Abstract: Digital Image Correlation is a non-contact optical technique for measuring the full-field deformation on the surface of a deforming specimen. The technique was initially used in quasi-static experiments, but with the development of high speed digital cameras is used also in dynamic experiments. This use of the Digital Image Correlation technique in several dynamic experiments is presented. This includes the compression and tensile split Hopkinson bar tests, a shear test for specimens made of sheet metal, a dynamic punch test, tensile test of Kevlar cloth and Kevlar yarn, and an intermediate strain rate test in compression.
Abstract: There are many civil engineering structures that have different systems and required functions. Their design methods do not have consistent design concepts. Thus, it has been pointed out the necessity of universal concepts on assumed external actions and risk for various structures and on the required level of safety. In order to meet those demands, a research committee as part of Japan Society of Civil Engineers summarizing the basic concepts of impact resistance design. This paper introduces several design methods of structures subjected to impact loads, and presents the current status and remaining issues of establishing new performance-based design methods.
Abstract: Innovative researches using neutrons are being performed at the Materials & Life Science Experimental Facility (MLF) in the Japan Proton Accelerator Research Complex (J-PARC), in which a mercury target system is installed as MW-class pulse spallation neutron sources. In order to produce neutrons by the spallation reaction, proton beams are injected into the mercury target. At the moment when the intense proton beam hits the target, pressure waves are generated in mercury because of abrupt heat deposition. The pressure waves interact with the target vessel leading to negative pressure that may cause cavitation along the vessel wall, i.e. the interface between liquid and solid metals. Localized impacts by microjets and/or shock waves that are caused by cavitation bubble collapse impose pitting damage on the vessel wall. The pitting damage that degrades the structural integrity of the target vessel is a crucial issue for the high power mercury targets. Therefore, the mitigation techniques for the pitting damages and cavitation are needed to reach the MW-class pulsed spallation neutron sources.
Abstract: The impact modeling of a hot-formed component with tailored mechanical properties is studied to understand the influence of the thermal processing history and how the final properties of the component will affect its impact response. This paper presents a numerical study of the forming and quenching process and subsequent impact simulations. The processing simulations serve to predict the final microstructure and hardness distribution within a lab-scale B-pillar component that is processed using a tool with separate heated and cooled regions. A remapping algorithm is used to translate the results of the forming simulation to the impact simulation. A strain-rate sensitive material model is applied to model the response of these tailored microstructures during impact events. A comparison between a component that is fully hardened and a tailored component with regions of lower strength but increased ductility is presented in this work. Simulations that do not consider the onset of fracture predict superior peak impact load and energy absorption of the fully martensitic component due to its higher overall strength. However, the bainitic regions within the tailored component exhibit much higher ductility. Current work is addressing the introduction of failure criteria into simulations of tailored hot stamped components under impact loading for which the tailored component is expected to demonstrate superior resistance to cracking relative to the fully hardened component.
Abstract: The responses of three high strength steels under impact loading were examined, specifically on their strain rate dependence. Split Hopkinson pressure bar test was used in this study. Over a wide strain rate range, the Johnson-Cook model and modified Johnson-Cook mode were adopted to determine the strain rate hardening behavior of the materials. The group determined the material parameters for each metallic material tested. Obtained material parameters were used to predict the behavior of each steel at high strain rate region. The modified Johnson-Cook model was not able to represent well enough the plastic deformation behavior of steels, specifically the steel that exhibited strain softening behavior at high strain rate region.
Abstract: In this study, we conducted water hammer experiments in the tube which was periodically supported by various numbers of clamps, named periodic structure, initiated by a projectile impact. The parts of the polycarbonate (PC) tube supported by 1-7 steel clamps make the tube stiffer and heavier than the original PC tube and are expected to cause a filtering effect of the frontal frequency components in the water hammer. According to our experimental observations, we confirmed that higher frequency components more than 1 kHz in the wave front were attenuated and the peak strains in circumferential direction of the tube were decreased 20% from the original PC tube. Moreover, we conducted numerical simulations of the water hammer wave similar to the experimental setup. Numerical results also revealed that frontal peak is attenuated 22% through periodic structure.
Abstract: AZ31-based magnesium nanocomposites were produced by a disintegrated melt deposition technique, whereby different volume fractions of 50-nm Al2O3 nanoparticles (1.0v%, 1.4v% and 3.0v%) were used as reinforcement and added to AZ31 Mg alloy. A monolithic counterpart was also produced by the same process for comparison. Samples of these materials were subjected to dynamic tension at strain rates up to 1.2 103 s-1, using a split-Hopkinson Bar device. Compared to the quasi-static response, the monolithic and composite materials showed significantly increased yield stress and ductility under dynamic loading. The enhancement in yield stress with strain rate indicates rate sensitivity of the critical resolved shear stress for slip systems under tension. The addition of nanoparticles was found to reduce the grain size of the resulting material and increase the yield stress and ductility simultaneously, for both low and high rate loading.
Abstract: The effect of strain rate up to nearly = 102/s on the tensile stress-strain properties of isotropic fine-grained nuclear-grade graphite IG-11 was investigated. Cylindrical tensile specimens machined out of graphite bars were used in both static and dynamic tests. The dynamic tensile stress-strain curves up to fracture were determined using the split Hopkinson bar (SHB). The low and intermediate strain-rate tensile stress-strain relations up to fracture were measured on an Instron 5500R testing machine. It was demonstrated that the ultimate tensile strength increases slightly, while the fracture strain and absorbed energy up to fracture decrease dramatically with increasing strain rate. Macro and microscopic examinations revealed a slight difference in the fracture surfaces between the static and dynamic tension specimens.
Abstract: This study deals with the effect of intermittent impact loading on the SCC growth behaviour of SUS304 austenitic stainless steel. To this end, the Split-Hopkinson pressure bar (SHPB) experiments on SUS304 were performed first to establish the dynamic tensile stress-strain response at strain rates up to 700s-1. The analyses of dynamic stress intensity factors for wedge loading experiments on modified compact tension specimens, designated WLCT, were then performed by the finite element method. The SCC experiments with the intermittent impact loading for WLCT specimens of SUS304 by the use of the SHPB were mentioned and discussed as well.