Strain Rate Dependent Deformation Behavior Research Articles

Temperature and Strain Rate Related Deformation Behavior of UHMWPE Fiber-Reinforced Composites.

As they possess the qualities of high specific strength, high specific modulus, high specific energy absorption, and excellent designability, ultra-high molecular weight polyethylene (UHMWPE) fiber-reinforced composites have gradually replaced traditional materials such as ceramics and steel plates as the main ballistic protection materials. Using an improved test method, the uniaxial tensile tests of UHMWPE fiber-reinforced composites at two strain rates of 10-4 s-1 and 10-2 s-1 and a temperature range from -20 °C to 80 °C are carried out to study the effects of strain rate and temperature on the tensile behavior of UHMWPE fiber-reinforced composites. The experimental results indicate that the tensile responses exhibit nonlinear characteristics and the sensitivity of strain rate and temperature. The yield strength and modulus decrease with increasing temperature and increase with the increase in strain rate. A phenomenological viscoelastic constitutive model composed of a nonlinear spring and a nonlinear Maxwell element is proposed to characterize the temperature and strain rate dependent deformation behavior of UHMWPE fiber-reinforced composites before yielding. The results show that the model can accurately predict the tensile nonlinear viscoelastic responses of UHMWPE fiber-reinforced composites before yielding over a wide temperature range under quasi-static loading.

Strain rate dependent deformation behavior of BCC-structured Ti29Zr24Nb23Hf24 high entropy alloy at elevated temperatures

The mechanical behavior and deformation mechanisms of a body-centered cubic (BCC) Ti29Zr24Nb23Hf24 (at%) high entropy alloy (HEA) was investigated in temperatures and strain rates from 700° to 1100 °C and 10−3 to 10 s−1, respectively. The HEA exhibits a substantial increase in yield stress with increasing strain rate. The strain rate sensitivity (SRS) coefficient is ~3 times that of BCC alloy Nb-1Zr and even ~3.5 times that of pure Nb. This high SRS is attributed to the high Peierls stress of the HEA, which is about twice the Peierls stress of pure Nb. On the other hand, the flow stress exhibits a tendency from strain softening to strain hardening with the increase of strain rate especially at the relatively low temperatures. This behavior is explained by a change in dislocation motion from climbing to multiple slip with the increase of strain rate. Taking the specimen subjected to 800 ºC for example, dislocation walls formed at the early stage of deformation and at low strain rate of 10−3 s−1. In this case there is sufficient time to activate dislocations climb, which results in discontinuous dynamic recrystallization, and strain softening. However, when the strain rate amounts to 1 s−1, thermally activated processes such as dislocation climb are too sluggish. As a consequence, multiple slip systems are activated, and the dislocation interactions lead to the evolution of deformation bands, leading to strain hardening.

Microstructure and strain rate-dependent deformation behavior of PBF-EB Ti6Al4V lattice structures

Abstract Additive manufacturing techniques enable the fabrication of new lightweight components with tailored mechanical properties. Considering current application fields, components are often over-dimensioned since a lack of data regarding the mechanical properties under compression or tensile loading at high strain rates is present. In this work, the influence of various strain rates on the mechanical properties of electron beam powder bed fusion Ti6Al4V lattice structures was investigated. In order to capture the damage mechanisms that occurred, a single unit cell plane was considered. In terms of mechanical characterization, high-speed tensile tests at nominal strain rates from 0.025 to 250 s-1 were carried out. By the additional use of a high-speed camera system and subsequent digital image correlation, an investigation of material reactions during shortest test times was enabled. Based on the results, a positive strain rate dependency was identified for yield and ultimate tensile strength for both investigated lattice types. In detail, an increase in ultimate tensile strength of 16 % for BCC- and 20 % for F2CCZ-specimens could be detected.

Strain rate-dependent hardness and deformation behavior in the nanocrystalline/amorphous Ti2AlNb film

Abstract Building nanocrystalline/amorphous biphase nanostructure has recently emerged as a new strengthening-toughening route that can combine the strengthening benefits of nanocrystallinity and amorphization. Therefore, it is required exploring on the strain rate-dependent deformation behavior with above different nanostructure in certain system. Herein, three typical nanostructures: polycrystalline, uniform nanocrystalline/amorphous core-shell nanostructure and amorphous matrix with nanoclusters, have been achieved in the sputtered Ti2AlNb films by individually regulating the sputtering bias voltage (Ubias). Note that a high Ubias can promote formation of Nb–rich amorphous tissues via Al preferential resputtering and Nb segregation. Moreover, microstructural evolution and strain rate-dependent hardness and deformation behavior were further explored. Firstly, at a low Ubias (−40 V), Ti2AlNb film exhibited polycrystalline character; it yielded a relatively low hardness (~10.0 GPa) but a high strain sensitivity coefficient of 0.1505. Subsequently, the novel nanocrystalline/amorphous core-shell nanostructure consisting of Ti2AlNb nanocrystalline cores uniformly encapsulated by thin amorphous shells was achieved at a higher Ubias (−120 V); this nanostructure provided a remarkable hardness enhancement (~15.2 GPa) without sacrificing its ductility and intermediate strain sensitivity coefficient of 0.1382. Finally, a transition to amorphous matrix with nanoclusters occurred with further increasing Ubias to −200 V, thus yielding slight decrease in hardness (~12.5 GPa) and a minimum strain sensitivity coefficient of 0.0915 when shear-band deformation was activated.

Local and global tensile deformation behavior of AA7075 sheet material at 673oK and different strain rates

The constitutive behavior of AA7075 aluminum sheet like other metallic sheet materials is highly temperature and strain rate dependent at elevated temperatures. The uniaxial tensile test is commonly employed to characterize the temperature and strain rate dependent deformation behavior. However, the traditional uniaxial tensile testing and data analysis procedure treats the gauge length of sample as a uniformly-deforming (or homogeneous) region. This may not be accurate due to early localized deformation that is initiated in the gauge region at elevated temperatures. Treating the post-necking hardening behavior of uniaxial tensile samples has been an important task to expand the effective range of true stress-true strain curves at elevated temperatures for metal and alloys. To explore this issue, uniaxial tensile tests on AA7075-F sheets (with as-fabricated temper) were conducted at 673o K and four different test speeds. The test procedure also utilized an on-line 2-D digital image correlation (DIC) system method to obtain full-field strain map from the deforming gauge region of the test specimen. Local stress-strain curves at different points along the gauge length were then calculated based on the recorded local in-plane principal stress and strain values from macroscopic force data and current cross-section of the specimen at the chosen points. In this manner, the DIC strain analysis not only enabled the determination of stress-strain response but also the determination of strain rates at specific chosen points along the gauge length. The local stress-strain curves revealed significant deviation at an earlier stage of deformation from the global (or macroscopic) stress-strain response based on the entire gauge region. To estimate the constitutive behavior at different strain rates, a correction method to traditional stress-strain curves was applied to obtain a family of stress-strain curves at different constant strain rates based on the local stress-strain curves. Additionally, the material deformation response in the stress, strain and strain rate space in the 3D Cartesian coordinate system was also constructed for tests conducted at different stretching speeds. Further, a verification test was conducted to see if the above methodology could provide an accurate estimation of local deformation behavior of AA7075 sheet. Furthermore, the above experimental method was also coupled with the Gurson-Tvergarrd-Needleman (GTN) ductile void damage model and incorporated into ABAQUS-Explicit general purpose finite element (FE) analysis code via a VUMAT material subroutine. Good agreement was obtained for both the local and global stress-strain curves between the FE test simulation and experimental stress-strain results. The proposed experimental methodology and the resulting stress-strain-strain rate behavior is believed to be applicable in describing the strain rate-dependent constitutive behavior of AA7075-F sheet metals at 673o K.

Modeling the strain rate-dependent constitutive behavior in nanotwinned polycrystalline metals

Experimental studies have demonstrated that both strain rate and temperature influence the mechanical behavior of nanostructured metals significantly. In this work, a theoretical model is developed to describe the strain-rate-dependent constitutive behavior of nanotwinned polycrystalline metals. The athermal flow stress and thermal-activated flow stress are both considered in modeling the plastic deformation of a nanotwinned metal. Numerical results are consistent with the experimental results, showing that the present model can well describe the strain rate-dependent deformation behavior of nanotwinned polycrystalline copper. Henceforth, the constitutive behaviors of nanotwinned copper at different strain rates and temperatures can be predicted, which will be useful for optimizing the dynamic mechanical properties at various temperatures for nanotwinned metals.

Strain-Rate Dependent Deformation Behavior in WC-10 wt%Ni <sub>3</sub>Al Cermet Micropillars Under Uniaxial Compression

The strain-rate dependent deformation behavior in WC-10%Ni3Al cermet micropillars was studied by in-situ uniaxial compression. The results show that compression at high strain rates exhibits a relatively smooth stress-strain response compared with that at low strain rates. Microstructural characterization suggests that at high strain rates, grain boundary sliding is the dominant mechanism of deformation, while at low strain rates, dislocation annihilation plays a dominant role. The strain rate sensitivity index also varies with strain rate, confirming that the dominant deformation mechanisms have changed.

Dynamic spherical indentation of strain hardening materials with and without strain rate dependent deformation behavior

Dynamic spherical indentation of elastic-plastic solids is examined in the light of finite element simulations. The deformation behavior of indented solids is described by two constitutive models – elastic-plastic strain hardening with or without strain rate dependent deformation. Numerical results of dimensionless global field parameters that describe the deformation response, such as mean contact pressure, contact area, and plastic work, are presented for a range of indentation velocity and elastic–plastic material properties. The dynamic contact response is interpreted in terms of a dimensionless indentation parameter, the normalized indentation strain, and the strain hardening and strain rate dependent material properties. It is shown that generalized solutions of global field parameters can be obtained in terms of the indentation parameter and indentation strain over a certain range of indentation depth. The results of this study provide new insight into the role of strain hardening and strain rate dependent deformation on the dynamic contact response of elastic-plastic solids.

In-situ TEM observation of [formula omitted] twin-dominated deformation of Mg pillars: Twinning mechanism, size effects and rate dependency

To investigate the mechanism of {101¯2} twinning in magnesium (Mg) single crystal and its influence on mechanical size effects and strain rate dependent deformation behavior, in-situ microcompression of Mg [21¯1¯0] pillars of various sizes from 0.5 μm to 4 μm was carried out in a scanning electron microscope (SEM) and also in a transmission electron microscope (TEM), covering strain rates from 10−4 to 10−2 s−1. The in-situ observations directly showed that the pile-up of prismatic <a> dislocations acts as local stress concentration for the twin nucleation. Preceding the twin nucleation, the lead dislocation from the dislocation pile-up cross-slips to the basal plane and dissociates into partial dislocations, one of which trails a stacking fault (SF) behind. The twin nucleus of a finite size formed at the junction between prismatic <a> dislocations and basal SFs and subsequently propagated rapidly across the pillar. The present in-situ observations reveal that not only the dislocation pile-up but also the dissociation reaction of <a> dislocations play critical roles in the nucleation of {101¯2} twins. Furthermore, the {101¯2} twinning exhibits a relatively strong size effect in terms of the twin nucleation stress (size exponent n = 0.7). This pronounced size effect may arise from the fact that the precursor to twin nucleation, namely dislocation pile-up and junction formation, depends more strongly on the crystal size than the ordinary dislocation source operation. Moreover, a noticeable effect of the strain rate is that a higher rate (10−2 s−1) promotes the activation of basal slip within the{101¯2} twin. While the twin nucleation occurs more easily at a high strain rate, here the twin growth rate cannot cope with the applied strain rate, so that strain energy accumulation increases with applied strain. When the twin grows to reach the required twin thickness for basal slip, the basal slip promptly activates within the twinned region to release the accumulated strain energy and plastic deformation swiftly catches up with the applied strain rate.

Constitutive model for high temperature deformation behavior of Ti–Zr–Ni–Be bulk metallic glass in supercooled liquid region

A constitutive equation based on a free volume model, that describes the strain rate dependent deformation behavior of bulk metallic glasses (BMGs) within the supercooled liquid region, has been modified in this paper in order to reproduce the stress increment that occurs due to crystalline phase formation during lengthy exposure to high temperature in compression deformation. A comparison of the simulated results obtained from finite element analyzes with the compression test results for Ti–Zr–Ni–Be BMG alloy has been conducted to determine the validity of the proposed model. Plastic deformation modes such as Newtonian and non-Newtonian viscous flows of this BMG alloy were found to be reproduced well by the finite element method simulations combined with the free volume based constitutive relations and to show a phenomenon of stress increment deviated from the steady state. Therefore, the constitutive relations introduced here are expected to allow accurate reproduction of the high temperature behavior and better estimation of the formability of BMG alloys.

Spherical indentation of elastic–plastic solids

The finite–element method is used to perform an accurate numerical study of the normal indentation of an elastic–plastic half–space by a rigid sphere. The effects of elasticity and strain–hardening rate of the half–space are explored, and the role of friction is assessed by analysing the limiting cases of frictionless contact and sticking friction. Indentation maps are constructed with axes of contact radius a (normalized by the indenter radius R and the yield strain of the half–space. Competing regimes of deformation mode are determined and are plotted on the indentation map: (i) elastic Hertzian contact; (ii) elastic–plastic deformation; (iii) plastic similarity regime; (iv) finite–deformation elastic contact; and (v) finite–deformation plastic contact. The locations of the boundaries between deformation regimes change only slightly with the degree of strain–hardening rate and of interfacial friction. It is found that the domain of validity of the rigid–strain–hardening similarity solution is rather restricted: it is relevant only for solids with a yield strain of less than 2 x 10 −4 and a / R