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This work deals with the strain-rate dependent characterization of paper under uniaxial tension at high strain-rates. Experiments were performed involving a Split Hopkinson bar for high strain-rate testing, comparing the results with conventional quasi-static tests. Tests were conducted in a strain-rate range between 0.0083 and 212 s−1, which is equivalent to testing velocities between 0.0003 and roughly 13.6 m/s. For the first time the change in tensile behaviour of paper is comprehensively characterized and modelled, using the Cowper-Symonds model for strain-rate hardening. The experimental tests showed that the tensile strength as well as the initial stiffness were gradually increasing with increasing strain-rate. The increase in tensile strength between the lowest and the highest strain-rate was 58% on average whereas the mean increase in stiffness between these two strain-rates was almost 115%. Regarding the fracture strain, it was observed that it significantly decreases with increasing strain-rate. While the average fracture strain of the quasi-static tests was at roughly 6% it was close to 3% for the dynamic tests. In case of the Split Hopkinson bar tests, high-speed videos of the samples were made to determine their elongation via target tracking and digital image correlation (DIC). We found that strain localization, which is a highly relevant mechanism for quasi-static tensile failure, is likely related to short term plastic creep of the material as strain localization nearly entirely disappears at high loading rates of paper. [ABSTRACT FROM AUTHOR]

The split Hopkinson bar has been widely used for testing the dynamic mechanical properties of various materials. However, there are still difficulties in ensuring stress uniformity when testing some low wave impedance materials such as soft materials and brittle materials and difficulty in achieving true multi-axial and multi-directional synchronous loading experiments and other issues. To overcome these issues, this paper uses an electromagnetic Hopkinson bar for experimental research and introduces its principle and implementation process. Finite element analysis and experimental methods were used to verify the feasibility of the electromagnetic Hopkinson bar in addressing the relevant issues. The superiority of the new device was demonstrated by its capability in performing compression experiments on two typical low wave impedance materials: polymethacrylates (PMMA) and polyurethane (PU), and two-way tension experiments on 2024 aluminum alloy. The results indicate that the electromagnetic Hopkinson bar can effectively solve the problem of uniform stress in low wave impedance materials testing, and can achieve synchronous loading of complex stress paths. [ABSTRACT FROM AUTHOR]

The transition from ductile to brittle for metals is usually encountered during fast machining operations, in low-temperature environments, and in all situations involving very high strain rates. Traditionally, classical material models used in the dynamic structural analysis focus on the plastic-stress-versus-strain rate. As a result, those models cannot incorporate sudden changes in failure strains and strengths triggered by material behavior transition. The ability to predict realistically the dynamic behavior of structures based on physical constitutive equations depends on having a comprehensive understanding of such drastic changes in material behavior. This transition is described by the DAMP-PLAST model, incorporating constitutive equations and governed by the shear band speed parameter at a finite time. After the development of the equation set, the material model is tested in regard to its ability to produce three distinguished material responses: elastic-plastic, elastic–plastic with strain-rate sensitivity, and brittle with increased dynamic failure strength. The study identifies metal dynamic brittleness linked to a critical loading rate influencing the plastic flow process. Based on this relation, the identification of the critical loading rate using split Hopkinson bar apparatus, and material constitutive equation is proposed. [ABSTRACT FROM AUTHOR]

The 6061-T651 aluminum alloy was strengthened by torsional deformation with the torsion angles of 90°, 180° and 360°. The quasi-static and dynamic compressive properties of the samples before and after torsional deformation were investigated. The results show that the grain size of the samples remains unchanged at first with the increase of the torsion angle, and then begins to decrease. However, the Kernel average misorientation(KAM) value continues to increase with the increase of the torsion angle. The quasi-static and dynamic compression experiments show a slight increase in the yield strength of the samples with the increase of the torsion angle. For the same torsion angle, the dynamic yield strength is significantly higher than the yield strength at quasi-static loading. The strain rate experiment shows that the yield strength of the sample increases with the increase of strain rate, but compared with the untorsional sample, the strain rate sensitivity of the 360° torsional sample is significantly reduced. Based on the experimental data, the parameters in the Cowper-Symonds constitutive model were fitted. The stress-strain curves obtained from this model are in good agreement with the experimental results.

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The transition from ductile to brittle for metals is usually encountered during fast machining operations, in low-temperature environments, and in all situations involving very high strain rates. Traditionally, classical material models used in the dynamic structural analysis focus on the plastic-stress-versus-strain rate. As a result, those models cannot incorporate sudden changes in failure strains and strengths triggered by material behavior transition. The ability to predict realistically the dynamic behavior of structures based on physical constitutive equations depends on having a comprehensive understanding of such drastic changes in material behavior. This transition is described by the DAMP-PLAST model, incorporating constitutive equations and governed by the shear band speed parameter at a finite time. After the development of the equation set, the material model is tested in regard to its ability to produce three distinguished material responses: elastic-plastic, elastic–plastic with strain-rate sensitivity, and brittle with increased dynamic failure strength. The study identifies metal dynamic brittleness linked to a critical loading rate influencing the plastic flow process. Based on this relation, the identification of the critical loading rate using split Hopkinson bar apparatus, and material constitutive equation is proposed.

Dynamic characterization of cement-based composites is crucial for understanding material behavior. When exposed to highly dynamic loading conditions, the strain-rate dependence of material causes the material response to differ significantly from that under quasi-static loading conditions. In this paper, a numerical investigation on the dynamic tensile behavior of cement-based materials. A gravitational split Hopkinson tension bar was used to characterize the dynamic tensile behavior of cement-based at high strain-rates. The commercial finite element software LS-Dyna is adopted to conduct the computations. The material specifications of cement-based are characterized by the Karagozian & Case (K&C) concrete model that accounts for shear dilation, strain-rate dependence, and strain softening. The model accuracy is verified with available experimental results in the form of strain signals, strain-rates, and tensile strengths. It was found that the results computed with the automatic generation version of K&C are slightly different from the experimental ones. Therefore, to achieve better agreement, the model was extended by calibrating a few parameters of the K&C material formulation. Finally, the simulation predictions were found to represent the experimental results with good agreement. [ABSTRACT FROM AUTHOR]

Background: Advanced testing methodologies and measurement techniques to identify complex deformation and failure at high strain rates have drawn increasing attention in recent years. Objective: The objective of the current study is the development of a novel combined tension–torsion split Hopkinson bar (TTHB) conceived to generate a combination of tensile and torsional stress waves in a single loading case, and to measure material data representative of real case impact scenarios. Methods: An energy store and release mechanism was employed to generate both the longitudinal and shear waves via the rapid release of a bespoke clamp assembly. A parametric study of the material and geometry of the clamp was implemented via numerical simulations to optimise critical aspects of the wave generation. Thin-walled tube specimens made of two metallic materials were utilised to examine the capability of the developed TTHB system by comparing the experimental measurements with those obtained from conventional split Hopkinson tension and torsion bars. Results: The experimental results demonstrate that the synchronisation of the longitudinal and torsional waves was achieved within 15 microseconds. Different wave rise time were obtained via the controlled release of the clamp using fracture pins of various materials. The analysis indicates that the developed TTHB is capable of characterising the dynamic behaviour of materials under tension, torsion, as well as under a wide range of complex stress states. Conclusions: The presented apparatus, testing and analysis methods allow for the direct population of the dynamic failure stress envelopes of engineering materials and for the accurate evaluation of existing and novel constitutive models. [ABSTRACT FROM AUTHOR]

• Weak, brittle, low impedance material characterized with metallic Split Hopkinson Bar. • Transmitter sensitivity increasable by local taper; can cause unwanted pulse-shaping. • Mismeasurements may become acute at higher bar velocities only. • Clamp-mass, -liner and extent of local taper contribute to mismeasurements. • Digital twin is key to pre-validate bar-setup for a given specimen and strain-rate range. The Split Hopkinson or Kolsky bar is one of the most popular devices when it comes to the mechanical characterization of material samples under high strain-rates. While testing of high impedance materials, such as metal alloys, is relatively straight forward, samples with low impedance pose certain challenges. The present work focuses on the detailed implementation of a high strain-rate tensile testing method for thin, low impedance, sheet-like materials by using the Split Hopkinson test principle. In order to find a suitable Split Hopkinson setup a digital twin was created using explicit finite element methods. With the help of the digital twin, the design of the transmission bar and the sample holders including the friction liners were explored. The numerical model indicated, that a hollow transmission bar with a moderate tapering (hollow bar 1.0) is suited for the characterization of low impedance materials over a wide strain-rate range. Furthermore, this setup has to be combined with an asymmetrical sample holder configuration (heavier on the incident side and lighter on the transmission side) and aluminum friction liners to return accurate results. This numerically derived setup was validated against experimental tests on paper, representative of low impedance, sheet-like materials. [ABSTRACT FROM AUTHOR]

Under static loading, measuring experimentally the mode II and mixed mode fracture toughness of composite materials and adhesive joints is well standardised. However, under dynamic loading, no standard procedure has been defined yet. Therefore, this paper proposes an experimental methodology to measure the mode II and mixed mode interlaminar fracture toughness of composite materials and adhesive joints. The methodology is based on a modified split Hopkinson compression bar. Two different data reduction schemes are explored and compared, one based on measuring the crack length, and another based on measuring the force from the strains in the transmitted bar. The two data reduction methods provided considerably different results. By using the method based on measuring the force, the mode II and mixed mode fracture toughness for both interlaminar and adhesive joints decreased for higher strain rates, while the opposite was found with the other approach. The method based on the crack length measurement was deemed to be unreliable due to the difficulties in measuring it. • Experimental study of the mode II and mixed mode fracture toughness under dynamic loading. • Interlaminar fracture toughness of composite materials and adhesive joints is explored. • Two different data reduction methods are employed and compared. • Significant strain rate is found, but results are dependent on the data reduction method. • Advantages and limitations of the data reduction methods are discussed. [ABSTRACT FROM AUTHOR]

The aim of this paper is to study the effect of strain rate on the compressive behavior of the highly cross-linked RTM6 epoxy resin used in advanced aerospace composites. Dynamic compression tests were performed using a split Hopkinson pressure bar, along with reference quasi-static compression tests, to cover a strain rate range from 0.001 to 1035 s−1. Special attention was paid to the optimization of the test methodologies in order to obtain material data free of bias related to the use of different load introduction techniques and sample geometries over the considered strain rate range. In addition, the use of full-field 3D deformation measurements allowed the validation of traditional test and material assumptions. A novel self-alignment tool was developed to enable perfect interfacial contact during compression loading. The 3D digital image correlation technique was used to measure the instantaneous deformation of the sample during compression at different strain rates. Results showed a pronounced strain rate sensitivity of the RTM6 epoxy in compression. The peak yield strength increased with increasing strain rate, while the elastic modulus and Poisson's ratio in compression were independent of the strain rate. The barreling of the sample in compression, quantified by the barreling ratio, showed an increase during the progression of the compression tests. However, the barreling ratio significantly decreased with the increasing strain rate. Finally, it was shown that neglecting the significant volume change in the yield stages gave rise to a non-negligible underestimation of the strength of the material. [ABSTRACT FROM AUTHOR]

The concrete of the deep mine shaft lining often produces a phenomenon similar to rock-burst, and basalt fiber has good crack resistance and is widely used in engineering construction. From the perspective of concrete dynamics, this paper conducts dynamic tests on ordinary concrete and basalt fiber reinforced concrete (BFRC) by using a Split Hopkinson Bar (SHPB) with a diameter of 80 mm. To accurately assess the brittleness of concrete under rock-burst, this paper comprehensively considers the energy evolution characteristics of concrete before and after the peak, and establishes a brittleness index evaluation method based on the full stress–strain curve that can reflect the deformation and failure process of concrete. The evolution law of total energy, elastic strain energy and dissipated energy in the failure process of specimens under impact load is explored. The results show that the higher the impact rate, the more the total energy absorbed by the concrete, and the incorporation of basalt fiber changes the internal pore structure of the concrete, making the total absorbed energy and dissipated energy of BFRC larger, but the effective energy ratio is smaller. Meanwhile, under the same impact air pressure, the brittleness index of BFRC is lower than that of ordinary concrete (PC). The brittleness index and fractal dimension of concrete have a positively correlated with strain rate. The incorporation of basalt fiber can improve the toughness of concrete and improve the ability of concrete to resist damage. The research results are expected to provide a reference for the brittleness evaluation of concrete under impact load. [ABSTRACT FROM AUTHOR]

Two-dimensional carbon fiber reinforced silicon carbide (2D-C/SiC) composites are well-known for the high-temperature performance in aeronautical and aerospace fields, but they are susceptible to foreign object damage in service. Therefore, the high-temperature performance of pre-impacted 2D-C/SiC is closely connected with flight security. In this study, the pre-impact testing was conducted using a stress-reversal split Hopkinson bar, and the pre-impact energies were set from 0.1J to 0.9J. Subsequently, the pre-impacted specimens were heated for a period in the temperature-keeping procedure from 20 °C to 1600 °C. Oxidation kinetics of the pre-impacted 2D-C/SiC specimens were obtained based on the weight loss-temperature trends. Finally, the specimens were compressed under high temperatures at the strain rates from 10−4/s to 1000/s, so the residual compressive strength (RCS)-temperature trends were achieved. Hence, the correlation between the weight loss and the RCS was established. Interestingly, the pre-impact strengthening effect on the compressive strength was found. The results revealed that the RCS of 2D-C/SiC is affected by various parameters, providing new sights into the high-temperature damage tolerance of ceramic matrix composites. [Display omitted] • High-temperature damage tolerance of 2D-C/SiC composites. • Oxidation mechanism is not only controlled by the temperature. • Pre-impact gives rise to the increase in strength to some extent. • Strain rate effect on the strength is reduced by the increasing temperature-keeping duration. [ABSTRACT FROM AUTHOR]

The influence of high strain rate on the forming limit curves of AA6082-T6 aluminum alloy sheets, 1 mm in thickness, was investigated by means of dynamic tensile and hemispherical punch tests carried out until the onset of sample failure using a Split Hopkinson Bar (SHB) system, at a punch speed of 7500 and 12000 mm/s. The equipment used was designed according to the standard EN ISO 12004-2 but properly scaled to adapt it to the SHB apparatus. A finite element analysis was performed to quantify the strain rate values reached during tests (around 600 and 1000 s-1 associated at the punch speeds of about 7500 and 12000 mm/s, respectively). The limit strains were determined on different sample geometries undergoing deformation in the range from the uniaxial and balanced biaxial stress states typical for sheet metal forming processes. The major and minor strain distributions, acquired through an accurate image analysis by analyzing the deformation of a previously imprinted grid on the specimen surface, allowed to plot the dynamic forming limit curves at different high strain rate values. Such forming limit curves were compared with those provided by samples deformed under quasi-static loading condition, using the same equipment and specimen geometries, in order to evaluate the influence of the loading rate on the formability. It was observed that the formability of AA6082-T6 aluminum alloy strongly improves as the strain rate increases from quasi-static to dynamic conditions. [ABSTRACT FROM AUTHOR]

Full understanding of the thermomechanical behaviors of materials at high strain rates and high temperatures are of great importance from not only scientific meaning but also practical value in engineering structure design and safety assessment. Great efforts have been made for abilities of operation the split Hopkinson bars, the most popular technique for experimental determination of mechanical behaviors of materials over the strain rates from 102 to 104 s−1 over the past 70 years, at high temperatures since 1960s. A review of experiment work is presented in this paper to give an overview of the development of experimental techniques at high temperatures based on Hopkinson bar systems. The principles of the split Hopkinson bar requires the loading bars avoiding temperature gradient or keeping relatively low temperature when performing high temperature testing. Techniques such as performing temperature gradient corrections, rapid heating or using special designed automatically assembled systems were proposed by researchers to enable the operation of the split Hopkinson bars at temperature as high as possible. Moreover, to the application of high speed photographic technique for capturing the dynamic deformation process of the specimen in high temperature Hopkinson bar testing, some key issues of eliminating the strong thermal radiation induced lights oversaturation and de-blurring of images due to insufficient exposure at high temperature and high strain rate condition, as well as fabrication of high contrast speckle pattern for high temperature digital image correlation measurement were also proposed. The technique can now enable the split Hopkinson bar testing to be performed at high temperature up to 1873 K under the loading conditions of compression or tension with the in situ observation and full field measurement of deformation as well. The paper concludes with summaries of the most important achievements and highlighting of the prospects, trends and remaining challenges for future research. [ABSTRACT FROM AUTHOR]

Background: There are a variety of approaches that can be employed for Hopkinson bar compression testing and there is no standard procedure. Objectives: A Split-Hopkinson pressure bar (SHPB) testing technique is presented which has been specifically developed for the characterisation of hazardous materials such as radioactive metals. This new SHPB technique is validated and a comparison is made with results obtained at another laboratory. Methods: Compression SHPB tests are performed on identical copper specimens using the new SHPB procedures at Imperial College London and confirmatory measurements are performed using the well-established configuration at the University of Oxford. The experiments are performed at a temperature of 20 ∘C and 200 ∘C. Imperial heat the specimens externally before being inserted into the test position (ex-situ heating) and Oxford heat the specimens whilst in contact with the pressure bars (in-situ heating). For the ex-situ case, specimen temperature homogeneity is investigated both experimentally and by simulation. Results: Stress-strain curves were generally consistent at both laboratories but sometimes discrepancies fell outside of the inherent measurement uncertainty range of the equipment, with differences mainly attributed to friction, loading pulse shapes and pulse alignment techniques. Small metallic specimens are found to be thermally homogenous even during contact with the pressure bars. Conclusion: A newly developed Hopkinson bar for hazardous materials is shown to be effective for characterising metals under both ambient and elevated temperature conditions. [ABSTRACT FROM AUTHOR]

Two-dimensional carbon fiber reinforced silicon carbide composites (2D-C/SiCs) exhibit excellent mechanical properties at high temperature. However, the weak interfacial performance limits range of their applications. In the present work, interlaminar shear strength (ILSS) of 2D-C/SiC was investigated. By using an industrial camera and an acoustic emission (AE) detection system, quasi-static tests at the shear strain rates from 10−5/s to 10−3/s were conducted. Strain contours revealed the damage evolution process. Peak frequencies of AE signals were clustered into three groups, corresponding to matrix damage, interfacial debonding and fiber fracture. Dynamic tests at the shear strain rates of 200/s and 600/s were conducted using a modified split Hopkinson bar (SHPB). The dynamic deformation phenomenon was captured by a high-speed camera. The high-speed images and digital image correlation (DIC) strain contours revealed the damage initiation under dynamic loading. Damage morphologies were observed by a scanning electron microscope (SEM). The real-time images and damage morphologies explained the mechanisms of shear strain-rate effect on ILSS. The proposed experimental method elicited a fresh perspective on designing dynamic interlaminar shear experiments. Moreover, the interlaminar shear performance over a wide range of shear strain rates enhanced our understanding of the strain-rate sensitivity of compressive strength and tensile strength of 2D-C/SiCs. Image 1 [ABSTRACT FROM AUTHOR]

The tensile testing of a needle-punched nonwoven fabric is presented. A high-sensitivity Split-Hopkinson Tensile Bar device was specifically designed for this purpose. The strain gauge measurements were combined with high-speed photography and Digital Image Correlation to analyse the deformation micromechanisms at high strain rates. The experimental set-up allowed to determine the wave propagation velocity of the as-received nonwove fabric, the evolution of the strain field with deformation and the wave interaction inside the fabric. The deformation was accommodated by the same micromechanisms observed during quasi-static tensile testing and ballistic impact, which comprised fibre straightening, rotation and sliding. Heterogeneous strain fields were developed in the nonwoven fabric as a result of the non-linear pseudoplastic response of the fabric and the internal dissipation due to the frictional deformation micromechanisms, preventing the propagation of high magnitude strain waves into the specimen. Additionally, the output forces were analysed to determine the influence of high-strain rates in the mechanical response of the nonwoven fabric, finding an increment of the stiffness for low applied strains under dynamic loading. These findings provide the basis to develop strain-rate dependent constitutive models to predict wave propagation in needle-punched nonwoven fabrics when subjected to impact loads. [ABSTRACT FROM AUTHOR]

The paper presents the results of investigation on X30MnAlSi26-4-3 austenitic steel subjected to dynamic compression using the split Hopkinson pressure bar. The strain rate was 3700 s−1. The compression test was also carried out without the use of breaking rings and then true strain was about 0.3. The split Hopkinson pressure bar test take only few milliseconds to complete during which time it is impossible to transfer the excess heat out of the specimen, therefore the test must be carried out in adiabatic conditions and so the increase of the temperature caused by the work of plastic deformation had to be calculated. The stepping load method was used in order to evaluate the effect of adiabatic heating on the properties of steel which allowed to maintain the isothermal deformation conditions. The paper presents the comparison of results obtained during deformation under adiabatic and isothermal conditions in correlation to structure changes occurring in course of dynamic compression.

The particle velocity and stress in the striker and bar generated by the striker impact on a bar with different general impedance are formulated based on one-dimensional assumptions. Departure of the impact-generated stress wave towards the rear end of the striker and arrival of the release wave from the rear to the front of the striker constitute one impact cycle. In cases where Z s ≤ Z b (Z is the general impedance, and subscripts 's' and 'b' denote striker and bar, respectively), only one impact cycle takes place because the striker is stationary or separated from the impact surface after the first impact cycle. As a result, only a single (primary) pulse is observed in the bar and striker. In the case where Z s > Z b, however, multiple impact cycles take place because the striker is compressing the bar continually after the first cycle. As a result, a series of step-wise residual pulses follow the primary pulse in the bar and striker. The magnitudes of the stress and particle velocity in the bar and striker calculated using the formulated equations are quantitatively consistent with the results of the numerical simulations, verifying the formulated one-dimensional equations. The equations formulated in this study may be useful for better understanding the various wave interaction phenomena that take place in a pseudo-one-dimensional impact system and for modifying/designing an impact system. [ABSTRACT FROM AUTHOR]

The compressive strength of concrete under biaxial dynamic compression The compressive strength of two concretes under biaxial dynamic compressive stress was investigated. A biaxial split Hopkinson bar was used, that applied the dynamic load to the test specimen in the form of two load waves perpendicular to each other. For reference, the static compressive strengths under biaxial load with different stress ratios of the two load axes were determined in a triaxial testing machine. The results indicate a partially superposition of the two strength‐increasing effects of multi‐axial compressive stress and increased strain rate. In particular, clear differences can be seen in the fracture and crack pattern for the different load types. [ABSTRACT FROM AUTHOR]

The dynamic mechanical behavior of aluminum alloy 2519A-T9I6 has been investigated by a split Hopkinson bar at temperature range from 298 to 523 K and at strain rates of from 103 to 5 × 103 s−1 in this study. The results show that both the strain rate and test temperature have a significant effect on the dynamic behavior and microstructure evolution of aluminum alloy 2519A-T9I6. Under similar strain rate, the strengthening effect of strain rate decreases with the increase in the test temperature. Moreover, the dynamic yield strength at above 423 K drops sharply at the strain rate of about 5000 s−1, which is attributed to the flow softening caused by decomposition of θ′ precipitates at high strain rate and high temperature. [ABSTRACT FROM AUTHOR]

Basalt fiber reinforced composites have a great potential to be used for high performance automotive and aeronautical components. Since these components might be subjected to impact loads, studying the behavior of basalt fiber composites at high strain rates is, therefore, important. The aim of this paper is to study the tensile behavior of woven basalt fiber reinforced epoxy composites at high strain rates in both warp and fill directions. Experiments were carried out using a split Hopkinson tensile bar facility. During the tests, full-field strain measurements were performed using high speed stereo digital image correlation. Reference quasi-static experiments were also performed. As such, the strain rates covered in this study ranged from 0.0006 s−1 to approx. 150 s−1. Results showed that the woven basalt epoxy composite was strain rate sensitive in both warp and fill directions. An increase in stiffness, Poisson's ratio, ultimate tensile strength, and ultimate tensile strain was observed with the increase of strain rate. Post mortem analysis of the fractured specimens using scanning electron microscopy revealed that the failure morphology was independent of the strain rate. Delaminations were present at all strain rates, however, a more homogeneous distribution of delaminations over the gauge section was observed with the increase of strain rate. Compared to conventional composites, the tested woven basalt epoxy composite showed excellent energy absorption capabilities at high strain rates, which can make this material a good candidate for impact resistant components. • High strain rate tensile behavior of woven basalt fiber reinforced epoxy composites was investigated. • Reference quasi-static experiments were also performed to compare the tensile behavior at low and high strain rates. • High speed 3D DIC was used to measure the full field displacements and strains on the surface of the sample. • Stiffness, Poisson's ratio, maximum tensile strength, and maximum tensile strains increased with increasing the strain rate. • Failure morphology was found to be (to a high degree) independent of the strain rate. [ABSTRACT FROM AUTHOR]

Abstract Mechanical engineering and its scientific principles constitute an essential core in medical science. Currently, different composite resins are widely used as restorative dental materials. However, their lack of adequate strength and toughness has led to research that is aimed at improving the mechanical properties of dental composites. In the present study, the behavior of three different dental materials is investigated under static and dynamic loading conditions. In the experimental tests, a split Hopkinson pressure bar is utilized which corresponds to the most commonly used experimental setup for examining material behavior under a high rate of loading. The examined dental composites experience impacts during their service life and also during car accidents or sport injuries. Hence, in the study, impact loading is modeled in an experiment. A series of compression tests is conducted from low to high strain rates up to 40 s − 1 , and the dynamic elastic moduli of three different dental composites are measured. Furthermore, studies on the compressed surface of the dental composite specimens are performed to improve the analysis with respect to the hardness of the materials. The responses of the examined composites to dynamic loadings verify the impact resistance of the materials. The results indicate the load-carrying capabilities of the dental composites. These results can be used for materials development and existing computational models. [ABSTRACT FROM AUTHOR]

The Influence of water saturation on the static and dynamic compressive strength of concrete The influence of water saturation on the static and dynamic compressive strength of concrete was investigated. Two different specimen geometries with defined degrees of water saturation were tested. The principle of the split Hopkinson bar was used for the dynamic compressive strength tests. It was found out that the compressive strength of concrete decreases with increasing degree of saturation, both in the static and the dynamic load cases. The amount of the reduction with increasing water saturation is the same under static and dynamic loading. Thus it can be stated that the influence of the water saturation is independent of the strain and loading rate. The dry specimens failed more explosively with an increased number of cracks and fragments, compared to the water‐saturated ones. [ABSTRACT FROM AUTHOR]

Tensile strength is an important material property for rocks. In applications where rocks are subjected to dynamic loads, the dynamic tensile strength is the controlling parameter. Similar to the study of static tensile strength, there are various methods proposed to measure the dynamic tensile strength of rocks. Here we examine dynamic tensile strength values of Laurentian granite (LG) measured from three methods: dynamic direct tension, dynamic Brazilian disc (BD) test, and dynamic semi-circular bending (SCB). We found that the dynamic tensile strength from direct tension has the lowest value, and the dynamic SCB gives the highest strength at a given loading rate. Because the dynamic direct tension measures the intrinsic rock tensile strength, it is thus necessary to reconcile the differences in strength values between the direct tension and the other two methods. We attribute the difference between the dynamic BD results and the direct tension results to the overload and internal friction in BD tests. The difference between the dynamic SCB results and the direct tension results can be understood by invoking the non-local failure theory. It is shown that, after appropriate corrections, the dynamic tensile strengths from the two other tests can be reduced to those from direct tension.

The dynamic behavior of the lithium-ion battery is evaluated by simulating the full battery system and each corresponding component, including the jellyroll and thin-foil electrodes. The thin-foil electrodes were evaluated using a novel design of split Hopkinson tensile bar (SHTB), while the jellyroll was evaluated using the split Hopkinson pressure bar (SHPB). A new stacking method was employed to strengthen the stress wave signal of the thin-foil electrodes in the SHTB simulation. The characteristic of the stress–strain curve should remain the same regardless of the amount of stacking. The jellyroll dynamic properties were characterized by using the SHPB method. The jellyroll was modeled with Fu-Chang foam and modified crushable foam and compared with experimental results at the loading speeds of 20 and 30 m/s. The dynamic behavior compared very well when it was modeled with Fu-Chang foam. These studies show that the dynamic characterization of Li-ion battery components can be evaluated using tensile loading of stacked layers of thin foil aluminum and copper with SHTB methodology as well as the compressive loading of jellyroll using SHPB methodology. Finally, the dynamic performance of the full system battery can be simulated by using the dynamic properties of each component, which were evaluated using the SHTB and SHPB methodologies.

The tensile testing of a needle-punched nonwoven fabric is presented. A high-sensitivity Split-Hopkinson Tensile Bar device was specifically designed for this purpose. The strain gauge measurements were combined with high-speed photography and Digital Image Correlation to analyse the deformation micromechanisms at high strain rates. The experimental set-up allowed to determine the wave propagation velocity of the as-received nonwove fabric, the evolution of the strain field with deformation and the wave interaction inside the fabric. The deformation was accommodated by the same micromechanisms observed during quasi-static tensile testing and ballistic impact, which comprised fibre straightening, rotation and sliding. Heterogeneous strain fields were developed in the nonwoven fabric as a result of the non-linear pseudoplastic response of the fabric and the internal dissipation due to the frictional deformation micromechanisms, preventing the propagation of high magnitude strain waves into the specimen. Additionally, the output forces were analysed to determine the influence of high-strain rates in the mechanical response of the nonwoven fabric, finding an increment of the stiffness for low applied strains under dynamic loading. These findings provide the basis to develop strain-rate dependent constitutive models to predict wave propagation in needle-punched nonwoven fabrics when subjected to impact loads.

Highlight • Symmetric split Hopkinson compression and tension tests using synchronized electromagnetic stress pulse generators. • The measured forces and velocities at the pressure bar/specimen interfaces are indeed identical and simultaneous. • Easy control of the synchronism and identity of generated stress pulses by the parallel connection of the stress pulse generators in the LC discharge circuit. • The formulas to calculate the stress, strain, and strain rate of the tested specimen in the symmetric test. • A novel dynamic loading technique to create the pure Mode I fracture in DCB test. Abstract This work presents an original impact testing arrangement and its applications using two identical synchronized stress pulses to load symmetrically the specimen sandwiched between the two identical Hopkinson pressure bars. In order to obtain two synchronized stress pulses, two identical electromagnetic stress pulse generators connected to the same LC discharge circuit are used. This symmetric impact loading configuration might be easily interchanged into a compressive as well as a tensile version because of the versatility of the electromagnetic stress pulse generators. In order to validate this apparatus, two materials (copper alloy and aluminum alloy) are tested in compression and in tension using this new experimental setup. The measured forces and velocities at the pressure bar/specimen interfaces are indeed identical and simultaneous. The derived material properties of tested materials are compared with those of the traditional split Hopkinson bars and a good agreement is found. A new symmetric Double Cantilever Beam (DCB) test using this experimental device is also presented. It allows overcoming the difficulties of only one moving side in previous work, which avoids an induced mode II component in a DCB test supposed to be a pure mode I testing. [ABSTRACT FROM AUTHOR]

Highlights • Present applications of SHPB in characterization of various cement-based materials. • Details of experimental tests on different concrete and UHPC under high rate of loading. • Data and information about specimen and fiber geometries on experimental studies. • Summarized results of numerical simulations on cement-based materials under test by SHPB. Abstract Our living standards are substantially influenced by the advent and development of new materials. The need for stronger building materials has led to useful innovations such as the fortification of concrete to produce Ultra-High-Performance Concrete (UHPC). Since numerous structures constructed by various types of concrete and UHPC, experience high rate of loading in their service life, reliable databehavior of this material is an important issue. In the current study, the behavior of different reinforced and unreinforced concrete, and also various UHPCs with and without fiber are comprehensively reviewed based on experiments by Split Hopkinson Pressure Bar (SHPB). The investigated parameters for the tests performed included matrix constituents, temperature condition, sample size, fiber material, and loading rate. The available results indicate that Ultra-High-Performance Fiber Reinforced Concrete (UHPFRC) is the most favorable material for utilization in impact-resistance structures. Since the fibers show crucial role in the dynamic strength, fibers material and their aspect ratio should be selected precisely. Applications of numerical simulations can significantly accelerate the material development process, therefore results of computational constitutive modeling are also presented in this paper. The summarized data can be used not only for comparison and the improvement of load carrying capacities of cement-based materials, but also can be beneficial in the development of new computational models. [ABSTRACT FROM AUTHOR]

Highlights • The study has been focused of the effect of high strain rates on sheet formability in balanced biaxial stretching condition. • An experimental equipment, based on the Nakazima test standard, has been designed and then mounted on a direct Hopkinson bar. • Dynamic tensile tests on AA6082 sheet samples have been carried out using the same direct Hopkinson bar. • The strain distribution over the dumbbell-like and Nakazima specimens has been evaluated by means of optical methods. • The tested alloy exhibits a relevant increment in the strain to failure when passing from quasi-static to high rate of loading. Abstract The knowledge of formability is important for the success of sheet metal forming processes. Notwithstanding they can involve relatively high speeds, higher than the quasi-static regime ones, few studies considering the strain rate effect on formability are available in literature. In the present paper, a study on the effect of strain rate on formability of AA6082 aluminum alloy, with a thickness of 1 mm, has been carried out. In order to evaluate the formability under dynamic loading conditions an experimental equipment, based on the Nakazima test, mounted on a direct tension-compression split Hopkinson bar (SHB), has been designed. The standard SHB method was also used to perform tensile to study the high strain rate behavior of AA6082 sheets. The results have been compared with the ones obtained in quasi-static conditions and engineering and true stress and strain data have been collected. A fast camera has been used to acquire images of the sheet samples during tests to obtain the true strain distribution. The study has been focused of the effect of high strain rates on sheet formability in balanced biaxial stretching condition. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

Full-field strain and strain-rate fields of rock materials under dynamic compression were studied by the high-speed three-dimensional digital image correlation (3D-DIC) method. A series of dynamic tests was conducted on Hawkesbury sandstones using a split Hopkinson pressure bar (SHPB) at three different strain rates. The real-time images of the loaded specimen were captured by two high-speed cameras at a frame rate of 200,000 frames per second (fps) with a resolution of 256 × 256 pixels. Wave propagation, dispersion and radial inertial effect on the specimen were found by DIC results. The strain rate vibration pattern on the specimen, which was visualised by DIC, found to be dependent on the input waveform. A recovery of strain in the post-peak stage was detected on the specimen by DIC, which is unrevealed in the traditional one-dimensional theory method (i.e., strain gauge signals). The results showed that strain localisation initiated from the interface of the bar and specimen with the order of tensile, shear and vertical. The initiation of crack from strain localisation is found rate-independent. Comparison between 2D- and 3D-DIC in strain measurement of the same experiment showed that the error in the strain obtained by 2D-DIC could be up to 32%. [ABSTRACT FROM AUTHOR]

Additively manufactured (AM) products use two scanning strategies to create the central core region and the inner region near the free-surface circumference i.e. the shell. Manufacturers usually machine the shell to meet standard mechanical property testing. However, it's essential to determine how the shell affects an AM product's mechanical properties. This study focused on the dynamic tension and compression mechanical properties of an AlSi10Mg alloy made using the laser powder bed fusion technique. Machined and non-machined samples with and without shells of different build sizes and orientations underwent testing using split Hopkinson bar systems. Machined products displayed anisotropic dynamic behavior, favoring vertically built samples for maximum dynamic stress and horizontally built samples for elongation. However, samples with a shell showed apparent isotropic dynamic behavior due to pore distribution in the shell regime. The high porosity in the shell region resulted from the high intensity of heating energy input during scanning. Differences in shell shape between Z (load parallel to the direction of the building) and XY (load tangential to the direction of the building) samples and a slower, repeated laser scanning process in the shell region contributed to differences in porosity distribution between orientations. High porosity negatively impacted mechanical behavior and reduced dynamic mechanical properties. [ABSTRACT FROM AUTHOR]