Papers by Keyword: Hopkinson Bar

Abstract: Carbon fiber composite material with light weight, high strength, corrosion resistance and other characteristics of its impact damage mechanism is different from the traditional metal materials. In this paper, the quasi-static compression of carbon fiber composites was carried out by using a material testing machine to analyze the damage mechanism. The Hopkinson bar technology was used to test the dynamic mechanical properties. The damage mechanism of the carbon fiber composites under dynamic compressive loading was studied. Stress - Strain relationship of composites under Quasi - static and dynamic compressive load. It is found that the main failure mode of out-of-plane direction of carbon fiber composite laminates is brittle shear failure, while the in-plane failure mode shows the properties of brittle materials.

Abstract: In the paper, experimental analysis of dynamic plastic behaviour of different structural materials at high and very high strain rates up to 10⁵ s^-1 in various modes at lowered, room and elevated temperatures, is presented. The paper describes laboratory set ups for testing material specimens, based on various modifications of the Hopkinson bar technique. The paper focuses the attention on experimental tests and their interpretation. The results reported here will be useful for the designers working on the dynamic behaviour of building and mechanical protective structures made of different materials such as steels, aluminium alloys, ceramic and sand bricks, mortars, woods (pine, birch and aspen), ceramics and concrete fireproof and soils (sand, clay and sandy soil clay).

Abstract: A novel experimental method was proposed for characterizing the energy absorbing capability of composite materials during the progressive crushing process under impact loading. A split Hopkinson pressure bars system was employed to carry out the progressive crushing tests under impact loading. The stress wave control technique was used to avoid the inhomogeneity of dynamic stress field in the specimen. The progressive crushing behavior was successfully achieved by using a coupon specimen and anti-buckling fixtures. With increasing strain rate, the absorbed energy during the crushing process slightly decreased, whereas the volume of the damaged part clearly increased regardless of material type. Consequently, the energy absorbing capability decreased with increasing loading rate. The effects of material composition, such as fiber type, matrix type and fabric pattern, on energy absorbing capability were also investigated by using the proposed method.

Abstract: An experimental analysis was performed to model transverse impact of free-free and free supported square duralumin beams loaded at different locations along their length. The applied impact load was obtained from tests carried out on a single Hopkinson pressure bar equipped with a high speed camera. The experimental set-up consisted of an Hopkinson measuring bar that is brought in contact with the beam. In this one-point impact experiment, a cylindrical striker, fired by the air gun, impacts the Hopkinson bar and generates stress waves that travel along the bar and impinge upon the aluminum beam. The stress waves are recorded by strain gages mounted on the Hopkinson measuring bar. These are used to calculate the applied load on the beam. Dynamical displacements of the impact zone of tested beam were recorded by the high speed camera. The dynamic experiments show that the plastic deformation, adjacent to the impact location, is due to combined dominant bending and stretching modes. Most of the plastic deformation is confined to the impact zone of tested beams. The plastic strain magnitude and distribution near the impact zone is similar for all tested impact locations, but higher for the more symmetrical impacts. The conversion of impact energy into kinetic, elastic strain energy and plastic dissipation work is characterized for various impact locations along the specimen of beam.

Abstract: A novel experimental method has been developed to evaluate the mode I crack growth behavior of adhesively bonded joints under impact loading. The split Hopkinson pressure bar (SHPB) technique and the digital image correlation (DIC) technique was employed to evaluate the crack growth behavior. To reduce the dynamic effects by controlling loading input of the SHPB apparatus, the fracture toughness was determined precisely based on static evaluation formula. To contrive the testing set-up, high loading rate was kept until the arrest of crack. The fracture toughness of titanium alloy/epoxy adhesively bonded joints during crack propagation was obtained successfully by using present method.

Abstract: In this work, Mode I dynamic fracture experiments are conducted on pre-cracked three point bending specimens by using modified split-Hopkinson pressure bar. Two sets of specimens with different initial textures are considered here: one set of the specimens are machined from a hot rolled AZ31B Mg alloy plate with a bigger grain size. The others are treated by four pass of equal channel angular pressing (ECAP) after they are cut from the initial material. They are with the finer grain size. Digital image correlation (DIC) technique is used to determine the strain contours around the crack tip and electron back scatter diffraction (EBSD) is employed to analyze the texture evolution after tests. It is found that the dynamic fracture toughness of finer grain specimen is higher than that of coarse grain specimen. The fracture toughness of both sets of specimens is enhanced by increasing the loading rates. Texture analysis shows the formation of tensile twinning in the ligament ahead of the crack tip in the coarse grain specimen but no sign in fine grain specimen. The brittle features e. g. cleavage planes and twinning lamellas are observed on the fracture surface of coarse grain specimen by scanning electron microscope (SEM). However, the relative ductile features such as micro-voids surrounding by tear ridges present on the fracture surface of fine grain specimen.

Abstract: Polyvinyl butyral (or PVB) is commonly used as interlayer in architectural laminated glass and windshield in automobiles for its strong binding, optical clarity, adhesion to many surfaces, toughness and flexibility. A modified in-situ Hopkinson bar system is used to measure the tensile properties of the PVB with the strain rates of 30~100 s^-1. In this system, a high impedance striker tube with the rubber pulse shaper is use to generate a long loading pulse of 50ms. Two X-cut quartz piezoelectric force transducers are sandwiched between the specimen and two bars respectively to directly measure the dynamic loading forces, and the strain field of the specimen is calculated by the Digital Image Correlation (DIC) method via photos obtained by the high speed camera. The local deformation of the full-field specimen was clearly displayed and the fracture strain of the specimen was evaluated. The results show that the tensile strengths of the PVB increase with increasing loading strain rates.

Abstract: The spall fracture of steel fiber reinforced concretes (SFRC) is investigated with the strain wave profiles in buffer bar behind the specimen bar in a large size Hopkinson pressure bar equipment. The experimental results indicate that the spall strength of SFRC is related to the steel fiber volume ratio, the compressive strength of concretes and the load rate (impact velocity). The spall strength of SFRC empirical formula shows the relationship between these factors. The conclusion that the steel fiber has the effect to improve the ability to prevent spall fracture is of real importance in the correlative numerical simulation and protective engineering design.

Abstract: The dynamic compression test was carried out for the AZ91 Magnesium alloy of as-cast and aging state with a split Hopkinson pressure bar, and the dynamic behavior has been investigated. Finally the fracture surface of samples at the different strain rates was analyzed by scanning electron microscope (SEM). The results show that compared with the as-cast AZ91, it is more impressible for the solid solution and age-treatment AZ91 at the similar strong strain rates, while the stress-strain curve has the duality of positive and negative effects as the variation of strain rate.

Abstract: In the process of acceleration dynamic measurement, the measurement results have quite large dynamic errors, due to the resonance of acceleration sensors. Using the system identification theory and combining with the dynamic calibration experimental data of acceleration calibration system based on Hopkinson bar, the dynamic model of the sensor is deduced, the dynamic compensation filter is designed. Simulation results show that the compensation filter can broaden the work band effectively. It improves the system dynamic response speed by nine times and advances the measuring accuracy of the test system.