In the present study, newly developed two-stage split Hopkinson pressure bar method for constant deformation rate has been proposed. Based on the one dimensional stress wave theory two kinds of arrangement of systems are investigated. Since the flow stress of a specimen is changed with strain, the strain rate during high velocity deformation is generally changed when a constant amplitude incident wave is applied. Therefore, a buffer of identical material to a specimen is provided and is deformed by impact. In the method, the stress wave to deform the buffer and the constant amplitude stress wave are superposed as the incident wave for the constant deformation rate test. Dimensional requirements for the buffer and the test piece are fully discussed together with mechanical impedance and sectional area of respective components of the system. Numerical simulations are carried out to identify the new method and availability of the method is confirmed.
Crash safety of a vehicle is based on impact energy absorbed by crashing the structure in a situation that a cabin is protected. Side impact is extremely dangerous for passengers of vehicles. Energy absorption capabilities of the vehicle in side impact are low because there is little room for large deformation of the safety element to absorb impact energy. On the other hand, weight reduction of the vehicle is also important for the fuel efficiency. The purpose of this study is to develop a light and effective energy absorbing member for side impact. In this paper, an energy absorbing tubular member made by a combination of a thin-walled circular tube and a thin-walled square tube was proposed. The effect of the cross-sectional shape of the tubular member on energy absorption capacities under bending impact was investigated by the experiment and the analysis of the finite element code LS-DYNA. From the obtained results, bending strength decreased significantly when the cross section of the member which was subjected to impact bending load was crushed in flat shape. Therefore, energy absorption capacities were able to be improved by preventing flattening deformation of the cross section. Energy absorption capacities of the tubular member were able to be improved by changing its cross-sectional shape without increasing of the weight.
Constant strain rate tests were made on SS400 steel at high strain rates from about 2×103 to 2×104 /s, where a dynamic flow stress depended strongly on the strain rate. In order to evaluate quantitatively the strain rate dependence of the dynamic flow stress, decrement strain rate tests were performed at the strain rates above 1×104/s. Also, to detect the effect of the strain on the strain rate dependence of the flow stress, the rate reduction tests were conducted at the constant strain rate of about 1×104/s at strains of 0.09, 0.16, 0.23 and 0.31, respectively. The flow stress drop caused by the rate reduction was assumed to be a response to an instantaneous strain rate. The results of measurement were treated on the basis of a theory of thermally activated process. The strain rate dependence of the dynamic flow stress which appeared strongly at high strain rates was due to the stress dependence of the activation energy E(τ) for the formation of kink pairs as the dislocation surmounted the Peierls potential barriers. The activation volume decreased with increasing the strain, which meant the strain rate dependence of the dynamic flow stress increased further as the strain increased.
In a gas circuit breaker of an electric power substation, a long operating rod made of an insulating material such as grass fiber reinforced plastics is employed, and when the breaker is operated, the rod was subjected to a strong impulsive compression load. In designing of the operating rods, therefore, the precise knowledge of their dynamic buckling loads is required. In this study, the dynamic elastic buckling behavior of GFRP rods the ends of which were tightly clamped was experimentally investigated. In the experiments, elastic buckling loads of slender GFRP rods in a wide range of loading velocities were measured by using the three different type testing machines: universal testing machine, hydraulic servo system and drop weight type impact loading machine. From the obtained experimental results, an empirical criterion for the dynamic buckling load of the GFRP rod was proposed in terms of the impact velocity and the slenderness ratio. Furthermore, we may show possibility that the proposed criterion can successfully describe buckling behaviors of long rods of many sorts of engineering materials with the same material parameters.
Indentation is widely used to evaluate mechanical properties of structural materials. It has been known that a measured hardness value increases with increase of the indentation strain rate. By using this inconvenient phenomenon, a new method for determining of constants for Cowper-Symonds constitutive equation by using a sharp indentation is proposed in this research. The first report evaluates the influence of strain rate on strain field, and the later report will propose the determining method based on the principle which is shown in the first report. The first report shows that a finite element model with Cowper-Symonds constitutive equation can simulate the strain rate effect such as a load increase with strain rate increase and a penetration increase during a holding time, and then the authors investigate on several parameters which may affect indentation results for a material, which shows dependence of strain rate. The investigated parameters are a critical strain, an indenter velocity, an indenter angle and a roundness of indenter tip. The obtained results are the following; the maximum strain rate appears near indentation crater rim and the strain value at this area is close to the value of the critical strain which describes important strain region for indentation load. A sharper indenter angle produces higher strain rate and makes indentation load higher due to strain rate effect. The roundness of indenter tip decreases the strain rate under the indenter, but an error caused by the roundness of indenter tip can be easily corrected with the conventional technique.
Instrumented indentation is widely used to investigate the elastic and plastic properties of mechanical materials. During an indentation experiment, a rigid indenter penetrates normally into a homogeneous solid, where the indentation load and the displacement are continuously recorded during loading and unloading. It was known that the micro-indentation test was strongly affected by the strain rate in the materials with strain rate dependence of strength. In the second report, the new approach for the evaluation of the strain rate dependence of materials was developed using the loading curvature-displacement relation obtained from the sharp indentation. Therefore, we attempted to use the response surface method in order to determine the constants for Cowper-Symonds constitutive equation (dynamic constants). First, the relationship between the dynamic constants and the loading curvature was constructed by the response surface, and then those were determined using the inverse analysis. It was confirmed that the dynamic constants could be obtained by substituting the result of the micro-indentation for the unknown materials into the response surface up to the strain rate of 102 s-1. By using this method, it is possible to evaluate the strain rate dependence of materials in such a strain rate range.
In order to use functionally graded foam materials (FGFM) effectively as an impact absorbing material, the impact behavior of FGFM which have density distribution in compressional direction was investigated with finite element analysis. FGFM was assumed to be laminated structure consisting of several homogeneous foams with different densities. The compressive stress-strain relations depending on the densities in FGFM were linearly approximated in three distinct deformation regions: elastic region, plateau stress region and densification region. Theoretical analysis of the static compression and finite element analysis on the dynamic compression were performed for the FGFM. The results of finite element analysis revealed that FGFM having high density in near the impact end can absorb a large amount of impact energy without increasing the maximum fixed end load from that of the homogeneous foam.
From the viewpoint of improving both the crash safety and the fuel efficiency, various shaped thin-walled structures have been utilized as energy absorbers of automobiles such as front side members and crash boxes. In this study, quasi-static (≅0.03mm/s), low-speed (6.67mm/s) and impact (3~4m/s) crushing tests of some thin-walled regular polygonal tubes were performed by the testing devices of Amsler, Instron and the dropping-weight types respectively for the crush angle φ = 0, 10, 20, and 30° to investigate the effects of strain rate, crush angle and number of corners on the crushing behavior. The material is annealed S25C that shows high strain rate dependence. The transition from the axial collapse mode to the bending one occurred at certain crush angle between 10° and 20° for all the polygonal tubes and the testing-speed conditions treated in this paper. Once the transition occurs, the mean buckling load abruptly decreases and is not so sensitive to the crush angle under the bending collapse mode. The mean buckling load under the axial collapse mode greatly increases with the strain rate, while that under the bending one hardly grows.
In this paper, the formula of four-point bending (Four-point bending test of the beam having a circular cross section, FPBc) test and three-point bending (Three-point bending test of the beam having a circular cross section，TPBc) test by use of short cylinder typed specimen is proposed to evaluate the flexural strength of rock or concrete, the strength of bedding plane of sedimentary rock and construction joint of concrete. Firstly, the stress distribution within cylinders with various lengths is analyzed by the three dimensional Finite Element Method. Then the formula is proposed based on the analyzed results for the four-point bending (Four-point bending test of the beam having a rectangular cross section, FPBr) test and the three-point bending (Three-point bending test of the beam having a rectangular cross section, TPBr) test of ASTM, JIS by the use of the beam having a rectangular cross section and the FPBc and TPBc tests. The specimen with 5-10cm in diameter and 10-20cm in length is used in the FPBc and TPBc tests. This dimension is that used in uniaxial compression test of concrete or rock drilled core. Secondly, a series of the FPB and TPB tests by use of both type of specimen is performed by the use of granite specimen with various lengths, then it is shown that the flexural strength is evaluated by the proposed formula and compared with that by the conventional one. Finally, it is made clear that the suggested three-point bending (TPBc) by use of the cylinder typed specimen is available for evaluating the flexural strength of concrete and rock easily.
Multiwalled carbon nanotubes in uncured ultraviolet ray curing resin suspension were aligned by applying traveling electric field. In general, the size of the area where the multiwalled carbon nanotubes were continuously aligned by applying electric field is restricted by an interelectrode distance. In this study, the restriction of the alignment area size was removed by adoption of the multiple-electrode and application of the multi-phase voltage. The multiple-electrode used to the experiment was constructed by 24 long and slender electrodes. A cover glass was put on the multiple-electrode, and the suspension was dropped on the cover glass. Rectangular and eight-phase voltage was applied to the multiple-electrode. The phase differences between voltages applied to one and the neighboring electrodes were ±45 degrees. The multiwalled carbon nanotubes in the suspension were aligned in much wider area than the interelectrode distance by applying the traveling electric field generated by the multi-phase voltage. In the latter half of this paper, the theoretical background for the experiment based on the method proposed by the author was shown. Furthermore the influences of the number of phases in the multi-phase voltage and the suspension thickness on fluctuation of the carbon nanotube orientation were theoretically clarified.
Effects of doping 1,2-diphenoxyethane (DPE) to the Ru(bpy)3(PF6)2 light emitting layer of a solid state electro chemical luminescence (SSECL) device, fabricated by simply paste and coating techniques in air, on its luminance efficiency have been investigated. Device of which molar ratio of DPE / Ru(bpy)3(PF6)2 in the light emitting layer is 0.2 showed 5.6times stronger luminescence than the DPE / Ru(bpy)3(PF6)2 - free device. Increase of the DPE / Ru(bpy)3(PF6)2 molar ratio increase the crystalline phase, and the light-emitting layer of high resistivity is obtained. Appropriate doping ratio of DPE / Ru(bpy)3(PF6)2 can allow both high luminance and low current operations of the device.
This paper presents the details of carbonation tests conducted on cross-section cutting-off girders from a reinforced concrete (RC) bridge and proposes a method to predict the service life of an RC bridge based on the extent of deterioration associated with corrosion of the reinforcing bars due to carbonation and chloride ion attack. Service life prediction is a crucial part of systematization of bridge maintenance. Bridges are typically exposed to a range of environmental conditions over their service lives. Deterioration of RC bridges due to carbonation may occur and may have a significant effect on service life. Chloride ion attack may also influence the deterioration process if the bridge is located close to the sea. Many previous studies have examined the effect of carbonation on RC bridges. However, this paper describes the first known application of carbonation tests to cross-section cutting-off girders from a bridge. A flowchart is presented for the proposed method for predicting the service life of an RC bridge based on the extent of deterioration due to carbonation and chloride ion attack. The results show that the main factor in the deterioration of the bridge has been carbonation and that chloride ion attack has also contributed to the deterioration of the bridge. The end of the service life was defined as the point at which the cumulative amount of steel corrosion reached a critical value of Q = 75 mg/cm2. The service life of the bridge was predicted to be approximately 81 years (remaining life of approximately 9 years) on the sea side and mountain side and approximately 142 years (remaining life of approximately 70 years) on the bottom side. The service life for the bottom side was predicted to be longer because of repair work done on the bottom side.