Split Hopkinson Bar Apparatus Research Articles

Dynamic mechanical properties and failure mechanisms of rock-like materials containing main and auxiliary holes with various arrangement inclinations and spacing ratios

As a typical rock defect, circular holes significantly influence rock masses, diversifying their failure modes and ultimately weakening their bearing capacities. To study the effects of hole arrangement inclination and spacing ratio on the mechanical properties and failure mechanisms of rock-like specimens, dynamic uniaxial compressive tests were conducted by employing a modified split Hopkinson bar apparatus and digital image correlation. The results reveal a V-shaped trend in the peak stress, Young’s modulus, and absorbed energy proportion as the hole arrangement inclination increases from 0° to 90°. However, the peak strain is insensitive to changes in the hole arrangement inclination and spacing ratio. The hole spacing ratio has no significant effect on the Young’s modulus, but it causes a gradual reduction in the proportion of energy absorbed in the specimens. Moreover, shear and tensile cracks emanating from the main hole dominate the failure mode of specimens with main and auxiliary holes, as demonstrated by the resultant displacement and vertical strain contours. Rock bridges experience shear failure as the hole arrangement inclination increases from 15° to 75°, whereas the rock bridges of specimens with various hole spacing ratios tend to fail owing to tensile cracks. The interaction between main and auxiliary holes with an arrangement inclination of 0° is the weakest, suggesting that tunnel groups should be arranged perpendicularly to the direction of dynamic loading.

Dynamic mechanical behavior of sandstone with filled joints at varying angles under coupled triaxial static and dynamic loading

Natural or artificially formed rock composite structures with consecutively filled joints at varying angles are frequently disturbed by coupled static and dynamic loads. Understanding the impacts of the confining pressure and joint angle on the dynamic mechanical behavior of rocks is important for the stability of rock mass engineering structures. In this study, seven joint angles and four confining pressures were considered. Dynamic triaxial compressive tests of sandstone with consecutively filled joints were conducted using a split Hopkinson bar (SHPB) apparatus. The test results showed that varying degrees of “strain rebound” phenomena were observed in the stress–strain curves, implying that the jointed samples in the failure stage could still store more elastic energy, which may trigger rock burst accidents in practical engineering. Moreover, the dynamic compressive strength and elastic deformation modulus were proportional to the confining pressure, but showed a trend of slight decrease followed by a noticeable increase with an increase in the joint angle from 0° to 90°. The combined effects of axial pre-stress and confining pressure resulted in different trends in the dynamic peak strain. The plastic deformation moduli of samples with smaller joint angles were insensitive to changes in the confining pressure and smaller joint angles, whereas those of samples with larger joint angles were sensitive to changes in the confining pressure and larger joint angles. In addition, the experimental tendency of the dynamic compressive strength under coupled triaxial static and dynamic loading was in good agreement with the theoretical derivation.

Controlled interlaminar crack initiation in thin notched laminated thermoplastic composites under impact loading: A first step towards unitary layer recovery

The recycling of composite materials is nowadays a major challenge as it is systematically associated with a downcycling due to the reduction of the fiber length. However, recent works demonstrated that the delamination-based recovery of the unitary layers of laminated structures with preserved mechanical properties is possible under dedicated loading conditions. The presented work aims at investigating this innovative process and in particular the interlaminar crack initiation from a straight notch milled in the material by using an impact loading; further crack extension to recover the complete lamina being the following step of the process. A Split Hopkinson Bar setup has been used in a Direct Impact configuration and a dedicated test setup has been implemented to impose translational and rotational kinematics to the impacted side of the tested specimens in order to enable crack initiation and propagation over a few millimeters. Imposing translational kinematics to the impacted side of the specimen has been shown to induce cracks in a reproducible manner. Numerical simulation results explain the more favorable conditions provided by the translational kinematic.

Dynamic splitting response and mechanical effect of joint angle in sandstone containing various weak-filling joints

Weak-filling joints substantially degrade the integrity and load-bearing capacity of rock masses, thus increasing the complexity and unpredictability of their mechanical response behavior. One of the main mechanical properties of jointed rocks is their splitting performance, which heavily influences their failure mode. To study the effects of joint angles on splitting mechanical properties, we conducted dynamic splitting tests on sandstone samples with seven different inclination angles of weak-filling joints. These tests were conducted using the Brazilian disc method and a modified split Hopkinson bar apparatus. The splitting mechanical response, energy rate characteristics, joint angle damage factors, failure modes, micromorphology characteristics, and the mechanical effects of joint angles were systematically analyzed. The findings suggested that varying joint angles led to different initial damage levels in the rock mass, which can be described by a power exponential function. The observed relationship between peak load and joint angle was corroborated by integrating a damage parameter into the theoretical derivation of the mechanical effect of the joint angle. Weak-filling joints reduced the brittle failure characteristics and peak loads of the samples but enhanced their ability to deform. Furthermore, the energy reflection rate exhibited an inverse relationship with the joint angle, which is contrary to the trends observed in energy transmission and absorption rates. Additionally, both macroscopic and microscopic morphology analyses confirmed that the joint angle primarily determined the fracture trajectory of the samples, causing a gradual shift in the failure modes of jointed samples from shear to tensile failure.

Stored Torque Torsional Split Hopkinson Bar Apparatus

Torsional Split Hopkinson Bar (TSHB) apparatus is extensively used for obtaining shear response of the materials in the strain rate range of 102 s-1 to 104 s-1. Even though the stored torque TSHB is widely used, its design and development technique is not readily available. The present work is focused on discussing the design, development and commissioning of Stored Torque TSHB apparatus. Design considerations of structural components of the experimental setup are discussed. Specimen design considerations, data acquisition system requirement and analysis procedure are presented. Physics of the experiment as well as the fundamental equations involved in obtaining shear stress - shear strain response of the specimen material are presented in detail. Further, typical observations on high strain rate shear behavior of composites are given.

Numerical Design of a Thread-Optimized Gripping System for Lap Joint Testing in a Split Hopkinson Apparatus.

Currently, few experimental methods exist that enable the mechanical characterization of adhesives under high strain rates. One such method is the Split Hopkinson Bar (SHB) test. The mechanical characterization of adhesives is performed using different specimen configurations, such as Single Lap Joint (SLJ) specimens. A gripping system, attached to the bars through threading, was conceived to enable the testing of SLJs. An optimization study for selecting the best thread was performed, analyzing the thread type, the nominal diameter, and the thread pitch. Afterwards, the gripping system geometry was numerically evaluated. The optimal threaded connection for the specimen consists of a trapezoidal thread with a 14 mm diameter and a 2 mm thread pitch. To validate the gripping system, the load-displacement (P-δ) curve of an SLJ, which was simulated as if it were tested on the SHB apparatus, was compared with an analogous curve from a validated drop-weight test numerical model.

Dynamic response of Advanced Placed Ply composites

This work investigates the high strain rate behavior of AP-PLY composites. The large representative volume elements and brittle nature of this material necessitated the use of a bespoke Split-Hopkinson bar apparatus. AP-PLY and baseline laminates were subjected to tensile loading at strain rates of 30 s−1. Results were compared with quasi-static data to evaluate whether the laminate architecture introduced any strain rate dependency. In addition, the dynamic experiments were simulated using a multiscale modeling framework, providing further insights into the micromechanisms governing material behavior. The moduli of the AP-PLY composites were found to be strain rate independent, however, strengths were found to be marginally higher than those of their baseline counterparts. At high strain rates, the strain concentrations induced by the geometry of the individual tapes at through thickness undulations and tow boundaries were less significant due to reduced out-of-plane tow straightening and delamination. As a result, no reduction in AP-PLY strength in comparison to the baseline laminates was obtained. These differences in deformation micromechanisms led to an improvement of the damage tolerance when subjected to dynamic loading.

Characterizing multi mechanical behaviors for epoxy-like materials under wide strain rate range

Polymers, extensively used in aerospace and transport industries, mostly present distinct strain rate dependent. Their mechanical behaviors under dynamic loading conditions have not been well characterized by succinct formulations for practical engineering. In this work, the mechanical behaviors of a representative epoxy are investigated under strain rates from 0.001 s−1 to 5100 s−1 using Instron 5848 and split Hopkinson bar apparatus, for its constitutive behaviors and efficient formulation to characterize their multi-mechanical behaviors. The materials present high rate-dependence and share similar mechanical behaviors under wide strain rates including multiple characteristics: peak stress followed by post-yielding stress softening, plateau stress and strain hardening. The results from the strain-controlled experiments facilitate to analyze the damage degradation after peak stress formulated by a modified Weibull model. A succinct constitutive model containing only eight parameters is then constructed with three parallel components: Weibull model, Maxwell model and power function, which can capture well the multi-mechanical characteristics. It is easily further extended to other polymer and composites from literatures with stronger characterization capacity by less parameters.

On the Development of New Test Techniques to Measure the Tensile Response of Materials at High and Ultra-high Strain Rates

BackgroundThere is a lack of reliable methods to obtain valid measurements of the tensile response of high performance materials such as fibre composites, ceramics and textile products at high rates of strain.ObjectiveWe propose and assess two new test techniques aimed at measuring valid tensile stress versus strain curves at high and ultra-high strain rates.MethodsWe conduct detailed, non-linear explicit Finite Element (FE) simulations of the transient response of the test apparatus and specimen during the tests and we develop simple analytical models to interpret the test measurements. We consider two test techniques: one based on the split Hopkinson bar apparatus, and suitable for strain rates of up to 1000 /s, and a second technique relying on projectile impact and aimed at measurements at strain rates higher than 1000 /s.ResultsThe simulations are successfully validated using test data at strain rates of order 200 /s and then used to predict the test performance at strain rates up to approximately 5500 /s. We find that both techniques can give valid stress versus strain curves across a wide range of strain rates.ConclusionsWe identify the limits of both techniques and recommend optimal measurement strategies for dynamic testing of materials with different ductility.

Performance characteristics of Hopkinson’s set-up pneumatic launcher

The paper presents a performance characteristics of a pneumatic launcher, which is an important element of the split Hopkinson bar set-up (SHPB) at the Department of Military Engineering and Infrastructure (the Military University of Technology in Warsaw) for the purpose of dynamic strength tests of construction materials. The process of experimental calibration of the launcher for selected loading bar-projectiles is shown. Two types of compression during direct impact tests were also used simultaneously to investigate the behaviour of metallic samples with the use of this launcher as well as the Hopkinson measuring bar: the first — a short cylindrical sample, including a miniature (small diameter) sample, and the second — a long cylindrical sample (Taylor test). The relationships describing the stress and strain state as a function of strain rate for the first type of the experiment and engineering empirical formulas for the second type of the research were given.

High strain rate compression behaviour of 3D printed Carbon-PA

In the last few years, Fused Filament Fabrication is growing in the industrial field for the manufacture of final products by using new materials with high mechanical performances. Among those, one of the strongest is Carbon-PA. This is a composite material made by Nylon thermoplastic matrix filled with short carbon fibers reinforces. The aim of this work is to investigate its mechanical properties in static and dynamic conditions. Cylindrical specimens were produced by extruding the material in the three main printing directions. Then, uniaxial quasi-static and dynamic compression tests have been performed to evaluate its strain rate sensitivity. Dynamic tests have been carried out through a direct Split Hopkinson Bar setup with a pulse-shaping technique. The results show a compression behaviour dependent on the printing direction and strain rate. The behaviour of Carbon-PA was different between static and dynamic condition, passing from ductile to brittle. Moreover, a tomography analysis was carried out on the samples to evaluate the voids distribution.Graphic abstract

Experimental and numerical investigation on forming limit curves of AA6082 aluminum alloy at high strain rates

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.

Dynamic inter-fibre failure of unidirectional composite laminates with through-thickness reinforcement

Unidirectional composite laminates reinforced in the thickness direction with Z-pins are tested in tension and compression with different off-axis angles at high loading rate. A split Hopkinson bar setup and a high speed imaging system have been used for these dynamic tests. It is found that the inter-fibre strength was reduced by Z-pinning in tension and shear dominated failure modes, and the reduction from dynamic loading conditions is greater than that in quasi-static. However, the laminate strength was not significantly influenced by Z-pinning when loaded predominantly in compression. This difference was caused by the transition of failure mechanisms around Z-pins. The stiffness of the laminate was not affected by Z-pins at elevated strain rates, since the increased epoxy stiffness helped to make up for the stiffness loss due to reduced fibre volume fraction resulting from Z-pins.

Experiments on wave propagation in soft materials

AbstractThe wave propagation in a group of soft resin based (3d printed) materials is investigated by using a Split Hopkinson Bar setup. The principal assumptions and the role of pulse shaping, as an approach to achieve constant strain rates and dispersion reduction, is discussed. The measured dynamic elastic modulus is presented and indicates that flexible resins have a dynamic elastic modulus of 20 to 40 MPa.

Combined shear/tension testing of fibre composites at high strain rates using an image-based inertial impact test

Testing fibre composites off-axis has been used extensively to explore shear/tension coupling effects. However, off-axis testing at strain rates above 500 s-1 is challenging with a split Hopkinson bar apparatus. This is primarily due to the effects of inertia, which violate the assumption of stress equilibrium necessary to infer stress and strain from point measurements taken on the bars. Therefore, there is a need to develop new high strain rate test methods that do not rely on the assumptions of split Hopkinson bar analysis. Recently, a new image-based inertial impact test has been used to successfully identify the transverse modulus and tensile strength of a unidirectional composite at strain rates on the order of 2000 -1. The image-based inertial impact test method uses a reflected compressive stress wave to generate tensile stress and failure in an impacted specimen. Thus, the purpose of this study is to modify the image-based inertial impact test method to investigate the high strain rate properties of fibre composites using an off-axis configuration. For an off-axis specimen, a combined shear/tension or shear/compression stress state will be obtained. Throughout the propagation of the stress wave, full-field displacement measurements are taken. Strain and acceleration fields are then derived from the displacement fields. The kinematic fields are then processed with the virtual fields method (VFM) to reconstruct stress averages and identify the in-plane stiffness components G12 and E22.

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.

Simulation of the plastic response of Ti–6Al–4V thin sheet under different loading conditions using the viscoplastic self-consistent model

In this article, the capacity of the Visco-Plastic Self-Consistent model (VPSC) as a constitutive model for the simulation of cold-rolled Ti–6Al–4V under a diverse set of loading conditions is investigated. The model uses information about the material crystallographic texture and grain morphology obtained with EBSD together with a grain constitutive model that is sensitive to strain rate and temperature. The ability of the model to capture the macroscopic response and underlying microstructural phenomena is critically assessed considering various sets of hardening parameters, obtained applying different fitting procedures to tensile experiments in different directions on the sheet plane. In order to validate the fitted parameters, the obtained model is applied to the simulation of experiments in a wide range of testing conditions, including uniaxial, plane strain and simple shear loading modes at strain rates ranging from 10−4s−1 to 103s−1 and temperatures between −10°C and 70°C. High strain rate experiments were performed using a Split Hopkinson Bar setup, with specimens designed to achieve the desired loading mode, while isothermal experiments were performed with the help of a fluid cell to keep a constant temperature. A good agreement between experimental and simulation results is obtained.

Influence of Fabrication Method on Tensile Response of Split Hopkinson Bar-Sized Specimens

Abstract The influence of the specimen fabrication method on results of tension experiments on miniature ductile metal samples was investigated. 2024-T351 aluminum specimens were fabricated using traditional machining methods (milling) to two surface roughness specifications and using electrical discharge machining (EDM). The recast layer was left on the EDM specimens. The surface roughness of all samples was documented. The influence of edge-breaking the samples (rounding the sharp edges) was also investigated. Tests were conducted at two nominal strain rates: 1.0 s−1 using a servohydraulic load frame and 1000 s−1 using a tension split Hopkinson bar (SHB) apparatus. Results showed that the fabrication method influenced both the ultimate tensile stress and the elongation at failure. For milled specimens, the ultimate tensile stress and elongation decreased with increasing surface roughness. EDM specimens had slightly lower ultimate stresses but ductility in line with finely milled samples. Ultimate stresses of milled specimens with broken edges were lower than those with unbroken edges.

Strain rate change tests with the Split Hopkinson Bar method

In this paper, methods to produce rapid strain rate changes for strain rate sensitivity measurements in Split Hopkinson Bar arrangements are presented and discussed. Two different cases are considered: a strain rate change test within the high strain rate region in compression, and a tension test incorporating a large strain rate jump directly from the low strain rate region to high strain rates. The former method is based on the loading wave amplitude manipulation, while the latter method is based on the incorporation of a low strain rate loading device into a Tensile Split Hopkinson Bar apparatus.

Dependency of dynamic interlaminar shear strength of composites on test technique used

Studies are presented on dependency of dynamic interlaminar shear (ILS) strength on the experimental technique used for a typical plain weave E-glass/epoxy composite. Dynamic ILS strength was determined based on two experimental techniques, namely torsional split Hopkinson bar (TSHB) apparatus using thin walled tubular specimens and compressive split Hopkinson pressure bar (SHPB) apparatus using single lap specimens. The results obtained from these techniques are compared. In general, it is observed that dynamic ILS strength for composites obtained by TSHB testing using thin walled tubular specimens is lower than the dynamic ILS strength obtained using single lap specimens in compressive SHPB. The issues involved in TSHB testing of thin walled tubular specimens made of composites are discussed and the reasons for reduced dynamic ILS strength using thin walled tubular specimens are highlighted. Finite element analysis (FEA) of thin walled tubular specimens made of composite and resin subjected to quasi-static torsional loading is presented. Using FEA results, the reasons for lower ILS strength of composite thin walled tubular specimens are substantiated.