Selecting a suitable constitutive equation which takes account of the precise strain rate sensitivity of the material is, in general, very important for dynamic-crash analyses. Tanimura-Mimura (T-M) model is one of a few constitutive equations which are able to describe the specific characteristics of many ferritic steels that the strain rate sensitivity in flow stress decreases with increasing plastic strain, however, it has not been examined well how the crash analyses are improved when this model is employed. In this paper, both experimental and numerical analyses of dynamic three-points bend test of thin steel sheet hollow column were conducted at the impact velocity of 4m/s in order to evaluate the effectiveness of T-M model. In the numerical analyses, Cowper-Symonds (C-S) model and strain rate insensitive (I-S) model as well as T-M model were employed, and their calculated results were compared with the experimental result. The calculated load-stroke relations by T-M model showed good agreement with the experimental ones over the whole stroke, on the contrary C-S model was in agreement only in the small stroke region. Shape profiles of the buckling portions of columns in the experiment were compared with the simulated results based on each model. The width of the buckling zone was getting smaller in the order of T-M model, I-S model and C-S model, and T-M model was found to give the best agreement with the experiment since its low strain hardening profiles in stress-strain relations at high strain rates promote the localization in the buckling zone. From these results, T-M model can be considered to improve accuracy of numerical simulation, especially in the case that significant strain localization such as buckling occurs.
Microstructures and mechanical properties of ultra fine-grained low carbon steels fabricated by equal channel angular pressing (ECAP) were investigated. Several specimens statically annealed for 5 hours at temperatures of 773-873K were studied simultaneously. Electron back-scattering diffraction (EBSD) measurements were carried out for microstructure observation. Differential scanning calorimeter (DSC) curves were obtained for studying the thermal stability of specimens. The initial grain size of ∼10μm in the ferrite-pearlite steel was refined to ∼0.2μm by 4 passes of ECAP. The tensile strength of the as-ECAPed specimen was around 2 times higher than that of the initial normalized specimen, although the ductility decreased by half after the process. At annealing temperature of 773-823K, tensile strength and yield stress decreased as the average grain size of each specimen increased. However the grain growth by annealing was not significant. At annealing temperature of 873K, the nonuniform grain growth occurred suddenly, and as a result the tensile strength decreased. On the Hall-Petch relation diagram, the gradient of the diagram over the average grain size of 1mm was slightly different from that of under 1μm. In addition, the nonuniform radical grain growth at the temperature of 873K was in accord with the appearance of the exothermic peak in the DSC curves. The accumulation of the local strain in the 823K-annealed specimen was investigated by the kernel average misorientation (KAM) approach using EBSD, and it is suggested that accumulation of the local strain in the 823K-annealed specimen is one of the causes of the nonuniform radical grain growth at the temperature of 873K.
In order to investigate the effect of surface treatment on fatigue properties of martensitic type stainless steel, SUS420J2, rotating bending fatigue tests were carried out using the steels surface-treated by three kinds of conditions. The surface treatments tested were CrN coating, radical nitriding and duplex treatment of CrN coating after the radical nitriding, and the results were discussed in comparison with those of quenched and tempered steel. Fatigue strengths were markedly increased by CrN coating, nitriding and duplex treatment. However, there was no or little effect of CrN coating to nitrided steel on the fatigue strength. Surface fracture yielded in quenched and tempered and CrN coated steels, while fractures occurred from inclusions at the subsurface of specimens in nitrided and duplex treated steels. The growth of an internal crack was suppressed at the boundary of fish eye to the direction to specimen surface by the nitriding in nitrided and duplex treated steels. The ligaments between the fish eye boundary and specimen surface showed a brittle fracture in both steels, especially in duplex treated steel there was a few steps on the ligament meaning the coalescence of cracks initiated from droplets at specimen surface and inclusion in subsurface, independently.
This article shows the environmental degradation in the fatigue strength of a glass fiber/unsaturated polyester composite, and proposes how to improve the environmental durability of the composite. First, the specimens of the composite are exposed to high temperature and in some cases moisture for long periods of time, and then the reduction in the fatigue strength of them is evaluated by performing fatigue tests. Next, to clarify the degradation of the matrix resin and the effect of the degradation on the fatigue strength of the composite, two viscoelastic properties, namely storage modulus and loss tangent, of the matrix resin are measured with DMA (Dynamic Mechanical Analyzer) before and after environmental exposure. In addition, the specimens broken in the fatigue tests are observed with SEM (Scanning Electron Microscope) in order to investigate the debonding of the interface between the fibers and the matrix resin and the effects of the debonding on fatigue strength. In the case of the composite, the fatigue strength of the composite is reduced by exposing it in high-temperature and moist environment. The reduction occurs especially in the low-cycle fatigue range, and hence it can be caused by the environmental degradation in the fatigue strength of the matrix resin rather than the interface. Since the degradation is easily confirmed by measuring the loss tangent of the matrix resin, measuring the loss tangent could be utilized to improve the environmental durability of the fatigue strength of the composite.
To improve the mechanical properties of biological decomposition plastics, in this study, laminated reinforce plastics (LBRP) were made using AZ31 magnesium alloy as reinforcement, Bionolle as the base material, and an adhesion bonding agent based on cyanoacrylate and epoxy resin. The LBRP were laminated as one-layer and as two-layer composites using AZ31 magnesium alloy and Bionolle. The tensile strength of a layer of clad material increased with an increase in the volume fraction of Mg alloy, and was corresponding to rule of mixtures with mother and reinforced material. The strengths of the two-layer LBRP indicated were much greater than that assumed by the mixtures rule. Because the adhesion bonding agent flowed and bonded into the grooves of the conversion coating film, the tensile strength of the LBRP increased along with an increase in the arithmetical mean roughness on the conversion coating film. As a functional characteristic, a vibration test was carried out for these LBRP made with AZ31 magnesium alloy reinforcement, which has an excellent attenuation characteristic. In the result, the damping ratio and amplitude ratio for a layer of clad material are better than these for a base material. Therefore, the effect of the attenuation characteristic of AZ31 magnesium alloy is proved as beneficial in composites.
The relaxation behavior of clamping force in friction-type FRP bolted joints is one of the significant subjects in application of FRP to structures. The aim of this paper is to obtain the factors of relaxation behaviors in the GFRP bolted joints based on the experimental results. The relaxation behaviors are due to both steel bolt and GFRP laminate. Steel bolt dominates the relaxed force immediately after initial loading but not reloading. The relaxation factors of GFRP laminate except matrix polymer could be both laminate surface geometry and laminate bond, considering the variance of relaxed force and more relaxed force for bonded laminate than single laminate observed in the experimental results. Boltzmann superposition principle can be hardly applicable to the relaxed forces for initial loading and reloading, suggesting that less relaxed force for reloading could be obtained by the strain hardening of bolt threads through the decrease of cross section area due to shear plastic deformation and that surface geometry could be modified by plastic and irreversible viscous deformation considering the viscoplastic property of matrix polymer. More relaxed force could be obtained for the surface geometry farther from the flat surface and the surface geometries could be categorized into the surface with either equal height protuberances or random height ones. Laminate bond dominates the relaxed force and its lower viscosity could give relaxed force more. The viscosity of laminate bond could be more variable than matrix polymer and increase through the modification of structure by plastic and irreversible viscous deformation. The minimum elapsed time could be required to complete the relaxation behavior for the flat surface and the bond viscosity as high as matrix polymer.
Due to the increasing of the environmental issues, in recent years, natural fiber reinforced biodegradable polymers, green-composites, has attracted attention. Within these green-composites, natural fiber reinforced PLA has attracted considerable attention. The disadvantages of natural fiber reinforced PLA are low heat resistant and low strength. Crystallization of PLA has been the subject of research and development in injection-moulding, which can be achieved by addition of nucleating agent and annealing process. The annealing process of PLA has not been applied to natural continuous fiber reinforced PLA, because it is difficult to control mould temperature during annealing process. In this study, jute continuous fiber and polylactic acid were used and moulded by electromagnetic induction heating system (IH system). This system allows heating and cooling of the mould surface instantaneously. Therefore the temperature of the mould surface can be arbitrarily controlled. The nucleating agent was added to the PLA to enhance crystallization. The IH system was applied to this PLA for maintaining the crystallization temperature. Heat-resistant and mechanical property of jute continuous fiber reinforced PLA were measured. As a result from the temperature of deflection under load, highly crystallized PLA composites had higher heat resistance than low crystallized PLA composites. Temperature of deflection under load of highly crystallized PLA composites was 123.5°C and bending strength was 75.9 MPa.
Montmorillonites (MMT) treated with various organic amines were added to the acrylic rubber in the polymerization process. Effects of the kind of organic amines, the contents of added MMT, and the method of the addition of MMT to the acrylic rubber on the dispersion state of MMT in the matrix and on the mechanical property of the obtained rubber were studied. As a result, the remarkable improvement of the mechanical property was achieved when 5 wt% of MMT organically treated with the laurylamine was added, although the deterioration of the mechanical property was observed at higher MMT contents. On the other hand, the mechanical property of the sample, to which MMT was added in the solution state, was improved gradually with the MMT content without showing a significant deterioration. Although the addition of organically treated MMT having a hydrophobic surface was added in the aqueous emulsion polymerization system, an excellent dispersion state was achieved when 5 wt% of MMT treated with laurylamine was added. Moreover, comparing with the friction property, the heat resistance, and the water resistance of the acrylic rubber added with MMT to those of the acrylic rubber added with the white carbon, it was proved that the addition of MMT significantly improve various physical and practical properties of the acrylic rubber.
The surface structures made of conventional asphalt pavements prevents rainfall from infiltrating into ground. In recent years, frequency of flood occurrence has been increasing by an increase of rainfall outflow into of rivers and sewers in urban areas. The rainfall storage and infiltration properties of permeable pavements, which are made of porous asphalt, have been attracted attention as a control measure of flood in urban areas. Previous study makes clear that rainfall infiltration properties of permeable pavements are strongly dependant on the behaviors of unsaturated zones in the pavement. In order to estimate the effects of permeable pavements on flood control that were shown by the experimental results, we have developed the simulation method that analyzes the air and seepage flow simultaneously. But, there have been few data about the flood control properties of permeable pavement for rainfall having inconstant intensities. This paper shows the rainfall storage and infiltration properties of permeable pavements for inconstant rainfall intensity by experiments. And we examine whether the simulation method we have developed in order to estimate the flood control properties of permeable pavements is valid for the inconstant rainfall. This study shows the design method of the permeable pavement structures which are appropriate for various precipitation probabilities and heavy downpours.
Natural fibers are usually used as reinforcement in green composites. Especially, the use of slivers of natural fibers is anticipated for increasing composites' stiffness and strength. However the slivers have various fiber orientation angles and often involve fiber fluctuation. This paper describes effects of fiber orientation angle and fluctuation on Young's modulus and tensile strength of the so-called fully green composites. The composites were reinforced with slivers of high-strength natural fibers extracted from curaua plants. For this study, a fabrication method called ‘direct method’ was applied for obtaining sliver-based green composites with various fiber orientation angles and fluctuation. Then optical micrographs of composites with fiber fluctuation were obtained. After the micrographs were divided into many fine segments, the fiber orientation angle in each segment was measured. Results show that the tensile strength depends on autocorrelation coefficients expressing the degree of fluctuation in fiber orientation, as well as the fiber orientation angles. However, the Young's modulus was dependent only on the angles. Furthermore, the Young's modulus of the composites was predicted using classical lamination theory. In addition, a statistical concept was applied to an orthotropic analysis for prediction of the Young's modulus. The predicted Young's moduli showed better agreement with the experimental results, than that of the predicted values based on a definite orthotropic theory.