Split Hopkinson Tensile Bar Tests Research Articles

Dynamic tensile properties of a novel high strength and high toughness bolt steel: Constitutive models and microstructures

With the continuous increase of depth in underground engineering, rock bursts or rock explosions can cause instantaneous shock load and result in rock engineering disasters. To solve these problems, a novel high strength and high toughness (HSHT) bolt steel with constant resistance and large deformation is proposed. This study aims to investigate the dynamic constitutive models and microstructure of HSHT steel under extreme loading conditions. A series of dynamic tension tests were conducted using a Split Hopkinson Tensile Bar (SHTB) system with a high-speed camera. The results revealed that the HSHT steel exhibited a positive strain rate effect. As the strain rate increased from 1000 s−1 to 5000 s−1, the yield strength and ultimate strength of the HSHT steel increased gradually. However, no obvious changes in the uniform elongation. Specifically, the yield strength, tension strength, and uniform elongation of strain rate at 5000 s−1 were 1100 MPa, 1170 MPa, and 30%, respectively. According to the results of the SHTB tests, a modified Johnson-Cook model was proposed. Additionally, we investigated the microstructure of HSHT steel at a high strain rate through microscopic characterization techniques. A large number of fine and deep dimples were observed near the fracture of HSHT steel under dynamic loading. Moreover, the HSHT steel demonstrates a significant presence of nano-twins, which effectively impedes crack propagation both within the grain interior and along grain boundaries. The above findings reveal the relationship between mechanical behavior and microstructure in HSHT steel within dynamic loading conditions, providing valuable insights for the design of HSHT steel in underground engineering.

Tensile performance of normal and high-strength structural steels at high strain rates

The split Hopkinson tensile bar (SHTB) tests are carried out for three structural steels, i.e., Q355 steel, Q460 steel and Q620 steel, for the increasing demand on the mechanical properties of steels at high strain rates. Four strain rate levels, i.e., 1000 s−1, 2000 s−1, 3000 s−1 and 4000 s−1, are used in the tests. The corresponding quasi-static tensile loading tests are also performed for comparison. The observation of fracture morphology and the metallographic organization is also conducted to reveal the mechanism of steel under high strain rate loading. Subsequently, a stress–strain relationship model is proposed based on the modified Johnson–Cook model, and the parameters are calibrated by data from both quasi-static tensile tests and SHTB tests. The results show that the effect of strain rate hardening on the strength of steel is significant, and the proposed model can well describe the mechanical behaviour of these three structural steels at high strain rates.

Strain-Rate Effect on Anisotropic Deformation Characterization and Material Modeling of High-Strength Aluminum Alloy Sheet

Aluminum alloy sheets are widely applied as structure components in automotive, aircraft and other industries to realize lightweight. Nowadays, many high strain rate forming techniques have been developed to improve their formability and widen their application. To ensure the reliability of the aluminum alloy structure components under high strain rate conditions, it is imperative to develop a thorough understanding of the alloy’s mechanical properties. In this paper, taking high-strength 6XXX aluminum alloy sheet as an example, the anisotropic deformation characterization and corresponding material models at various strain-rate conditions are investigated systematically. The material hardening curves and anisotropic plastic yielding stresses were achieved based on the quasi-static uniaxial tensile test and the split Hopkinson tensile bar tests. In this study, the Johnson–Cook hardening model and two anisotropic yield functions are applied to well describe the strain-rate-dependent anisotropic plastic deformation behavior. In addition, the fractographic characterization of the fractured samples at various strain-rate conditions are measured and compared. The study systematically investigates the influence of strain rate on the anisotropic deformation behavior of the high-strength aluminum alloy sheets and gives the basic experimental data for their application in engineering fields in the future.

Orthotropic elastic–plastic–damage model of beech wood based on split Hopkinson pressure and tensile bar experiments

The hardwood is a natural composite, which finds applications in many fields such as an important building material. Attention was paid to the accurate description of the European beech wood under high strain rates in order to constitute a reliable input for the finite element method. It was done on the basis of split Hopkinson tensile bar tests, taking also into consideration the previously reported pressure bar tests to capture the tensile–compressive failure asymmetry, which was incorporated through the stress triaxiality. Strains measured on the steel bars for specimens oriented in all principal loading directions were used to calibrate the fully orthotropic behaviour of elasticity, plasticity and fracture. The concept of continuum damage mechanics was applied to the damage accumulation in order to simulate the crack propagation within explicit finite elements realistically.

Laser Metal Deposition of Inconel 718 Alloy and As-built Mechanical Properties Compared to Casting.

In the last years, powder-based Laser Metal Deposition (LMD) has been attracting attention as a disruptive Additive Manufacturing (AM) technique for both the fabrication and restoration of Inconel 718 components, enabling to overcome current limitations faced by conventional manufacturing processes in terms of manufacturing costs, tool wear, and lead time. Nevertheless, the uncertainty related to the final mechanical performance of the as-built LMD parts limits a wider adoption of such technology at industrial level. This research work focuses on the mechanical characterization of as-built Inconel 718 specimens through split Hopkinson tensile bar tests performed at different strain rate conditions. The influence of laser power on the final mechanical behavior of the as-built tensile samples is discussed and compared with the mechanical response of as-cast ones. The as-built specimens exhibit a high internal density (i.e., 99.92% and 99.90% for 300 W and 400 W, respectively) and a more ductile behavior compared to the as-cast ones for every evaluated strain rate condition. The strain hardening capacity of the as-built samples increases with the laser power involved in the LMD process, reaching an average Yield Strength of 703 MPa for specimens realized at 400 W and tested at 800/s.

A Study on the Effects of Specimen Geometry on Measurement Accuracy of Dynamic Constitutive Properties of Metals Using SHTB

Determination of dynamic tensile response of materials has been a challenge because of experimental difficulty. The split Hopkinson tensile bar (SHTB) is one of the most widely used devices for characterization of various materials under dynamic-tensile loading conditions. Since one-dimensional wave propagation in bars is disturbed by specimens and grips, however, SHTB measurement accuracy may not be guaranteed. This means that the stress–strain curve of the specimen that is calculated using strains at bars may not indicate the real stress–strain relation of the specimen. In this study, simulations for the SHTB test were carried out to investigate the effects of thread pitch, specimen length, specimen diameter, and thread inner diameter of the specimen on the measurement accuracy for two types of metals with medium and high yield strengths. Finally, specimen shapes are recommended for accurate measurement of the stress–strain relation of tantalum and tungsten carbide.

Effect of build orientation on the quasi-static and dynamic response of SLM AlSi10Mg

Selective Laser Melting (SLM) allows the fabrication of complex geometries with high resolution and robust mechanical properties. However, the manner of manufacture – melting of metallic powder with a laser power source – affects microstructure and results in mechanical anisotropy. While some studies have sought to characterise the microstructure and performance of SLM AlSi10Mg, the dynamic response, particularly with regard to anisotropic effects, remains relatively undefined. To overcome this deficit AlSi10Mg specimens were fabricated using SLM with three different build orientations, and quasi-static and dynamic split-Hopkinson tensile bar tests were performed to characterise the tensile properties of the material at strain rates ranging from 3.33 x 10-2 to 2.4 x 103 s-1. The microstructure of as-manufactured specimens and fracture surfaces of failed specimens were analysed. Quasi-static and dynamic results showed little difference between build orientations with regard to strength, but components loaded perpendicular to the build direction were found to be more ductile than other build orientations. Significant scatter was observed in dynamic results, suggesting no strain rate sensitivity of the material in the tested strain rate range. Build orientation was found to affect fracture surface morphology of dynamically tested specimens due to fracture paths following melt pool boundaries. These results assist in the characterisation of the anisotropic effects of build orientation on quasi-static and dynamic behaviours of SLM AlSi10Mg towards the further commercial adoption of the manufacturing technique and material.

Direct dynamic tensile study of concrete materials based on mesoscale model

Dynamic tensile tests of concrete materials at high strain rates are primarily splitting experiments and spalling experiments at present, however, certain limitations exist in these methods to indirectly study dynamic tensile mechanical behaviour of concrete materials. Thus it is necessary to perform direct dynamic tensile study of concrete materials with a large-diameter split Hopkinson tensile bar (SHTB). Concrete is considered as a two-phase composite material which composed of coarse aggregates and cement matrix in this paper, then cylindrical and dumbbell-shaped three-dimensional mesoscale concrete models are established and applied to SHTB simulation research. Three contact modes between concrete specimen and member bars are discussed based on existing experiments and our numerical simulation analysis. The results show that established finite element models can effectively simulate SHTB tests and mesoscale concrete models also reflect proper tensile behaviour at high strain rates, which also provides convenience to mesoscopic numerical simulation research of many other heterogeneous materials. At last, a proper contact mode is chosen to perform mesomechanics analysis of concrete materials subjected to four types of tensile impact wave, then five mesoscale concrete models with different aggregates content are established and corresponding numerical simulation analysis are conducted.

Fracture mechanism and model of 18Ni maraging steel

ABSTRACT18Ni maraging steel is normally used to make key components by machining processes. It is necessary to understand the fracture mechanism and behaviour of 18Ni maraging steel for the study and further optimisation of machining processes. In this paper, the Split Hopkinson Tensile Bar tests accompanied by quasi-static mechanical tests are carried out to relate the stress triaxiality, deformation temperature and strain rate with the fracture strain. During the building up of the fracture model, it is noticed that the change of fracture mechanism from 600°C to 900°C brings about a non-linear relationship of fracture strain with the deformation temperature. This results in a large error in fitting the Johnson–Cook (J-C) fracture model, so a modified fracture model is put forward.

Intelligent optimization design of specimen on sheet materials for Split Hopkinson Tensile Bar tests

The Split Hopkinson Tensile Bar (SHTB) is one of the most widely used methods to study the high strain rate behavior of materials. For these experiments usually dogbone-shaped sheet specimens are used. However, there’s no agreement on the exact dimensions. In the present study, mechanism of the influence of specimen responses on accuracy of SHTB experiments was investigated with finite element program ABAQUS (Explicit). Indicators which can evaluate the measurement accuracy of specimens are proposed based on this. Orthogonal test is designed to establish the sample database for back-propagation (BP) neural network, which is adopted to fit the non-linear mapping from structure parameters to accuracy indicators of specimen. Optimal design of structure for sheet specimen is obtained with Genetic Algorithm (GA) according to the fitness of individual determined by trained and qualified BP neural network. At last, numerical simulations are adopted to verify the validity of the optimal structure for sheet specimen. The result of this study can provide recommendations for specimen design and data reliability analysis in Split Hopkinson Tensile experiments.

Prediction of Fatigue Resistance in Lead-Free Ni-Doped SAC 1205 Solder Alloys Using a Rate-Dependent Material Model and Subjected to Drop Tests

In this paper, a novel Ni-doped SAC 1205 (Sn–1.2Ag–0.5Cu) solder alloy is investigated to determine its mechanical behavior at a high strain rate. The Ni-doped SAC 1205 solder alloy has never been discussed in terms of high-strain-rate testing. The purpose of the SAC 1205 solder material doped with Ni is to enhance the mechanism of the solder alloy and delay the formation of the brittle intermetallic compound. In the drop test standard (Joint Electron Device Engineering Council condition B), the test vehicle was subjected to impact loading with a peak acceleration of 1500G and a time duration of 0.5 ms. Because the test board supported the impact loading for a very short time, the solder joint material was considered to have a rate-dependent elastic–plastic behavior. The quasi-static mechanical behavior and dynamic behavior were estimated for the simulation modeling by conducting a quasi-static tensile test and a split Hopkinson tensile bar test. A finite-element model was used to analyze the stress and strain of the solder joint during the drop test. The experimental results revealed that Young’s modulus and yielding strength of the Ni-doped SAC 1205 solder alloy increased during the high-strain-rate testing. Therefore, a rate-dependent elastic–plastic model of the Ni-doped SAC 1205 solder alloy was designed and implemented to describe the real mechanical behavior of a solder joint at a high strain rate during a drop test. The primary failure mode occurs at the top of the solder joint (package side) during the drop test. The predicted fatigue model we provided was focused on the primary failure mode. Eventually, the fatigue life curve of the rate-dependent elastic–plastic model offered the greatest accuracy in predicting the solder joint fatigue life.

Numerical investigation into the stress wave transmitting characteristics of threads in the split Hopkinson tensile bar test

The Split Hopkinson tensile Bar (SHTB) is one of the most widely used methods to study various material behaviors under tensile loading and high nominal strain rate. Since the specimens and grips with thread may disturb the wave propagation between bars and specimen, the stress-strain relation of the specimen, which is calculated from the strains of the incident and transmitter bars, may not match with the stress-strain relation measured directly from the specimen. In order to ensure a high accuracy of SHTB system, specimen calibration is usually carried out. To ensure the no wave distortion in the thread region, the stress wave should be entirely transmitted from the incident bar to the transmitter bar in tests using the calibration specimen (not the real specimen) of the same material and the same diameter with the transmitter and incident bars. Therefore, the strain signals at the incident and transmitter bars need to be the same. This study investigated the wave transmit characteristics through threads in SHTB test. To investigate the effect of the thread on the wave transmission, a specimen with the same diameter of the incident and the transmitter bars is used in the SHTB test. The effects of the thread inner diameter, yield stress, thread type, and strike velocity on the wave propagation characteristics are investigated. Based on the analysis results for various conditions, a proper design guide for a thread shape to secure the measurement accuracy in the SHTB test is proposed.

A New Technique for Conducting Split Hopkinson Tensile Bar Test at Elevated Temperatures

In this paper, a new experimental technique is developed for conducting split Hopkinson tensile bar (SHTB) test at elevated temperatures, which includes a Quick Hook Joint to join the specimen and the loading bars and a system to heat, transmit and mount the specimen. The effectiveness of the Quick Hook Joint is demonstrated through numerical simulation. The cold contact time (CCT) experienced by the specimen is measured by laser sensors and the two cold contact processes are numerically simulated. The simulation results suggest no evident temperature decrease in the specimen gage section and no significant temperature increase in the loading bars due to thermal diffusion. Finally SHTB tests of a 45# steel are conducted at various temperatures to provide validations and to demonstrate the feasibility of the developed experimental apparatus and testing method.

Determination of Dynamic Compressive and Tensile Behavior of Rocks from Numerical Tests of Split Hopkinson Pressure and Tension Bars

FEM-based numerical testing systems of the split Hopkinson pressure bar (SHPB) and the split Hopkinson tensile bar (SHTB) are established to study the characteristics of rock materials under dynamic compressive and tensile loadings. First of all, the accuracy and applicability of the numerical testing system are validated and calibrated through comparison between the laboratory measurements and the simulation results. Subsequently, the dynamic behavior of rock is analyzed in detail with the numerical testing system followed by the underlying physical mechanism. For the SHPB tests, the simulation results demonstrate that the incident waveform is determined by the striker length, the striker shape and the pulse shaper. The dynamic increase factor (DIF) of the rock specimen varies with different impact velocities, which is attributed to the strain rate effect. The rock specimen size and bar size also have effects on the DIF. In addition, the interfacial friction between the rock specimen and the bars cannot be ignored. For the SHTB tests, it is found that the incident waveform is dependent on the striker tube length and the striker tube thickness. In addition, similar to the SHPB tests, the impact velocity, rock specimen size and bar size all have strong effects on the rock dynamic tensile strength.