Defect-induced asymmetrical mechanical behavior in shape memory zirconia: A phase-field investigation

An elastoplastic phase-field model is used to investigate the deformation mechanisms of yttria stabilized tetragonal zirconia in presence of defects. A remarkable tension-compression asymmetry is detected. A higher strength and a lower degree of transformation are observed in compression than in tension. Also, deformation mechanism is asymmetric depending on the crystal orientation. For some cases (other cases), phase transformation is absent in tension (in compression), while both transformation and plasticity are present in compression (in tension). Such tension-compression asymmetry is attributed to activation of different monoclinic variants with different Eigen strain tensors in tension versus compression. Results also reveal a higher degree of transformation and plasticity with lower onset stresses as the void size increases. Elliptic voids exhibit a directional effect with a maximum stress intensity factor of 5.6 MPa m^1/2 when the long semi-axis is diagonally oriented with respect to the loading direction, and this prediction is comparable to experiments.     

Thermal and strain rate sensitive compressive behavior of polycarbonate polymer - experimental and constitutive analysis

The mechanical behavior of polycarbonate (PC) polymer was investigated under the effect of various temperatures and strain rates. Characterization of polymer was carried out through uniaxial compression tests and split Hopkinson pressure bar (SHPB) dynamic tests for low and high strain rates respectively. The experiments were performed for strain rates varying from 10 ^?3 to 10³ and temperature range of 213 to 393 K. By conducting these experiments, the true stress–strain (SS) curves were obtained at different temperatures and strain rates. The results from experiments reveal that the stress–strain behavior of polycarbonates is different at lower and higher strain rates. At higher strain rate, the polymer yields at higher yield stress compared to that at low strain rate. At lower strain rate, the yield stress of the polymer increases with the increase in strain rate while it decreases significantly with the increase of temperature. Likewise, initial elastic modulus, yield and flow stress increase with the increase in strain rate while decreases with the increase in temperature. The yield stress increases significantly for low temperature and higher strain rates. On the basis of experimental findings, a phenomenological constitutive model was employed to capture the mechanical behavior of polymer under temperature and loading rate variations. The model predicted the yield stress of polymer at varying strain rate and temperature also it successfully predicted the compressive behavior of polymer under entire range of deformation.     

The effect of time and temperature on the mechanical behavior of a “plasticized” epoxy resin under different loading modes

Epoxy–Versamid specimens were loaded in tension, compression, and flexure at different strain rates and temperatures to determine mode of failure, yield stress and strain, and tangent and relaxation moduli. Stress-strain curves were used to define brittle, ductile, ductile-rubbery, and rubbery modes of behavior which prevailed in different temperature-strain rate regions. The time-temperature superposition principle was applied to yield stress, initial tangent moduli, and relaxation moduli data for all three types of loading. The transition regions, tangent and relaxation moduli, and shift factors were the same in tension, compression, and flexure. Thus the most convenient mode of loading can be used to determine the general time-temperature dependence. The ratio of compressive-to-tensile yield stress was almost constant over the entire ductile region. Flexural yielding data were used to predict yield stress in tension and compression, and stress relaxation master curves were shown to be related to elastic modulus vs. strain rate curves. The yielding phenomenon was interpreted using Eyring's theory of non-Newtonian viscoplastic flow. The apparent activation energy and activation volume were larger for tension than compression. A theory is offered to explain why yielding can occur in a cross-linked system.     

The effect of time and temperature on the mechanical behavior of epoxy composites

A crosslinked epoxy resin consisting of a 60/40 weight ratio of Epon 815 and Versamid 140 and composites of this material with glass beads, unidirectional glass fibers and air (foams) were tested in tension, compression and flexure to determine the effect of time and temperature on the elastic properties, yield properties and modes of failure. Unidirectional continuous fiber-filled samples were tested at different fiber orientation angles with respect to the stress axis. Strain rates ranged from 10^?4 to 10 in./in.-min and the temperature from ?1 to 107°C. Isotherms of tangent modulus versus strain rate were shifted to form master modulus curves. The moduli of the filled composites and the foams were predictable over the entire strain rate range. It was concluded that the time-temperature shift factors for tangent moduli and the time-temperature shift factors for stress relaxation were identical and were independent of the type and concentration of filler as well as the mode of loading. The material was found to change from a brittle-to-ductile-to-rubbery failure mode with the transition temperatures being a function of strain rate, filler content, filler type and fiber orientation angle, indicating that the transition is perhaps dependent on the state of stress. In the ductile region, an approximately linear relationship between yield stress and log strain is evident in all cases. The isotherms of yield stress versus log strain rate were shifted to form a practically linear master plot that can be used to predict the yield stress of the composites at any temperature and strain rate in the ductile region. The time-temperature shift factors for yielding were found to be independent of the type, concentration and orientation of filler and the mode of loading. Thus, the composite shift factors seem to be a property of the matrix and not dependent on the state of stress. The compressive-to-tensile yield stress ratio was practically invariant with strain rate for the unfilled matrix, while fillers and voids raised this ratio and caused it to increase with a decrease in strain rate. The yield strain of the composites is less than the unfilled matrix and is a function of fiber orientation and strain rate.     

Effects of modulation periods on mechanical properties of V/VN nano-multilayers

Metal/ceramic nano-multilayers, which possess the advantages of both metal and ceramics, have attracted intensive attention. In this work, the mechanical properties of V/VN nano-multilayers with different modulation periods (λs) were investigated using molecular dynamics simulations. The relationships between the microstructures and stress-strain (σ - ε) curves during in-plane tension and out-of-plane compression were explored, and the effects of λ were discussed. It shows that: (i) three kinds of V/VN nano-multilayers with different interfacial structures have different stabilities during deformation; (ii) dislocations nucleate firstly from the interfaces, propagate on {111} planes in VN layers, and deposit on the adjacent interface, and finally shift to V layers with the increase of applied strain; (iii) the nano-multilayer with larger λ has larger elastic modulus, elastic limit and lattice integrity, and strain hardening and cracks would appear in the nano-multilayers with λ?>?42.90?Å.     

Experimental characterisation and constitutive modelling of RTM-6 resin under impact loading

The response to mechanical loading of the thermosetting resin system RTM-6 has been investigated experimentally as a function of strain rate and a constitutive model has been applied to describe the observed and quantified material behaviour. In order to determine strain rate effects and to draw conclusions about the hydrostatic stress dependency of the material, specimens were tested in compression and tension at strain rates from 10⁻³ to 10⁴ s⁻¹. A Standard screw-driven tensile machine was used for quasi-static testing, with an ‘in house’ hydraulic rig and Hopkinson bars for medium and high strain rates, respectively. At all rates appropriate photography and optical metrology have been used for direct strain measurement, observation of failure and validation of experimental procedures. In order to enable the experimental characterisation of this brittle material at very high rates in tension, a novel pulse shaping technique has been applied. With the help of this device, strain rates of up to 3800 s⁻¹ have been achieved while maintaining homogeneous deformation state until specimen fracture in the gauge section of the tensile specimens. The yield stress and initial modulus increased with increasing strain rate for both compression and tension, while the strain to failure decreased with strain rate in tension. An existing constitutive model, the Goldberg model has been extended in order to take into account the nonlinear strain rate dependence of the elastic modulus. The model has been validated against 3-point impact bending tests of prismatic RTM-6 beams.     

基于DIC系统的PE-HD力学性能

基于数字图像相关法(DIC)研究了加载速率和汽油浸泡时间对高密度聚乙烯(PE–HD)材料拉伸性能的影响。实验表明,PE–HD材料的屈服应力随加载速率的增加而增加,但随加载速率的增大,其对屈服应力的影响逐渐减小;断裂伸长率随加载速率的增加明显减小;屈服应变不随加载速率的改变而改变;计算出各加载速率下材料的拉伸弹性模量大小。经汽油处理过的PE–HD屈服应力和拉伸弹性模量随浸泡时间快速下降随后趋于稳定,材料的强度降低到一个稳定的值。     

Experimental and numerical investigation of the dynamic response of a ZrB2-SiC ceramic under uniaxial compression

The dynamic experimental tests were performed on cylindrical zirconium diboride-silicon carbide ceramic specimens under the uniaxial compression from 519 to 2861 s⁻¹. The effect of the strain rate on the dynamic response of a ZrB₂-SiC ceramic was investigated using experimental and numerical methods. A significant increase on the dynamic compressive strength, elastic modulus, and the dynamic tensile strength was found with the increase of the strain rate. The damage process and fracture pattern of the ZrB₂-SiC ceramic exhibited a significant strain-rate dependence under the dynamic compression. The strain rate-dependent elastic modulus and tensile strength were introduced into Johnson–Holmquist (JH-2) model to predict the dynamic compression behavior of the ZrB₂-SiC ceramic. The simulation results of the dynamic compressive strength, stress–strain relation, and fracture patterns were in good accordance with the dynamic experimental results.     

Tensile yield in phenolphthalein polyether ketone

Uniaxial tension tests to the yield point were performed on phenolphthalein polyether ketone (PEK-C) from room temperature to near the glass transition temperature (T_g) at a constant rate of 0.02 min^?1. At room temperature, some measurements were also made at strain rates from 0.002 min^?1 to 2 min^?1. Yield stress was a linear function of temperature and log strain rate. The temperature and the strain rate dependence of yield stress could be modeled using Eyring theory. Yield energy was found to be a linear function of temperature. Young's modulus, yield strain, elastic strain, and plastic strain all decreased with temperature. © 1994 John Wiley & Sons, Inc.     

Mechanical properties of nanosilica/polypropylene composites under dynamic compression loading

This article presents results on the dynamic mechanical properties of PP‐SiO₂ nanocomposites, with nanosilica contents of 1, 3, and 5% by weight, at various strain rates using a Split Hopkinson Pressure Bar (SHPB) apparatus. The specimens were prepared using a hot compression technique. The dynamic mechanical characteristics, of PP‐SiO₂ nanocomposites, are illustrated in terms of stress–strain curves, up to nearly 1100 s⁻¹ of strain rates. From the results, the yield stress, compression modulus, and compressive strength of the composites, were significantly influenced by the strain rates and nanosilica contents. The values of strain rate sensitivity, and dissipation energy of the composites at various strain rates, were also determined. It was found that the strain rate sensitivity, and the dissipation energy, increased with increasing strain rates. In addition, it was observed that the composites experienced more severe damage under a high strain rate loading, compared to a low strain rate loading. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Molecular dynamics predictions of thermal and mechanical properties of thermoset polymer EPON862/DETDA

We use molecular dynamics (MD) to perform an extensive characterization of the thermo-mechanical response of a thermoset polymer composed of epoxy EPON862 and curing agent DETDA. Our simulations, with no adjustable parameters, show that atomistic simulation can capture non-trivial behavior of amorphous thermosets including the role of polymerization degree, thermal history, strain rate and temperature on the glass transition temperature (T_g) and mechanical response (including ultimate properties) and lead to predictions in quantitative agreement with experiments. We find a significant increase in T_g, Young’s modulus and yield stress with degree of polymerization while yield strain is significantly less sensitive to it. For structures cured beyond the gel point (percolation of a 3D network) conversion degree and temperature affect yield stress in a similar way with yield stress linearly dependent on T−T_g; however, we find non-linear and non-universal relationship below the gel point. Our results show that a relative small variation in polymerization degree (∼5%) can explain the spread in experimental measurements of T_g and elastic constants available in the literature.     

Tensile yield in poly(chlorotrifluoroethylene) and poly(vinylidene fluoride)

Uniaxial tension tests to the yield point were performed on poly(chlorotrifluoroethylene) (PCTFE) and poly(vinylidene fluoride) (PVF2) from room temperature to near the melting point at a strain rate of 2 min^?1. At room temperature and at least two elevated temperatures, measurements were also made at strain rates from 0.02 to 8 min^?1. The properties of these polymers were found to be similar to those of other semicrystalline polymers. In the absence of other transitions, yield energy was found to be a linear function of temperature extrapolating to zero near the melting temperature. The ratio of thermal to mechanical energy to produce yielding is smaller than for glassy polymers. Yield stress is a linear function of log strain rate. The ratio of yield stress to (initial) Young's modulus is about 0.03 at room temperature for both polymers. Yield stress is a linear function of unstrained volume. Yield strain, elastic, and plastic strain all initially increase with temperature, but PCTFE shows a decrease with temperature starting at about 100°C, thus behaving like a glassy amorphous polymer in this region.     

Plastic deformation of polyethylene crystals as a function of crystal thickness and compression rate

Plastic deformation of polyethylene (PE) samples with crystals of various thickness was studied during uniaxial compression with initial compressive strain rates of 5.5×10⁻⁵, 1.1×10⁻³ and 5.5×10⁻³ s⁻¹. Samples with a broad range of crystals thickness, from usual 20 up to 170 nm, were obtained by crystallization under high pressure. The samples underwent recoverable compression below the compression ratio of 1.05-1.07. Following yield, plastic flow sets in above a compression ratio of 1.12. At a compression rate of 5.5×10⁻⁵ s⁻¹ the yield stress increases with the increase of crystal thickness up to 40 nm. For crystals thicker than 40 nm the yield stress levels off and remains constant. This experimental dependence was compared with the model developed on the basis of classical crystal plasticity and the monolithic nucleation of screw dislocations from polymer crystals. In that model contrary to the experimental evidence, the yield stress does not saturate with increase of crystal thickness. The activation volumes determined from strain rate jump experiments and from stress relaxation for crystals thicker than 40 nm are nearly constant at a level of 8.1 nm³. This activation length agrees very well with 40 nm for crystal thickness above which the yield stress levels off. It is proposed, as shown in a companion communication, that for PE crystals thicker than 40 nm two other modes of dislocation emission in the form of half loops of edge and screw dislocations begin to govern the strain rate, which no longer depend on lamella thickness.     

Finite strain behavior of poly(ethylene terephthalate) (PET) and poly(ethylene terephthalate)-glycol (PETG)

Uniaxial and plane strain compression experiments are conducted on amorphous poly(ethylene terephthalate) (PET) and poly(ethylene terephthalate)-glycol (PETG) over a wide range of temperatures (25-110 °C) and strain rates (.005-1.0 s⁻¹). The stress-strain behavior of each material is presented and the results for the two materials are found to be remarkably similar over the investigated range of rates, temperatures, and strain levels. Below the glass transition temperature (θ_g=80 °C), the materials exhibit a distinct yield stress, followed by strain softening then moderate strain hardening at moderate strain levels and dramatic strain hardening at large strains. Above the glass transition temperature, the stress-strain curves exhibit the classic trends of a rubbery material during loading, albeit with a strong temperature and time dependence. Instead of a distinct yield stress, the curve transitions gradually, or rolls over, to flow. As in the sub-θ_g range, this is followed by moderate strain hardening and stiffening, and subsequent dramatic hardening. The exhibition of dramatic hardening in PETG, a copolymer of PET which does not undergo strain-induced crystallization, indicates that crystallization may not be the source of the dramatic hardening and stiffening in PET and, instead molecular orientation is the primary hardening and stiffening mechanism in both PET and PETG. Indeed, it is only in cases of deformation which result in highly uniaxial network orientation that the stress-strain behavior of PET differs significantly from that of PETG, suggesting the influence of a meso-ordered structure or crystallization in these instances. During unloading, PETG exhibits extensive elastic recovery, whereas PET exhibits relatively little recovery, suggesting that crystallization occurs (or continues to develop) after active loading ceases and unloading has commenced, locking in much of the deformation in PET.     

The effects of thermomechanical history and strain rate on antiplasticization of PVC

Antiplasticization is mechanically characterized by an increase in the polymer stiffness and/or yield strength upon the incorporation of a small amount of a low-molecular weight diluent. It is attributed to hindrance of the local β-relaxation motions of the polymer. Here, we have studied the effects of thermal treatment, plastic deformation, and strain rate on the antiplasticization of the yield stress of a 95 wt% poly(vinyl chloride)/5 wt% dioctyl phthalate (PVC/5 wt% DOP) compound. Two thermal treatments were applied to the materials - cooling to room temperature from above T_g by a quench or by a slow oven-cool anneal. When compressed at low to moderate strain rates, antiplasticization was observed in the annealed (physically aged) PVC/5 wt% DOP but not in the quenched (unaged) PVC/5 wt% DOP. Load-unload-reload compression cycles revealed that antiplasticization can be erased by plastic strain; the anomalously high yield stress of PVC/5 wt% DOP observed in the first load cycle softens to a value lower than that of the neat PVC in subsequent cycles. The results indicate that disordered, high free volume microstructural states, obtained either from thermal quenching or from plastic straining, liberate the beta motions of the PVC molecule which, in turn, erase antiplasticization of the yield stress. Earlier work on the rate-dependence of yield has demonstrated that beta motions must be stress-activated in order to yield neat PVC when deformed at high strain rates (>100/s). Hence, we have characterized the rate-dependence of the antiplasticization of the yield stress by testing the annealed materials in uniaxial compression over a wide range of strain rates (10⁻⁴/s-3000/s). Antiplasticization was observed in PVC/5 wt% DOP in the low strain rate regime where beta motions are free in neat PVC but hindered in PVC/5 wt% DOP; however, the antiplasticization (elevation of yield stress) gradually diminished with increasing strain rate.     

Viscoelastic characterization of medium consistency pulp suspensions

The dynamic viscoelastic properties of pulp suspensions having consistencies, c_m3 ranging from 2-13% were measured using a Weissenberg Rheogoniometer. A special reservoir-type parallel plate fixture was designed to minimize oozing and compression during sample loading. Data for the complex modulus, G*, were obtained for a chemical thermal mechanical pulp and a pine sulfate pulp as functions of strain and frequency. Results for the elastic part of the modulus, G', show a relative insensitivity to frequency over the range l0 ^-* to 5 ^s-'. The applied strain has a significant effect, in some cases reducing the modulus by half. This effect was most pronounced at the lower consistencies where it is postulated that the suspension consists of small flocs loosely connected by individual fibres. At small strains, the linkages are disturbed elastically, whereas, as the strain increases, the connections are broken and must reform. At higher consistencies there is a continuous network of fibres, and this breakage-reformation mechanism is not active. This view is supported by nonlinear effects observed in the suspensions (harmonic stresses produced in response to a single frequency strain) which are largest at the low consistencies. The measured yield stress values, as determined from the storage modulus data, are shown to agree with earlier studies.     

Visco-elasto-plastic constitutive model of adhesives under uniaxial compression in a range of strain rates

Determining the constitutive model of adhesives enables the prediction of the mechanical response of hybrid structures in which the adhesive is used. In this study, the stress–strain behavior of a two-component structural adhesive was first measured in uniaxial compression experiments at the strain rates ranging from 0.001 to 2600 s⁻¹ for developing a constitutive model. It was found that the compressive response, including elastic modulus, yield strength, and plastic flow stress, is substantially influenced by the strain rate. In plastic deformation, the strain rate sensitivity is not constant but varies with the rate; moreover, strain softening and hardening dominate the plastic deformation at low and high strains, respectively. A visco-elasto-plastic constitutive model was then proposed for the adhesive, integrating the strain rate sensitivity that was quantified by empirical equations. The model which was validated can reliably represent the strain rate dependent compressive behavior of the adhesive. © 2020 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48962.     

Plastic deformation and damage of polyoxymethylene in the large strain range at elevated temperatures

The deformation behaviour of polyoxymethylene has been studied in plane strain compression at temperatures from 120 °C up to 165 °C and in uniaxial tension and simple shear at 160 °C for strain rates from 10⁻⁴ to 1 s⁻¹. In uniaxial tension the stress-strain behaviour was determined by a novel video-controlled testing system. The measurements showed that there was a very significant evolution of volumetric strain, indicating that damage mechanisms play a key role in the plastic deformation behaviour.All tests showed similar deformation stages with a short region of visco-elastic behaviour followed by a rounded yield point. The von Mises equivalent yield stress for these tests showed a linear relationship with logarithmic strain rate, suggestive of an Eyring type thermally activated process. After yielding, all stress-strain curves showed a long plastic deformation regime, which in shear occurred at constant stress. In plane strain compression there was also only a very small increase in stress, in contrast to uniaxial tension where very significant strain hardening was observed at high strains, which is attributed to the onset of structural changes.     

Delayed yielding of epoxy resin under tension,compression, and flexure. I. Behavior under constant strain rate

Series of loading tests were carried out on epoxy resin specimens, at varying constant strain rates, under tension, compression, and flexure. The stress–strain relationship revealed a distinct yielding stage followed shortly by a post-yielding region of decreasing load. In all cases, results indicate linearity between yield stress and log strain rate, in accordance with Eyring's theory of viscous flow. For specimens unloaded close to the yield point, photoelastic observations revealed a residual pattern parallel to the theoretical principal shear stresses. These results, supported by additional data from other works, indicate a viscoplastic deviatoric stress-biased diffusional mechanism as the dominant factor in the yielding of an amorphous crosslinked epoxy system.