properties of nanocrystalline NiTi shape memory alloys (SMAs) change drastically with grain size. The present contribution develops a constitutive model to reproduce the grain size dependent superelastic behavior and tensile–compressive asymmetry observed in the experiments of nanocrystalline NiTi SMAs. Effects of grain size are incorporated in the developed model by introducing the intrinsic length scale accounting for the transformation hardening as well as the grain-core and grain-boundary phase. In this work, nanocrystalline NiTi SMA is regarded as a two-phase composite material made of inclusions of the grain-core phase dispersed in the grain-boundary phase acting as a matrix. A transformation function allowing for the description of fine-grain strengthening mechanism and tensile–compressive asymmetry is proposed. In the grain-core phase, the evolution law for transformation strain during the forward and reverse transformation is determined. Besides, the constitutive relation of the grain-boundary phase is assumed to be linearly elastic. Based on the equivalent secant bulk and shear modulus of the grain-core and grain-boundary phase, the stress–strain relationship of nanocrystalline NiTi SMAs is derived by using the extended Mori–Tanaka method. Comparisons between experimental and predicted results demonstrate that the proposed model has the ability to reproduce the grain size dependent deformation and asymmetric stress–strain behavior under tension and compression of nanocrystalline NiTi SMAs. In detail, it is found that critical transformation stresses for forward and reverse transformations, dissipation energy density, transformation strain hardening, and maximum transformation strain are sensitive to the grain size and stress states.

A modified JH2 model with improved strength model, damage evolution, and equation of state for rock under impact and blasting loads

Rock fragmentation by blasting is widely applied in geotechnical engineering, requiring the development of the rock damage constitutive model. A modified Johnson-Holmquist Ceramic (M-JH2) model, which adds an initial yield strength surface, is proposed to characterize the rock damage process under blasting loads. In addition, a new damage scale factor was proposed to capture the strain hardening and gradual softening behaviors. Tensile and compressive damages are treated separately to describe the tensile-compressive asymmetry, and the volumetric plastic strain is introduced into the compressive damage. Moreover, the EOS of the original JH2 model is modified by including unloading and reloading paths. To calibrate the model, benchmark examples were established in the LS-DYNA with the M-JH2 models, such as single element tests, Split Hopkinson Pressure Bar test, rock blasting fracture, and crater blasting field test. The results show that the M-JH2 model agrees well with the classic experiments. The M-JH2 model improves the JH2 model in strain hardening, gradual softening, and the sensitivity difference of damage evolution between tension and compression in a Brazilian disc dynamic splitting test.

Strength evaluation of CFRP structure of high pressure hydrogen tank based on mesoscale analysis

For designing with rigorous cost reduction and safety, high pressure hydrogen tanks manufactured from carbon fiber reinforced plastic (CFRP) of fuel cell vehicles, a method that can precisely predict the burst pressures of the tank is urgently needed. CFRP layers of tanks are formed with complicated mesostructured consisting of carbon fiber bundles and matrix resin using a filament winding process. A methodology that approaches by using cross shaped specimens to present fiber bundle conditions in CFRP layer of the tank is scrutinized. From the analysis and experiment with the specimen, it was found that strain rate dependency and tensile compression asymmetry of the resin have a great influence in addition to the fiber strength in the fiber direction on the CFRP fracture process. Burst pressures were predicted by CFRP mesoscale model with the load and boundary condition from macroscale analysis of conventional axisymmetric continuum model of the tank and validated through an actual tank burst test. As the results, we found the burst pressure of tank could be accurately predicted based on carbon fiber ruptures in the mesoscale model.

Tensile-compressive asymmetry in extruded AZ31B rod and its effect on Profilometry-based Indentation Plastometry (PIP)

This paper concerns Tensile-Compressive Asymmetry (TCA) in a Mg-based alloy. Strong TCA is in general unusual, but previous publications have indicated that it can arise with the AZ31(B) alloy (in extruded rod or rolled plate form), such that there is a factor of up to two between yield stress values obtained by testing the same sample (in the same direction) in tension and in compression. This is confirmed in the present work. It has also been previously established that this effect is associated both with strong crystallographic texture and with the plasticity involving at least some deformation twinning. It is thus more likely with hexagonal metals, such as Mg, since their lower crystallographic symmetry (compared with cubic metals) favours this type of deformation. The microstructural evolution, and operative deformation mechanisms, that give rise to the observed TCA effects are investigated in detail here and at least most of the observed mechanical characteristics are rationalised. Finally, the Profilometry-based Indentation Plastometry (PIP) methodology for inferring stress-strain curves from indentation tests is applied to this material. As expected, full TCA characteristics cannot be obtained via PIP, but it is shown that there is a well-defined relationship between curves inferred in this way and the full set of uniaxial test outcomes.

Tensile-Compressive Asymmetry in Extruded Az31b Rod and its Effect on Profilometry-Based Indentation Plastometry (Pip)

Tensile-Compressive Asymmetry in Extruded Az31b Rod and its Effect on Profilometry-Based Indentation Plastometry (Pip)

Analytical investigation of an energy harvesting shape memory alloy–piezoelectric beam

This work presents an analytical investigation of a cantilever shape memory alloy (SMA)/piezoelectric laminated composite beam subjected to a tip load with possible temperature variation. The beam consists of two piezoelectric layers of identical thickness bonded to a superelastic SMA core exhibiting asymmetric tensile–compressive behavior. Two loading scenarios are considered where either the temperature of the beam or the applied tip load is varied, while the other parameter is fixed. In each case, the load results in deformation of the SMA core and the piezoelectric layers bonded thereto, leading to the generation of an electric charge. The deformation of the SMA material is modeled based on an extended version of the ZM constitutive relations for SMAs that accounts for intrinsic tensile–compressive asymmetry. Geometric and force equilibrium considerations are used to identify the correct sequence in which different solid-phase structures develop within the superelastic core during a complete loading–unloading process. Temperature-dependent moment and shear force equations are then derived and used in investigating the electrical and mechanical properties of the beam. The proposed model is validated against 3D finite element simulations, and the general trend of output voltage variation with temperature is shown to be consistent with experimental data.

A Temperature-Dependent Model of Shape Memory Alloys Considering Tensile-Compressive Asymmetry and the Ratcheting Effect.

Tensile-compressive asymmetry and the ratcheting effect are two significant characteristics of shape memory alloys (SMAs) during uniaxial cyclic tests, thus having received substantial attention in research. In this study, by redefining the internal variables in SMAs by considering the cyclic accumulation of residual martensite, we propose a constitutive model for SMAs to simultaneously reflect tensile-compressive asymmetry and the cyclic ratcheting effect under multiple cyclic tests. This constitutive model is temperature dependent and can be used to reasonably capture the typical features of SMAs during tensile-compressive cyclic tests, which include the pseudo-elasticity at higher temperatures as well as the shape-memory effect at lower temperatures. Moreover, the proposed model can predict the cyclic mechanical behavior of SMAs subjected to applied stresses with different peak and valley values under tension and compression. Agreement between the predictions obtained from the proposed model and the published experimental data is observed, which confirms that the proposed novel constitutive model of SMAs is feasible.

Constitutive Modeling of Porous Shape Memory Alloys Using Gurson–Tvergaard–Needleman Model Under Isothermal Conditions

Based on the Gurson–Tvergaard–Needleman (GTN) model, a constitutive relationship considering both the effects of strain hardening and hydrostatic stress for porous shape memory alloys (SMAs) is proposed. To capture the relationship between microscopic and mesoscopic behaviors, a representative volume element (RVE) containing an array of spherical voids is presented. In this paper, an approximate solution including strain hardening exponent [Formula: see text] is deduced by considering the porous SMA as a two phase composite with the SMA matrix and the second phase representing voids. The model parameters, [Formula: see text] and [Formula: see text], accounting for interactions between voids are investigated to take into account their influences on strain hardening, critical phase transformation stress and yield surface. In addition, the evolution equations of phase transformation are derived and then applied to the simulation of porous Ni–Ti SMAs with a porosity of 13%. Using the calibrated GTN model parameters, the critical phase transformation stress closer to experimental data is obtained. The predictions of stress–strain curve by the proposed constitutive model are found to be in excellent agreement with published experimental data and finite element results. The results prove that the model is capable of reproducing the features of porous SMAs such as superelasticity, tensile-compressive asymmetry and internal loops under uniaxial even combined loading conditions. A conclusion is drawn that the present constitutive relationship is powerful and useful for the analysis of porous SMAs.

Comparison of High-Temperature Compression and Compression-Comp ressionFatigue Behavior of Magnesium Alloys DieMag422 and AE42.

Due to their high strength-to-weight-ratio, magnesium alloys are very attractive for use in automotive engineering. For application at elevated temperatures, the alloys must be creep-resistant. Therefore, the influence of the operating temperature on the material properties under quasistatic and cyclic load has to be understood. A previous study investigated tensile-tensile fatigue behavior of the magnesium alloys DieMag422 and AE42 at room temperature (RT). The aim of this study was the comparison of both alloys regarding compression, tensile, and compression-compression fatigue behavior. The quasistatic behavior was determined by means of tensile and compression tests, and the tensile-compression asymmetry was analyzed. In temperature increase fatigue tests (TIFT) and constant amplitude tests (CAT), the temperature influence on the cyclic creeping (ratcheting) behavior was investigated, and mechanisms-relevant test temperatures were determined. Furthermore, characteristic fracture mechanisms were evaluated with investigations of the microstructure and the fracture surfaces. The initial material was analyzed in computed tomographic scans and energy dispersive X-ray (EDX) analyses.

The Influence of Casting Microvoids on the Constitutive Behavior of As-Quenched Al-Cu-Mn Alloy

Microvoids, a common casting defect, have a significant effect on the mechanical properties of as-quenched aluminum alloys. In this study, low-pressure casting and quenching processes were conducted on an Al-Cu-Mn alloy to investigate the porosity variation, which was characterized by using a high-precision densimeter and x-ray microtomography. Mechanical properties were tested at elevated densities varying from 2.7828 to 2.8114 g/cm3. The Voce behavior and linear hypotheses were proposed on the relationship between the yield strength and density. A tensile constitutive model was established and validated with experimental data, and the difference between tension and compression was compared. With a decrease in density, the yield stress declined, and the tensile–compressive asymmetry increased. The ideal density, $$\rho_{0}$$, and the ideal saturated stress, $$\sigma_{\text{s}}$$, of the as-quenched alloy were predicted. A comparison between the quantity and volume of microvoids showed that larger pores were the main factor affecting the porosity of the as-quenched alloy. The growth of single microvoid and the coalescence of several microvoids caused an increase in porosity under tensile stress.

A tensile-compressive asymmetry model for shape memory alloys with a redefined martensite internal variable

In this paper, through redefining the martensite internal variable of shape memory alloys (SMAs) based on the Brinson model, we propose the one-dimensional phenomenological constitutive model for SMAs to predict asymmetric behavior during tension and compression. Compared with other models, the presented model is much more continuous and efficient by using a novel expression to describe the variation in the martensitic fraction. The numerical results show that our model can successfully predict the asymmetric behaviors of SMAs during multiple tensile-compressive cycles under arbitrary temperatures. Moreover, the proposed model expands its applicable scopes by revealing the internal sub-loops of SMAs, which are caused by incomplete martensitic transitions during tensile-compressive processes. The results of numerical examples show reasonable agreement with the experimental data, which confirm the reliability of the proposed model in applications.

A model for shape memory alloy beams accounting for tensile compressive asymmetry

A new analytical model is derived for cantilever beams made from superelastic shape memory alloy and subjected to tip load. The deformation of the beam is described based on Timoshenko beam theory using constitutive relations that account for asymmetric shape memory alloy response in tension and compression. Analytical moment and shear force equations are developed and the position of the neutral axis and the different solid phase regions in the beam are tracked throughout a full loading–unloading cycle. Validation of the proposed model is carried out against data from the literature and from the finite element analysis considering tensile–compressive asymmetry in shape memory alloy behavior.

Bending model for a laminated composite cantilever beam with multiple embedded shape memory alloy layers presenting tensile-compressive asymmetry

A new model is proposed for a composite laminated beam comprising multiple alternating shape memory alloy (SMA) and elastic layers. The model fully considers asymmetry in SMA behavior, which is found to significantly influence the behavior of the laminates. Moreover, the equations governing the response of the SMA-reinforced beams are derived for a complete loading-unloading cycle considering a Timoshenko beam deformation model combined with well-established constitutive relations for SMAs. The derivation procedure involves first identifying the solid phase structure of the beam for a given applied load, followed by integration of the stress and strain in a cross section to obtain moment and shear force equations. The influence of temperature, as well as of layer thickness and material properties on the bending response of the beam is investigated. The results obtained using the proposed model are found to agree with finite element simulations.