Texture evolution and dynamic mechanical behavior of cast AZ magnesium alloys under high strain rate compressive loading

In this study, texture and compressive mechanical behavior of three cast magnesium alloys, including AZ31, AZ61 and AZ91, were examined over a range of strain rates between 1000 and 1400 s⁻¹ using Split Hopkinson Pressure Bar. Texture measurements showed that after shock loading, initial weak texture of the cast samples transformed to a relatively strong (00.2) basal texture that can be ascribed to deformation by twinning. Furthermore, increasing the aluminum content in the alloys resulted in increase in the volume fraction of β-Mg₁₇Al₁₂ and Al₄Mn phases, strength and strain hardening but ductility decreased at all strain rates. Besides, it was found for each alloy that the tensile strength and total ductility increased with strain rate. By increasing the strain rate, the maximum value of strain hardening rate occurred at higher strains. Also, it is suggested that a combination of twinning and second phase formation would affect the hardening behavior of the cast AZ magnesium alloys studied in this research.     

Fatigue-creep interaction performance of Incoloy 825 nickel-based superalloy at 650 °C

The fatigue-creep interaction performance of Incoloy 825 nickel-based superalloy at 650 °C was investigated through introducing the tensile, compressive, and tensile-compressive strain hold time at the controlled total strain amplitude Δϵ_t = 0.3 %∼0.7 %. The results show that the Incoloy 825 nickel-based superalloy exhibits the cyclic hardening behavior, the cyclic hardening behavior followed by cyclic softening behavior and the cyclic hardening behavior followed by cyclic stability during the cyclic deformation with tensile strain hold time, while the alloy exhibits the cyclic hardening behavior and the cyclic hardening behavior followed by the cyclic stability during the cyclic deformation with compressive and tensile-compressive strain hold time. The relationship between both plastic and elastic strain amplitudes with reversals to failure for the alloy shows a single slope linear behavior, which can be described by the Coffin-Manson and Basquin equations, respectively. The deformation mechanism of the alloy under three loading condition of fatigue-creep interaction is mainly the planar slip. In addition, under three loading condition of fatigue-creep interaction, the cracks initiate and propagate in the transgranular mode.     

Particle stabilization of plastic flow in nanostructured Al-1 %Si Alloy

A nanostructured Al-1 %Si alloy containing a dispersion of Si particles in ultrapure aluminum (99.9996 %) was produced by heavy cold rolling to study the effect of second-phase particles on the occurrence of plastic instability during tensile testing of a nanostructured metal. Tensile tests were conducted on the as-deformed sample and on samples after recovery annealing treatments. Work hardening and strain rate sensitivity were studied by tensile test at different strain rates, and deformed and annealed samples were characterized by transmission electron microscopy. By comparing the observed behavior with that of nanostructured commercial purity Al, a significant increase in the tensile elongation was found. This is related to a homogenization of the deformation microstructure due to the presence of finely dispersed Si particles. As a result localized deformation is reduced and tensile ductility increased.     

The uniaxial stress versus strain response of pig skin and silicone rubber at low and high strain rates

The uniaxial compressive responses of silicone rubber (B452 and Sil8800) and pig skin have been measured over a wide range of strain rates (0.004–4000 s⁻¹). The uniaxial tensile response of the silicone rubbers was also measured at low strain rates. The high strain rate compression tests were performed using a split-Hopkinson pressure bar made from AZM magnesium alloy. High gain semi-conductor strain gauges were used to detect the low levels of stress (1–10 MPa), and a pulse shaper increased the rise time of dynamic loading on the specimen. The experiments reveal that pig skin strain hardens more rapidly than silicone rubbers and has a greater strain rate sensitivity: pig skin stiffens and strengthens with increasing strain rate over the full range explored, whereas silicone rubber stiffens and strengthens at strain rates in excess of 40 s⁻¹. A one term Ogden strain energy density function adequately describes the measured constitutive response of each solid, and a strategy is outlined for determining the associated material constants (strain hardening exponent and a shear modulus). The strain rate sensitivities of the pig skin and two silicone rubbers are each quantified by an increase in the shear modulus with increasing strain rate, with no attendant change in the strain hardening exponent. It is shown that the Mooney-Rivlin model is unable to describe the strong strain hardening capacity of these rubber-like solids.     

Tensile and work hardening properties of low carbon dual phase strip steels at high strain rates

Abstract

As a result of their unique combination of strength and ductility dual phase steels play an important role in reducing weight in automobile components and improving crashworthiness. The purpose of this paper is to quantify the crash performance of dual phase steels, as defined by the influence of low and high strain deformation rates (0·001 s^-1 and 100 s^-1 respectively), on the tensile and work hardening properties of a range of commercial dual phase products. The objective is to establish whether dual phase steels maintain their desirable mechanical property characteristics of low yield strength, high tensile strength and high work hardening rates during plastic deformation under the application of a high strain rate loading. The results confirmed that the yield/proof strength and tensile strength increased with increasing volume fraction of second phase constituents and increasing strain rate. In particular, a dual phase steel with a microstructure consisting of a significant volume fraction (>10–15%) of additional second phase material (bainite) is shown to display superior energy absorption properties. However, this is accompanied by poor ductility and work hardening characteristics.     

The variation of strain rate sensitivity exponent and strain hardening exponent in isothermal compression of Ti–6Al–4V alloy

The deformation behavior in isothermal compression of Ti–6Al–4V alloy is investigated in the deformation temperatures ranging from 1093 K to 1303 K, the strain rates ranging from 0.001 s⁻¹ to 10.0 s⁻¹ at an interval of an order magnitude and the height reductions ranging from 20% to 60% at an interval of 10%. Based on the experimental results in isothermal compression of Ti–6Al–4V alloy, the effect of processing parameters and grain size of primary α phase on the strain rate sensitivity exponent m and the strain hardening exponent n is in depth analyzed. The strain rate sensitivity exponent m at a strain of 0.7 and strain rate of 0.001 s⁻¹ firstly tends to increase with the increasing of deformation temperature, and maximum m value is obtained at deformation temperature close to the beta-transus temperature, while at higher deformation temperature it drops to the smaller values. Moreover, the strain rate sensitivity exponent m decreases with the increasing of strain rate at the deformation temperatures below 1253 K, but the m values become maximal at a strain rate of 0.01 s⁻¹ and the deformation temperature above 1253 K. The strain rate affects the variation of strain rate sensitivity exponent with strain. Those phenomena can be explained reasonably based on the microstructural evolution. On the other hand, the strain hardening exponent n depends strongly on the strain rate at the strains of 0.5 and 0.7. The strain affects significantly the strain hardening exponent n due to the variation of grain size of primary α phase with strain, and the competition between thermal softening and work hardening.     

An approximate method of determination of maximum strain hardening levels in metals under nonproportional low-cycle loading

Using the findings of analysis of deformation curves for metallic materials under static and cyclic loading, an approximate method is put forward for the determination of maximum strain hardening levels in the case of non-proportional low-cycle loading with strain monitoring. Based on the correlation between strain hardening data obtained from the static and proportional and nonproportional cyclic deformation curves, an approximate analytical relationship is built up which allows for predicting maximum strain hardening levels under nonproportional low-cycle loading. __________ Translated from Problemy Prochnosti, No. 2, pp. 29–38, March–April, 2006.     

Fracture energy of UHP-FRC under direct tensile loading applied at low strain rates

This research investigates the fracture energy of ultra-high performance fiber reinforced concretes (UHP-FRC) under direct tensile loading applied at relatively low strain rates. Nine UHP-FRC series incorporating three types of steel fibers (straight, end-hooked, and twisted fibers), each in three different fiber volume fractions, are tested under uniaxial tensile loading at four different strain rates, ranging from 0.0001 s⁻¹ to 0.1 s⁻¹. Particular attention is given to clearly distinguish between the dissipated energy during the strain hardening and softening portions of the loading regime. The test results show that: 1) the fracture energy is mainly influenced by a parameter, termed fiber factor, which is a function of the fiber volume fraction and slenderness, and 2) all three types of UHP-FRCs exhibit increases in fracture energy with increasing strain rates. The observed strain rate sensitivity of the fracture energy suggests it is likely associated with the strain sensitive micro-cracking that occurs during fiber pull-out.     

Cyclic strain rate effect on martensitic transformation and fatigue behaviour of an austenitic stainless steel

In this study, the effect of strain rate on the cyclic behaviour of 304L stainless steel is investigated to unveil the complex interrelationship between martensitic phase transformation, secondary hardening, cyclic deformation and fatigue behaviour of this alloy. A series of uniaxial strain controlled fatigue tests with varying cyclic strain rates were conducted at zero and non‐zero mean strain conditions. Secondary hardening was found to be closely related to the volume fraction of strain‐induced martensite which was affected by adiabatic heating due to increasing cyclic strain rates. Tests with lower secondary hardening rates maintained lower stress amplitudes during cyclic loading which resulted in longer fatigue lives for similar strain amplitudes. Fatigue resistance of 304L stainless steel was found to be more sensitive to changes in strain rate than the presence of mean strain. The mean strain effect was minimal due to the significant mean stress relaxation in this material.     

Compressive properties of nanoparticle modified epoxy resin at different strain rates

Epoxy resins often exhibit high strength yet are often brittle, especially at high strain rates. Block copolymer modified epoxy resins have generated significant interest since it was demonstrated that the combination could lead to nanostructured thermosets through the self-assembly of the block copolymer. Such nanostructured epoxies exhibit increased ductility without the significant loss in yield strength exhibited by traditional rubber-modified epoxies. In this study, the effect of different nanoscale additives on the compressive yield strength of a model epoxy resin has been studied. In the first case, a block copolymer styrene-b-butadiene-b-polymethylmethacrylate (SBM) was added to the model epoxy resin. In the second case, carbon nanotubes (CNTs) were added. In the final case, both additives were mixed simultaneously with the epoxy resin. The compressive mechanical behavior of these materials has been investigated over a wide range of strain rates (0.001–3500 s⁻¹). The yield behavior was found to fit the cooperative yield model proposed by Fotheringham and Cherry.     

Impact behavior and constitutive model of nanocrystalline Ni under high strain rate loading

To investigate mechanical properties and deformation mechanisms of nanocrystalline materials under high strain rate, dynamic impact tests for nanocrystalline Ni bulk prepared by high-energy ball milling combined with compaction and hot-pressure sintering were carried out under different high strain rates on Split Hopkinson bar. Compared with the testing results under quasi-static strain rate, the nanocrystalline Ni has higher strength under high strain rate. Meanwhile, the impact stress–strain curves exhibit rate-dependence strength and light strain hardening behavior. Subsequently, a mechanism of dislocation gliding in combination of grain boundary sliding was discussed and a constitutive model was built under high strain rate loading based on the mechanism. The predictions of the constitutive model under high strain rates show good agreements with the experimental data. Finally, the properties of the nanocrystalline Ni were discussed in detail.     

Work hardening associated with ?-martensitic transformation, deformation twinning and dynamic strain aging in Fe-17Mn-0.6C and Fe-17Mn-0.8C TWIP steels

The tensile properties of carbon-containing twinning induced plasticity (TWIP) steels and their temperature dependence were investigated. Two steels with carbon concentrations of 0.6% and 0.8% (w/w) were tensile-tested at 173, 223, 273, 294, and 373 K. Three deformation modes were observed during tensile testing: ?-martensitic transformation, deformation twinning, and dynamic strain aging. The characteristic deformation mode that contributed to the work hardening rates changed with the deformation temperature and chemical compositions. The work hardening rate in the carbon-containing TWIP steels increased according to the deformation modes in the following order: ?-martensitic transformation > deformation twinning > dynamic strain aging.     

The effects of aging treatment and strain rates on damage evolution in AA 6061 aluminum alloy in compression

AA 6061 aluminum alloy in T4, T6 and T8 temper were subjected to quasi-static compressive loading at a strain-rate of 3.2 × 10⁻³ s⁻¹ and dynamic compressive loading at strain-rates between 7.0 × 10³ and 8.5 × 10³ s⁻¹. The effects of strain rates and temper condition on the deformation behavior of the alloy are discussed. Under the quasi-static loading, deformation was relatively homogeneous and controlled by strain hardening, which is more pronounced in the naturally aged than the artificially aged alloys. Thermal softening played a dominant role under impact loading leading to strain localization along narrow bands called adiabatic shear bands (ASBs). Both deformed bands consisting of aligned second phase particles and transformed bands consisting of fine recrystallized grains were observed. The average size of the recrystallized grains in the transformed bands is about 600 nm and varies slightly depending on the temper condition. The fine grains are suggested to form by dynamic recrystallization. The T4 alloy showed the highest propensity for thermal softening, strain localization and cracking under impact loading while the T8 alloy showed the least tendency. The degree of recrystallization in the transformed band is influenced by temper condition with T8 alloy having the highest fraction of unrecrystallized grains inside the transformed bands. This is related to the temperature rise in the transformed bands that was estimated to be highest in the T4 alloy and lowest in the T8 alloy. The combined effects of high temperature and severe strain inside the transformed bands caused dissolution of second phase particles and induced microstructural changes that resulted in less silicon inside the transformed bands than in the adjacent region.     

Superelasticity,energy dissipation and strain hardening of vimentin coiled-coil intermediate filaments: atomistic and continuum studies

Vimentin coiled-coil alpha-helical dimers are elementary protein building blocks of intermediate filaments, an important component of the cell’s cytoskeleton that has been shown to control the large-deformation behavior of eukaryotic cells. Here we use a combination of atomistic simulation and continuum theory to model tensile and bending deformation of single alpha-helices as well as coiled-coil double helices of the 2B segment of the vimentin dimer. We find that vimentin dimers can be extended to tensile strains up to 100% at forces below 50 pN, until strain hardening sets in with rapidly rising forces, approaching 8 nN at 200% strain. We systematically explore the differences between single alpha-helical structures and coiled-coil superhelical structures. Based on atomistic simulation, we discover a transition in deformation mechanism under varying pulling rates, resulting in different strength criteria for the unfolding force. Based on an extension of Bell’s theory that describes the dependence of the mechanical unfolding force on the pulling rate, we develop a fully atomistically informed continuum model of the mechanical properties of vimentin coiled-coils that is capable of predicting its nanomechanical behavior over a wide range of deformation rates that include experimental conditions. This model enables us to describe the mechanics of cyclic stretching experiments, suggesting a hysteresis in the force–strain response, leading to energy dissipation as the protein undergoes repeated tensile loading. We find that the dissipated energy increases continuously with increasing pulling rate. Our atomistic and continuum results help to interpret experimental studies that have provided evidence for the significnificance of vimentin intermediate filaments for the large-deformation regime of eukaryotic cells. We conclude that vimentin dimers are superelastic, highly dissipative protein assemblies.     

应变率突增对混凝土动态拉伸破坏影响的细观模拟

实际工程中混凝土结构往往遭遇多次动态荷载作用或在承受一定初始损伤荷载基础上再承受不同应变率的动态荷载.建立了哑铃型混凝土三维细观数值模型,模拟不同名义应变率单独作用下混凝土材料的单轴动态拉伸破坏行为,又分别对混凝土单轴拉伸应力应变曲线上升段和软化段的应变率突增行为开展了细观模拟,初步分析了应变率突增行为对动态拉伸破坏强...     

Modeling high-temperature tensile deformation behavior of AZ31B magnesium alloy considering strain effects

The hot tensile deformation behaviors of AZ31B magnesium alloy are investigated over wide ranges of forming temperature and strain rate. Considering the effects of strain on material constants, a comprehensive constitutive model is applied to describe the relationships of flow stress, strain rate and forming temperature for AZ31B magnesium alloy. The results show that: (1) The effects of forming temperature and strain rate on the flow behaviors of AZ31B magnesium alloy are significant. The true stress–true strain curves exhibit a peak stress at small strains, after which the flow stress decreases until large strain, showing an obvious dynamic softening behavior. A considerable strain hardening stage with a uniform macroscopic deformation appears under the temperatures of 523 and 573 K. The strain hardening exponent (n) increases with the increase of strain rate or the decrease of forming temperature. There are not obvious strain-hardening stages when the forming temperature is relatively high, which indicates that the dynamic recrystallization (DRX) occurs under the high forming temperature, and the balance of strain hardening and DRX softening is easy to obtain. (2) The predicted stress–strain values by the established model well agree with experimental results, which confirm that the established constitutive equation can give an accurate and precise estimate of the flow stress for AZ31B magnesium alloy.     

Cyclic hardening and fatigue behavior of stainless steel 304L

This article discusses cyclic hardening and fatigue behaviors of stainless steel 304L, the behavior of which is greatly influenced by prior loading. Effects of loading sequence, mean strain and mean stress, and pre-straining (PS) were investigated using constant amplitude as well as step and random loading tests. Contrary to common expectations, fatigue lives in strain-controlled mean strain tests were significantly affected by the mean strain, in spite of mean stress relaxation. PS induced considerable hardening and led to different results on fatigue life, depending on the test control mode. Secondary hardening was observed in some tests, characterized by a continuous increase in the stress response. Possible mechanisms for this behavior are also discussed. To correlate fatigue life data of a material such as stainless steel with strong deformation history effect, it is shown that a damage parameter with both stress and strain is required. The Fatemi–Socie (FS) parameter as such a parameter is shown to correlate the data under different control modes and loading conditions.     

Effect of strain rate on quasistatic tensile flow behaviour of solution annealed 304 austenitic stainless steel at room temperature

Room temperature tensile test results of solution annealed 304 stainless steel at strain rates ranging between 5 × 10⁻⁴ and 1 × 10⁻¹ s⁻¹ reveal that with increase in strain rate yield strength increases and tensile strength decreases, both maintaining power–law relationships with strain rate. The decrease in tensile strength with increasing strain rate is attributed to the lesser amount of deformation-induced martensite formation and greater role of thermal softening over work hardening at higher strain rates. Tensile deformation of the steel is found to occur in three stages. The deformation transition strains are found to depend on strain rate in such a manner that Stage-I deformation (planar slip) is favoured at lower strain rate. A continuously decreasing linear function of strain rate sensitivity with true strain has been observed. Reasonably good estimation for the stress exponent relating dislocation velocity and stress has been made. The linear plot of reciprocal of strain rate sensitivity with true strain suggests that after some critical amount of deformation the increased dislocation density in austenite due to the formation of some critical amount of deformation-induced martensite plays important role in carrying out the imposed strain rate.     

Failure of AA 6061 and 2099 aluminum alloys under dynamic shock loading

Microstructural aspects of the deformation and failure of AA 6061 and AA 2099 aluminum alloys under dynamic impact loading are investigated and compared with their responses to quasi-static mechanical loading in compression. Cylindrical specimens of the alloys, heat-treated to T4, T6 and T8 tempers, were subjected to dynamic compressive loading at strain rates of between 2800 and 9200 s⁻¹ and quasi-static compressive loading at a strain rate of 0.0032 s⁻¹. Plastic deformation under the dynamic impact loading is dominated by thermal softening leading to formation of adiabatic shear bands. Both deformed and transformed shear bands were observed in the two alloys. The shear bands offer preferential crack initiation site and crack propagation path in the alloys during impact loading leading to ductile shear fracture. While cracks propagate along the central region of transformed bands in AA 6061 alloy, the AA 2099 alloy failed by cracks that propagate preferentially along the boundary region between the transformed shear bands and the bulk material. Whereas the AA 2099 alloy shows the greatest propensity for adiabatic shear banding and failure in the T8 temper condition, AA 6061 alloy is most susceptible to formation of adiabatic shear bands and failure in the T4 temper. Deformation under quasi-static loading is dominated by strain hardening in the two alloys. Rate of strain hardening is higher for naturally aged AA 6061 than the artificially aged alloy, while the strain hardening rate for the AA 2099 alloy is independent of the temper condition. The AA 2099 alloy shows a superior mechanical behaviour under quasi-static compressive loading whereas the AA 6061 shows a higher resistance to impact damage.     

The compressive behavior of isocyanate-crosslinked silica aerogel at high strain rates

Aerogels are low-density, highly nano-porous materials. Their engineering applications are limited due to their brittleness and hydrophilicity. Recently, a strong lightweight crosslinked silica aerogel has been developed by encapsulating the skeletal framework of amine-modified silica aerogels with polyureas derived by isocyanate. The mesoporous structure of the underlying silica framework is preserved through conformal polymer coating, and the thermal conductivity remains low. Characterization has been conducted on the thermal, physical properties and the mechanical properties under quasi-static loading conditions. In this paper, we present results on the dynamic compressive behavior of the crosslinked silica aerogel (CSA) using a split Hopkinson pressure bar (SHPB). A new tubing pulse shaper was employed to help reach the dynamic stress equilibrium and constant strain rate. The stress-strain relationship was determined at high strain rates within 114–4386 s⁻¹. The effects of strain rate, density, specimen thickness and water absorption on the dynamic behavior of the CSA were investigated through a series of dynamic experiments. The Young’s moduli (or 0.2% offset compressive yield strengths) at a strain rate ∼350 s⁻¹ were determined as 10.96/2.08, 159.5/6.75, 192.2/7.68, 304.6/11.46, 407.0/20.91 and 640.5/30.47 MPa for CSA with densities 0.205, 0.454, 0.492, 0.551, 0.628 and 0.731 g cm⁻³, respectively. The deformation and failure behaviors of a native silica aerogel with density (0.472 g cm⁻³), approximately the same as a typical CSA sample were observed with a high speed digital camera. Digital image correlation technique was used to determine the surface strains through a series of images acquired using high speed photography. The relative uniform axial deformation indicated that localized compaction did not occur at a compressive strain level of ∼17%, suggesting most likely failure mechanism at high strain rate to be different from that under quasi-static loading condition. The Poisson’s ratio was determined to be 0.162 in nonlinear regime under high strain rates. CSA samples failed generally by splitting, but were much more ductile than native silica aerogels.