Compressive split-Hopkinson Bar Research Articles

Experimental and virtual testing of mode II and mixed mode crack propagation under dynamic loading

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.

Impact-dynamic properties of aromatic hyperbranched polyester/RTM6 epoxy nanocomposites

Present study investigates the influence of aromatic hyperbranched polyester (AHBP) nanofillers on the impact-dynamic properties of RTM6 epoxy. The strain rate dependence of the energy absorption capacity of neat RTM6 epoxy and RTM6 nanocomposites with filler weight ratios of 0.1%, 1%, and 5% was determined through compression testing. Additionally, static and dynamic tensile tests were performed on these materials. Universal testing machines were used for the quasi-static tests, split Hopkinson bar setups for the high-strain rate experiments. Furthermore, dynamic single-edge notch bending (SENB) tests were conducted to measure fracture toughness using an adapted split Hopkinson compression bar setup. In addition to global deformation measurements, full-field deformation measurements were also performed in all experiments using the digital image correlation (DIC) technique. The results indicate that the energy absorption capacity, tensile strength, fracture strain, and toughness are significantly influenced by both strain rate and filler content of the RTM6 epoxy.

The Influence of the Strain Rate and Prestatic Stress on the Dynamic Mechanical Properties of Sandstone-A Case Study from China.

A series of conventional dynamic uniaxial compressive (CDUC) tests and coupled static dynamic loading (CSDL) tests were performed using a split Hopkinson compression bar (SHPB) system to explore the variable dynamic mechanical behavior and fracture characteristics of medium siltstone at a microscopic scale in the laboratory. In the CDUC tests, the dynamic uniaxial strength of the medium sandstone is rate-dependent in the range of 17.5 to 96.8 s-1, while the dynamic elastic modulus is not dependent on the strain rate. Then, this paper proposes a generalized model to characterize the rate-dependent strength from 17.5 to 96.8 s-1. In the CSDL tests, with increasing initial prestatic stress, the dynamic elastic modulus and dynamic strength increase nonlinearly at first and then decrease. The results show that two classical morphological types (i.e., Type I and Type II) are observed in the dynamic stress-strain response from the CDUC and CSDL tests. By scanning electron microscopy (SEM), microscopic differences in the post-loading microcrack characteristics in the behavior of Type I and Type II are identified. In Class I behavior, intergranular fracture (IF) usually initiates at or near the grains, with most cracks deflected along the grain boundaries, resulting in a sharp angular edge, and then coalesces to the main fracture surface that splits the specimen along the direction of stress wave propagation. In contrast, Class II behavior results from the combined IF and transgranular fracture (TF).

Mechanical properties of steel wires of 1670 MPa, 1770 MPa, and 1860 MPa grades under high strain rates and temperatures

In this study, the mechanical behavior under high strain rates and high temperatures of 1670 MPa, 1770 MPa, and 1860 MPa grades steel wires were investigated. Quasi-static tensile experiments (10−4 s−1), split Hopkinson compression bar experiments (102–103 s−1) and high-temperature tensile experiments were conducted (20–900 °C). A total of 89 samples were tested for the study. Stress-strain curves were obtained for different strain rates and different temperatures. The experiments indicated that the strain rate had an obvious influence on the behavior of high-strength steel wires, both the yield and ultimate strength increased with increasing strain rate. The strength ratio and strain rate sensitivity coefficients of high-strength steel wires were investigated for high strain rates. The mechanical properties of the steel wire are significantly reduced under high temperatures. The Cowper-Symonds (C-S) and Johnson-Cook (J-C) constitutive models were determined using experimental data. By contrasting the proposed models with the experiment data, the precision of the constitutive models was confirmed.

Study on mechanical properties of (1 % PVA fiber +1 % Steel fiber) PS‐ECC under the coupling effects of elevated temperature and dynamic loads

AbstractThe coupling effect of temperature and impact loads can cause the concrete strength to decay and lead to the failure of the structure that cannot reach the designed load‐bearing strength. In this study, the evolution laws of dynamic compressive strength, dynamic increase factor (DIF), and absorbed energy of (1% polyvinyl alcohol [PVA] fiber +1% steel fiber) PS‐ECC (engineered cementitious composite) were obtained by a split Hopkinson compression bar test. The conclusions can be concluded: the dynamic compressive strength, DIF, and absorbed energy of PS‐ECC largely show a tendency to decrease with increasing temperature; however, the recovery effect occurs at 200°C due to the accumulation of steam inside the specimen to counteract the impact force. Then, 200°C becomes a threshold value, and the sensitivity of PS‐ECC to strain rate shows a steep drop when the temperature exceeds 200°C. Compared to 200°C, the growth rate of dynamic compressive strength, DIF, and energy absorption capacity from strain rate 50–80 s−1 decreases by 50.65, 55.24, and 45.71% at 300°C, the growth rate decreases by 55.87, 56.5, and 48.45% at 400°C. The impact energy absorption of PS‐ECC was provided mainly by PVA fiber. The energy absorption of PS‐ECC is almost the same at 300 and 400°C for the same strain rate due to PVA fiber burning disappears.

Dynamic mechanical properties of FGH4097 powder alloy and the applications of its constitutive model

As a typical multiphase alloy, FGH4097 has good lasting strength (thermal stability and thermal fatigue properties), excellent wear resistance and heat resistance. It has been widely used in aerospace and other hot-end parts manufacturing fields. Most of the existing researches on the mechanical properties of FGH4097 alloy are carried out at low strain rate. There is no effective research on high temperature dynamic mechanical properties of FGH4097. Through the impact compression test of FGH4097 under different strain rates and different temperature conditions by the split Hopkinson compression bar equipment, the corresponding true stress-true strain curve was obtained, the strain rate sensitivity and temperature sensitivity of the material were analyzed. The specific heat capacity of FGH4097 alloy at different temperatures was measured by a laser thermal conductivity meter, and the adiabatic temperature rise of the alloy at different temperatures and strain rates was studied. The isothermal true stress-strain curve of the alloy was obtained by calculating the thermal softening coefficient. Two constitutive equations, Johnson-Cook and Power-Law, were introduced to fit and characterize the dynamic mechanical properties of FGH4097 alloy at different strain rates and temperatures, and the corresponding material constitutive models were obtained. Comparing the fitting accuracy of the two models, it is found that the fitting accuracy of the Power-Law model is higher, with an average error of less than 8%. The cutting simulation of FGH4097 alloy was carried out by AdvantEdge software, and the effects of strain and strain rate on the machinability of FGH4097 alloy were investigated.

Experimental Study on the Dynamic Mechanical Properties of a Jointed Rock Mass under Impact Loading

The propagation law of blast stress waves in jointed rock masses is closely related to the characteristics of the joint filling medium. The water content and thickness of the filling medium affect the propagation characteristics of the blast stress wave in the joint surface. Based on similarity theory, mortar-red clay-mortar composite specimens with different moisture contents (36%, 39%, 42%, and 45%) and medium thicknesses (3 mm, 6 mm, and 9 mm) were prepared with red clay as the filling medium. The dynamic uniaxial compression test of the specimen under different strain rates was carried out by using a variable cross-section split Hopkinson compression bar with a diameter of 50 mm. The effects of strain rate, moisture content, and medium thickness on the stress wave propagation characteristics, dynamic stress-strain curve, peak stress, crushing energy consumption density, and crushing characteristics of mortar-red clay-mortar composite specimens were analyzed. The results show that the blocking effect of joints on stress waves increases significantly with increasing filling medium thickness and water content. With increasing strain rate, the peak stress and energy of the composite specimen increase and there is an obvious strain rate effect. With the increase in the thickness and moisture content of the filling medium, the peak stress, dynamic elastic modulus, energy consumption density per unit volume, and crushing degree of the specimen gradually decrease and the nonlinear deformation is more obvious. There is a good linear relationship between the peak stress and the thickness and moisture content of the filling medium. The existence of joints greatly weakens the damage effect of stress waves on the specimens behind the joints, and with the increase in the thickness and water content of the filling medium, the degree of damage to the specimens behind the joint surface decreases under the same impact pressure and the weakening effect becomes increasingly significant.

Experimental and numerical studies of NiTi dynamic fracture behaviors under the impact loading

The mechanical behaviors of NiTi shape memory alloy under the impact loading raised many interests in the fields of engineering and science, whereas its fracture process has not been well understood so far. In this work, multi-scale experiments and numerical simulations were conducted to comprehensively study the dynamic fracture performances of NiTi alloy. The impact crack experiments were carried out with the utilization of split Hopkinson compression bar, which showed the variation of crack propagation rate versus impact velocity. The Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) were employed to observe the fracture surfaces. The micro images revealed that the fracture behaviors were manifested as quasi-brittle fracture features. By taking the plastic evolution into consideration, a novel constitutive model based on Boyd and Lagoudas phase transition formulation was proposed, which overcomes the limitation of simultaneously describing super-elastic and plastic behaviors of NiTi. With the novel constitutive model, the crack propagation process was accurately reproduced with numerical results in good agreements with our experimental observations. Furthermore, the representative volume element (RVE) with Voronoi tessellation was employed to numerically study the grain size effect on intergranular fracture toughness. It was demonstrated that the reduction of grain size promoted the hinderance effect of intergranular fracture, which led to an evident improvement of intergranular fracture toughness in model with a smaller grain size.

Experimental study on dynamic recrystallization of titanium alloy Ti6Al4V at different strain rates

The dynamic recrystallization condition and law of Ti6Al4V alloy at all strain rates are investigated through thermal processing simulation test, split Hopkinson compression bar test and milling test. The theoretical calculation of the experimental results and the microstructure observation showed that the dynamic recrystallization become possible under the conditions of high strain rate and low strain rate. The model uses two expressions to describe the material constitutive characteristics of different critical strain value intervals; the subprogram written in Fortran language is imported into AdvantEdge FEM software, and the finite element analysis of John-Cook constitutive and improved constitutive is carried out respectively. The comparative study of simulation proves that the improved constitutive equation is closer to the high temperature and high impact environment of high-speed cutting of titanium alloy, which has certain guiding significance for high-speed cutting of titanium alloy.

Preparation of impact-resistant functional polyurea coatings and effect of γ-ray irradiation on its microstructure and performance

New polyurea coatings were synthesized using aromatic diphenylmethane diisocyanate, large molecular weight amino-terminated polyether, and appropriate additives. Fourier transform infrared (FTIR) spectroscopy and field emission scanning electron microscopy were used to examine the effects of γ-ray irradiation with different dose rates and cumulative doses on the microstructure of the new polyurea coatings. The variations of mechanical properties of the polyurea coatings before and after irradiation were studied using an electronic universal testing machine and a split Hopkinson compression bar. FTIR spectroscopy showed that the raw materials for preparing polyurea coatings had reacted completely, and polyurea had been synthesized. An analysis of the mechanical properties showed that the mechanical behavior of the new polyurea changed gradually from rubbery under a quasi-static load to glassy under a dynamic load, showing excellent impact resistance. The characteristic absorption peaks of the FTIR spectrum of polyurea did not change significantly before and after irradiation. However, the color of the polyurea changed from light yellow to dark red with decreasing dose rate or increasing cumulative dose, which is related to the radiation oxidation time. Some white fragments appeared after the polyurea was irradiated with γ-rays. The number of white fragments increased with decreasing dose rate or increasing cumulative dose, which may be caused by the soft segment embrittlement of polyurea under radiation oxidation. A comparison of the mechanical properties under different γ-ray irradiation conditions found that the newly synthesized polyurea coatings had good resistance to γ-ray radiation. Moreover, their performance was improved to a certain extent after irradiation. The performance of irradiated polyurea was optimum when the irradiation dose rate was 2 KGy/h, and the cumulative dose was 220 KGy.

Pressure and temperature dependent dynamic flow and failure behavior of PMMA at intermediate strain rates

Polymethylmethacrylate (PMMA) has been widely used as transparency in aerospace and automobile engineering to enhance structural reliability under impact loading such as bird strike events. This study investigates the confined compressive behaviour of PMMA at quasi-static 0.01 s−1, medium rate 1-100 s−1 and intermediate strain rates 100-1000 s−1 from room temperature to elevated temperatures. Constant intermediate strain rate loading is achieved by using pulse shaping technique on a bespoke in-house developed split Hopkinson compression bar equipped with a high-speed camera and an environmental chamber. The material presents significant strain rate, temperature and pressure sensitivities. The failure mode at intermediate strain rate changes from brittle fragmentation without confinement to adiabatic shear banding with medium lateral confinement (65.3 MPa), which can be seen from the post-peak slope in the constitutive behavior, melting flow PMMA filaments observed in high speed photography and microstructural analysis. A series of intermediate strain rate experiments at elevated temperatures indicate that the higher temperature results in ductile failure for corresponding pressures. The temperature and strain rate dependent Drucker-Prager (DP) model is found to describe the response of PMMA. The brittle-ductile transition and pressure dependent model provide a better understanding of PMMA at intermediate strain rates, which is appropriate for engineering applications.

Experimental Study on Mechanical Properties of High Performance Hybrid Fiber Concrete for Shaft Lining

In order to solve the problem of highly brittle shaft lining under dynamic loading, a combination of hybrid fiber concrete mixed with steel and polypropylene fiber is proposed to make shaft lining. C60, the concrete commonly used in shaft lining, was selected as the reference group. The static mechanical properties, dynamic mechanical properties, and crack failure characteristics of the hybrid fiber concrete were experimentally studied. The test results showed that compared to the reference group concrete, the compressive strength of the hybrid fiber-reinforced concrete did not significantly increase, but the splitting tensile strength increased by 60.4%. The split Hopkinson compression bar results showed that the optimal group peak stress and peak strain of the hybrid fiber concrete increased by 58.2% and 79.2%, respectively, and the dynamic toughness increased by 68.1%. The strain distribution before visible cracks was analyzed by the DIC technology. The results showed that the strain dispersion phenomenon of the fiber-reinforced concrete specimen was stronger than that of the reference group concrete. By comparing the crack failure forms of the specimens, it was found that compared to the reference group concrete, the fiber-reinforced concrete specimens showed the characteristics of continuous and slow ductile failure. The above results suggest that HFRC has significantly high dynamic splitting tensile strength and compressive deformation capacity, as well as a certain anti-disturbance effect. It is an excellent construction material for deep mines under complex working conditions.

Design and Simulation of Insert to Measure Dynamic Sliding Friction in a Split-Hopkinson Pressure Bar

Interfacial friction is a key aspect to understanding and modelling dynamic processes in which materials interact. However, friction is a complex phenomenon that depends on a multitude of factors, including sliding velocity. Understanding how friction behavior changes as a function of sliding rate is thus crucial for accurately simulating dynamic processes. Recent literature has shown that the split-Hopkinson pressure bar can be adapted for friction measurements associated with high sliding rates. The present work introduces an insert designed to be transferrable between a quasi-static load frame and a compression split-Hopkinson bar, enabling friction measurements across a wide range of sliding velocities (10-4 – 20 m/s). Here, the split-Hopkinson pressure bar setup is modelled using a multiphysics research code (FLAG), developed at Los Alamos National Laboratory (LANL), to identify and reduce potential issues in the configuration prior to experimental implementation.

Effect of Strain Rate on the Plastic Deformation and Fracture of 90W-7Ni-3Fe Alloy Prepared by Liquid-Phase Sintering

In this study, the effect of the strain rates on the plastic deformation behavior and fracture mechanism of 90W-7Ni-3Fe heavy alloy prepared by liquid-phase sintering was investigated. Stress–strain tests were carried out in tensile and compression under quasi-static condition with the strain rate ranging from 10−3 to 1 s−1. Dynamic test was examined using a compression split Hopkinson bar at strain rates ranging from 1000 to 1700 s−1 in order to evaluate the effects of high strain rate on dynamic plastic deformation behavior. Quasi-static tests results show that the strength and hardness of this alloy increase with increasing strain rate. Results indicate that the as-sintered 90W-7Ni-3Fe heavy alloy has obvious strain-hardening characteristic under tensile or compression deformation under quasi-static condition. At this time, strain hardening is the main factor. Nevertheless, strain-hardening and strain-softening behaviors of the specimens occur simultaneously under dynamic compression condition. The effect of compressive stress on the work hardening is more obvious. A strong relevance of work-hardening effect on strain rate is also found. And the fracture mode of quasi-static and dynamic compression samples is also different.

Compression, Tension and Shear Testing of Fibrous Composite with the Split Hopkinson Bar Technique

The Split Hopkinson Bar (SHB) technique is used for high strain rate testing of T800/F3900 composite in compression, tension and shear. Digital Image Correlation (DIC) is used for measuring the full-field deformation on the surface of the specimen by using Shimadzu HPV-X2 high-speed video camera. Compression tests have been done on specimens machined from a unidirectional laminate in the 0°and 90° directions. Tensile tests were done in the 90° direction. Shear tests were done by using a notched specimen in a compression SHB apparatus. To study the effect of strain rate, quasi-static testing was also done using DIC and specimens with the same geometry as in the SHB tests. The results show that the DIC technique provides accurate strain measurements even at strains that are smaller than 1%. No strain rate effect is observed in compression in the 0° direction and significant strain rate effects are observed in compression and tension in the 90° direction, and in shear.

Strain measuring accuracy with splitting-beam laser extensometer technique at split Hopkinson compression bar experiment

Abstract An accuracy problem of strain measurement at compression split Hopkinson compression bar experiments with a splitting-beam laser extensometer was considered. The splitting-beam laser extensometer technique was developed by Nie et al. to measure strain of a specimen during its tension under a high strain rate loading condition. This novel concept was an inspiration for the authors to develop own laser extensometer system, which allows for simultaneous and independent measurement of displacement of bar ends between which a compressed material specimen is placed. In order to assess a metrological property of this measuring system, a wide range of high strain rate experiments were performed, including tests with various sample materials (Al 5251, Cu OFE) with different rate of strain, and with the use of two bars material. A high accuracy of the developed laser extensometer was found in measurement of specimen strain, for which uncertainty is not greater than 0.1% and, for a typical specimen dimension, the maximum permissible error is 4.5 μm.

Tensile Split Hopkinson Bar Technique: Numerical Analysis of the Problem of Wave Disturbance and Specimen Geometry Selection

Abstract A method of tensile testing of materials in dynamic conditions based on a slightly modified compressive split Hopkinson bar system using a shoulder is described in this paper. The main goal was to solve, with the use of numerical modelling, the problem of wave disturbance resulting from application of a shoulder, as well as the problem of selecting a specimen geometry that enables to study the phenomenon of high strain-rate failure in tension. It is shown that, in order to prevent any interference of disturbance with the required strain signals at a given recording moment, the positions of the strain gages on the bars have to be correctly chosen for a given experimental setup. Besides, it is demonstrated that - on the basis of simplified numerical analysis - an appropriate gage length and diameter of a material specimen for failure testing in tension can be estimated.

Constitutive modeling for Ti-6Al-4V alloy machining based on the SHPB tests and simulation

A constitutive model is critical for the prediction accuracy of a metal cutting simulation. The highest strain rate involved in the cutting process can be in the range of 104–106 s–1. Flow stresses at high strain rates are close to that of cutting are difficult to test via experiments. Split Hopkinson compression bar (SHPB) technology is used to study the deformation behavior of Ti-6Al-4V alloy at strain rates of 10–4–104s–1. The Johnson Cook (JC) model was applied to characterize the flow stresses of the SHPB tests at various conditions. The parameters of the JC model are optimized by using a genetic algorithm technology. The JC plastic model and the energy density-based ductile failure criteria are adopted in the proposed SHPB finite element simulation model. The simulated flow stresses and the failure characteristics, such as the cracks along the adiabatic shear bands agree well with the experimental results. Afterwards, the SHPB simulation is used to simulate higher strain rate(approximately 3×104 s–1) conditions by minimizing the size of the specimen. The JC model parameters covering higher strain rate conditions which are close to the deformation condition in cutting were calculated based on the flow stresses obtained by using the SHPB tests (10–4–104 s–1) and simulation (up to 3×104 s–1). The cutting simulation using the constitutive parameters is validated by the measured forces and chip morphology. The constitutive model and parameters for high strain rate conditions that are identical to those of cutting were obtained based on the SHPB tests and simulation.

Effects of Weave Styles and Crimp Gradients in Woven Kevlar/Epoxy Composites

This research investigates the effects of weave styles and crimp gradients (CGs) on the damage tolerance levels and energy absorption capacities of woven fabric reinforced polymer (WFRP) composites. A comparative study was conducted to determine the specific failure mechanisms including fiber/matrix cohesive failures, matrix cracking, fiber breakage and fiber buckling resulting from static and dynamic loading events. The tests included flexure, short beam shear, drop impact, flexure-after-impact, ballistic impact, and split Hopkinson compression bar (SHCB) and were performed on 20-ply Kevlar/epoxy WFRP laminates. Laminates of three different Kevlar fabric weave styles were fabricated using plain, 2 × 2 twill and 4H satin weaves. A fourth laminate was constructed having a mixture of weave styles forming a hybrid crimp gradient construction. The experimental results demonstrated (1) that weave style selections and CGs can positively influence the spatial and temporal distributions of stress resulting from severe loading events and (2) that the fiber/matrix cohesive zone stresses that often lead to delaminations can be reduced. Accordingly, the dependence of mechanical performance on weave styles, crimp contents, and CGs can be exploited to increase the damage tolerance levels and energy absorption capacities in WFRP composites.

Design of an Innovative System for Wave Generation in Direct Tension–Compression Split Hopkinson Bar

The split Hopkinson bar is a well-known apparatus for performing tests at high strain-rate on engineering materials. The specimen is placed between two long bars and quickly loaded by a pressure wave, which travels at sound speed along the input bar, passes through the specimen, then is partly reflected and partly transmitted to the output bar. In the classical version, the input wave is generated by the impact of a striker bar against the input bar at a given velocity. In the direct version of the Hopkinson bar, the wave is generated by pre-loading and releasing a portion of the input bar. However, with this approach the rise time of the pressure wave is usually longer respect to the classical version. In this paper, an innovative wave generation system is presented, which exploits the abrupt shear fracture of a thin brittle disc. In this way, the rise time of the incident waves can be close to that obtained by the classical impact-based Hopkison bars (80 µs instead of 50 µs). Moreover, both tension and compression tests can be carried out with the same experimental system.