Design of an Impact Loading Machine Based on a Flywheel Device: Application to the Fatigue Resistance of the High Rate Pre-straining Sensitivity of Aluminium Alloys

This paper presents a device that has been designed for tensile loading at medium impact rates (up to 10³ s^–1) and for performing either interrupted or failure tests. This machine allows us to apply prescribed pre-straining to the specimen, and then apply subsequent loading histories such as impact fatigue. Two specimen loading systems are considered, which make it possible to carry out tests with various ranges of force and various durations of time. A multi-CCD camera system is triggered by a chosen threshold from the force signal. The system is dedicated to the displacement measurement and gives both qualitative and quantitative information about the stretching mechanism leading to fracture. To illustrate the performance of the device, experimental results concerning impact tensile tests at a strain rate of about 300 s^–1 are presented, as well as consecutive impact-fatigue tests on two aluminium alloys.     

A study of localisation in dual-phase high-strength steels under dynamic loading using digital image correlation and FE analysis

Tensile tests were conducted on dual-phase high-strength steel in a Split-Hopkinson Tension Bar at a strain-rate in the range of 150–600/s and in a servo-hydraulic testing machine at a strain-rate between 10^?3 and 10⁰/s. A novel specimen design was utilized for the Hopkinson bar tests of this sheet material. Digital image correlation was used together with high-speed photography to study strain localisation in the tensile specimens at high rates of strain. By using digital image correlation, it is possible to obtain in-plane displacement and strain fields during non-uniform deformation of the gauge section, and accordingly the strains associated with diffuse and localised necking may be determined. The full-field measurements in high strain-rate tests reveal that strain localisation started even before the maximum load was attained in the specimen. An elasto-viscoplastic constitutive model is used to predict the observed stress–strain behaviour and strain localisation for the dual-phase steel. Numerical simulations of dynamic tensile tests were performed using the non-linear explicit FE code LS-DYNA. Simulations were done with shell (plane stress) and brick elements. Good correlation between experiments and numerical predictions was achieved, in terms of engineering stress–strain behaviour, deformed geometry and strain fields. However, mesh density plays a role in the localisation of deformation in numerical simulations, particularly for the shell element analysis.     

Techniques for measuring stress-strain relations at high strain rates

The qualitative dependence of the mechanical behavior of some materials on strain rate is now well known. But the quantitative relation between stress, strain and strain rate has been established for only a few materials and for only a limited range. This relation, the so-called constitutive equation, must be known before plasticity or plastic-wave-propagation theory can be used to predict the stress or strain distribution in parts subjected to impact stresses above the yield strength. In this paper, a brief review of some of the experimental techniques for measuring the stress, strain, strain-rate relationship is given, and some of the difficulties and shortcomings pointed out. Ordinary creep or tensile tests can be used at plastic-strain rates from 10^?8 to about 10^?1/sec. Special quasi-static tests, in which the stress- and strain-measuring devices as well as the specimen geometry and support have been optimized, are capable of giving accurate results to strain rates of about 10²/sec. At higher strain rates, it is shown that wave-propagation effects must be included in the design and analysis of the experiments. Special testing machines for measuring stress, strain and strain-rate relationships in compression, tension and shear at strain rates up to 10⁵/sec are described, and some of the results presented. With this type of testing machine, the analysis of the data requires certain assumptions whose validity depends upon proper design of the equipment. A critical evaluation of the accuracy of these types of tests is presented.     

Evaluation of the Material Properties of an OFHC Copper Film at High Strain Rates Using a Micro-Testing Machine

The material properties of an oxygen-free high thermal conductivity (OFHC) film with a thickness of 0.1 mm were evaluated at strain rates ranging from 10⁻³/s to 10³/s using a high-speed material micro-testing machine (HSMMTM). The high strain-rate material properties of thin films are important especially for an evaluation of the structural reliability of micro-formed parts and MEMS products. The high strain-rate material testing methods of thin films, however, have yet to be established to the point that the testing methods of larger specimens for electronics, auto-body, train, ship, and ocean structures are. For evaluation, a new type of HSMMTM was developed to conduct high-speed tensile tests of thin films. This machine is capable of testing at a sufficiently high tensile speed with an electromagnetic actuator, a novel gripping mechanism, and an accurate load measurement system. The OFHC copper film shows high strain-rate sensitivity in terms of the flow stress, fracture elongation, and strain hardening. These measures increase as the tensile strain rate increases. The rate-dependent material properties of an OFHC copper film are also compared with those of a bulk OFHC copper sheet with a thickness of 1 mm. The flow stress of an OFHC copper film is relatively low compared to that of a bulk OFHC copper sheet in the entire range of strain rates, while the fracture elongation of an OFHC copper film is much larger than that of a bulk OFHC copper sheet. A quantitative comparison would provide material data at high strain rates for the design and analysis of micro-appliances and different types of micro-equipment.     

A Combined Theoretical/Experimental Approach for Reducing Ringing Artifacts in Low Dynamic Testing with Servo-hydraulic Load Frames

One of the most challenging tasks facing computer-aided engineering (CAE) analysis is the acquisition of accurate tensile test data that spans quasi-static to low dynamic (10^?5/s?≤ $ \overset{.}{\varepsilon } $ ≤5?×?10²/s) strain rates ( $ \overset{.}{\varepsilon } $ ). Critical to the accuracy of data acquired over the low dynamic range is the reduction of ringing artifacts in flow data. Ringing artifacts, which are a consequence of the inertial response of the load frame, are spurious oscillations that can obscure the desired material response (i.e. load vs. time or load vs. displacement) from which flow data are derived. These oscillations tend to grow with increasing strain rate and peak at the high end of the low dynamic range on servo-hydraulic tensile test frames. Common practices for addressing ringing are data filtering, which is often problematic since filtering introduces distortion in smoothed material data, or trial-and-error design of test specimen geometries. This renders techniques for reducing ringing based upon the mechanics of the load frame and optimization of tensile specimen geometry quite attractive. In the present paper, relationships between load, stress wave propagation, and specimen geometries are addressed, to both quantify ringing and to develop specimen designs that will reduce ringing. A combined theoretical/experimental approach for tensile specimen design was developed for reducing ringing in flow data over the low dynamic range of strain rates (10^?5/s≤ $ \overset{.}{\varepsilon } $ ≤5?×?10²/s). The single camera digital image correlation (DIC) method was used to measure the displacement fields and strain rates with specimens resulting from the combined theoretical/experimental approach. While the approach was developed on a specific commercial load frame with a TRIP steel subject to a two-step quenching and partitioning heat treatment (Q&P980), it is readily adaptable to other servo-hydraulic load frames and metallic alloys. The developed approach results in a 90 % reduction in ringing artifact (with no filtering) in a tensile flow curve for Q&P980 at $ \overset{.}{\varepsilon}\kern-4pt $ = 5?×?10²/s. Results from split Hopkinson bar tests of Q&P980 were performed at $ \overset{.}{\varepsilon } $ = 500/s and compare favorably with the test data generated by the developed testing approach. Since the Q&P980 steel represents a new generation of advanced high strength steels, we also evaluated its strain rate sensitivity over the low dynamic range.     

Load-Inversion Device for the High Strain Rate Tensile Testing of Sheet Materials with Hopkinson Pressure Bars

A high strain rate tensile testing technique for sheet materials is presented which makes use of a split Hopkinson pressure bar system in conjunction with a load inversion device. With compressive loads applied to its boundaries, the load inversion device introduces tension into a sheet specimen. Two output bars are used to minimize the effect of bending waves on the output force measurement. A Digital Image Correlation (DIC) algorithm is used to determine the strain history in the specimen gage section based on high speed video imaging. Detailed finite element analysis of the experimental set-up is performed to validate the design of the load inversion device. It is shown that under the assumption of perfect alignment and slip-free attachment of the specimen, the measured stress–strain curve is free from spurious oscillations at a strain rate of 1,000 s^?1. Validation experiments are carried out using tensile specimens extracted from 1.4 thick TRIP780 steel sheets. The experimental results for uniaxial tension at strain rates ranging from 200 s^?1 to 1,000 s^?1 confirm the oscillation-free numerical results in an approximate manner. Dynamic tension experiments are also performed on notched specimens to illustrate the validity of the proposed experimental technique for characterizing the effect of strain rate on the onset of ductile fracture in sheet materials.     

Characterization of shock accelerometers using Davies bar and laser interferometer

In this paper, we propose a novel method for evaluating the frequency response of shock accelerometers using Davies bar and interferometry. The method adopts elastic wave pulses propagating in a thin circular bar for the generation of high accelerations. The accelerometer to be examined is attached to one end of the bar and experiences high accelerations of the order of 10³∼10⁵ m/s². A laser interferometer system is newly designed for the absolute measurement of the bar end motion. It can measure the motion of a diffuse surface specimen at a speed of 10⁻³ ∼10⁰ m/s. Uncertainty of the velocity measurement is estimated to be±6×10⁻⁴ m/s, proving a high potential for use in the primary calibration of shock accelerometers. Frequency characteristics of the accelerometer are determined by comparing the accelerometer's output with velocity data of the interferometry in the frequency domain. Two piezoelectric-type accelerometers are tested in the experiment, and their frequency characteristics are obtained over a wide frequency range up to several ten kilohertz. It is also shown that the results obtained using strain gages are consistent with those by this new method. Paper was presented at the 1992 SEM Spring Conference on Experimental Mechanics held in Las Vegas, NV on June 8–11.     

Technique for Combined Dynamic Compression–Shear Testing of PBXs

We modify the split Hopkinson pressure bar and propose a compression–shear experimental method to investigate the dynamic behavior of polymer-bonded explosives (PBXs). The main apparatuses used include an incident bar with a wedge-shaped end and two transmission bars. We employ Y-cut quartzes with a rotation angle of 17.7° to measure the shear force and an optical system for shear strain measurement. A PBX with a density of 1.7 g/cm³ is investigated using the proposed method. Experimental results show that the specimen endures both compression and shear failure. Compression failure stress rises, and shear failure stress decreases as the strain rate increases. The sequences of shear and compression failure times are various at different strain rates. Based on the maximum shear failure criterion, we conclude that these phenomena are related to the experimental loading path.     

Miniaturized Compression Test at Very High Strain Rates by Direct Impact

A modified miniaturized version of the Direct Impact Compression Test (DICT) technique is described in this paper. The method permits determination of the rate-sensitive plastic properties of materials up to strain rate ∼10⁵ s⁻¹. Miniaturization of the experimental setup with specimen dimensions: diameter d _S = 2.0 mm and thickness l _S = 1.0 mm, Hopkinson bar diameter 5.2 mm, with application of a novel optical arrangement in measurement of specimen strain, makes possible compression tests at strain rates from ∼10³ s⁻¹ to ∼10⁵ s⁻¹. In order to estimate the rate sensitivity of a low-alloy construction steel, quasi-static, Split Hopkinson Pressure Bar (SHPB) and DICT tests have been performed at room temperature within the rate spectrum ranging from 5*10⁻⁴ s⁻¹ to 5*10⁴ s⁻¹. Adiabatic heating and friction effects are analyzed and the final true stress versus true strain curves at different strain rates are corrected to a constant temperature and zero friction. The results have been analyzed in the form of true stress versus the logarithm of strain rate and they show two regions of a constant rate sensitivity : relatively low up to the strain rate threshold ∼50 s⁻¹, and relatively high above the threshold, up to strain rate ∼4.5*10⁴ s⁻¹.     

Quasi-Static and Dynamic Large Strain Shear-Tension Testing

A new shear specimen is designed, evaluated and tested quasi-statically and dynamically. The specimen consists of a long cylinder having a horizontal gauge section created by two diametrically opposed semi-circular slots machined parallel to the longitudinal axis. This geometry imposes a rather uniform stress state, close to pure shear in the gauge section. Quasi-static and dynamic tension-shear tests up to a strain rate of 10⁴ 1/s were carried out on 1020 cold-drawn steel specimens. The obtained stress–strain curves and ductility were validated numerically. The new specimen can be used to study the shear mechanical characteristics of a material using tensile testing.     

Optimizing material strength constants numerically extracted from taylor impact data

Advanced design requirements have dictated a need for the mechanical properties of materials at high strain rates. Mechanical testing for these data poses a significant problem for experimentalists. High-speed testing machines have a limited capability at rates approaching 10²/s. The split Hopkinson pressure bar is the most reliable alternative for rates approaching 10⁴/s. Plate impact experiments are capable of generating strain rates of 10⁸/s and higher. The Taylor impact test occupies a place of particular importance by providing data at strain rates on the order of 10⁴/s–10⁵/s. The issue at present is extracting the data. This paper provides a method for obtaining dynamic strength model material constants from a single Taylor impact test. A polynomial response surface is used to describe the volume difference (error) between the deformed specimen from the Taylor test and the results of a computer simulation. The volume difference can be minimized using an optimizer, with the result being an optimum set of material constants. This method was applied to the modified Johnson-Cook model for OFHC copper. Starting from a nominal set of material constants, the iterative process improved the relative volume difference from 23.1 percent to 4.5 percent. Other starting points were used that yielded similar results. The material constants were validated by comparing numerical results with Taylor tests of cylinders having varying aspect ratios, calibers and impact velocities.     

三维DIC在GFRP锚杆拉伸力学性能测试中的应用

随着玻璃纤维增强塑料(GFRP)广泛运用于轨道交通及边坡加固中,其力学性能的测试显得尤为重要。基于三维数字图像相关(DIC)方法,研究了三种不同加载速率下GFRP锚杆的拉伸力学性能,分别得到了应力-应变曲线及弹性模量、抗拉强度、最大力总延伸率等参数。实验结果表明,螺旋状变截面锚杆的轴向应变呈非均匀分布,由三维DIC得到的锚杆弹性模量结果一致性优于传统的接触式引伸计方法,GFRP锚杆几乎呈线性分布的应力-应变曲线表明该材料为典型的脆性材料。     

Noncontacting strain measurements during tensile tests

The application of an innovative noncontracting Doppler laser extensometer is presented. True axial strain has been measured during tensile tests conducted on stainless-steel metal sheets over a range of strain rates (from 10^–4 to 10² 1/s) and temperatures (from –40°C to 400°C). The laser radiation scattered at the surface of the specimen is recorded during the duration of the experiment. The signals are then used to determine the evolution of the axial strain, which is subsequently combined with the load signal to construct the stress-strain curve for the material. Excellent agreement has been obtained between the total elongation predicted by the laser measurements and the actual values measured from the specimens. This technique offers several advantages over traditional strain-measuring technologies.     

Characterization of Anisotropic Polymeric Foam Under Static and Dynamic Loading

An orthotropic polymeric foam with transverse isotropy (Divinycell H250) used in composite sandwich structures was characterized at various strain rates. Uniaxial experiments were conducted along principal material axes as well as along off-axis directions under tension, compression, and shear to determine engineering constants, such as Young??s and shear moduli. Uniaxial strain experiments were conducted to determine mathematical stiffness constants, i. e., C _ij . An optimum specimen aspect ratio for these tests was selected by means of finite element analysis. Quasi-static and intermediate strain rate tests were conducted in a servo-hydraulic testing machine. High strain rate tests were conducted using a split Hopkinson Pressure Bar system built for the purpose using polymeric (polycarbonate) bars. The polycarbonate material has an impedance that is closer to that of foam than metals and results in lower noise to signal ratios and longer loading pulses. It was determined by analysis and verified experimentally that the loading pulses applied, propagated along the polycarbonate rods at nearly constant phase velocity with very low attenuation and dispersion. Material properties of the foam were obtained at three strain rates, quasi-static (10^?4 s^?1), intermediate (1 s^?1), and high (10³ s^?1) strain rates. A simple model proposed for the Young??s modulus of the foam was in very good agreement with the present and published experimental results.     

High Strain Rate Torsion Properties of Ultrafine-Grained Aluminum

Mechanical properties of most metallic materials can be improved by reducing their grain size. One of the methods used to reduce the grain size even to the nanometer level is the severe plastic deformation processing. Equal Channel Angular Pressing (ECAP) is one of the most promising severe plastic deformation processes for the nanocrystallization of ductile metals. Nanocrystalline and ultrafine grained metals usually have significantly higher strength properties but lower tensile ductility compared to the coarse grained metals. In this work, the torsion properties of ECAP processed ultrafine grained pure 1070 aluminum were studied in a wide range of strain rates using both servohydraulic materials testing machines and Hopkinson Split Bar techniques. The material exhibits extremely high ductility in torsion and the specimens did not fail even after 300% of strain. Pronounced yield point behavior was observed at strain rates 500 s⁻¹ and higher, whereas at lower strain rates the yielding was continuous. The material showed slight strain softening at the strain rate of 10⁻⁴ s⁻¹, almost ideally plastic behavior at strain rates between 10⁻³ s⁻¹ and 500 s⁻¹, and slight but increasing strain hardening at strain rates higher than that. The tests were monitored using digital cameras, and the strain distributions on the surface of the specimens were calculated using digital image correlation. The strain in the specimen localized very rapidly after yielding at all strain rates, and the localization lead to the development of a shear band. At high strain rates the shear band developed faster than at low strain rates.     

New method for testing composites at very high strain rates

A method was developed for testing and characterizing composite materials at strain rates in the 100 to 500 s^?1 regime. The method utilizes a thin ring specimen, 10.16 cm (4 in.) in diameter, 2.54 cm (1 in.) wide and 6–8 plies thick. This specimen is loaded by an internal pressure pulse applied explosively through a liquid. Pressure is measured by means of a calibrated steel ring instrumented with strain gages. Strains in the composite specimen are measured with strain gages. Strains in the calibration and specimen rings are recorded with a digital processing oscilloscope. The equation of motion is solved numerically and the data processed by the mini-computer attached to the oscilloscope. Results are obtained, and plotted by an X-Y plotter in the form of a dynamic stress-strain curve. Unidirectional 0-deg, 90-deg and 10-deg off-axis graphite/epoxy rings were tested at strain rates up to 690 s^?1. Times to failure ranged between 30 and 60 μs. The 0-deg properties which are governed by the fibers do not vary much from the static ones with only small increases in modulus. The 90-deg properties show much higher than static modulus and strength. The dynamic in-plane shear properties, obtained from the 10-deg off-axis specimens, are noticeably higher than static ones. In all cases the dynamic ultimate strains do not vary much from the static values.     

M-shaped Specimen for the High-strain Rate Tensile Testing Using a Split Hopkinson Pressure Bar Apparatus

An experimental technique is proposed to determine the tensile stress–strain curve of metals at high strain rates. An M-shaped specimen is designed which transforms a compressive loading at its boundaries into tensile loading of its gage section. The specimen can be used in a conventional split Hopkinson pressure bar apparatus, thereby circumventing experimental problems associated with the gripping of tensile specimens under dynamic loading. The M-specimen geometry provides plane strain conditions within its gage section. This feature retards necking and allows for very short gage sections. This new technique is validated both experimentally and numerically for true equivalent plastic strain rates of up to 4,250/s.     

PP/PA共混高聚物在高应变率下的热粘弹性本构关系和时温等效性

采用Instron1342电液伺服试验机和改进的SHPB技术对以113材料为例的PP/PA共混高聚物进行了应变率在10-4~103 s-1宽广范围,温度为25、40、60、80 ℃下的一维应力力学性能试验。结果表明,这类共混高聚物的力学响应对温度和应变率都是敏感的。以朱-王-唐非线性粘弹性本构方程来描述这类PP/PA共混高聚物的力学响应,并拟合得到了其热粘弹性本构参数,理论预言与试验结果在应变小于7%时吻合良好。对113材料20~80 ℃温度范围内不同应变率下的试验结果进行分析,证明PP/PA共混高聚物存在率温等效关系,提高温度和增加作用时间（减小应变率）的效果相当,反之,降低温度与减少作用时间（提高应变率）的效果相当。通过引入量纲一参量Z,使应变率d/dt、温度T这2个参量归结为统一的Z参量,从而得到了体现时温等效性的统一曲线。     

The strain-rate effect and the incremental plastic wave in copper

An experimental investigation was conducted to determine the degree of sensitivity of commerically pure copper to strain rate and to note the effect of this sensitivity on the velocity of propagation of shearing strain in copper. Thin-walled cylindrical specimens of copper were loaded in torsion to eliminate the effects of radial inertia. All specimens were annealed and then cold worked in torsion to obtain necessary specimen uniformity. Quasi-static tests were performed on short-length specimens to determine the shearing stress-strain curve of copper at a very low strain rate. The strain-rate sensitivity of copper at low strain rates, from 3×10^?4/sec to 5/sec, was tested by loading short specimens at a very slow continuous rate and then suddenly increasing the strain rate. A quasi-static test was also performed to determine the effect of creep on prestressed copper. Dynamic tests involving strain rates up to 500/sec were performed on long specimens with a torsional impact machine. Specimens were tested under stress-free and prestressed initial conditions. The prestressed specimen was loaded at a slow, continuous rate before impact to avoid the undesirable effects of creep which would have occurred with a static preload. Results from the quasi-static tests showed that copper is noticeably sensitive to strain rate in the low strain-rate regions, but that the sensitivity becomes almost constant as the strain rate is increased. Results from the dynamic tests showed that large strains propagated at speeds which agreed well with speeds predicted by the strain-rate-independent theory of plastic-wave propagation. The lower-level strains in the prestressed specimen, however, propagated at much higher speeds than are predicted by the strain-rate independent. Because radial-inertia effects were not present, this discrepancy in measured and predicted speeds for low-level strains must be due to the strain-rate sensitivity of copper.