Split Hopkinson Pressure Bar Facility Research Articles

Static and impact behaviour of recycled aggregate concrete under daily temperature variations

Abstract Concrete continuously suffers from impact loading and variations in daily temperature. In this study, tests on static compressive, static flexural, and impact compressive concrete specimens with different replacement percentages of recycled coarse aggregate (RCA), additional water, and cyclic temperatures were carried out to investigate the mechanical behaviour of recycled aggregate concrete (RAC). The static tests were conducted using a universal mechanical testing machine, and the impact tests were then applied using a 100-mm diameter split Hopkinson pressure bar facility. The failure mode, compressive strength, flexural strength, brittleness coefficient, strain-strain curve, and dynamic increase factor (DIF) were analysed and discussed. The results showed that (1) the influence of the replacement percentage of RCA and additional water on the static behaviour of RAC mainly depend on the effective water-to-cement ratio, RCA strength, and bond strength between the mortar and RCA, (2) the strain rate has a significant effect on the impact performance of RAC, and (3) the cyclic temperature significantly weakens the material properties of RAC. Finally, based on an empirical formula given by Comite Euro-International du Beton (CEB), a modified CEB model was proposed to accurately estimate the relationship between the strain rate and DIF of RAC.

Experimental Investigation of the Confined Behavior of Dry and Wet High-Strength Concrete: Quasi Static Versus Dynamic Loading

This paper presents an experimental investigation of the behavior of high-strength concrete (f c28 = 80 MPa) under very high pressure and quasi-static and dynamic loadings. Concrete specimen is inserted in an elastic, high-strength steel ring and subjected to an axial compression. In order to evaluate the confining pressure acting on concrete the ring is instrumented with multiple hoop strain gauges glued on its outer lateral surface. Dry and wet specimens are tested in both static and dynamic loadings in order to assess the influence of the free water content and the strain rate. Dynamic test are performed with a large diameter (80 mm) split Hopkinson pressure bar facility, adapted to test samples of large size (required for testing concrete material) at high stress levels. A strong effect of the free water content and the strain rate is observed in both deviatoric and volumetric behaviors of high-strength concrete. Both, the shear strength and volumetric stiffness increase with the increase of loading rate. One the other hand, free water has a beneficial effect on the volumetric stiffness and a strong negative influence on the deviatoric strength of high-strength concrete. The behavior of high-strength concrete was also compared to that of an ordinary concrete (f c28 = 30 MPa), tested in a similar experimental campaign. In both static and dynamic conditions, dry specimens of the two concretes reach approximately the same deviatoric stress level. However, when the specimens are saturated with free water, high-strength concrete exhibits a deviatoric strength much greater than ordinary concrete.

Mechanical Characterisation of Aluminium Alloy 7449-T7651 at High Strain Rates and Elevated Temperatures Using Split Hopkinson Bar Testing

The mechanical behaviour of a 7 series high-strength aluminium alloy that is mainly used by the aerospace industry is under investigation. Aluminium alloy AA7449-T7651 is mechanically tested at high strain rates (SRs), from 1000 to 10,000 s−1 and from room temperature up to elevated temperatures (80, 140, 200, and 300°C). The mechanical characterisation is performed in compression by means of an experimental Split Hopkinson Pressure Bar facility. The material’s flow stress (σ n ) and maximum stress are significantly affected by both temperature and SR Nowadays, where the numerical simulation of several mechanical applications and processes that involve high deformation rates is a requirement the mechanical behaviour of the material should be described accordingly. The experimental data are utilised to calculate the SR hardening coefficients of two commonly used material models: Cowper Symonds and Johnson-Cook.

Split Hopkinson bar testing to study failure waves in brittle materials

Evaluation of the mechanical properties of brittle materials submitted to high strain rates is fundamental for a deep understanding of events that take place during ballistic impacts or high-speed mechanical processes. In special, brittle materials are an important component of ballistic protection systems. A number of tests were carried out on a short split Hopkinson pressure bar facility to obtain more information about the dynamic crack propagation on an experimental alumina-niobia composition. Strain rates as high as 1825 s -1 were measured during dynamic testing of ceramic resistance. Stress and deformation history were obtained from the stress-strain curve. The fractured samples were analyzed by scanning electron microscopy (SEM). The images showed wave marks on the alumina surface. The electronic instrumentation acquired the spalling time leading to crack velocity values that are consistent with Broek's theory.

Testing and Response of Large Diameter Brittle Materials Subjected to High Strain Rate

Tests on large diameter specimens using a split Hopkinson pressure bar (SHPB) facility are essential to obtain representative dynamic characteristics of engineering materials such as rocks, concrete, and steel fiber-reinforced concrete (SFRC). However, direct application of the conventional SHPB technique to measure the dynamic behavior of these materials has serious limitations, especially if the size of the specimen is relatively large. The background to material testing using the SHPB facility is explored in this paper. The experimental technique and analytical waveform analysis, together with their attendant problems, are presented. Waveform oscillations and nonhomogeneous deformation of brittle specimens are major problems in test data originating from the conventional SHPB technique. These problems are identified and addressed; techniques for circumventing such problems are described. The design and commissioning of a short-length 75 mm diameter SHPB is presented. To overcome a number of problems associated with such a large diameter short length facility, a novel loading method for eliminating oscillation and maintaining uniform deformation of specimens is proposed. Experimental waveforms are presented for brittle engineering materials to underscore the validity of the technique.

Determination of moisture effects on impact properties of composite materials

Many applications of structural materials involving composites include impact or dynamic loading in a humid environment. Composite materials are known to degrade when subjected to humid conditions, and therefore the humidity confounds the difficulty of determining the high strain rate behavior of composites. Several researchers have found that water absorption by composites causes degradation of matrix dominated quasi-static properties. However, very little is known of the effect of absorbed moisture on the high strain rate properties of polymer matrix composites, that are useful in the automotive, aerospace, and naval applications of composite structures. A Split-Hopkinson Pressure Bar facility is used herein to study the effect of absorbed moisture in high strain rate tests (200–1200/s) of a unidirectional IM7/8551-7 graphite/epoxy composite. The study includes dry, medium, and saturated moisture conditions. The tests show significant variation of high strain rate properties from static properties, and the reasons are identified. In addition, a better understanding of the effect of the matrix and fiber/matrix interface on the high strain rate properties of composites is achieved.

Material behaviour in conditions similar to metal cutting: flow stress in the primary shear zone

This paper is aimed at determining the flow stress in metal cutting. A major problem in finding the flow stress is the intense circumstances under which deformation takes place in the chip root. Large deformations are imposed on the workpiece material at high speed in a very small area. This results in mechanical material behaviour far removed from that encountered in conventional material tests. Therefore, to determine the flow stress in cutting, it is necessary to determine the conditions under which the material is being deformed firstly. Hereafter, a material test has been developed that can be used in conditions similar to those found in cutting. Based on the obtained information on the deformation conditions in cutting as found in previous work [J. Mater. Process. Technol, in press], a split Hopkinson pressure bar (SHPB) facility could be constructed for mechanical testing of materials at high strain rates and elevated temperatures. From measurements with this SHPB facility, it appeared that both strain rate and temperature have a considerable influence on the flow stress of metals. Combining the information on the deformation conditions in the primary shear zone with the measured constitutive behaviour, the flow stress of the metals in the primary shear zone is found.

Specimen Geometry Effects on High-Strain-Rate Testing of Graphite/Epoxy Composites

Numerous papers have been published that include high-strain-rate experimental data obtained from Split Hopkinson Pressure Bar (SHPB) tests. There have been no standardized test specimens with differing dimensions and geometry. For this reason, it can be difficult to compare results of tests performed by other researchers. The SHPB facility was used in this study to determine if specimen geometry has an effect on the results of high-strain-rate compression tests. The material used in all of the tests is a unidirectional IM7/8551-7 graphite/epoxy composite. The experiments discussed herein show the effect of varying the length-to-diameter (L/D, or aspect) ratio of the specimen, as well as the effect of changing from the more typical cylindrical to a square/rectangular specimen geometry. The L/D study examined 84 cylindrical specimens with L/Ds from 0.5 to 2.0, with D = 0.261 in., and tested in both the 1 and 2 directions. The geometry study used 42 square/rectangular specimens and tested in the 1, 2, and 3 directions. The results of both studies are compared, and no statistically significant effect of either L/D or geometry could be found.

Specimen geometry effects on high-strain-rate testing of graphite/epoxy composites

Numerous papers have been published that include high-strain-rate experimental data obtained from Split Hopkinson Pressure Bar (SHPB) tests. There have been no standardized test specimens with differing dimensions and geometry. For this reason, it can be difficult to compare results of tests performed by other researchers. The SHPB facility was used in this study to determine if specimen geometry has an effect on the results of high-strain-rate compression tests. The material used in all of the tests is a unidirectional IM7/8551-7 graphite/epoxy composite. The experiments discussed herein show the effect of varying the length-to-diameter (L/D, or aspect) ratio of the specimen, as well as the effect of changing from the more typical cylindrical to a square/rectangular specimen geometry. The L/D study examined 84 cylindrical specimens with L/Ds from 0.5 to 2.0, with D = 0.261 in., and tested in both the 1 and 2 directions. The geometry study used 42 square/rectangular specimens and tested in the 1, 2, and 3 directions. The results of both studies are compared, and no statistically significant effect of either L/D or geometry could be found.