Split Hopkinson Pressure Bar Impact Research Articles

Dynamic responses of hybrid lightweight composite sandwich panels with aluminium pyramidal truss cores

This paper experimentally and numerically investigates the free vibration, quasi-static compressive and split Hopkinson pressure bar impact responses of hybrid composite pyramidal truss sandwich panels. Such sandwich panels made of carbon fibre composite face sheets and aluminium alloy pyramidal truss cores are fabricated using an interlocking and adhesive bonding approach. Modal tests and quasi-static compression tests are conducted. A good consistency for natural frequencies, modal shapes and static stress–strain curves of the specimen with the same specification is obtained, which ensures the good repeatability of the present specimens. Considering the effect of strain rate, a series of split Hopkinson pressure bar tests combined with numerical simulations is carried out to investigate their dynamic compression responses. A good agreement between simulation results and experimental data is observed, which shows that the adopted split Hopkinson pressure bar testing device and the modified Johnson-Cook model are reasonable and reliable. Results show that the dynamic compression modulus and strength of specimen are strongly influenced by the relative density of the truss cores and much higher than the corresponding static compression modulus and strength. Furthermore, it is also revealed that all the specimens have excellent energy absorption performance, which may have greatly advantage in shock isolation application.

Crack propagation and dynamic properties of coal under SHPB impact loading: Experimental investigation and numerical simulation

To investigate the crack propagation and dynamic mechanical properties of coal under high strain rate loading, 24 sets of Brazilian disk (BD) coal specimens with vertical and horizontal beddings were made, and tests were conducted by a split Hopkinson pressure bar (SHPB) system. Under different impact velocities, a high-frame and high-resolution camera was employed to capture the fracture process, and two high-dynamic strain gauges were used to record the stress pulse signals simultaneously. Using one-dimensional stress wave theory, the dynamic mechanical properties of coal with different bedding directions under different impact velocities were analyzed and discussed. Experimental results indicate that bedding directions not only have a major influence on dynamic mechanical properties such as dynamic tensile strength, strain rate and strain energy but also have a great influence on the crack propagation path. Then, based on the image processing technique and fractal method, cracks conforming to the fractal have been proven, and the results further illustrate that the fractal dimension of cracks on the coal surface increased in the fracture process under SHPB loading. Finally, a numerical model based on bond-based Peridynamic theory was proposed to simulate crack propagation and dynamic mechanical properties of coal under the SHPB test, and the displacement-strain-stress fields were also calculated, which further reveal the fracture mechanism and dynamic behavior of coal under different impact loading conditions.

Experimental study on dynamic mechanical property of cemented tailings backfill under SHPB impact loading

Cemented tailings backfill (CTB) have increasingly been used in recent years to improve the stability of mining stopes in deep underground mines. Deep mining processes are often associated with rock bursting and high-speed dynamic loading conditions. Therefore, it is important to investigate the characteristics and dynamic mechanical behavior of CTB. This paper presents the results of dynamic tests on CTB specimens with different cement and solid contents using a split Hopkinson pressure bar (SHPB). The results showed that some CTB specimens exhibited one to two lower stress peaks after reaching dynamic peak stress before they completely failed. The greater the cement-to-tailings ratio is, the more obvious the strain reaction. This property mainly manifested as follows. First, the dynamic peak stress increased with the increase of the cement-to-tailings ratio when the impact velocity was fixed. Second, the dynamic peak stress had a quadratic relationship with the average stress rate. Third, the cement-to-tailings ratio could enhance the increase rate of dynamic peak stress with strain rate. In addition, the dynamic strength enhancement factor K increased with the increase of strain rate, and its value was larger than that of the rock samples. The failure modes of CTB specimens under low-speed impact were tensile failure and X conjugate shear failure, where were nearly the same as those under static uniaxial and triaxial compression. The CTB specimens were crushed and broken under critical strain, a failure mode similar to that of low-strength concrete. The results of the experimental research can improve the understanding of the dynamic mechanical properties of CTB and guide the strength design of deep mining backfills.

Impact resistance of shear thickening fluid/Kevlar composite treated with shear-stiffening gel

In this work, shear-stiffening gel (STG) was introduced into shear thickening fluid (STF)-impregnated-Kevlar® woven fabric (Kevlar/STF) to improve the impact resistance. The STF filled within the yarns of Kevlar and the STG covered the Kevlar/STF to form Kevlar/STF/STG composite. The STG in the Kevlar/STF/STG not only protected STF but also improved the impact resistance of the fabric because of its excellent shear-stiffening characteristics. A series of experiments including the yarn pull-out test, the split Hopkinson pressure bar impact test, rod penetration test, and knife cutting test were carried out to verify the enhancement effect. The improvement mechanism of the impact resistance for the Kevlar/STF/STG was studied. Under the similar anti-impact performance, the Kevlar/STF/STG possessed lower weight than the Kevlar and its strong impact resistance originated from the synergetic effect among the STF, STG and Kevlar. Therefore, the Kevlar/STF/STG exhibited broad potential in the soft body armor.

Transient dynamic behavior of polypropylene fiber reinforced mortar under compressive impact loading

The dynamic characteristics and the constitutive relationship, of polypropylene fiber reinforced mortar (PFRM) materials, were investigated under compressive impact loading. The impact tests were carried out using an improved Split Hopkinson Pressure Bar (SHPB) equipment installed with confining pressure device. Both the static tests and SHPB impact experiments of PFRM were conducted to study the effect of water–cement ratios, fiber volume fractions, and strain rates, on the blast resistance performance of PFRM. The compressive strength, the dynamic elastic modulus, the toughness as well as the ductility of PFRM were analyzed. Experimental results show that the dynamic performance of PFRM materials is significantly affected by strain rates. Nevertheless, the strain rate effect decreases as the strength of mortar increases. Polypropylene fibers are able to improve the impact toughness. The dynamic constitutive law of PFRM was finally derived based on an improved Zhu–Wang–Tang (ZWT) dynamic constitutive model taking into account the material damage. The dynamic stress–strain curves from experiments can be well predicted by the proposed constitutive law which, consequently, can be used to describe the dynamic properties of PFRM materials.

Dynamic behaviour of normal-strength carbonate aggregate concrete at temperatures up to 800°C

The impact resistance of normal-strength carbonate concrete at elevated temperatures up to 800°C was experimentally studied using a split Hopkinson pressure bar (SHPB). The SHPB impact tests focused on the effects of concrete design strengths, high temperature and strain rate on the dynamic behaviour of concrete at elevated temperatures. The stress–time and strain–time curves of the tested specimens were recorded to analyse the impact behaviour of concrete at elevated temperatures. The influences of static compressive strength, high temperature and strain rate on the dynamic peak strength of concrete are discussed. The test results show that the strain rate effect of concrete at elevated temperature is significantly different from that of concrete at room temperature. Within the scope of the SHPB impact tests in this work, the strain rate effects of concrete at 200°C and 300°C are obviously higher than at room temperature. Concrete heated to 400°C was found to recover to around the same level of the dynamic strength increase factor (DIF) values of concrete at 20°C. The strain rate effects of concrete at 600°C and 800°C deteriorated sharply compared with those of concrete at room temperature and lower than 600°C. This paper also shows that a previously recommended formula for the DIF is not suitable for predicting the dynamic behaviour of concrete at temperatures higher than 400°C.

The Deformation Measurement and Analysis on Meso-Structure of Aluminum Foams During SHPB Test

Abstract The split Hopkinson pressure bar (SHPB) is most widely used to measure the dynamic mechanical properties of materials. Such a testing methodology implies the assumption of uniform deformation during an impact test. However, the experimental verification of this assumption for aluminum foams has, as yet, been unreported. In this paper, a test system combining the SHPB with a high-speed digital camera was designed and constructed to study the problem. In the system, the synchronization between SHPB and the high-speed digital camera, the lighting, and the surface treatment of specimen are established. The deformation of meso-structure of aluminum foams during the SHPB impact was observed successfully; furthermore, the localized strains along the specimens were measured quantitatively. Experimental results show that the deformation is non-uniform; that means the assumption of uniform deformation for aluminum foam is not well satisfied. Therefore, a need exists for some modifications to characterize the dynamic mechanical properties of aluminum foams by SHPB.

Experimental study on dynamic properties and constitutive model of polypropylene fibre concrete under high-strain rates

An improved Split Hopkinson pressure bar (SHPB) test device was used to study the dynamic characteristics and the constitutive relationships of polypropylene fibre-reinforced concrete (PFRC). The uniaxial SHPB impact experiments of PFRC with five fibre contents under different strain rates were conducted. The effects of strain rates and fibre contents on concrete strength and toughness were then analysed according to the experimental stress–strain curves. The constitutive model of PFRC was obtained according to the damage viscoelastic constitutive model and fitting of test data. Experimental results show that PFRC has non-linear properties and is sensitive to strain rates under impact loads. Dynamic strength and toughness of PFRC grow non-linearly following a parabolic trend as the fibre contents increasing. The dynamic stress–strain relationship of PFRC can be well described by the damage viscoelastic constitutive model in this paper.

Dynamic compressive behavior of unidirectional IM7/5250-4 laminate after thermal oxidation

Fiber reinforced high temperature polymer matrix composites are currently gaining wide usage in aircraft structures, especially in airframe and engine inlet casing. The failure of composites in worst-case operational conditions mandates the extensive investigation of the mechanical behavior, and the durability in long-term performance and service life under thermal oxidation. In this work, unidirectional IM7 carbon fiber reinforced high-temperature BMI resin composite (IM7/5250-4) were isothermally aged in air for 2 months at 195 °C and 245 °C, respectively. The dynamic behavior of thermally aged composites was investigated on a split Hopkinson pressure bar (SHPB) in three principal directions. The results indicate that thermal oxidation leads to significant reduction in both stiffness and strength of the composites. Optical micrographs of fracture surface and failure pattern of composite after SHPB impact reveals oxidation induced debonding along the fiber–matrix interface due to oxygen diffusion under long-term exposure to elevated temperatures.

Testing Techniques to Eliminate Frictional Effect and to Achieve Constant Strain-Rate in Split Hopkinson Pressure Bar Impact Compression System

In order to obtain the intrinsic stress – strain relationship by split Hopkinson pressure bar (SHPB) impact compression system, two testing techniques to eliminate the frictional effect and hold the prescribed constant strain-rate are demonstrated. The extrapolation method eliminating frictional effect at the tool – specimen interface was exhibited. Several specimens with different height were used for changing the initial diameter / height ratio in the method, where the circular plates were laminated. In order to maintain the constant strain-rate during compression, the incident pulse was adjusted to elevate in accordance with the compression force, where the tapered cylindrical striker bar was applied. Test material is a fully annealed pure aluminum. The intrinsic stress – strain relationship at strain-rate of 1000 /s was determined and the strain-rate sensitivity was also determined. The numerical simulation of the SHPB impact compression system was also performed using the dynamic explicit finite element method. The additional evidence for these experimental techniques to accomplish the elimination of friction and the constant strain-rate was shown.