Split-Hopkinson pressure bar method based on one-dimensional stress wave theory has been the most reliable and versatile testing method for materials at high rate of deformation, and generation of various kinds of incident wave for the respective tests such as strain rate jump, constant strain rate, and reverse loading, is inevitable requirement in the method. Since the ramped incident wave is fundamental to suppress high frequency vibrations and to obtain the smooth transmitted wave, various kinds of methods have been investigated. In the present paper elastic springs were focused as a buffer to generate ramped incident wave. It was confirmed that an elastic spring could be used instead of traditional metallic buffer. It was also found that the ramped wave with gradual increase at the beginning could be generated by combination of two springs together with the two-stage SHPB method previously proposed by the author. Application of the present method for impact impression test is discussed.
In this study, the propagation behavior of multiple stress waves propagating in the stepped bar under various mechanical conditions was investigated by numerical analysis and a series of impact experiments. When an incident stress wave with short rising time passes through the connection surface between input and extended bars, low frequency vibration appears on the head of the incident wave. It was found that from the simulation using extremely long extension bar that the vibration on the wave front was caused by the wave dispersion during propagation. The influence of the geometrical effect of the extension bar and its mechanical impedance effect was also investigated. The effect of the cross-section difference between input and extension bars is greater than that of mechanical impedance, especially when the rising time of incident stress wave is short. Using analytical results, a series of trial experiments for the extension of wave duration were performed. We succeeded to extend the duration of the incident stress wave to twice length.
In order to evaluate the strain rate dependence of the dynamic flow stress of 1070 Aluminum, high strain rate tests are performed at strain rates ranging from about 4.4×103/sec to 2.2×104/sec and strain rate reduction tests are also conducted at the strain rates of about 1×104/sec and 2×104/sec, respectively. A gradual increase in the flow stress is observed in the high strain rate. A simplified model for dislocation kinetics under a dynamic plastic deformation is used to consider the deformation mechanism in the above strain rate range. An equation derived from the model can describe the both at lower strain rates where the motion of dislocation is controlled by a thermally activated process and at higher strain rates where the effect of a phonon viscosity drag against the dislocation motion becomes explicit. However, the results of the reduction tests detect that the increase in the flow stress observed in this experiment is not attributed to the rate dependence of the viscous drag against the dislocation motion. The strain rate region conducted on high strain rate tests is still controlled by the thermally activated process and the gradual increase in the flow stress can be expressed by the stress dependence of the activation volume. The transition in the rate controlling mechanism seems to be in the higher strain rate side.
In order to investigate the effect of strain rate and inner fluid on deformation behavior of the formed polyethylene (PE) film, the fluid-structure interaction analysis of the cell structure was carried out using SPH (Smoothed particle hydrodynamics)-FEM (Finite element method) analysis model. The analysis model was composed of cell walls, inner fluid and rigid walls. The cell walls were meshed using shell elements, and the inner fluid was replaced with the SPH particles. Numerical simulations of the compressing behavior of the formed PE film were performed at strain rates of 5.0×100, 5.0×101 and 5.0×102 s-1. Air-filled and no-air models were prepared to investigate the effect of the inner air on the compressive properties. Even if the strain rate dependence was not defined for the constituent materials, it was confirmed that the effect of flow resistance of the residual inner fluid on the strain rate dependence of structural strength is remarkable when the internal fluid flows out. Therefore, in the quasi-static strain rate range, the strength increases due to the flow resistance of the residual inner fluid at the deformation and fracture of cell wall, which induces high strain rate sensitivity of the formed PE film. On the other hand, the increase in the strength due to the flow resistance of the residual inner fluid has reached the upper limit since the internal fluid does not flow out in the impact strain rate region. Thus, the strength of the formed PE film shows low strain rate sensitivity. From the above results, it was suggested that the deformation of the foamed PE film is influenced by the variation of flow resistance of the residual inner fluid with the strain rate.
In this paper, the influence of striker radius on impact response for Charpy specimen was investigated by means of a dynamic finite element analysis. Three-dimensional explicit finite element analysis was performed by considering the contact stiffness between the Charpy specimen and striker according to the Hertzian contact theory. Charpy impact tests were conducted with instrumented striker for a mild steel. The inertial oscillation in contact load between striker and specimen obtained by finite element analysis was almost consistent with the experimental data measured by instrumented Charpy tests. The striker radius affected the magnitude and period of the peak inertia load. It was found that the magnitude of the peak inertia load increase with increasing in striker radius. On the other hand, the dynamic stress field near the V-notch in the Charpy specimen was not affected by striker radius when the time elapsed more than two times of the oscillation period.
The objective of this study is to obtain data that contributes to the development of a damage-reducing method for reinforced concrete columns under severe accidents involving shock phenomena. Experimental investigations were conducted to evaluate the effects of long/short fiber reinforcement on the damage of reinforced mortar (RM) columns subjected to high-velocity projectile impact. The following two types of RM column specimens were supplied for high-velocity impact tests: (i) RM columns strengthened with two-directional aramid fiber sheets (AFRP sheets) lining; and (ii) RM columns of which the mortar was replaced with polyvinyl-alcohol fiber reinforced cementitous composite (PVA-FRCC). The results showed that the lining strengthening with two-directional AFRP sheets remarkably reduces the exfoliation of mortar even under the condition that an impact velocity is almost 1 000 m/s. PVA-FRCC also possessed good damage-reducing performance under an impact velocity of almost 600 m/s, but the exfoliation occurred under that of almost 1 000 m/s.
In this research, we conducted impact fatigue tests in which the magnitude of the impact stress and the duration time of the stress were varied on the smooth specimens of JIS SS400 by using a repeated impact tensile loading machine. To investigate the difference between the behavior of impact fatigue and that of standard fatigue up to about 50 000 times at room temperature, two types fatigue testing machine were used. To continuously obtain the true strain of the specimen during the impact fatigue test, the minimum diameter of the specimen was measured using a two dimensional displacement measurement system. The relationship between the ratio of number of cycles and true strain was investigated. It became apparent that the fracture mode transition from the ductile type to the crack type when the number of cycles to failure was around 10 000 times. It was also found that impact fatigue was lower in fatigue life than standard fatigue when the number of cycles to failure exceeded 10 000 times. As a result of observation of the fractography by SEM, dimples patterns were observed in the cross-sectional of ductile type fracture specimen, and striations patterns were observed in the cross-sectional of crack type fracture specimen.
We clarify elevated temperature strength and deformation mechanisms of the eutectic Al-Si alloy (AC8A) matrix composites reinforced by 15 vol% and 20 vol% FeCrSif and then construct the constitutive equation of the elevated temperature deformation. Compression test was carried out by strain rate rapid change method on the materials under the conditions of the temperatures from 648 to 723 K and the strain rates between 3 ×10-5 and 1×10-3 s-1. As the results, the flow stress of all aluminum alloy matrix composites was increased with increasing volume fraction of the reinforcements. Threshold stress, which was calculated using a stress exponent of 5, decreased with increasing temperature. The values of the activation energies for FeCrSif/AC8A were close to the activation energy of the lattice diffusion for pure aluminium. Experimental results suggested that the dominant deformation mechanism in FeCrSif/AC8A was the climb-controlled dislocation creep controlled by the lattice diffusion. By using the constitutive equation obtained in this study, it is possible to estimate the elevated temperature strength of FeCrSif/AC8A.
Carbon Fiber Reinforced Plastics (CFRP) with laminated construction has excellent mechanical properties, while interlaminar delamination can be easily induced by external impact. To prevent delamination and to improve the impact resistance of CFRP, the addition of interlayers with high toughness material has been reported. Recently, Carbon Fiber Reinforced Thermoplastics (CFRTP) using thermoplastic as a matrix is expected to be applied to the automotive industry instead of CFRP, which require curing time. Thermoplastic elastomer (TPE) are expected to contribute to improve the impact and vibration damping properties of CFRTP because TPE has rubber like elasticity without chemical crosslink and can be molded in the same way as thermoplastics. In this study, CF/PA12 laminates with PA12 elastomer layers were molded and the effects of the quantity of PA12 elastomer layers on its mechanical properties were clarified. PA12 elastomer has a good impregnation property just as PA12, and the absorbed energy in impact test and the loss factor in vibration test of CF/PA12 laminates are improved by addition of PA12 elastomer layers.
CFRTP (Carbon Fiber Reinforced Thermoplastics) are expected to be used for lightweight component parts in the automobile industry, due to their properties such as high specific strength and high specific stiffness. Because of their expensive cost, however, their application to mass-produced vehicles is limited and expected to be used based on the concept of multi-material combined with conventional materials such as aluminum and steel. For the adaptation of a multi-material body, joining technologies for dissimilar materials are required, and a reasonable and practical high performance joining method is expected to be developed. Resistance spot welding is the well-used method for joining panels and bodies in automobiles using metal materials. If the resistance spot welding can be used for joining dissimilar materials of CFRTP and aluminum alloy, there is no need to introduce new equipment and it promotes the adoption of CFRTP. In this study, in order to develop a resistance spot welding technology of CFRTP and aluminum alloy, the influences of current, welding time and electrode force on lap-shear strength were evaluated and the joining mechanism was clarified. In the resistance spot welding of CFRTP and aluminum, the dielectric breakdown of resin, which is a matrix of CFRTP, occurs at the initial stage of energization. After the decomposition of the resin by the high temperature with the dielectric breakdown, an electric path of carbon fibers in CFRTP was created, and direct resistant heating of the carbon fibers melts the surrounding matrix resin. Therefore, larger welding area and fracture load is obtained in the case of longer welding time, larger current and lower electrode force. The shear strength of 17.3MPa was obtained by the resistance spot welding of CFRTP and aluminum with silane coupling treatment.