Strain Rate Tensile Research Articles

The Behaviour of 2169 Steel Under Uniaxial Stress and Uniaxial Strain Loading

A series of plate impact and uniaxial stress compression experiments have been undertaken on 2169 steel, a nitrogen strengthened austenitic stainless steel. The shock experiments ranged from impact stresses of 3.7–17.8 GPa, recording the wave profiles of the shock and spall behaviour observed at the rear target surfaces. Target and impactor configurations were varied to observe the effects of tensile strain-rates achieved at spall planes. Spall strength was observed to vary linearly with impact stress and tensile strain-rate although was less rate sensitive than 300 series steels suggestive of an influence of the shock induced microstructure on void nucleation. A fourth power relationship between strain-rate in the shock front and shock stress was observed similar to many other materials. The linear coefficient A relating the two is discussed in terms of dislocation mobility. Uniaxial stress compression experiments were undertaken on two grain sizes of the same material at a range of strain-rates from ~10−4 to 103 s−1. The data is consistent with other published data on 2169 steel showing a clear grain-size dependence on flow stress and higher flow stresses than other common austenitic steels.

Unfolding of membrane ruffles of in situ chondrocytes under compressive loads.

Impact loading results in chondrocyte death. Previous studies implicated high tensile strain rates in chondrocyte membranes as the cause of impact-induced cell deaths. However, this hypothesis relies on the untested assumption that chondrocyte membranes unfold in vivo during physiological tissue compression, but do not unfold during impact loading. Although membrane unfolding has been observed in isolated chondrocytes during osmotically induced swelling and mechanical compression, it is not known if membrane unfolding also occurs in chondrocytes embedded in their natural extracellular matrix. This study was aimed at quantifying changes in membrane morphology of in situ superficial zone chondrocytes during slow physiological cartilage compression. Bovine cartilage-bone explants were loaded at 5 μm/s to nominal compressive strains ranging from 0% to 50%. After holding the final strains for 45 min, the loaded cartilage was chemically pre-fixed for 12 h. The cartilage layer was post-processed for visualization of cell ultrastructure using electron microscopy. The changes in membrane morphology in superficial zone cells were quantified from planar electron micrographs by measuring the roughness and the complexity of the cell surfaces. Qualitatively, the cell surface ruffles that existed before loading disappeared when cartilage was loaded. Quantitatively, the roughness and complexity of cell surfaces decreased with increasing load magnitudes, suggesting a load-dependent use of membrane reservoirs. Chondrocyte membranes unfold in a load-dependent manner when cartilage is compressed. Under physiologically meaningful loading conditions, the cells likely expand their surface through unfolding of the membrane ruffles, and therefore avoid direct stretch of the cell membrane. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:304-310, 2017.

Microstructural and mechanical characterization of aluminum-lithium alloy 2060 welded by fiber laser

In this study, with constant welding parameters, butt joints consisting of 2.0 mm thick 2060-T8 Al-Li alloy have been fabricated by laser beam welding. A fiber laser using eutectic alloy AA4047 filler wire was employed. The joints' microstructural evolution and the effects of strain rates on the joints' mechanical behaviors were researched. Standard tensile specimens extracted from the welded plates were tested at room temperature with strain rates ranging from 10−4 to 10−1 s−1. Extensive microhardness measurements were also conducted to investigate the relationships between strain rates and microhardness distributions after fractured. It was found that solute segregations of Si and Cu occurred in the grain boundaries in the heat affected zone and fusion zone. Mechanical properties and fracture morphology showed close relationships with the tensile strain rate. The nondendritic equiaxed zone was the minimum hardness and weakest region on the whole joint.

Liquid Zn assisted embrittlement of advanced high strength steels with different microstructures

In the present study, liquid metal embrittlement (LME) phenomenon during high temperature deformation was investigated for 3 grades of Zn-coated high strength automotive steel sheets consisting of different phases. Hot tensile tests were conducted for each alloy to compare their LME sensitivities at temperature ranges between 600 and 900 °C with different strain rates. The results suggest that Zn embrittles all the Fe-alloy system regardless of constituent phases of the steel. As hot tensile temperature and strain rate increase, LME sensitivity increases in every alloy. Furthermore, it is observed that the critical strain, which is experimentally thought to be 0.4% of strain at temperatures over 700 °C, is needed for LME to occur. It is observed via TEM work that Zn diffuses along grain boundaries of the substrate alloy when the specimen is strained at high temperatures. When the specimen is exposed to the strain more than 0.4% at over 700 °C, the segregation level of Zn at grain boundaries seems to become critical, leading to occurrence of LME cracks.

Microstructure- and Strain Rate-Dependent Tensile Behavior of Fiber Laser-Welded DP980 Steel Joint

DP980 steels were butt-welded by fiber laser welding. The microstructures, microhardness distribution, and tensile behavior of the joint were investigated. The results showed that the fusion zone (FZ) consisted of fully martensite with higher hardness compared to the base metal (BM). A softened zone (20 HV0.2 drop) was produced in heat-affected zone due to martensite tempering during the laser welding. The ultimate tensile strength (UTS) and yield strength (YS) of the laser-welded joint were not degraded compared to BM with the existence of softened zone. The UTS and YS of the welded joint increased with the increase of tensile strain rate. The work hardening exponents of the BM and welded joint showed weak positive strain rate dependence. The deformation of softened zone was restrained by the hardened FZ during loading, resulting in a higher work hardening rate of softened zone than that of BM. The failure of welded joint occurred in the BM instead of softened zone. The fracture surfaces of the joint exhibited typical ductile fracture over strain rate from 0.0001 to 0.1 s−1.

Influence of the tensile strain rate on the mechanical properties and phase composition of VNS 9-Sh TRIP steel

The influence of the strain rate on the mechanical properties and the phase composition of a sheet VNS 9-Sh (23Kh15N5AM3-Sh) TRIP steel is studied during static tension. The strain rate is changed in the range from 8.3 × 10–5 to 25 × 10–3 s–1. The dependence of the mechanical properties on the strain rate is found to be nonlinear. The TRIP effect is most pronounced at a strain rate of (8.3–17) × 10–5 s–1. In this strain rate, the deformation martensite content increases significantly, from 50 to 87%, as is detected by X-ray diffraction.

Structural evolution of uniaxial tensile-deformed injection molded poly(ɛ-caprolactone)/hydroxyapatite composites

It is essential to examine the mechanisms of plastic deformation of polymer composites under external loads and large strains, especially if the material is intended to be used in a dynamic environment. This work investigated the variation of structure as well as the properties of poly(ɛ-caprolactone) (PCL) deformed under different tensile draw ratios and strain rates. The PCL/HA composites were prepared by melt mixing the PCL with up to 10 wt% HA in a twin-screw extruder. The deformation behavior of the PCL/HA composites revealed a strong correlation between the mechanical response and the accompanying structural transformations. It was found that the strain rate and stretching ratio played important roles in modulating the molecular orientation and crystallization of the PCL/HA composites. The increase in strain rate from 0.2 to 100 mm/min led to the variation of crystallinity from 56.81 to 67.50%. With an increase of the strain rate, the chain extension rate along the stretching direction increased faster than the chain relaxation, which improved the orientation of the polymer chains. The crystallinity and orientation of the deformed PCL/HA composites increased with an increase in draw ratio. The composites also possessed enhanced yield strength resulting from an increased strain rate. POLYM. COMPOS., 2015. © 2015 Society of Plastics Engineers

High Temperature High Strain-Rate Tensile and Compressive Deformation Behaviors of Cu-Zn-Sn-Al Alloy

The present work gives a systematic study on the high temperature and high strain-rate deformation behaviors of a two-phase α/β Cu-Zn-Sn-Al alloy, by combining the split Hopkinson bar experiments and microstructural investigations. The results show that under high strain-rate, both the dislocation slip and deformation twins within the α phase contribute to the plastic strengthening of Cu-Zn-An-Al alloy, resulting in the strain-rate-hardening effect. As the deformation temperature increases, the shapes of the stress-strain curves are mainly influenced by the temperature-softening effect and the dynamic recrystallization of the α phase. Finally, material constants regarding the strain-rate-hardening and temperature-softening effects are determined, based on the Johnson-Cook constitutive model. The results show that compared with other metallic materials, the present Cu-Zn-Sn-Al alloy has a relatively stronger strain-rate-hardening effect and weaker temperature-softening effect.

High-Strain-Rate Response of a Specially-Made Copper Sample

In the present study, a systematic study on both the high strain-rate tensile and compressive deformation behaviors of a specially-made copper sample have been carried out at different high temperatures, by using the split Hopkinson bar experiments. The Johnson-Cook constitutive model was used to model the high strain-rate responses of the specimen at high temperatures. The results showed that compared with other metallic materials, the specially-made copper sample had a relatively stronger strain-rate-hardening effect and weaker temperature-softening effect. Evolution of the microstructure suggests that under high strain-rate, both the dislocation slip and deformation twins contribute to the plastic strengthening of the copper specimen, resulting in the strain-rate-hardening effect. And the dynamic recrystallization behavior plays an important role during the high strain-rate deformation process at the high temperatures.

Temperature and strain rate effects on the mechanical behavior of dual phase steel

The effect of temperature and strain rate on the mechanical behavior of a commercial dual phase steel (DP 800) has been investigated experimentally by uniaxial tensile tests in this study, covering temperatures (−60°C to 100°C) and strain rates (1×10–4 to 1×102s−1) encompassing conditions experienced in automotive crash situations. Yield and ultimate tensile strength, ductility, temperature effects and strain rate sensitivity have been determined and discussed. It was found that the Voce equation [σ=σs−(σs−σ0)exp(−εθ0/σs))] can be satisfactorily applied to describe the tensile flow curves by means of a modified Kocks–Mecking model. In this model the parameter θ0 is fixed, whereas both σ0 and σs consist of athermal and thermal stress components. The athermal component is only weakly dependent on temperature through the elastic shear modulus μ. The thermal stress component is governed by temperature and strain rate. Statistical analysis based on the experimental data has allowed all parameters in the Voce equation to be quantified.

Strain rate sensitivity and deformation activation volume of coarse-grained and ultrafine-grained TiNi alloys

Two TiNi alloys, Ti49.4Ni50.6 and Ti50Ni50, are subjected to equal-channel angular pressing (ECAP) resulting in the formation of a homogeneous ultra-fine grained microstructure. Tensile tests and strain rate jump tests are carried out in the temperature range of 25–400 °C to measure mechanical properties and strain rate sensitivity of both alloys before and after ECAP processing. Effect of grain size on mechanical behavior, strain rate sensitivity and mechanisms operating during plastic deformation of both alloys in the given temperature range is discussed.

Effects of cold-treatment and strain-rate on mechanical properties of NbTi/Cu superconducting composite wires.

During design and winding of superconducting magnets at room temperature, a pre-tension under different rate is always applied to improve the mechanical stability of the magnets. However, an inconsistency rises for superconductors usually being sensitive to strain and oversized pre-stress which results in degradation of the superconducting composites’ critical performance at low temperature. The present study focused on the effects of the cold-treatment and strain-rate of tension deformation on mechanical properties of NbTi/Cu superconducting composite wires. The samples were immersed in a liquid nitrogen (LN2) cryostat for the adiabatic cold-treatment, respectively with 18-hour, 20-hour, 22-hour and 24-hour. A universal testing machine was utilized for tension tests of the NbTi/Cu superconducting composite wires at room temperature; a small-scale extensometer was used to measure strain of samples with variable strain-rate. The strength, elongation at fracture and yield strength of pre-cold-treatment NbTi/Cu composite wires were drawn. It was shown that, the mechanical properties of the superconducting wires are linearly dependent on the holding time of cold-treatment at lower tensile strain-rate, while they exhibit notable nonlinear features at higher strain-rate. The cold-treatment in advance and the strain-rate of pre-tension demonstrate remarkable influences on the mechanical property of the superconducting composite wires.

Achieving superplasticity in a Bi-Sn alloy processed by high-pressure torsion

A Bi-Sn alloy was processed by high-pressure torsion (HPT) at room temperature for 10 turns to achieve a homogeneous fine microstructure. Microhardness observations showed that the refined microstructure experienced self-annealing during storage at room temperature. Following HPT processing and storage at room temperature for 35 days, a sample had excellent superplastic properties when testing at an initial tensile strain rate of 1.0 × 10-4 s-1. The enhanced superplastic flow behavior of the fine-grained Bi-Sn alloy was attributed to the homogeneously distributed Bi-Sn and Sn-Sn boundaries after HPT processing since these are preferred sites for grain boundary sliding.

On the Cold Drawing of Poly-Ether-Ether-Ketone (PEEK) Tubes

The mechanical behaviour commonly referred as cold drawing (CD) is a phenomenon that occurs once the yield point is reached, in a thermoplastic material subjected to a uniaxial tensile test at constant strain rate. In effect, after yielding the sample undergoes a neck in its cross-sectional area, which then propagates along the entire useful length of the sample while the applied force remains constant. This phenomenon occurs in amorphous or semi-crystalline materials in a wide range of temperatures below the glass transition temperature. Understanding this phenomenon involves knowing what deformation mechanisms operate and how they are related to the morphology of the material. In this work, on one hand, the mechanical response of poly-ether-ether-ketone (PEEK) thermally aged at 25 and 135°C for 30 day is measured. In previous researches PEEK aged at 130 and 135°C undergoes cold drawing when subjected to a tensile strain rate of 1,7 10-3 s-1 at 25°C. In addition, the average stress and the ultimate elongation of the plateau depend on the temperature and the duration of the annealing. On the other hand, wide angle X ray diffraction, density measurements, differential thermal analysis, shape memory analysis and complex impedance measurement allow determine changes in the amorphous and crystalline phases due to thermal aging and cold drawing effects.

Effect of S-phase on the Hydrogen Induced Phase Transition and Hydrogen Embrittlement Susceptibility in AISI 304 Stainless Steel

S-phase produced by low temperature plasma carburizing treatment was studied for the feasibility in reducing the susceptibility to hydrogen embrittlement (HE) of metastable AISI 304 stainless steel. It is demonstrated by slow strain-rate tensile (SSRT) tests that samples with S-phase coatings have higher resistance to HE comparing with untreated counterparts. Meanwhile, “in situ” EBSD investigations have shown that hydrogen induced phase transition (HIPT) is suppressed by S-phase coatings, which is beneficial to improving the resistance to HE.

The influence of temperatures and strain rates on the mechanical behavior of dual phase steel in different conditions

This study deals with the mechanical behavior of DP steel. A commercial dual phase steel (DP 800) was strained to 3.5% followed by annealing at 180°C for 30min to simulate the pressing of the plates and the paint-bake cycle involved in the manufacturing process of automobile body structures. The effect of temperature and strain rate on the mechanical behavior of this material was investigated by uniaxial tensile tests, covering applicable temperatures (−60°C to 100°C) and strain rates (1×10−4 to 1×102s−1) experienced in automotive crash situations. Yield and ultimate tensile strength, ductility, temperature effects and strain rate sensitivity as well as strain hardening rate have been determined and discussed.

Comparison of twinning evolution with work hardening ability in twinning-induced plasticity steel under different strain rates

Abstract For Fe–22Mn–0.6C (wt%) twinning-induced plasticity (TWIP) steel with grain sizes of 35 and 170 μm, both the ultimate tensile strength and uniform elongation decrease with increasing the tensile strain rates (from 10 −4 to 10 0 s −1 ). The density of twin boundary (DTB) is presumed to be the major factor influencing mechanical properties. Accordingly, a parameter 2/( t + s ) ( t and s represent the thickness and spacing of deformation twins), also the inverse of the mean free path for dislocation slip, is proposed to characterize the planar-oriented twinned structure in infinite grain size ( D » t ≈ s ). It also evaluates the effect of DTB on the work hardening ability and ductility of the TWIP steel.

Effect of High Tensile Strain Rate on the Evolution of Microstructure in Fe-Mn-C-Al Twinning-Induced Plasticity (TWIP) Steel

Fe-17.8Mn-0.52C-0.5Al TWIP steel has been investigated under high-strain rate conditions. Twinning along with stacking faults and high dislocation densities in the austenite matrix has been evaluated by X-ray diffraction line profile analysis and transmission electron microscopy. The samples strained at 100 s−1 show a gradient in the evolution of the dislocation density along the gage length except the fracture end where the density shows a decrease. In case of the samples strained at 1 s−1, the evolution of density shows attainment of a near-saturation stage. Electron backscatter diffraction analysis shows that the decrease in the dislocation density as well as near-saturation stage is due to dynamic recovery as well as dynamic recrystallization at region near the fracture end. The dynamically recrystallized grains are related to the deformed matrix through twin relationship.

Spiropyran as a Mechanochromic Probe in Dual Cross-Linked Elastomers

We study the mechanical activation of spiropyran (SP) in a doubly cross-linked polyurethane elastomer. Besides chemical cross-linking, the elastomer comprises polytetrahydrofuran as soft segments and hydrogen-bonding 2-ureido-4-pyrimidone (UPy) as hard segments. The material shows two color changes because of the ring-opening reaction of SP to merocyanine (MC) at strained state and the isomerization about the methane bridge of MC at relaxed state. Increasing tensile strain rate leads to stiffer and stronger elastomer as well as earlier activation of SP. The activation point of SP to MC always coincides well with strain hardening of the stress–elongation curves. We further use the two-color transitions of SP to study the fracture of the elastomer during crack propagation.

Effect of pre-strain on hydrogen embrittlement of high strength steels

The effect of the pre-strain on hydrogen embrittlement (HE) of high strength steels was investigated by slow strain-rate tensile (SSRT). The specimens were electrochemically hydrogen-charged in 0.5mol/L H2SO4 with 1g/L CH4N2S at room temperature for 24h. It is found that the amount of hydrogen increases linearly with the exponent of pre-strain, i.e., CH=11.963+0.00385exp(1.144εP). The HE susceptibility of the alloy initially decreases and then increases with increasing pre-strain. The ultimate tensile strength for the hydrogen-charged specimen is highest in the 3% pre-strain, which is interpreted by competition between strain hardening and Hydrogen Enhanced Decohesion (HEDE) mechanism. The fracture mode shows a mixed mode fracture of quasi-cleavage and intergranular fracture for hydrogen-charged specimens while the fracture mode exhibits dimples for hydrogen-free specimens.