Hydrostatic pressure dependence of the mechanical behavior of polymers is studied, using constitutive modeling of the yield surface described by the first and the second invariants of stress and the nonassociated flow rule that satisfies the incompressible hypothesis. An internal variable theory of rate-independent plasticity is presented, which incorporates isotropic hardening as a function of accumulated plastic strain. After the determination of material constants under uniaxial tension and compression, the model shows that the von Mises-type effective stress - plastic strain curves under multiaxial load are quite different from those under uniaxial load. The model is compared with the experimental results of uniaxial tension and compression obtained by Spitzig and Richmond and those of torsion obtained by Silano et al. under high pressure.
Improvement of mechanical properties in thermoplastics is experimentally investigated by solid-state extrusion at elevated temperatures. Hot extruded rods of thermoplastics such as polypropylene (PP), polyoxymethylene (POM), high-density polyethylene (HDPE), polycarbonate (PC) and polymethyl methacrylate (PMMA) are cooled rapidly under constrained strain recovery. Rods with 6 to 24mm in diameter can be extruded at elevated temperatures below the melting temperature and the glass transition temperature. A high degree of molecular orientation is observed for semicrystalline polymers extruded below the melting temperature. Uniaxial tensile tests, compressive tests and shear tests are made at room temperature. The stress-strain curves are affected by extrusion processing conditions, such as extrusion ratio, extrusion temperature and constrained strain recovery. Young's modulus and tensile strength of amorphous polymers in the extrusion direction is improved more than those in solid-state extruded rods cooled under free strain recovery. The maximum values can become approximately 6 to 7 times larger than those of virgin materials. The mechanical properties are improved with increasing extrusion ratio for crystalline and amorphous polymers, and decreasing extrusion temperature for amorphous polymers. The tensile modulus of extruded polymers is greater than the compressive modulus and the difference comes to be greater with increasing extrusion ratio. Shear strength of extruded rods is also higher than that of virgin materials.
The fracture behavior of cracks in the vicinity of a HIP diffusion bonded interface between JIS designation SS400 mild steel and C1100 copper was investigated. The cracks were oriented parallel to the diffusion bonded interface and a tensile load was applied in a direction perpendicular to the interface for a range of test temperatures from 20°C to -180°C. Material dissimilarity led to a complex mode of crack opening, and failure occurred either at the crack or at the material interface depending on the position of the crack relative to the interface and the test temperature. The differences in behavior were investigated using a three-dimensional finite element analysis (FEA). The strain energy density criterion was applied to predict the occurrence and direction of crack propagation by lowstress brittle fracture at extremely low temperatures. It is suggested in this paper that fracture initiation at the interface edge can be predicted based on the total strain energy density in that region.
The effects of small defects on the fatigue strength of Ti-6Al-4V were investigated in tension-compression fatigue tests. To simulate the defects, holes having 50, 200, 400, 760 and 1000μm diameters were introduced onto the surface of a series of specimens (Series A). Another series of specimens (Series B) were prepared to investigate the effect of a burr or a pre-crack which was introduced at the edge of the artificial hole. For Series A specimens, the fatigue limit was defined as the threshold for crack initiation from the edge of the hole. For Series B specimens, the fatigue limit was defined as the threshold for crack propagation from the burr or the tip of the pre-crack. The results of the fatigue tests for R=-1 indicated that the fatigue limits of Series B specimens were 20-60MPa lower than those of Series A specimens. In contrast to steels, non-propagating cracks did not form at the holes, a fact associated with the relatively high stress level for crack initiation from hole in Ti-6Al-4V. It was also found that the √area parameter model underestimated the fatigue limit of Series A specimens, but the model accurately predicted the fatigue limit defined by crack-growth threshold for Series B specimens. In addition it was noted that the presence of a burr resulted in a decrease in fatigue life, since the burr facilitated the crack initiation process.
Fluid flow in rock mass is controlled by geometry of fractures which is mainly characterized by roughness, aperture and orientation. In this study we measure the aperture of rock fractures using a high resolution confocal laser scanning microscope (CLSM). Digital images of the aperture are acquired under applying five stages of uniaxial normal stresses. The error range is less than 0.125μm. It has been difficult to check the aperture change under stress application for the same specimen continuously. Our method can characterize the response of aperture. Results of measurements show that roughness geometry of fracture bears no uniform aperture. That is, it is changed in nonuniform manner under the different stress levels: Some parts bear a smaller aperture because of the applied stress, while some parts with very narrow aperture develop new cracks or shear displacement because of no space of aperture reduction. Laboratory permeability tests are also conducted to evaluate changes of permeability related to aperture variation due to different stress levels. The results do not imply a simple reduction of hydraulic conductivity under increase of the normal stress. This suggests that the mechanical aperture is different from the hydraulic aperture which is an effective conduit of fluid flow along a fracture. It is shown that the hydraulic aperture is slightly smaller than the mechanical aperture. Clearly the flow channels are changed due to the local change of geometry under the applied stress. The hydraulic conductivity does not follow the cubic law. This means that a parallel plate model is not suitable to express the hydraulic conductivity including local fracture geometry.
Domain structures and rheological properties of immiscible polymer blends composed of components having almost the same viscosities in a parallel-plate (PP) geometry are studied in comparison with those in a coneplate (CP) geometry. Both steady states and transient states after step increase of shear rate are examined. At steady states, it was confirmed that the domain size becomes smaller inversely proportional to the distance from the center (i. e., shear rate) along the radial direction of PP, consistent with the data in CP compared at the same shear rate. The related rheological data are also consistent. At transient states after the step increase of shear rate, there was no apparent long-range interference in the transient behaviors undergoing in different time scales at different positions along the radial direction of PP. The structure change and the related excess stresses of immiscible polymer blends with the nearly same viscosities under shear flow in PP are consistent with those in the CP geometry.
For cleaning-up the water quality of artificial non-fecal drainage, surface-treated porous bodies made of glass beads were used as carriers for microorganisms which have biological activity for the mitigation of pollutants in the water. The influence of chemical species used for coating the bodies and also the texture of the treated surface on the effectiveness of the mitigation of pollution was examined. Coating with calcium silicate, iron oxide and calcium oxide decreased CODMn of the artificial drainage significantly, thus indicating effectiveness for the mitigation of the pollution. The size of cavities which were generated as voids in the three dimensional arrangement of the differently-sized glass beads also affected the effectiveness of the pollution mitigation though the effectiveness was limited. It was concluded that chemical composition of the surface was the most important factor in acting as the carrier of the microorganisms.