Compression Split Hopkinson Pressure Bar Research Articles

On the design of unconventional testing machines for engineering testing – the case study of advanced joining processes unit

Abstract Structural adhesive joining has become a widely used joining method for various types of structures, thus avoiding more conventional joining methods. In this manner, adhesive materials may be present in structures which suffer loading conditions that may not be normally considered in the design phase, such as high-strain rate conditions or creep and torsional loading. In these situations, adhesive properties and mechanical behaviour are sometimes not well understood, thus requiring special machine testing setups that can only be built for the specific application. Therefore, this paper provides a brief overview of proposed machine architectures for a torsion testing apparatus, a tensile & compressive Split Hopkinson Pressure Bar (SHPB), a drop-weight apparatus and a three station creep testing machine with a climactic chamber. Each testing apparatus has been designed by graduate students during their master thesis projects, and facilitated in the production of interesting and relevant scientific output in the field of structural adhesive behaviour.

A comparative study on the dynamic loading response of heat-treated soft and medium hard rocks

The present study assesses the effects of thermal exposure on the strength, stiffness, and fracture propagation in different rocks subjected to impact loading. The dynamic loading response of soft (synthetic die stone) and medium hard (natural marble) rocks are compared in this study. In the experiments, Split Hopkinson Pressure Bar (SHPB) tests are carried out on the heat-treated rock samples. A high-speed camera records the whole failure process of these specimens, and digital image correlation (DIC) is employed to track the evolution of strain during SHPB compression testing. The variation of mechanical properties of intact rocks with heating temperature is characterized by various thermal zones depending on the transition temperature, which identifies a significant shift in the impact loading response due to heating-induced micro-crack formation. Rocks made of synthetic materials and marble have transition temperatures between 100 and 150 °C and 500 – 650 °C, respectively. In the pre-transition regime, the dynamic compressive strength slightly rises or stays almost unchanged, whereas in the transitional regime, compressive strength falls drastically, and it does not show any vital changes with increasing treatment temperatures in the post-transitional regime. For synthetic rocks and marble, the critical heating durations are 10 and 5 h, respectively, and strength changes are relatively minimal after these heating durations. With increasing temperature (from 30 °C to 300 °C), the fracture pattern of synthetic specimens shifts from shear-tensile-dominated to tensile-dominated followed by shear-dominated. For marble specimens, the fracture pattern shifts from shear-tensile-dominated to shear-dominated as the temperature rises from room temperature to 650 °C.

Compressive Split Hopkinson Pressure Bar Apparatus: Part I - Design and Development

Optimum utilization of composite materials envisages its use under static as well as dynamic loading conditions. Hence, there is a need to assess the behavior of composite materials at high strain rates. Split Hopkinson Pressure Bar (SHPB) apparatus is widely used to assess high strain rate behavior of composites. Even though SHPB apparatus is widely used, its design methodology and development technique are not readily available. Design and devel- opment details of a compressive SHPB apparatus for characterizing mechanical properties of different materials at high strain rate loading are presented. Brief theoretical background necessary for the design of SHPB apparatus is presented based on one-dimensional wave propagation theory in elastic bars. Next, brief working principle of the apparatus and schematic arrangement are presented. Different components of the compressive SHPB appa- ratus are identified. Design, development and commissioning of the apparatus are discussed. Further, different instruments used and the calibration technique used are presented.

Compressive Split Hopkinson Pressure Bar Apparatus: Part II- Experimental Results

Investigations on high strain rate behavior of a typical unidirectional glass / epoxy composite under compressive loading are presented. Compressive Split Hopkinson Pressure Bar (SHPB) apparatus was used for the studies. Compressive properties were evaluated along longitudi- nal, transverse and thickness directions in the strain rate range of 548 - 2645 per sec. It is observed that the compressive strength is enhanced at high strain rate loading compared with those at quasi-static loading. Studies were also carried out on 10° and 45° off-axis specimens. In-plane shear strength was determined based on 10° off-axis compressive results. It is observed that the in-plane shear strength is increased at high strain rate loading compared with that at quasi-static loading. But it remains nearly constant with increasing strain rate at high strain rates.

Design and Performance of Layered Heterostructure Composite Material System for Protective Armors.

A new layered heterostructure composite material system (TC4 as front layer and 2024Al alloy as back layer) was developed and analyzed for its design and performance in terms of an enhanced absorption capability and anti-penetration behavior. The Florence model for energy absorption was modified, so that it can be utilized for the layered heterostructure composite material system with more efficacy. Numerical simulation through Ls-Dyna validated the analytical model findings regarding the energy absorption of the system and both were in good agreement. Results showed that two ductile materials with diverse properties, the hardness gradient and varied layer thickness joined together, specifically behaved like a unified structure and exhibited elastic collision after slight bending, which is possibly due to the decreased yield strength of the front layer and increased yield strength of the second layer. To validate the analytical and numerical findings, the samples of the layered heterostructure composite material system were subjected to a SHPB (Split Hopkinson pressure bar) compression test. The deformation behavior was analyzed in the context of the strain energy density and stain rate sensitivity parameter at different strain rates. The encouraging results proposed that two ductile materials with a hardness gradient can be used as an alternate structure instead of a brittle-ductile combination in a layered structure.

EXPERIMENTAL INVESTIGATION OF THE EFFECT OF STRAIN RATE ON THE CRUSHING STRENGTH OF A CELLULAR CONCRETE

The strain rate dependent compressive strength of an autoclaved aerated concrete (AAC) having a density of 600 kg m-3 was experimentally investigated between quasi-static and high strain rates (2x10-3-~4150 s-1) through quasi-static and dynamic compression, confined compression and indentation tests. High strain rate equilibrium and direct impact non-equilibrium compression tests in conjunction with the high strain rate confined compression and indentation tests were conducted in a compression Split Hopkinson Pressure Bar. The experimental results showed two different regions of the compressive strength-dependency on the strain rate: a low-strain rate-dependent region from quasi-static to ~18 s-1 and a high-strain ratedependent region from ~18 s-1 to ~1000 s-1. The switch of the failure mode from the single axial cracking at quasi-static strain rates to the extensive axial and circumferential cracking at increasing strain rates was ascribed to both the axial and radial inertia. The dynamic increased factor (DIF=dynamic strength/static strength) showed an abrupt increase after ~18 s-1 as similar with the compressive strength. The mean confined and indentation strength values also increased as the velocity increased, while the mean confined compression strength values were shown to be comparable with the dynamic compressive strength values. The inertia and strain rate contributions to the enhancement of DIF until about 1000 s-1 were predicted by taking the quasistatic indentation strength as the full confinement strength.

Compressive and tensile behavior of AA2014-T6 under different strain rates and different temperatures

This article investigates the quasi-static and dynamic mechanical response of AA2014-T6 alloy under uniaxial compression and tension loading. Influence of strain rate (10−4 to 2×103 s−1) and temperatures (25 °C to 250 °C) are also presented in this study. Static experiments have been performed on a conventional UTM, while the dynamic experiments were performed using tensile and compressive split Hopkinson pressure bar setup. It was observed that the plastic flow stress of the material follows the positive strain rate sensitivity over the wide range of strain rates for both kind of loadings. However, it follows the negative sensitivity with increase in temperature. Experimental results also showed that the flow stress gradually decreased uniformly with temperature increases up to 200 °C and beyond that, it is reduced drastically. Also the true fracture strain increased from 14.8 % to 51.5 % when temperature was increased from 25 °C to 250 °C. Material strength achieved during the uniaxial compression experiments is found to be higher than the strength measured during the tension experiments. For numerical modelling purposes, the Johnson-Cook constitutive model parameters were also evaluated to predict the plastic flow behavior of AA2014-T6 under different strain rates and temperatures. A good agreement between experimental observations and model predictions is observed under different strain rates and different temperatures.

Quasi-static and dynamic Brazilian testing and failure analysis of a deer antler in the transverse to the osteon growth direction

The transverse tensile strength of a naturally fallen red deer antler (Cervus Elaphus) was determined through indirect Brazilian tests using dry disc-shape specimens at quasi-static and high strain rates. Dynamic Brazilian tests were performed in a compression Split-Hopkinson Pressure Bar. Quasi-static tensile and indirect Brazilian tests were also performed along the osteon growth direction for comparison. The quasi-static transverse tensile strength ranged 31.5–44.5 MPa. The strength increased to 83 MPa on the average in the dynamic Brazilian tests, proving a rate sensitive transverse strength. The quasi-static tensile strength in the osteon growth direction was however found comparably higher, 192 MPa. A Weibull analysis indicated a higher tensile ductility in the osteon growth direction than in the transverse to the osteon growth direction. The microscopic analysis of the quasi-static Brazilian test specimens (tensile strain along the osteon growth direction) revealed a micro-cracking mechanism operating by the crack deflection/twisting at the lacunae in the concentric lamellae region and at the interface between concentric lamellae and interstitial lamellae. On the other side, the specimens in the transverse direction fractured in a more brittle manner by the separation/delamination of the concentric lamellae and pulling of the interstitial lamellae. The detected increase in the transverse strength in the high strain rate tests was further ascribed to the pull and fracture of the visco-plastic collagen fibers in the interstitial lamellae. This was also confirmed microscopically; the dynamically tested specimens exhibited flatter fracture surfaces.

High temperature and strain-rate response of AA2124-SiC metal matrix composites

This paper presents the results of the study of the dynamic impact behaviour of the AA2124-SiC Metal Matrix Composite (MMC) material with different particle reinforcement sizes and qualities using a compressive Split-Hopkinson Pressure Bar (SHPB) apparatus. Mechanical tests were performed at strain rates 1000 s−1, 2000 s−1, and 3000 s−1 and at temperatures of room temperature (25 °C), 100 °C and 200 °C. Microstructural analyses were carried out on the samples pre and post-compression experiments to study the fracture characteristics and mechanisms of the MMC materials. The flow stress-strain, strain rate, temperature effects and deformation mechanism were investigated. The backscattered electron images show that a higher strain rate of deformation induces the formation of denser and smaller grain size of CuAl2 precipitates, especially in composites with smaller SiC particle sizes. Temperature has posed a minor effect on the microstructural change. Heating the samples close to a solution treatment temperature, and then followed by an air quenching has resulted in a fine dispersion of CuAl2 precipitates, as well as a high saturation for the materials with a higher volumetric fraction of SiC reinforcement. The SHPB compression results reveal that the 225XF material has developed the highest stress among all materials. For materials with a higher volumetric fraction of SiC reinforcement (225XE and 225XF), cracks and failures have appeared in the samples during high strain rate and high-temperature compression experiments, which is believed to be caused by the increased brittleness of the material as a result of intensified oxide phases.

Finite element modelling and experimentation of plain weave glass/epoxy composites under high strain-rate compression loading for estimation of Johnson-Cook model parameters

Split Hopkinson pressure bar (SHPB) is a widely used experimental technique for testing materials at a high strain-rate. SHPB testing of fiber-reinforced polymer (FRP) composite samples have become common in recent decades. Specimen geometry is one of the essential parameters that can ease sample fabrication and help in-situ analysis when altered as per convenience. Further, developing a computationally accurate model would reduce the need for multiple experimental iterations, thus saving cost and time. As part of the current study, high strain-rate tests using a compressive SHPB setup were conducted on glass/epoxy (GE) laminated composite samples of both square prismatic and cylindrical shape along the through-thickness direction experimentally. Negligible differences were observed due to the change in specimen geometry. Further, we know that the woven fabric consists of yarns, which are a bundle of fibers. The strain-rate dependency of the epoxy's material property is well known, but very limited information is available about the strain-rate dependency of yarn's properties. The effect of strain-rate dependency of the yarn was analysed numerically on Abaqus software using a square prismatic shaped yarn-level finite element composite model. The homogenised properties of the yarn were taken to be reflective of a high fiber volume fraction composite. The bulk epoxy's and yarn's material property were defined using the Johnson-Cook (JC) constitutive model, and the failure mechanism was defined using the JC damage model. One of the primary reasons for using this model is its availability and ease of implementation in almost every finite element analysis (FEA) simulation software. Three cases were analysed numerically, first keeping both yarn and bulk matrix property strain-rate dependent, second keeping the strain-rate dependency only in the bulk matrix, and in third case both yarn and matrix were kept strain-rate independent. It was observed that completely strain-rate dependent model showed comparable peak stress and strain values with the experimental result than other models.

Young's Modulus Calculus Using Split Hopkinson Bar Tests on Long and Thin Material Samples.

Young’s modulus is a key parameter of materials. The method of its calculation in the current paper is concerned with the mismatch of the mechanical impedance at the bar/specimen interface for a compression SHPB (split Hopkinson pressure bar) test. By using long and thin specimens, the signal recorded in the transmission bar presents itself as a multistep signal. The ratio between the heights of two successive steps represents the experimental data that are considered in the formula of the elastic modulus this article is devoted to. The oscillatory nature of the real signals on the horizontal or quasi-horizontal segments prevents a precise determination of the two successive step heights ratio. A fine tuning of this value is made based on the characteristic time necessary for the signal to rise from one level to the next one. The FEM (Finite Element Method) simulations are also used in calculation of the Poisson coefficient of the tested complex concentrated alloy.

Energy Dissipation and Particle Size Distribution of Granite under Different Incident Energies in SHPB Compression Tests

To investigate energy dissipation and particle size distribution of rock under dynamic loads, a series of dynamic compression tests of granite specimens were conducted using a conventional split-Hopkinson pressure bar (SHPB) device with a high-speed camera. The experimental results show that the dissipated energy increases linearly with an increasing incident energy, following two different inclined paths connected by a critical incident energy, and the linear energy dissipation law in the dynamic compression test has been confirmed. This critical incident energy was found to be 0.29–0.33 MJ/m3. As the incident energy was smaller than the critical incident energy, the rock specimens remained unruptured after the impact. When the incident energy was greater than the critical incident energy, the rock specimens were ruptured or fragmented after the impact. In addition, the experimental results indicate that the dissipated energy and energy consumption ratio of a rock specimen, either unruptured or fragmented, increase with an increasing strain rate. Furthermore, it was found that fragment sizes at each mesh decrease with an increasing incident energy; that is, fragmentation becomes finer as incident energy increases.

Determination of the material model and damage parameters of a carbon fiber reinforced laminated epoxy composite for high strain rate planar compression

The progressive failure of a 0°/90° laminated carbon fiber reinforced epoxy composite was modeled in LS-DYNA using the MAT_162 material model, including the strain rate, damage progression and anisotropy effects. In addition to conventional standard and non-standard tests, double-shear and Brazilian tests were applied to determine the through-thickness shear modulus and the through-thickness tensile strength of the composite, respectively. The modulus reduction and strain softening for shear and delamination parameters were calibrated by low velocity drop-weight impact tests. The rate sensitivities of the modulus and strength of in-plane and through-thickness direction were determined by the compression tests at quasi-static and high strain rates. The fidelity of the determined model parameters was finally verified in the in-plane and through-thickness direction by the 3D numerical models of the Split Hopkinson Pressure Bar compression tests. The numerical bar stresses and damage progressions modes showed acceptable correlations with those of the experiments in both directions. The composite failed both numerically and experimentally by the fiber buckling induced fiber-matrix axial splitting in the in-plane and the matrix shear fracture in the through-thickness direction.

High strain-rate dynamic compressive behavior and energy absorption of distiller’s dried grains and soluble composites with paulownia and pine wood using a split Hopkinson pressure bar technique

Novel bio-based composite wood panels (CWPs) that consisted of distiller’s dried grains and solubles (DDGS) flour adhesive bound to a wood filler/reinforcement were subjected to high strain-rate compression loading, and their behavior was investigated. Specimens of DDGS-Paulownia wood (PW) or DDGS-pinewood (Pine) composites made using DDGS with fractions of 10%, 15%, 25%, and 50% were tested at high strain-rates using a modified compression Split Hopkinson Pressure Bar (SHPB). Both DDGS-PW and DDGS-Pine composites displayed strain-rate sensitivity, and DDGS-PW had a 25% fraction, which showed the highest ultimate compressive strength of 655 MPa at approximately 1600/s. The 90%-PW had the highest specific energy of 19.24 kJ/kg at approximately 1600/s when loaded via dynamic compression. The CWPs constructed of DDGS-PW had higher strength and energy absorption than DDGS-Pine with the exception of the 50% DDGS composites.

A rate- and pressure-dependent damage-plasticity constitutive model for rock

A new damage-plasticity constitutive model which takes account of the effect of confining pressure and strain rate on the strength and post-peak behaviour is proposed for rock subjected to dynamic loading. The model treats the compressive and tensile damages, both related to plastic and volumetric deformations, separately. The strain rate effect is considered by incorporating a newly developed dynamic increase factor formula, which reflects the pure rate sensitivity of rock, into the yield surface via radial enhancement approach. Fractionally-associated flow rule and lode-angle dependency are employed to capture the shear dilation behaviour of rock. The new model is then implemented into LS-DYNA via a user defined material subroutine to simulate the mechanical response of rock under various loading scenarios including triaxial compression, direct tension, Split Hopkinson Pressure Bar compression, oval-nose projectile penetration and explosive blast action, reported in literature. The robustness and accuracy of the new model are demonstrated by validating against available experimental results and by benchmarking with the reported simulations.

Effect of strain rate on macro- and micro-failure characteristics of marble from SHPB tests

A rock's microstructural characteristics reflect its macroscopic mechanical properties. Here, the effect of impact load on the microstructural characteristics of fractured rock is investigated, based on the split Hopkinson pressure bar compression test results for marble, and using a scanning electron microscope (SEM) to analyse fracture morphology. SEM results show that rock fractures under impact load are typical brittle failures. The fracture morphology reveals mainly a transgranular fracture mode, also containing intergranular fracture characteristics. Fractural microstructure exhibits strain rate effect, and with increased impact load, the fracture surface details become rougher and the quantity of scattered crystal particles increases, while the transgranular features become more apparent, the results of which are consistent with the macroscopic mechanical test results on rock.

NUMERICAL MODELLING OF WAVE SHAPES DURING SHPB MEASUREMENT

The paper aims at the numerical simulation of the wave propagation in compressive Split Hopkinson Pressure Bar (SHPB) experiment. The paper deals with principles of SHPB measurement, optimisation of a numerical model and techniques of pulse shaping. The parametric model of the typical SHPB configuration developed for LS-DYNA environment is introduced and optimised (in terms of element size and distribution) using the sensitivity study. Then, a parametric analysis of a geometric properties of the pulse shaper is carried out to reveal their influence on a shape of the incident pulse. The analysis is algorithmized including the pre- and post-processing routines to enable automated processing of numerical results and comparison with the experimental data. Results of the parametric analysis and the influence of geometric properties of the pulse shaper (diameter, length) on the incident wave are demonstrated.

Dynamic Mechanical and Microstructural Properties of Outburst-Prone Coal Based on Compressive SHPB Tests

Understanding the dynamic mechanical behaviors and microstructural properties of outburst-prone coal is significant for preventing coal and gas outbursts during underground mining. In this paper, the split Hopkinson pressure bar (SHPB) tests were completed to study the strength and micro-structures of outburst-prone coal subjected to compressive impact loading. Two suites of coals—outburst-prone and outburst-resistant—were selected as the experimental specimens. The characteristics of dynamic strength, failure processes, fragment distribution, and microstructure evolution were analyzed based on the obtained stress-strain curves, failed fragments, and scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR) images. Results showed that the dynamic compressive strength inclined linearly with the applied strain rate approximately. The obtained dynamic stress-strain responses could be represented by a typical curve with stages of compression, linear elasticity, microcrack evolution, unstable crack propagation, and rapid rapture. When the loading rate was relatively low, fragments fell in tension. With an increase in loading rates, the fragments fell predominantly in shear. The equivalent particle size of coal fragments decreased with the applied strain rate. The Uniaxial compressive strength (UCS) of outburst-prone coal was smaller than that of resistant coal, resulting in its smaller equivalent particle size of coal fragments. Moreover, the impact loading accelerated the propagation of fractures within the specimen, which enhanced the connectivity within the porous coal. The outburst-prone coal with behaviors of low strength and sudden increase of permeability could easily initiate gas outbursts.

Study on the constitutive model of porous titanium alloy

Porous titanium (Ti) alloys usually need high-speed precision machining before they are put into practical applications. Therefore, it is necessary to study the materials’ mechanical properties under conditions of larger strain rate and higher temperature in order to understand the processing mechanism and determine the appropriate processing parameters. In this study, the mechanical properties of porous titanium alloys of two porosities were first investigated by quasi-static compression tests and split Hopkinson pressure bar tests under different conditions. Then, the constitutive models of the flow stress were fitted. From the results of quasi-static compression tests, it was found that the static mechanical properties of porous titanium alloys decreased rapidly with increase in porosity. From the result of the split Hopkinson pressure bar tests, the strain rate sensitivity and temperature sensitivity of porous titanium alloys were determined. The mechanical properties of porous titanium alloys increased with strain rate increase in the range 1000–3000 s−1. The alloys exhibited a temperature-softening effect at 100°C, while they had a temperature-hardening effect at 300°C. The constitutive model could describe the strain-hardening rate of the material appropriately, but the accuracy of the simulated results needed to be improved compared with experimental values.

Thermo⁻Mechanical Behavior and Constitutive Modeling of In Situ TiB2/7050 Al Metal Matrix Composites Over Wide Temperature and Strain Rate Ranges.

The thermo–mechanical behavior of in situ TiB2/7050 Al metal matrix composites is investigated by quasi-static and Split Hopkinson Pressure Bar compression tests over a wide range of temperature (20~30 °C) and strain rate (0.001~5000 s−1). Johnson–Cook and Khan–Liu constitutive models determined from curve fitting and constrained optimization are used to predict the flow stress during deformation. In addition, another Johnson–Cook model calculated from an orthogonal cutting experiment and finite element simulation is also compared in this study. The prediction capability of these models is compared in terms of correlation coefficient and average absolute error. Due to the assumptions in orthogonal cutting theory, the determined Johnson–Cook model from cutting cannot describe the material deformation behavior accurately. The results also show that the Khan–Liu model has better performance in characterizing the material’s thermo–mechanical behavior.