Strain Temperature Dependence Research Articles

Modeling steady state rate- and temperature-dependent strain hardening behavior of glassy polymers

In this paper, we use molecular dynamics (MD) simulations to investigate the origin of strain hardening in glassy polymers, with a particular focus on the influences of strain rate and temperature. We demonstrate that strain hardening in uniaxial tension arises from bond stretching and is relaxed by bond rotation, characterized by the evolution of dihedral angles. Based on this rotational relaxation, strain rate plays a role analogous to temperature, resulting in a steady state behavior in the hardening region identifiable by strain rate and temperature. Following these observations, we propose a constitutive model to describe the rate- and temperature-dependent strain hardening behavior of glassy polymers using a viscous potential as a function of strain rate. To capture the elastic and yield behavior at small strains, we integrate this viscous potential with a conventional elasto-viscoplastic (EVP) model, giving rise to our V-EVP model. The V-EVP model is shown to be thermodynamically consistent and is validated by comparison to MD simulation results of glassy polymers undergoing uniaxial tension tests across a broad range of temperatures and strain rates.

Modeling of strain hardening behaviors of 6061 aluminum alloy considering strain rate and temperature effects

This paper focuses on the plastic strain hardening behaviors of 6061 aluminum alloy (AA6061), considering its dependence on strain rate and temperature. Quasi-static tensile tests were conducted over a wide temperature range (20 °C-300 °C), and the digital image correlation technique was used to capture the deformation process of specimens. Additionally, the uniaxial compressive mechanical response of AA6061 was tested utilizing quasi-static compression and split Hopkinson pressure bar experiments over a wide range of strain rates (0.001 s−1–6000 s−1). Experimental results showed that the flow stress decreased and increased nonlinearly with temperature and strain rate, respectively. A modified Johnson–Cook (MJC) model was developed by incorporating a temperature-dependent strain softening term, along with corrections to the temperature and strain rate product terms, to reproduce the nonlinear dependence. Furthermore, the MJC model was proven to be able to accurately reproduce the deformation of the specimens tested in Taylor impact experiments (strain rates up to 105 s−1), suggesting its effectiveness for interpolating and extrapolating stress-strain relationships. This model provides applicable modeling for materials with nonlinear rate sensitivity and temperature sensitivity, and it is expected to be applied in engineering problems involving ultra-high strain rates.

Experimental study and crystal plasticity modeling of additive manufacturing IN718 superalloy considering negative strain rate sensitivity behavior

Revealing underlying mechanisms of IN718 superalloy's negative strain rate sensitivity behavior at elevated temperatures is significantly important for its further applications in extreme conditions. This work investigates the microstructural mechanisms of the temperature-dependent strain rate sensitivity using experimental methods and crystal plasticity characterization model. Firstly, Electron backscatter diffraction (EBSD) and scanning electron microscopy (SEM) techniques are utilized to observe microstructural characteristics of additively manufactured IN718 alloy at different temperatures. The influence of solute precipitation and dislocation slip interaction on the mechanical properties is discussed. Subsequently, the representative volume element (RVE) model is constructed by using acquired orientation, grain size, and dislocation density data. Finally, the crystal plasticity model incorporating dynamic strain aging and dislocation density evolution is developed to characterize the relationship between strain rate sensitivity and temperature. The model can effectively simulate the transition of strain rate sensitivity of additive manufacturing materials. The results indicate that the increase in dislocation pinning caused by intensified carbon element precipitation at high temperatures is the primary reason of the strain rate sensitivity change observed in IN718 metal materials. The carbon precipitated at the dislocation nucleus induces serration flow in IN718 superalloy under high strain. This study contributes to understanding and characterizing the microscopic mechanisms of deformation in metal materials subjected to thermal environments.

Thermally processed phase change behavior on the stoichiometric efficacy of quaternary Se77.5-X Te20 Sn2.5 AgX (X = 2.5, 5, 7.5) thin films

Thin films of the quaternary metal chalcogenide have recently gained attention due to their improved electrical, optical, thermal, and structural properties. The structural, thermal, morphological, optical, and electrical properties of Ag-modified novel quaternary Se–Te–Sn–Ag thin film systems are investigated in this paper. Temperature-dependent structural, electrical, and thermoelectric properties are given special attention. From Differential Thermal Analysis/Thermogravimetric Analysis (DTA/TGA), the Ag content in this composition induces thermal stability and crystalline phase as well as glass phase transitions. High-temperature X-ray diffraction (HTXRD) patterns were used to investigate the temperature-dependent structural phase transition, grain size, lattice strain, and dislocation density. The morphological observations show that Ag concentration promotes the nucleation and growth processes of nanorod structures in as-deposited thin films. Optical characteristics reveal higher IR transparency in thin films and high refractive indexes in the range of 3.22–3.29. The as-deposited thin films resistivity decreases with increasing temperature and silver content. The Hall measurement corroborates the results obtained from the Seebeck measurements, indicating that the samples are n-type. The phase change switching mechanism of phase change devices is still unclear; there are two possible mechanisms: thermal and electronic models. This work proposes thermally processed phase transition behaviors in structural and electrical studies. The result of the above analyses shows that the quaternary Se–Te–Sn–Ag thin film will be a potential candidate for phase change device applications.

Effect of Cr content in temperature-dependent mechanical properties and strain hardening of a twinning-induced plasticity steel

This study aims at assessing and analyzing the effect on the mechanical behavior at temperatures between 25 °C and 350 °C of the Cr additions (5 % and 10 %) to a TWIP steel (Fe–22Mn-0.6C). We conducted tensile tests and characterized the microstructure after deformation by light microscopy, X-ray diffraction (XRD) and FEG-SEM-EBSD. Different temperature-dependent strengthening mechanisms were determined. Mechanical twinning, Dynamic Strain Aging (DSA), and slip bands were the predominant mechanisms, with a combination of them depending on the stacking fault energy (SFE). The SFE decreases with the chromium content and increases with temperature, influencing the percentage of mechanical twinning generated during plastic deformation and consequently, affecting the strain hardening rate (SHR). There exists a direct relationship between strain hardening rate with temperature of deformation and mechanical twinning. DSA was observed and its effect on strain hardening was analyzed in detail.

Solidification crack elimination in TIG welding of Al7075: Experimental investigation and modified RDG modelling

Although solidification crack susceptibility modelling has been a point of interest for many years in fusion welding of aluminum alloys, the underlying mechanism of solidification crack elimination is not yet fully understood. To contribute in closing this knowledge gap, the present study investigates and proposes a new modified Rappaz-Drezet-Gremaud (RDG) physical model in autogenous and heterogeneous TIG welding of Al7075 sheets. The core novelty of the new model is that it includes the shrinkage strain rate, which allows to better capture the effects of capillary pressure, coherency point, as well as final dendrite or grain size in the fusion zone. In agreement with the experimental results and previous models, the new model shows that the defining mechanism of eliminating solidification cracks in heterogeneous joints is the capacity of nanoparticles to initiate heterogeneous grain nucleation, altering the fusion zone grain morphology from dendritic to fine equiaxed, with only little impact of the resulting grain size. This leads to two main effects causing reduced solidification crack susceptibility: (1) a reduction in coherency point by about 15 % from 902 K to 879 K, which reduces strain accumulation on the coherent solid network; (2) a considerable increase in capillary pressure across the entire mushy zone, allowing to maintain the total pressure at void/liquid interfaces in the positive range, which prevents the growth of potentially formed voids between grains at any temperature during solidification. In contrast, dendrite widths below 68 μm are required to prevent solidification cracks in autogenous joints without TiC nanoparticles, indicating a greater role of grain refinement. Overall, the proposed model highlights the more determinant role of the temperature-dependent solidification shrinkage strain rate in solidification crack susceptibility as it is an order of magnitude greater compared to the temperature-independent thermal contraction strain rate that has been the main or sole focus of previous models.

Thermomechanical Modeling of Material Flow and Weld Quality in the Friction Stir Welding of High-Density Polyethylene.

A thermomechanical model of the friction stir welding (FSW) of high-density polyethylene (HDPE) was developed by incorporating a Coupled Eulerian-Lagrangian (CEL) approach. A Johnson Cook (JC) material model of HDPE was developed through experimentally generated strain-rate- and temperature-dependent stress strain data. Two sets of FSW process parameters with minimum and maximum weld defects were numerically modeled. The numerically calculated temperature distribution, material flow and flash and potential defects were validated and discussed with the experimental results. Tracer particles allowed to visualize the material movement during and after the tool had traversed from the specified region of the workpiece. Both numerical models presented similar maximum temperatures on the upper surface of the workpiece, while the model with high traverse speed and slow rotational speed had narrower shoulder- and heat-affected zones than the slow traverse, high rotational speed model. This contributed to the lack of material flow, hence the development of voids and worm holes in the high traverse speed model. Flash and weld defects were observed in models for both sets of process parameters. However, slow traverse, high rotational speeds exhibited smaller and lesser weld defects than high traverse, slow rotational speeds. The numerical results based on the CEL approach and JC material model were found to be in good agreement with the experimental results.

Effect of multilayered coating of single-mode optical fibers on distributed temperature and strain measurement in mortar specimens

This study aims to characterize the effects of multilayered coatings on the performance of distributed fiber optic sensors (DFOS) when the coatings experience softening and melting at high temperatures. Two strain sensors (B-DFOS and W-DFOS) and one temperature sensor (Y-DFOS) were calibrated. They were either embedded along the centerline of a mortar bar or bonded on the surface of the specimen. The Y-DFOS was found to be no longer strain-free at high temperature since the softened sheath, aramid yarns, buffer, and polymer coatings became viscous and adhered to their surrounding mortar above softening temperatures 263–320 °C. Both the B-DFOS and W-DFOS captured uneven strain distributions along the mortar specimen due to non-uniform temperature distribution, mortar heterogeneity, and temperature-dependent strain transfer efficiency. The W-DFOS showed higher measured strains than the calculated thermal-induced strains at 100 °C - 300 °C due to the high thermal expansion coefficient of the additional buffer layer. The B-DFOS gave smaller measured strains than the thermal-induced strains at 300 °C - 500 °C. The displacement calculated by integrating the measured strains along the length of each mortar specimen was related to the applied temperature by parabolic regression equations.

Thermal effects on fracture and the brittle-to-ductile transition.

The fracture behavior of brittle and ductile materials can be strongly influenced by thermal fluctuations, especially in micro- and nanodevices as well as in rubberlike and biological materials. However, temperature effects, in particular on the brittle-to-ductile transition, still require a deeper theoretical investigation. As a step in this direction we propose a theory, based on equilibrium statistical mechanics, able to describe the temperature-dependent brittle fracture and brittle-to-ductile transition in prototypical discrete systems consisting in a lattice with breakable elements. Concerning the brittle behavior, we obtain closed form expressions for the temperature-dependent fracture stress and strain, representing a generalized Griffith criterion, ultimately describing the fracture as a genuine phase transition. With regard to the brittle-to-ductile transition, we obtain a complex critical scenario characterized by a threshold temperature between the two fracture regimes (brittle and ductile), an upper and a lower yield strength, and a critical temperature corresponding to the complete breakdown. To show the effectiveness of the proposed models in describing thermal fracture behaviors at small scales, we successfully compare our theoretical results with molecular dynamics simulations of Si and GaN nanowires.

N-ZnO/p-CuZnS heterostructure deposited by spray pyrolysis for photosensor application in the UV region

In this work, n-ZnO/p-CuZnS heterojunction-based UV photodetector is fabricated using the spray pyrolysis technique. The current-voltage (I-V) characterization under dark condition demonstrated good rectifying behavior. For the fixed bias voltage, upon illumination with UV light (wavelength, λ = 385 nm), the current has increased. At 1 V bias, a responsivity of 0.255 A/W was recorded. Using the thermionic emission (TE) and Chueng models, the carrier transport properties of the photodetector were explained. Upon illumination, barrier height decreased from 0.89 to 0.79. Low-temperature Raman analysis was carried out to investigate the temperature-dependent strain between the layers. Photoluminescence study employed for defect analysis showed suppressed emissions indicating better absorption

Modeling the one-way and two-way shape memory effects of semi-crystalline polymers

Polymers of two-way shape memory effects (SMEs) are of great application potential ranging from intelligent actuators to artificial muscles due to their reversible shape response on the basis of an on-off stimulus such as a cooling-heating cycle, which motivates the development of theoretical tools to support their design. In the paper, a three-dimensional phase transition finite deformation constitutive model is proposed to phenomenologically describe the underlying mechanisms of the one-way and two-way SMEs in thermal-responsive semi-crystalline polymers. A unified transition equation is proposed to depict the volume changes of crystalline phase during both crystallization and melting transitions. The crystallization-induced elongation and melting-induced contraction behaviours in two-way SMEs are assumed to be realized by an elongation-contraction element and its temperature-dependent strain evolution rules. The overall constitutive model is then implemented into Mathematica, and the model predictions are compared to and agree well with the experimental results. The paper provides an efficient method on modeling the thermal-induced two-way SMEs under stress conditions in semi-crystalline polymers and detailed discussion about the influence of several parameters such as the crosslinking density, the applied external load and the cooling/heating rate.

Influence of Constitutive Models and the Choice of the Parameters on FE Simulation of Ti6Al4V Orthogonal Cutting Process for Different Uncut Chip Thicknesses

The constitutive model and its pertinent set of parameters are important input data in finite element modeling to define the behavior of Ti6Al4V during machining process. The present work focusses on comparing different constitutive models and the parameters sets available in literatures and investigating the quality of the predictions when varying uncut chip thickness (40 µm, 60 µm, 100 µm and 280 µm). In addition, temperature-dependent strain hardening factor along with strain softening phenomenon based reconstructed material model is proposed. The results from the numerical simulations are compared with experimental results available in literature. The comparison shows that the force values are highly influenced by constitutive models and the choice of parameters sets, whereas the chip morphologies are mainly influenced by the uncut chip thickness and constitutive models. This work justifies the need for an appropriate set of parameters and constitutive model that replicate the machining behavior of Ti6Al4V alloy for different cutting conditions.

A novel phenomenological constitutive model for Ti-6Al-4V at high temperature conditions and quasi-static strain rates

Phenomenological constitutive modeling of Ti-6Al-4V at temperatures between 923 and 1023 K under 0.0005–0.05 s−1 quasi-static rates is studied based on a phenomenological approach. For this purpose, the Johnson–Cook constitutive model is revisited. At low temperature conditions under moderate to high strain rates, the material’s stress–strain curves are the most similar to power-law function. Contrary to this, at high temperature conditions under low to moderate strain rates, the saturation-type function well describes the stress–strain curves. On the other hand, it is illustrated that the Johnson–Cook constitutive model is feeble to predict the material’s behavior correctly. Accordingly, in this study, a viscoplastic temperature-dependent constitutive model is developed. The strain rate hardening as well as thermal softening of the developed model is the same as the Johnson–Cook model. But a temperature-dependent strain hardening function is proposed in which both the saturation-type and power-law hardening behaviors of the material are implemented. In comparison with the Johnson–Cook model, the new constitutive model’s fidelity in capturing the titanium behavior is depicted. At last, by considering an Arrhenius-type phenomenological constitutive model, it is noted that the developed constitutive model has the best correctness in predicting the Ti-6Al-4V stress–strain behavior at high temperature conditions under quasi-static rates.

Thermally activated dislocation mechanism in Mo studied by indentation, compression and impact testing

Body-centered cubic metals like molybdenum and tungsten are interesting structural materials for high-temperature applications. These metals, are however, brittle at low homologous temperature, caused by the limited mobility of screw dislocations. In this study, the thermally activated deformation mechanisms in bcc Mo have been investigated using strain rate jump nanoindentation and compression tests as well as Charpy V-notch impact testing. The material shows a significant softening with increasing temperature and a maximum in strain rate sensitivity is found at the critical temperature, before decreasing again in the ductile regime. The activation volume, however, showed a distinct increase from about 5 b3 at the onset of the brittle to ductile transition temperature. Here we propose to use temperature-dependent nanoindentation strain rate jump testing and the activation volume as a complementary approach to provide some indication of the brittle to ductile transition temperature of bcc metals.Graphic

Topology optimization of shape memory polymer structures with programmable morphology

We present a novel optimization framework for optimal design of structures exhibiting memory characteristics by incorporating shape memory polymers (SMPs). SMPs are a class of memory materials capable of undergoing and recovering applied deformations. A finite-element analysis incorporating the additive decomposition of small strain is implemented to analyze and predict temperature-dependent memory characteristics of SMPs. The finite element method consists of a viscoelastic material modelling combined with a temperature-dependent strain storage mechanism, giving SMPs their characteristic property. The thermo-mechanical characteristics of SMPs are exploited to actuate structural deflection to enable morphing toward a target shape. A time-dependent adjoint sensitivity formulation implemented through a recursive algorithm is used to calculate the gradients required for the topology optimization algorithm. Multimaterial topology optimization combined with the thermo-mechanical programming cycle is used to optimally distribute the active and passive SMP materials within the design domain. This allows us to tailor the response of the structures to design them with specific target displacements, by exploiting the difference in the glass-transition temperatures of the two SMP materials. Forward analysis and sensitivity calculations are combined in a PETSc-based optimization framework to enable efficient multi-functional, multimaterial structural design with controlled deformations.

A thermo-viscoelastic model for particle-reinforced composites based on micromechanical modeling

Micromechanics-based constitutive models offer superior ability to estimate the effective mechanical properties for the composites, which greatly promote the computational efficiency in the multiscale analysis for composite structures. In this work, a thermo-viscoelastic model for particle-reinforced composites is proposed to estimate their thermal–mechanical coupling behaviors in terms of a micromechanics-based homogenization method in the time domain. The matrix and particles of the composites are modeled as “thermo-rheologically complex” viscoelastic materials. The temperature-dependent effective elastic strain energy ratios of particle to composite are proposed to evaluate the contributions of the matrix and particles. The thermo-viscoelastic model for the composites is then formulated by superposing the matrix and particle’s contributions. Finite element simulations based on the representative volume element models are employed to validate the constitutive model under various thermal–mechanical coupling loads. The effects of the loading rate, viscous parameter and particle content on the effective thermal–mechanical responses of the composites are also comprehensively discussed. The experimental data from literature are also employed to verify the constitutive model. The findings show that the proposed thermo-viscoelastic model can accurately predict the thermal–mechanical coupling behaviors for the particle-reinforced composites.

Temperature dependence of polarization and strain in Bi0.5Na0.5TiO3-SrTiO3 multilayer composites

Relaxor-ferroelectric phase transition materials exhibit excellent strain behavior under applied electric fields for utilization in actuator applications. However, the large strain of relaxor-ferroelectric phase transition materials can only be achieved in a narrow temperature range. Hence, in this study, composition-based multilayer samples and compositional gradient composite (CGC) samples were designed and fabricated to extend the usable temperature range of relaxor-ferroelectric materials. The raw materials used were 0.78(Bi0.5Na0.5TiO3)–0.22SrTiO3 and 0.72(Bi0.5Na0.5TiO3)–0.28SrTiO3. The multilayered sample containing two ceramics showed higher temperature-dependent strain and polarization than those of the single-composition ceramic. In addition, phase transition was observed in the CGC in the low- and high-temperature ranges. In the CGC sample, the compositional gradient characteristics of Sr were observed along the vertical direction owing to diffusion. This reduced the temperature-dependent unipolar strain of the sample (0.148–0.175%) in the range of −20 °C < T < 100 °C. This behavior can be attributed to the combined effect of the relaxor-ferroelectric phase transition and ferroelectric domain reorientation, as well as the relaxor electrostriction. The sintering temperature had a significant effect on the compositional gradient of Sr and the performance of the layered composites.

Influence of material constitutive models on thermomechanical behaviors in the laser powder bed fusion of Ti-6Al-4V

A transient three-dimensional thermomechanical model was developed to examine the evolution of thermal and mechanical fields in the laser powder bed fusion of Ti-6Al-4V alloy, particularly with respect to the development of plastic strain. A primary focus was to evaluate the influence of material constitutive models on predicted mechanical behaviors. Johnson-Cook (JC) constitutive material models were chosen to account for the contribution of strain, strain rate, and temperature on material responses. Three different modifications of the JC models were applied to assess the sensitivity of rate hardening and temperature-dependent strain hardening on mechanical prediction. Numerical results revealed that predicted stress and strain fields were highly sensitive to the rate-dependent term but minimally affected by the temperature-dependent strain hardening. The maximum differences in average stresses and plastic strains were 23% and 13% when comparing results between the JC model and the JC model without strain rate hardening, respectively. Moreover, it was found that strain rate hardening mostly occurred at intermediate temperatures, ≈ 1000 – 1600 K, during the continuous cooling phase. Finally, the present study emphasizes the importance of material constitutive models on mechanical behavior in laser powder bed fusion as well as providing insight for further exploration of process parameter optimization.

New Role of Relaxor Multiphase Coexistence in Potassium Sodium Niobate Ceramics: Reduced Electric Field Dependence of Strain Temperature Stability.

The influence of relaxor behavior on strain behavior is less investigated in potassium sodium niobate [(K, Na)NbO3, KNN] ceramics. Here, we report novel phenomena in the temperature-dependent strain behavior with the electric field of KNN-based ceramics with relaxation characteristics. The strain temperature stability is electric field dependent below the threshold electric field: temperature-dependent strain can be effectively improved by increasing the applied electric fields, while it remains almost electric field independent above the threshold electric field. Such a macroscopic property change can be well consistent with the following microscopic domain structure evolution. Little voltage dependence is found above a certain voltage by employing voltage-dependent piezoresponse hysteresis loops and domain switching under different temperatures, implying the contribution of domain behavior to the change of strain. Ergodic polar nanoregions (PNRs) are induced by the high-density domain walls among nanodomains in the relaxor samples, as revealed by the atomic-resolution polarization mapping with Z-contrast. The facilitated domain switching due to the lowered energy barrier and nearly vanished polarization anisotropy based on the PNRs with nanoscale multiphase coexistence can promote the electric field compensation for temperature effect. This work demonstrates the contribution of relaxor behavior to the electric field dependence of strain temperature stability in KNN-based ceramics.

Mechanical properties of 1670 MPa parallel wire strands at elevated temperatures

High-strength prestressing steel cables serve as main load-bearing components for cable-supported structures, and exposure to high temperatures may result in their prestress loss, leading to failure and even collapse of such structures. This paper experimentally investigates the mechanical properties of 7-wire parallel wire strands with a tensile strength of 1670 MPa. The thermal elongation tests and steady-state tensile tests are conducted on 39 strand specimens at different target temperatures. The complete stress-strain curves including rupture strain are obtained by using charge-coupled device camera systems. The temperature-dependent thermal strain, Young’s modulus, proportional limit, effective yield strength, ultimate strength, ultimate strain and rupture strain are recorded and compared to EC2, ACI 216 and other existing test data. The test results show that parallel wire strands experience obvious strain hardening before 300 °C, and their ductility is significantly improved after 400 °C. It is reasonable to define effective yield strength as the stress at a total strain of 2%. It is found that the existing design code and test data on single wires and twisted strands cannot be directly used for parallel strands. The EC2 provides conservative predictions for ultimate strain, proportional limit, yield strength of parallel wire strands, but fails to capture the full range of stress-strain curves of parallel strands due to ignoring strain hardening effect and underestimating rupture strain. The ACI 216 provides unconservative predictions for reduction factors of ultimate strength of parallel strands. Parallel wire strands have higher thermal strain, higher ultimate strain, higher rupture strain, lower reduction factor of proportional limit than single wires and twisted wire strands. Finally, calculation methods are proposed to mathematically determine the thermal strain, stress-strain curve and reduction factors of these properties, and dividing the stress-strain relationship into elastic, plastic, necking and rupture stage.