High Strain Rate Compression Tests Research Articles

Strain rate dependence of anisotropic compression behavior in porous iron with unidirectional pores

The strain rate dependence of anisotropic compression behavior in porous iron with cylindrical pores oriented in one direction was investigated. Through high strain rate (˜103 s−1) compression tests along the orientation direction of pores using the split Hopkinson pressure bar method, it was shown that the stress–strain curve exhibits a unique plateau-stress region where deformation proceeds with almost no stress increase. The appearance of the plateau-stress region is related to the buckling deformation of the iron matrix and provides superior energy absorption. However, for the middle (˜10−1 s−1) and low strain rates (˜10−4 s−1), compression along the same direction produces no such plateau region. In fact, in contrast to compression in the parallel direction, compression perpendicular to the orientation direction of pores produces no plateau-stress regions in any of the three strain rates.

Tensile, High Strain Rate Compression and Microstructural Evaluation of Lightweight Age Hardenable Cast Fe-30Mn-9Al-XSi-0.9C-0.5Mo Steel

Age hardenable, castable, and lightweight Fe-Mn-Al-C steels are currently being developed and evaluated for substitution of high strength low alloy steel and to meet MIL-PRF-32269 criteria. Two nominal Fe-30Mn-9Al-0.9C-0.5Mo steels were cast and modified with 1 and 1.4 wt.% silicon. Ageing, tensile, and high strain rate compression testing were performed on solution treated and aged samples of both chemistries. Each alloy was solution treated at 1050°C for 2 hours. Microstructures of the solution treated and aged alloys show primary austenite with less than 8 volume % ferrite. The solution treated hardness of the low silicon steel was 230 BHN and the high silicon alloy was 225 BHN. Specimens were aged at 530°C for up to 60 hours. Peak ageing occurs after 30 hours with a peak hardness of 371 BHN for the 1% silicon containing alloy and 377 BHN for the high silicon alloy. Tensile strengths of the 30 hour aged specimens were 1065 MPa (154 ksi) and 1080 MPa (156 ksi) for the low and high silicon alloys. High strain rate compression testing was conducted on solution treated and 10 hour aged 1% Si containing alloy. Compressive strength of the 1 wt.% Si alloy exceeded 1,500 MPa (217 ksi) at a strain rate of 3000 s.−1.

Constitutive Modelling of Mechanical Behaviour of a Ti-Alloy, Applicable in Metal Cutting

In order to simulate manufacturing processes, it is essential to have accurate information about mechanical behaviour of material for different deformation conditions depending on the type of the process. In finite element (FE) analysis based techniques for simulation, a constitutive equation is needed to model the mechanical behaviour of material. In the case of metal cutting, the Johnson and Cook (JC) flow stress model is the most suitable constitutive equation to be used in simulation since it contains the effects of strain, strain-rate and temperature. It is needed to evaluate the parameters and constants of the JC model to make it applicable in FE simulations. There are several ways to evaluate the parameters of the equation: experimental such as high strain-rate compression tests called “Split Hopkinson Pressure Bar” which is relatively complicated and expensive technique requiring special testing apparatus; and analytical approach based on Oxley’s theory. An integral method containing quasi-static compression and machining tests have been used in this paper to evaluate the JC equation parameters by fitting data from both tests for a Ti-alloy (Ti6Al4V). Finally the estimated JC model is validated by some other machining tests.

Aging effects on hardness and dynamic compressive behavior of Ti–55Ni (at.%) alloy

Materials for impact applications require a unique combination of high hardness, high dynamic compressive strength, and reasonable toughness ( i.e., suitable ductility). This paper presents the aging effects on the microstructural evolution, hardness, and compressive high-strain rate behavior of Ni-rich Ti–55Ni (at.%) alloy. Aging heat-treatments of Ti–55Ni between 400 °C and 1000 °C led to the following precipitation sequence: Ti 3Ni 4 → T 2Ni 3 → TiNi 3. Among these, Ti 3Ni 4 and T 2Ni 3 were found to be metastable phases since they transformed to TiNi 3 phase during longer aging times or higher aging temperatures. Initial investigations were focused on precipitation studies to obtain a time–temperature–transformation (T–T–T) diagram for Ti–55Ni. High-strain rate compression tests and hardness studies indicated that significant strengthening by Ti 3Ni 4 and Ti 2Ni 3 precipitates was accompanied by very low failure strains, i.e., low ductility. However, precipitation of TiNi 3 during high-temperature aging led to a good balance between strength (1400–2000 MPa) and ductility (10–20%).

Experimental Studies on Concrete Filled Steel Tubes under High Strain Rate Loading

A split Hopkinson pressure bar equipment was used to perform high strain rate compressive tests on concrete filled steel tubes (CFT) along with benchmark plain concrete cylinders for comparison. This paper reports the results of dynamic compressive strength, ultimate strain, residual strength and the secant stiffness. The dynamic increase factor (DIF) of CFT strength upon the increase of strain rate was obtained based on the logarithm linear fitting of the test results. The DIF of the CFT specimens can be relatively well estimated with conservatism using the simple superposition of dynamic uniaxial strength of concrete and steel. The confinement stress and the confinement coefficient under high strain rate loading are also discussed based on assumptions similar to those used for the static loading cases.

Compressive uniaxial properties of auxetic open cell PU based foams

AbstractThe mechanical properties of negative Poisson's ratio foams are described under compressive loading both from a numerical and experimental point of view. Different samples of open cell PU (polyurethane) based auxetic foams (i.e. with negative Poisson's ratio) were manufactured following a dedicated manufacturing process, using different compression ratios and temperature profiles. High strain rate and quasi static compressive loading tests were performed in order to benchmark the numerical model. A numerical technique based on loading simulations of statistical assemblies of cells has been adapted to represent the uniaxial compressive behaviour of the auxetic foams. The results can be applied to predict the auxetic foam's behaviour and solving problems related to protection and packaging.

High strain rate behavior of ultrafine-grained Al–1.5 Mg

The high strain rate behavior of ultrafine-grained Al–1.5 Mg alloy produced by the equal channel angular pressing technique is investigated in this work. High strain rate compression tests were performed in a split Hopkinson pressure bar to achieve nominal strain rates of around 2000 s −1 at temperatures ranging from 300 to 523 K. In addition to un-interrupted Hopkinson bar tests, strain rate change tests were carried out using a flange and sleeve arrangement on the incident bar. The results obtained at the high strain rates were compared with those at low strain rates from an earlier study. It was seen that at the strain rate of 2000 s −1 the transition from hardening to softening (with respect to the coarse grained alloy), was shifted to higher temperatures. The strain rate sensitivity for the ultrafine-grained alloy was higher as compared to the coarse grained alloy for the entire range of testing. The increased strain rate sensitivity and the average subgrain size measured after testing at 473 and 523 K, suggest an active role of grain boundaries in the process of deformation. The possible reasons for softening at the low strain rates and its suppression at the high strain rates are discussed.

Temperature Rise in Polymeric Materials During High Rate Deformation

Many polymeric materials undergo substantial plastic strain prior to failure. Much of this post yield deformation is dissipative and, at high strain rates, will result in a substantial temperature rise in the material. In this paper, an infrared (IR) detector system is constructed to measure the rise in temperature of a polymer during high strain rate compression testing. Temperature measurements were made using a high-speed mercury-cadmium-telluride (HgCdTe) single-element photovoltaic detector sensitive in the mid-infrared spectrum (6–12μm), while mechanical deformation was accomplished in a split Hopkinson pressure bar (SHPB). Two representative polymers, an amorphous thermoplastic (polycarbonate (PC)) and a thermoset epoxy (EPON 862/W), were tested in uniaxial compression at strain rates greater than 1000s−1 while simultaneously measuring the specimen temperature as a function of strain. For comparison purposes, analogous measurements were conducted on these materials tested at a strain rate of 0.5s−1 on another test system. The data are further reduced to energy quantities revealing the dissipative versus storage character of the post yield work of deformation. The fraction of post yield work that is dissipative was found to be a strong function of strain for both polymers. Furthermore, a greater percentage of work is found to be dissipative at high rates of strain (>1000s−1) than at the lower rate of strain (0.5s−1) for both polymers; this is consistent with the need to overcome an additional energy barrier to yield at strain rates greater than 100s−1 in these two polymers. The highly cross-linked thermoset polymer was found to store a greater percentage of the post yield work of deformation than the physically entangled thermoplastic.

A study on mechanical properties of carbon/phenolic composites

Carbon/phenolic (C/PH) composites have been widely used for impact protection and energy-absorption, the proper design of which require appropriate material characterization data and stress–strain relations suitable for wide range of strain-rate conditions. Firstly, sophisticated quasi-static and medium–high strain-rate compression tests are conducted to study the mechanical behaviors of C/PH composites, and one nonlinear viscoelastic constitutive model is proposed based on the experimental results. Light gas gun tests are subsequently conducted to derive the impact Hugoniot curve and high-pressure equation of state (EOS). Finally, the proposed material model and EOS are built into a cross-format centered difference scheme to preliminarily investigate the wave effects of C/PH composites. It is established that the viscoelastic model, the equation of state along with the 1-D difference code are very effective in capturing the mechanical properties of C/PH composites over a wide range strain-rates.

The high strain rate response of PVC foams and end-grain balsa wood

The uniaxial compressive responses of two polymeric foams (Divinycell H100 and H250) and balsa wood (ProBalsa LD7) have been measured over a wide range of strain rates, ranging from 10 −4 s −1 to 4000 s −1. These materials are widely used as cores for composite sandwich structures. The high strain rate compression tests were performed using a Split-Hopkinson Pressure Bar made from AZM magnesium alloy, with semi-conductor strain gauges used to measure the low levels of stress in the specimens. The experimental data for compressive strength as a function of strain rate are adequately approximated by power-law fits. The compressive yield strength of the H250 PVC foam and balsa wood doubles when the strain rate is increased from quasi-static rates (10 −4 s −1) to rates on the order of 10 3 s −1. In contrast, the H100 PVC foam displays only a small elevation in uniaxial compressive strength (about 30%) for the same increase in strain rate.

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.

Surface hot-shortness of 1045 forging steel with residual copper

Hot-shortness is brittleness in metals during high temperature deformation. In this study, surface hot-shortness of 1045 steels with residual copper was investigated. Eight 1045 steels with differing copper contents, from 0.09 to 0.39% (by weight), were tested. High strain rate compression tests of pre-bulged samples were used to simulate forging deformation conditions. It was found that oxidation time and temperature are critical parameters for the control of hot-shortness. The testing was divided into two stages: stage one, in which the steels were oxidized at different temperatures from 1100 to 1200 °C for 10 and 30 min and subsequently deformed to determine the critical temperature where the surface cracking becomes severe; and stage two, in which the steels were oxidized for 1, 3, 5, and 7 min at their critical temperature, and then deformed. In the stage one testing, all steels oxidized for 10 min and subsequently deformed exhibited a critical temperature. The critical temperature decreased with decreasing copper content, from 1160 °C for the steel with the highest copper content to 1110 °C for the steel with the lowest copper content. A simple model based on copper enrichment and depletion along surface grain boundaries is presented to explain these observations. Steels oxidized for 30 min and subsequently deformed did not exhibit severe cracking at any temperature, but the steel with the highest copper content that was deformed at 1140 °C exhibited cracking. In stage two testing, the results were less consistent. The steels with high copper content (0.39–0.32%) exhibited maximum cracking at shorter times, while for the steels with medium copper content (0.30–0.21%) the maximum cracking occurred at longer times. No steel exhibited cracking when oxidized at 1200 °C and subsequently deformed. The study shows that steels have a critical temperature at which cracking is severe. Steels oxidized and deformed above the critical temperature did not exhibit hot-shortness surface cracking. Hence, a forging practice that both maintains and deforms the steel above the critical temperature could reduce or eliminate surface hot-shortness due to residual copper.

Stress Wave Propagation Effects in Two- and Three-Layered Composite Materials

Multilayer materials consisting of ceramic and glass/epoxy composites have been subjected to high strain rate compression testing using the Split Hopkinson Pressure Bar. The samples were extensively strain gaged so that dynamic data were generated directly from the samples during testing. Output data from the experiments were compared with numerical simulations of the same experiments and good agreement was noted. It was found that the stress distribution within samples was quite inhomogeneous and that stresses were highest in the region of the bar-sample interface. The presence of a rubber interlayer between the ceramic and glass/epoxy decreased the stress in both components but dramatically increased the degree of stress in homogeneity.

Determination of the plastic work converted to heat using radiometry

An improved calibration method for infrared radiometers is developed that has been shown to increase the accuracy of temperature measurement. To validate this new calibration technique, high strain rate compression tests are performed on the aluminum alloy BS 2011 and high strain rate torsion tests are performed on the titanium alloy 90% Ti-6%Al-4%V. The adiabatic temperature rise that occurs during these tests is measured using an infrared radiometer and validated, in the case of the compression tests, using fast response thermocouples. The proportion of plastic work converted to heat, β, is found to increase with plastic strain for both materials, which is similar to previous research. These results challenge the classical assumption that β has a constant value of approximately 0.95.

High-temperature deformation behaviour of Ti6Al4V alloy evaluated by high strain-rate compression tests

The high-temperature deformation behaviour of Ti6Al4V alloy has been investigated using the split Hopkinson bar. The specimens were deformed under a constant strain-rate of 2×10 3 s −1 at an initial temperature varying from 700–1100°C at intervals of 100°C. In order to compare the effect of temperature on the flow characteristics, room-temperature tests were also performed to determine the basic mechanical properties. Parallel with this work, the fracture features and microstructures of the deformed specimens were studied by means of different microscopy techniques to understand the formation of adiabatic shear bands and the variations of dislocation features. In addition, for the purpose of describing the flow behaviour of this material in terms of strain, strain rate and temperature, a deformation constitutive equation is proposed. The present results demonstrate that temperature has a significant effect on the flow behaviour of the material. The strength of the material and the work-hardening coefficient decrease rapidly with an increase in temperature. Furthermore, the proposed constitutive equation describes and predicts excellently the observed flow response. From the fracture analysis localized adiabatic shearing forming due to impact is found to be favoured with Ti6Al4V alloy and increases with increasing temperature of deformation. Microstructural observations show that dislocation cells form under all conditions, and that the dislocation density and cell size vary with the temperature. A quantitative relationship between the flow stress and observed dislocation structure is characterized.

High Strain-Rate Compression Tests on Ice

High-speed uniaxial compression tests were conducted in the strain-rate range 10-1−101 s-1, at −11 °C, on freshwater ice and “Baltic” sea ice of mean salinity 2.4 ± 0.7 ppt. Two different testing m...

High strain-rate compression testing of a short-fiber reinforced aluminum composite

Compression behavior of 15–26 V f% Saffil ™ short-fiber reinforced Al-1.17wt.%Cu alloy metal matrix composites has been determined over a strain-rate range of approximately 10 −4 to 2×10 3 s −1. The strain-rate sensitivity of composite samples at 4% strain, tested parallel and normal to the plane of reinforcement, was found to be higher than that of unreinforced alloy in the strain-rate range studied. Quantitative analysis of fiber fragment lengths from samples tested to different strain levels showed that, at small strains, high strain-rate testing induced a relatively shorter fiber fragment length distribution in the composite compared to quasi-static testing. At quasi-static strain rates, the fiber strengthening effect was found to increase with increasing V f% and was higher in samples tested parallel to the planar random array. The observed anisotropy of the composite at quasi-static strain rates was also observed to continue into the high strain-rate regime. Microscopic observations on composite samples tested quasi-statically and dynamically to a range of strains showed that the major damage process involved during compression testing was fiber breakage followed by the microcracking of the matrix at relatively large strains. Fiber breakage modes were found to be mostly shearing and buckling.

Thermomechanical Properties of Polymers at High Rates of Strain

The objective of this project was to develop a suitable testing technique which will allow the measurement of the bulk surface temperature of a specimen during high strain-rate compression testing. A fast response infra-red radiometer was developed which has the capability of measuring bulk surface temperature changes with an estimated accuracy of ±2°C and with a response time of 1.4μs. The radiometer is capable of measuring temperature changes above 20°C, when the signal-to-noise ratio becomes greater than 1. The properties of polymeric materials are extremely temperature sensitive, particularly when the glass transition temperature is approached. Models such as Eyring predict the yield of a polymer at constant temperature; the yield stress being unaffected by temperature rises. Temperature as well as mechanical data will allow models to be revised to include strain and dynamic temperature terms to predict the flow under adiabatic conditions. Strain-rates of ca. 2500s -1 were achieved when testing specimens and this rate was obtained using a split Hopkinson pressure bar. A substantial number of preliminary tests were conducted in order to obtain a suitable specimen size which was then used in the temperature measurement process. Quasistatic, intermediate and high strain-rate tests were performed; the last utilised the radiometer for temperature measurement. An Eyring plot was constructed from which fundamental values for activation volumes and enthalpies were obtained. Full descriptions of the testing techniques used have been included and a brief photoelastic analysis has been carried out on a partially deformed specimen which shows molecular alignment.

Dynamic deformation of shock prestrained copper

Measurements of yield stress and strain hardening behavior at strain rates in the range 10 −3–10 4s −1 are reported for copper that has been previously shock deformed at a shock pressure of 10 GPa. Characterization of the mechanical properties and microstructure, using transmission electron microscopy, showed that the hardening induced by the shock wave is equivalent to that achieved when copper is deformed at low strain rates to strains between 20% and 30%. Deformation in the shock-deformed material is analyzed using the internal-state-variable model applied by Follansbee and Kocks. The model adequately describes the stress levels, the strain hardening rate, and the dependence of the flow stress on strain rate measured for high strain rate compression tests on the shock-deformed material.

快削鋼の被削性解析 （第１報） 低合金硫黄快削鋼の摩擦特性と切削機構との関連性

A split tool dynamometer was employed to investigate friction characteristics at the tool-chip interface. Compared to a standard steel, a calcium treated, resulfurized steel shows lower frictional stress near the chip leaving point and shorter chip contact length. The friction on the rake face can be expressed by a characteristic equation which is similar to that proposed by Shirakashi and Usui.On the other hand, high strain rate compression tests at elevated temperatures were carried out using the Hopkinson bar method to evaluate flow characteristics of the steels. On the basis of the empirical equations thus obtained, a simulation analysis was conducted to examine a correlation between the material characteristics and cutting mechanism. It shows that the difference in friction characteristics mainly causes the chip flow, temperature and cutting forces to change : the free-machining steel brings better machinability, though its flow properties are almost the same as those of the standard steel.