Hopkinson Bar

Split Hopkinson bar (SHB) techniques are commonly used to experimentally characterise materials at high strain-rates. One important aspect of high-strain rate characterisation using SHBs is the necessity to tailor the input pulse to the needs of the material to be tested. Here, a new method to shape the input pulse, specifically developed for tensile SHBs (SHTB), is presented. The new method overcomes several challenges of existing designs, allows for a controlled adjustment of the pulse rising time, and significantly reduces wave dispersion effects.

The split Hopkinson pressure bar (SHPB) technique has been employed to evaluate the dynamic squeeze flow behavior of viscous Newtonian fluids. In this paper, the conditions under which classic Hopkinson bar data analysis is applicable for fluid specimens are discussed in detail. Requirements include the development of a parabolic flow profile and associated pressure distribution across the specimen. The times required for these processes to occur are calculated and compared with the experimental timescale in order to establish a specimen design criterion for valid SHPB testing. To evaluate this design criterion, an isothermal squeeze flow model describing the behavior of a cylindrical fluid specimen which includes inertial forces is used to predict the experimental results for a model Newtonian fluid. Good agreement between the theory and the experiment is obtained for thin specimens (w1.0 mm) across a wide range of shear strain rates (over 10 5 s À1 ). As a result of this study, the conditions under which valid SHPB experimental results may be obtained for a Newtonian fluid specimen are identified.

A new test method to directly characterize fiber-matrix interface properties under high rate of loading has been developed. A tensile Hopkinson bar with a modified incident bar is used to load a microdroplet test specimen. Numerical simulations were carried out to design the test specimen geometry and validate data reduction procedures for the dynamic interface experiments. Stress wave propagation in an S-2 Glass/Epoxy microdroplet specimen was studied with different droplet sizes (100 mm - 200 mm) and fiber gage lengths (2 mm - 6 mm). Simulation results indicate that dynamic equilibrium can be maintained up to a displacement rate of 10 m/s. Dynamic microdroplet experiments were conducted at a displacement rate of 1 m/s on S-2 glass/epoxy interface. Experimental results and post-failure inspection of the fiber matrix interface showed that the new test method is effective in measuring high rate interface properties of composites.

The 3D image correlation technique is used for full field measurement of strain (and strain rate) in compression and tensile split Hopkinson bar experiments using commercial image correlation software and two digital high-speed cameras that provide a synchronized stereo view of the specimen. Using an array of 128×80 (compression tests) and 258×48 (tensile tests) pixels, the cameras record about 110,000 frames per second. A random dot pattern is applied to the surface of the specimens. The image correlation algorithm uses the dot pattern to define a field of overlapping virtual gage boxes, and the 3-D coordinates of the center of each gage box are determined at each frame. The coordinates are then used for calculating the strains throughout the surface of the specimen. The strains determined with the image correlation method are compared with those determined from analyzing the elastic waves in the bars, and with strains measured with strain gages placed on the specimens. The system is used to study the response of OFE C10100 copper. In compression tests, the image correlation shows a nearly uniform deformation which agrees with the average strain that is determined from the waves in the bars and the strains measured with strain gages that are placed directly on the specimen. In tensile tests, the specimen geometry and properties affect the outcome from the experiment. The full field strain measurement provides means for examining the validity and accuracy of the tests. In tests where the deforming section of the specimen is well defined and the deformation is uniform, the strains measured with the image correlation technique agree with the average strain that is determined from the split Hopkinson bar wave records. If significant deformation is taking place outside the gage section, and when necking develops, the strains determined from the waves are not valid, but the image correlation method provides the accurate full field strain history.

Dynamic properties of materials such as aramid and polyethylene fiber reinforced composites are rarely found in the literature, in spite of their significance in ballistic design. It is also difficult to find detailed descriptions of the testing techniques and procedures to characterize these materials. This paper describes a dynamic tensile testing technique for these composite materials, and discusses problems such as the specimen size and the clamping system. A full numerical simulation is performed in Autodyn-2D to elucidate what is happening during the experiment. Finally this testing technique is applied to show that reliable data can be obtained from the Hopkinson bar for aramid and polyethylene fiber reinforced composites. Stress-strain, stress versus rate of strain and strain versus rate of strain curves are included. The results indicate that for these materials the tensile strength rises with the strain rate while the maximum strain diminishes in the range studied.

During automotive related accidents, PVB plays an important role in both pedestrian and passenger protection as an interlayer of automotive windshield. In this paper, dynamic constitutive behavior of PVB material is thoroughly studied. Firstly, a set of dynamic compression impact experiments on PVB specimens using SHPB (Split Hopkinson Pressure Bar) method are conducted at strain rates from 700/s to 4500/s. Details of the constitutive response is analyzed based on the validation of experiment data. Stress-strain curve of PVB is then divided into two parts, i.e., "Compaction Stage" and "Hardening Stage". Dislocations and entanglements among molecules are major reasons for the two-stage phenomena. Constitutive behaviors are different in low and high speed impacts, leading to three times more energy absorption ability of PVB in high speed impact scenario. Further, data fitting models based on both MooneyeRivlin and Ogden Model are studied and then compared. MooneyeRivlin Model is found to be more appropriate to describe PVB material. Moreover, PVB is proved to be a rate-dependent material with the failure strength intensify factor b z 4. PVB material shows little viscoelasticity after comparison of the both models with and without the viscoelasticity part. Results offer critical experimental data, constitutive models and analysis of PVB material to further study of automotive crashworthiness and pedestrian/passenger protection.

The mixed system composed by a Sensing block system and a Hopkinson bar is a new method to investigate mechanical properties of materials under high strain rates. In practice, however, wave dispersion distorts pulse shape and may limit the accuracy of results. Using finite element simulat ion, it is shown in this work that a half sinusoidal pulse decreases wave dispersion. Moreover, it is also demonstrated that the higher the rise time the lower the wave dispersion. It is known that a half s inusoidal pulse with a great rise time may be obtained by inserting a deformable disk (pad) between the incident bar and the sensing block. This fact is verified in this work using an appropriate constitutive suitable for the rise time. Optimu m characteris tics are found for the sensing block, the most suitable material and dimensions. The use of deformable pad is also impo rtant when brittle materials are tested; this is because the increase in pulse rise time allo ws a gradual stress application .

Sandwich composites are widely used in many structural applications due to their superior bending stiffness, low weight and excellent thermal insulation, etc. Many innovative improvements have been made in the construction of sandwich composites in recent years to improve their impact resistance. Evaluation of dynamic properties of such laminates is of vital importance to the designer. In the current study, foam filled 3D integrated core sandwich composite laminates with and without additional face sheets were fabricated using vacuum assisted resin infusion molding process in multiple steps. For additional facing, plain weave S2-glass and twill weave carbon fabrics were used on top and bottom sides of the panels in four different monolithic (S2-glass only, carbon only) and hybrid (glass + carbon, one with S2-glass layer outside and other with carbon layer outside) combinations. High strain rate (HSR) response of integrated core sandwich composites was determined using a split Hopkinson pressure bar setup (SHPB) under compression loading at strain rates ranging from 742 to 1546 s −1 . Dynamic response of different types of samples was compared in terms of the peak stress, modulus, strain at peak stress and failure modes.

The development of an innovative apparatus, based on Hopkinson bar techniques, for performing large scale dynamic tests is presented and discussed. The activity is centered at the recently upgraded HOPLAB facility, which is basically a split Hopkinson bar with a total length of approximately 200 m, with bar diameters of 72 mm and where force pulses up to 2 MN and 40 ms duration can be generated and strain rates up to 50 s −1 can be achieved. Several modifications in the basic configuration have been introduced: twin incident and transmitter bars have been installed with strong steel plates at their ends where large specimens can be placed. A series of calibration and quan-tification tests has been conducted in order to prove the reliability of the experimental technique proposed. Moreover , real tests on concrete cylindrical samples of 200 mm diameter and of up to 400 mm length have been performed. Analyses of recorded signals indicate proper Hopkinson bar testing conditions and reliable functioning of the facility.

Constant strain rate, constant velocity and Hopkinson Pressure Bar compression tests were carried out on AerMet 100 martensitic steel between 1130 °C and 1250 °C spanning strain rates from 0.01 s À 1 to 4000 s À 1. The results were used to generate a predictive flow stress model over the entire range of test conditions. The effect of initial austenite grain size on flow stress was found to follow the Hall–Petch relationship. This dependency was then removed through innovative heat treatments. The morphology of the flow stress curves were also dependent on mechanisms of microstructural evolution which were controlled by the test method, strain rate and temperature. Friction and adiabatic heating also had a major contribution. A novel method was proposed in order to define the flow stress, which was then used to determine the work hardening exponent of the Zener–Hollomon equation. It was found that a deviation from the linear trend was observed in Hopkinson Pressure Bar tests and reasons were given. An artificial neural network approach was used to determine a more accurate predictive flow stress model which included the effects of test method, temperature, stain rate and initial austenite grain size. The method showed that it was possible to predict the flow stress between 50 and 2000 s À 1 where mechanical testing's results were absent.

This work presents an experimental characterization of the mechanical behaviour of the EK4 deep drawing steel. The experimental procedure encompasses spectrometry, metalography, tension testing and hardness measurements. Special attention is devoted to the derivation of the elastic and plastic parameters involved in the assumed constitutive model based on the anisotropic Hill-48 yield criterion. The simulation of the deformation process during the whole tensile test is subsequently performed with the aim of assessing the adequateness of the proposed methodology. It should be mentioned that the material parameters obtained with this procedure are the basic data for the modelling and experimental validation of different deep drawing applications presented in Part II of this work.

This paper presents the experimental measurements of a highly magnetoelastic material (Galfenol) under impact loading. A Split-Hopkinson Pressure Bar was used to generate compressive stress up to 275 MPa at strain rates of either 20/s or 33/s while measuring the stress-strain response and change in magnetic flux density due to magnetoelastic coupling. The average Young's modulus (44.85 GPa) was invariant to strain rate, with instantaneous stiffness ranging from 25 to 55 GPa. A lumped parameters model simulated the measured pickup coil voltages in response to an applied stress pulse. Fitting the model to the experimental data provided the average piezomagnetic coefficient and relative permeability as functions of field strength. The model suggests magnetoelastic coupling is primarily insensitive to strain rates as high as 33/s. Additionally, the lumped parameters model was used to investigate magnetoelastic transducers as potential pulsed power sources. Results show that Galfenol can generate large quantities of instantaneous power (80 MW/m3 ), comparable to explosively driven ferromagnetic pulse generators (500 MW/m3 ). However, this process is much more efficient and can be cyclically carried out in the linear elastic range of the material, in stark contrast with explosively driven pulsed power generators.

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 À1). 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 semiconductor 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 À1. 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.

The high-speed deformation behavior of a commercially available dual phase steel composition was studied by means of split Hopkinson bar testing in shear punch mode. The as-received cold-rolled material was subjected to a variety of heat treatment conditions to produce several different microstructures, namely ferrite plus pearlite, ferrite plus bainite and/or acicular ferrite, ferrite plus bainite and martensite, and ferrite plus different fractions of martensite. A strong effect of microstructure on both static and dynamic properties and on the dynamic factor (ratio of dynamic to static strength) was observed. According to the variation of dynamic factor with static strength, three groups of microstructures with three distinct behaviors were identified. Under the same static strength level, the classic dual phase microstructure was found to have the highest value of dynamic factor and therefore can absorb more dynamic energy than other microstructures.

The development of an innovative apparatus, based on Hopkinson bar techniques, for performing large scale dynamic tests is presented and discussed. The activity is centered at the recently upgraded HOPLAB facility, which is basically a split Hopkinson bar with a total length of approximately 200 m, with bar diameters of 72 mm and where force pulses up to 2 MN and 40 ms duration can be generated and strain rates up to 50 s −1 can be achieved. Several modifications in the basic configuration have been introduced: twin incident and transmitter bars have been installed with strong steel plates at their ends where large specimens can be placed. A series of calibration and quantification tests has been conducted in order to prove the reliability of the experimental technique proposed. Moreover, real tests on concrete cylindrical samples of 200 mm diameter and of up to 400 mm length have been performed. Analyses of recorded signals indicate proper Hopkinson bar testing conditions and reliable functioning of the facility. Keywords Dynamic material tests • Large scale tests • High strain rate • Modified Hopkinson bar • Twin bar Alessio Caverzan

A split Hopkinson pressure bar (two-bar set-up) has been modified to perform dynamic three-point bend tests to measure dynamic fracture toughness, and to understand the influences of various experimental parameters, as well as inertial effects, on the dynamic material response. Modeling and analysis of the dynamic three-point bend test, as loaded by a modified split Hopkinson pressure bar, is conducted. The effects of support motion, crack propagation and plastic contact stiffness on total sample deflection are investigated. The effects of crack propagation and plastic contact stiffness on the contribution of support motion to the total sample deflection are also investigated theoretically and experimentally in this paper. Further, the effects of crack propagation and plastic contact stiffness on impactor and sample load are also addressed.

Large mammalian antler is extremely tough and fracture resistant compared to other more brittle forms of skeletal bone. The ability of antler to resist fracture is associated with a decrease in material stiffness and yield strength and increased non-linear response, due in part to antler being fast growing, since they are typically shed and regrown annually. Since male Elk commonly engage in antler sparring as a means of making dominance displays, the ability to withstand large impacts suggest that antler may exhibit strain-rate dependent behavior even greater than skeletal bone. To evaluate this hypothesis, specimens of antler were tested in compression over a range of strain rates. Specimens were loaded either along or transversely to the osteonal growth direction, in wet and dry conditions. Results showed that antler exhibits higher compressive strengths at increased strain rates, and that strain rate and hydration are greater determinants of compressive strength than osteonal orientation. In addition, antler can sustain compressive strains a full order-ofmagnitude greater than in mammalian long bone. Failed specimens showed that a hierarchical chain of deformation mechanisms sustains the large bulk strains supported by antler. These mechanisms appear to be less brittle and more fibrous than those seen previously in skeletal bone.

Sandwich composites are widely used in many structural applications due to their superior bending stiffness, low weight and excellent thermal insulation, etc. Many innovative improvements have been made in the construction of sandwich composites in recent years to improve their impact resistance. Evaluation of dynamic properties of such laminates is of vital importance to the designer. In the current study, foam filled 3D integrated core sandwich composite laminates with and without additional face sheets were fabricated using vacuum assisted resin infusion molding process in multiple steps. For additional facing, plain weave S2-glass and twill weave carbon fabrics were used on top and bottom sides of the panels in four different monolithic (S2-glass only, carbon only) and hybrid (glass + carbon, one with S2-glass layer outside and other with carbon layer outside) combinations. High strain rate (HSR) response of integrated core sandwich composites was determined using a split Hopkinson pressure bar setup (SHPB) under compression loading at strain rates ranging from 742 to 1546 s −1 . Dynamic response of different types of samples was compared in terms of the peak stress, modulus, strain at peak stress and failure modes.

High-and low strain-rate compression experiments were conducted on epoxidized soybean oil (ESO)/clay nanocomposites with nanoclay weights of 0%, 5%, and 8%. A pulse-shaped split Hopkinson pressure bar (SHPB) was employed to conduct high strain-rate experiments. The pulse shaping technique ensures nearly constant-strain-rate deformation under dynamically equilibrated stresses in specimens such that accurate stress-strain curves at various high rates were obtained. A MTS 810 hydraulically driven materials testing system was used to obtain low strain-rate stress-strain curves. Strain-rate and nanoclay weight effects on the compressive properties of the nanocomposites were experimentally determined. A phenomenological strain-rate-dependent material model was presented to describe the stress-strain response. The model agrees well with the experimental data at both large and small strains as well as high and low strain rates.

The National Institute of Standards and Technology (NIST) has developed an electrical pulse-heated Kolsky Bar technique for measuring the constitutive response of metals at heating rates of up to 6,000 K/s and strain rates up to 10 4 s −1 . Under these conditions, which are approaching those found in high speed machining, thermally activated microstructural processes such as grain growth, solid state phase transformation and dislocation annealing can be bypassed, leading to unique non-equilibrium superheated microstructural states. Flow stresses can thus differ significantly from equilibrium high temperature conditions. This paper describes the NIST pulse-heated Kolsky bar technique in detail, including a thorough assessment of uncertainties in temperature and flow stress measurement.

Constant strain rate, constant velocity and Hopkinson Pressure Bar compression tests were carried out on AerMet 100 martensitic steel between 1130°C and 1250°C spanning strain rates from 0.01 s À 1 to 4000 s À 1. The results were used to generate a predictive flow stress model over the entire range of test conditions. The effect of initial austenite grain size on flow stress was found to follow the Hall-Petch relationship. This dependency was then removed through innovative heat treatments. The morphology of the flow stress curves were also dependent on mechanisms of microstructural evolution which were controlled by the test method, strain rate and temperature. Friction and adiabatic heating also had a major contribution. A novel method was proposed in order to define the flow stress, which was then used to determine the work hardening exponent of the Zener-Hollomon equation. It was found that a deviation from the linear trend was observed in Hopkinson Pressure Bar tests and reasons were given. An artificial neural network approach was used to determine a more accurate predictive flow stress model which included the effects of test method, temperature, stain rate and initial austenite grain size. The method showed that it was possible to predict the flow stress between 50 and 2000 s À 1 where mechanical testing's results were absent.

The 3D image correlation technique is used for full field measurement of strain (and strain rate) in compression and tensile split Hopkinson bar experiments using commercial image correlation software and two digital high-speed cameras that provide a synchronized stereo view of the specimen. Using an array of 128×80 (compression tests) and 258×48 (tensile tests) pixels, the cameras record about 110,000 frames per second. A random dot pattern is applied to the surface of the specimens. The image correlation algorithm uses the dot pattern to define a field of overlapping virtual gage boxes, and the 3-D coordinates of the center of each gage box are determined at each frame. The coordinates are then used for calculating the strains throughout the surface of the specimen. The strains determined with the image correlation method are compared with those determined from analyzing the elastic waves in the bars, and with strains measured with strain gages placed on the specimens. The system is used to study the response of OFE C10100 copper. In compression tests, the image correlation shows a nearly uniform deformation which agrees with the average strain that is determined from the waves in the bars and the strains measured with strain gages that are placed directly on the specimen. In tensile tests, the specimen geometry and properties affect the outcome from the experiment. The full field strain measurement provides means for examining the validity and accuracy of the tests. In tests where the deforming section of the specimen is well defined and the deformation is uniform, the strains measured with the image correlation technique agree with the average strain that is determined from the split Hopkinson bar wave records. If significant deformation is taking place outside the gage section, and when necking develops, the strains determined from the waves are not valid, but the image correlation method provides the accurate full field strain history.

Although clinical bone fractures occur predominately under impact loading (as occurs during sporting accidents, falls, high-speed impacts or other catastrophic events), experimentally validated studies on the dynamic fracture behavior of bone, at the loading rates associated with such events, remain limited. In this study, a series of tests were performed on femoral specimens obtained post-mortem from equine donors ranging in age from 6 months to 28 years. Fracture toughness and compressive tests were performed under both quasi-static and dynamic loading conditions in order to determine the effects of loading rate and age on the mechanical behavior of the cortical bone. Fracture toughness experiments were performed using a four-point bending geometry on single and doublenotch specimens in order to measure fracture toughness, as well as observe differences in crack initiation between dynamic and quasi-static experiments. Compressive properties were measured on bone loaded parallel and transverse to the osteonal growth direction. Fracture propagation was then analyzed using scanning electron and scanning confocal microscopy to observe the effects of microstructural toughening mechanisms at different strain rates. Specimens from each horse were also analyzed for dry, wet and mineral densities, as well as weight percent mineral, in order to investigate possible influences of composition on mechanical behavior. Results indicate that bone has a higher compressive strength, but lower fracture toughness when tested dynamically as compared to quasistatic experiments. Fracture toughness also tends to decrease with age when measured quasi-statically, but shows little change with age under dynamic loading conditions, where brittle "cleavage-like" fracture behavior dominates. c

In the present study a new insert design is presented and validated to enable reliable dynamic mechanical characterization of low strain-to-failure materials using the Split-Hopkinson Pressure Bar (SHPB) apparatus. Finite element-based simulations are conducted to better understand the effects of stress concentrations on the dynamic behavior of LM-1, a Zr-based bulk metallic glass (BMG), using the conventional SHPB setup with cylindrical inserts, and two modified setups-one utilizing conical inserts and the other utilizing a "dogbone" shaped specimen. Based on the results of these computational experiments the ends of the dogbone specimen are replaced with high-strength maraging steel inserts. This new insertspecimen configuration is expected to prevent specimen failure outside the specimen gage section. Simulations are then performed to validate the new insert design. Moreover, high strain-rate uniaxial compression tests are conducted on LM-1 using the modified SHPB with the new inserts. An ultra-high-speed camera is employed to investigate the changes in failure behavior of the specimens. Additional experiments are conducted with strain gages directly attached to the gage section of the specimens to determine accurately their dynamic stress-strain behavior.

This paper presents a part of an on-going study of light-weight shelters made of aluminium alloy AA5083-H116. A necessary prerequisite for numerical simulations of impact behaviour of such shelters is a calibrated numerical model. Slightly modified versions of the two Johnson-Cook models describing flow stress and fracture strain are applied. In addition to ordinary quasistatic tests with smooth specimens, these models demand tests at elevated temperatures and strain rates, and different triaxiality ratios. All tests to be presented in this paper are uniaxial tension tests performed in servohydraulic test machines or a Split-Hopkinson bar. The test programme involves approximately 100 specimens, and includes coupons from three different directions of the plate material, thereby taking account for the anisotropy. The AA5083 alloy is susceptible to dynamic strain ageing, and this phenomenon is shown to result in serrated stress-strain curves and negative strain rate sensitivity for a rather wide range of strain rates. In particular, the specimen geometry seems to have a strong influence on the serrated curves. The calibration shows that the Johnson-Cook-type models represent most of the observed material behaviour reasonably well, although the negative strain rate sensitivity is not adequately described.

The aim of this paper was to describe a method of analysing the test data recorded during a Hopkinson Bar bending test. This three-point dynamic bending test was designed for testing the strength of materials under dynamic loads. It is carried out on a specimen consisting of a beam placed on two supports, which is subjected to an impact. The use of Hopkinson Bar as supports makes it possible to determine the forces and displacements at these points. An analytical solution for the transient response of a long beam subjected to a transverse impact was used to determine the impact force and the displacement. This procedure applies for the first few instants when the motions generated by the impact have not yet reached the supports, and the mechanical state of the specimen is identical to that of an unsupported beam. It is suitable for use with quasi-brittle materials for which failure occurs at the very beginning of the test. The material strength is determined at the time of failure, which is characterised by a sudden decrease in the bending moment. The results of a test in which a quasi-brittle material was loaded up to failure are presented and analysed as outlined above. The results obtained confirm the relevance of the present method.

The main concern of this work is the mechanical characterization of adhesively bonded assemblies under dynamic shear loading ranging from quasi-static (10 À 4 s À 1 ) up to high (10 4 s À 1 ) strain rates. The doublelap shear sample is proposed and a bonding procedure is established. The assemblies are made of steel substrates bonded with an epoxy adhesive. Two surface treatments of the substrates are considered: ethanol and sand shooting. The shear strength and the failure strain are measured by taking into account the testing setups accuracy and the non-uniform distribution of the stress and strain fields in the overlap region. The sensitivity of the strength and the failure strain to the strain rate is highlighted; it is found that the failure strain decreases and the shear strength increases with the strain rate until reaching a maximum value then it drops for very high strain rates.