Porous materials have specific features of extremely light weight, high energy absorption, acoustic insulation, permeability, and so on. However the deformation mechanism of such materials is so complicated, since the material is composed of many slender and/or thin member elements with surrounding pores. This paper discusses the macroscopic deformation behavior of artificially fabricated porous-skeletal materials. The materials are designed by use of the computational geometry so-called ’Voronoi tessellation’ method, through which the high porosity is achieved by setting the line segments of the Voronoi polyhedra as beam-like thin rods, and the test materials are fabricated by 3-D printer. This technique enables us to compare the deformation mechanism on the same structure both by experiment and simulation. Four-point bending and compression tests are carried out to evaluate the macroscopic stiffness. It is observed that the stiffness depends not only on the porosity but also on the real size of pores/members. Finite element analyses are next implemented to examine the microscopic response of the material in the course of these processes. We discuss the mechanical effect of microscopic member elements on the macroscopic stiffness of the material.
Recently, a possibility of a new technique of CO2 storage in the ground, namely, CO2 storage as hydrate, has been under discussion. In the present study, the mechanical behavior of CO2 hydrate bearing sediments was investigated to provide the fundamental mechanical properties of CO2 hydrate sediments. Especially, time dependent behavior was focused on and a series of undrained triaxial tests were conducted using high pressure and low temperature triaxial apparatus. From the results, it is found that hydrate-bearing sand specimen exhibits acceleration creep under undrained condition while sand specimen does not. In addition, an elasto-viscoplastic model considering the effect of hydrate saturation is adopted to reproduce the test results.
A viscoplastic self-consistent (VPSC) numerical model for deformation texture prediction been developed and applied to the plane strain compression problems of the high temperature austenite steels having fcc lattice structure. Using the VPSC model, the parameter study of the strain rate sensitivity exponent (m-value) was conducted and the results were compared with those by an extended Taylor model (the viscoplastic constitutive law is employed while uniform strain assumption over grains in the representative volume element is adopted) and by the hot rolling experiments using Fe-70Ni fcc metal. Despite the small underestimation of brass texture, the results by VPSC model have revealed that m = 0.02 case successfully reproduced the similar texture with the experimental ones of 650 °C hot rolled Fe-70Ni sample, while m = 0.2 case reproduced 850 °C hot rolled sample texture. By contrast, the extended Taylor model overestimated the copper texture for any parameter cases. Therefore, the simulated deformation textures by the VPSC model agree better than those by the extended Taylor model. In addition, the effect of deformation magnitude on the texture formation has been analysed by using the VPSC model. The results find the more pronounced copper texture with increasing compressive strain, which agrees with the experimental results.
The dislocation-based crystal plasticity model has been extended to a model considering higher-order stress by explicitly considering the mechanical work induced by the strain gradient. Furthermore, a finite element (FE) analysis is performed for a single crystal with a long-period stacking ordered (LPSO) phase on the basis of the obtained model. The mesh dependence of the kink band formation is discussed from the viewpoints of the strain gradient, size effect and higher-order boundary condition. Then, it is shown that the mesh dependence of kink deformation in the FE analysis can be removed even when the size effect parameter is relatively small. The width of the kink band is determined by the intrinsic length scale. When the kink band occurs in the entire specimen, work hardening can be reproduced by appropriately defining the boundary conditions for slip. The effect of length scale ratio on the kink deformation is small. Moreover, it is shown that the disclination quadrupole structure in the kink band can be expressed qualitatively by using incompatibility of crystal slip.
Welding of metal and polymer is attracting example of a joining technique of different materials. However, it is expected that mechanical behavior of the polymer changes markedly due to heating exceeding the melting point and residual stress. Plates of polyamide and SUS304 were welded using the hot-air welding machine under different welding conditions. The influence of heating time, distance and cooling conditions on the bonding strength of the welded structure were evaluated by tensile test. The obtained results demonstrated that the bonding strength in some specimens became larger than yielding strength of PA depending on the welding condition. However, the tensile responses considerably varied even for specimens obtained from same welding condition. The situation of fracture was classified in three cases, namely, fracture of interface, fracture of polymer, and fracture of welding area. The first and third cases were observed when the total heat given to the polymer were large and small, respectively. The strain distribution around the welding area under small deformation could be a indicator of predicting the fracture pattern of the welded specimen.
A second-order homogenization method, in which strain gradient in macroscopic region is considered, has been proposed to evaluate the size effect on the macroscopic deformation. In this study, plane strain compression of a polycrystalline material is numerically investigated using an FE-based second-order homogenization method, and effects of size of macrostructure and friction on macroscopic non-uniform deformation is discussed. In the simulation of plane strain compression, the friction between an anvil and a specimen causes non-uniform deformation on the macroscopic region and the size of the specimen affects the compressive force and the macroscopic distribution of strain when the friction force works. The results find that under deformation with the strain gradient caused by the friction the compressive force becomes larger and the macroscopic distribution of strain becomes more inhomogeneous as the size of the specimen becomes larger. These size effects on the macroscopic region are related to deformation in microstructure that depends on the strain gradient in the macrostructure.
In order to investigate the fatigue strength of thin film comprising of copper (Cu) nano-elements which is fabricated by means of the glancing angle deposition (GLAD) technique, cyclic loading tests are carried out on the specimens where the thin film is sandwiched between dissimilar materials. The thin film does not fracture at 106 cycles in the strain range of 9.2×10-2, which is 30 and 90 times higher than the fatigue strength at 106 cycles of Cu solid thin film (grain size: 1000 nm) and bulk, respectively. This fatigue strength is brought about by superimposing the increase of allowable strain due to the shape of helical nano-elements (shape effect) and the improvement of material strength due to small grain size (size effect). The rates of strength increase due to the shape effect and the size effect are estimated to be 20 (film) - 40 (bulk), of which order has an agreement with the experimental results. In the relationship between the strain range and the number of cycles to failure (Δε - Nf curve), the gradient of the curve of the thin film comprising of Cu helical nano-elements is significantly smaller than those of the solid thin film and the bulk. This indicates that the thin film comprising of Cu nano-elements has brittle nature, which is induced by fine grains in the nano-elements.
Ankle Foot Orthosis (AFO) is widely prescribed for the purpose of assisting the gait of patients with spastic paralysis or flaccid paralysis. Carbon Fiber Reinforced Thermoplastics (CFRTP) using a thermoplastic resin as a matrix is expected to be used for AFO, since CFRTP-AFO can be adjusted after molding. Cost competitive CFRTP-AFO is expected to be developed by choosing a low-cost intermediate material and low cost molding method. Random prepreg sheets have been developed as an intermediate material of CFRTP which is excellent in formability and manufacturing cost. As a method of shaping intermediate material, there is diaphragm forming which is simple in equipment and excellent in mold cost. In this study, diaphragm forming and random carbon fiber reinforced polycarbonate prepreg sheets were applied to manufacture affordable CFRTP-AFO. The forming simulation of CFRTP-AFO by finite element analysis (FEM) was conducted to predict the formability. Young modulus of CFRTP prepreg sheets obtained by the tensile test at molding temperature was used for the FEM analysis.by obtaining the mechanical properties of CFRTP prepreg sheets at molding temperature. Wrinkle positions of CFRTP-AFO were qualitatively predicted by using FEM. The flexibility of CFRTP-AFO can be controlled by the number of laminated prepreg sheets which are used at the heel.
Formation of SiC in the isotropic pitch-based carbon fiber and its C/C composites was investigated. The carbon fiber heat-treated at 2000°C and the glass-like carbon derived from a resol type phenolic resin which was heat-treated at 2000°C were reacted with SiO gas at 1700°C for 5 hours. It was found that SiC crystal formed on the surface of the carbon fiber, and the part of the carbon fiber was broken. The D-band peak of raman spectrum of the carbon fiber became higher after the reaction, which may indicate the increase of defects on the surface of carbon fiber. The surface of glass-like carbon showed less damage than that of the carbon fiber, and the D-band peak decreased after the reaction. The C/C composites were also prepared from the same carbon fiber and phenolic resin, and heat-treated at 2000°C. The C/C composites were reacted with SiO gas in the same way, and it was found that the damage on the carbon fiber was low. It is considered that the carbon matrix derived from resin may protect the carbon fiber. SiC formation test was conducted for a commercially available rigid insulation, DON-1000-H, under SiO gas atmosphere at 1600 ∼ 2000°C for 5 hours. SiC was formed in both of the carbon fiber and the carbon matrix in DON-1000-H after the SiC formation test, and the weight of DON-1000-H increased by 1.9 ∼ 2.9%. The compressive stress at 5% displacement of DON-1000-H decreased by 33.3 ∼ 48.7 % after the SiC formation test. The higher the temperature of SiC formation atmosphere, the more changes of the weight and the compressive stress of DON-1000-H. High temperatures probably accelerate the formation of SiC which caused the damage on rigid insulations.