Takashi Ariyama

Viscoelastic-plastic behaviour with mean strain changes in polypropylene

The stress-strain curves and stress-relaxation curves of polypropylene are obtained by using a closed-loop, electrohydraulic, servo-controlled testing machine. Effects of mean strain changes on deformation behaviour are examined in a tension-compression mode under strain control at room temperature (18–23 °C). The hysteresis loops of three mean strains show a steady-state response from the stress-strain curves at a strain rate of 1 × 10−3 s−1 at a strain width of 5%, at a number of cycles of N=50 and at three mean strains (ɛm=0, + 1.0 and + 2.0%). The drop of stress at the mean strain of ɛm= -1.0% is larger in magnitude than that at ɛm=+1.0%; this is caused by the higher stress level at ɛm=- 1.0% as compared with the stress level at ɛm=+1.0%. From the results of stress amplitude and the stress drop behaviour, the magnitude of stress drop is hardly affected by the mean strain.

Plastic behavior of nuclear structural metals after cyclic prestraining

Abstract The plastic behavior of metals (SUS 304 stainless steel, 2 1 4 Cr-1Mo steel and Al-alloy) after cyclic prestraining is systematically and experimentally investigated in a state of plane stress that consists of axial and torsional stress (strain) components. A practical biaxial extensometer for use in cyclic and subsequently large strain tests is also developed. The stress level in reloading after cyclic prestraining is never below the corresponding initial stress-strain curve, regardless of the loading direction, even after cyclic softening. Initial plastic anisotropy does not disappear or become small due to cyclic prestraining, and may become strong, depending on the prestrain direction. The larger the cyclic strain hardening, smaller the work hardening coefficient in reloading. This suggests initial stress-strain relations should and can be used as basic data on materials when analyzing the strength and safety of a structure undergoing normal cyclic loading followed by abrupt overloading such as that occuring during an accident or earthquake.

Internal friction and electrical properties in phosphate glasses containing transition metal oxides

The internal friction and electrical properties of glasses of 4OFe2O3-6OP2O5 system and xV2O5-(100 −x)P2O5 system were measured. For internal friction, two peaks were observed in the temperature range of-100°C to 400°C at a frequency of about 1Hz. For iron oxide glasses, the area of low-temperature peak(at about 0°C)increased with increasing valence ratio Re=Fe2+/Fe2++Fe3+, and for vanadium oxide glasses those of low-temperature peak (at about 50°C to 200°C) increased with increasing the valence ratio V4+ /V4++V5+. Temperature dependence of frequency of dielectric loss peak and low temperature peak on internal friction showed a straight line, and its activation energy was 0.55 eV for iron oxide glasses and 0.32 eV for vanadium glasses. The peak observed at low temperatures was due to a hopping of localized electrons in small polaron.

Cyclic deformation behaviour and morphology of polypropylene

The viscoelastic-plastic behaviour and stress relaxation in polypropylene after uniaxial simple tension or cyclic preloading are studied by an electrohydraulic, servo-controlled testing machine. The stress-strain curve data of simple tension at different strain rates show that the magnitude of stress depends strongly on strain rate. The stress relaxation behaviour after simple tension and cyclic preloading indicates complex features and reveals differences in the deformation of molecular chains in polypropylene subjected to different cyclic preloadings. The stress-strain curves and the stress relaxation curves under three mean strains are fairly different from each other. The microstructural changes in samples subjected to different cyclic preloadings were determined by scanning electron microscopy (SEM). The results of the stress-strain curves and the stress relaxation behaviour are discussed in comparison with the observation of SEM fractographs.

Effects of hydrostatic extrusion on the mechanical and thermal properties of polypropylene

The structure change in spherulites for hydrostatically extruded polypropylene (PP) was studied by the use of internal friction measurements and thermophotometry tests. The onset temperature of the β-loss peak of the PP sample decreases with increasing reduction in area, R. For the extrudates below R=50%, the peak temperature of α-loss shifts to lower temperature. The α and β absorptions for the extrudates up to R=50% become broad and overlap with each other. The intensity of the β-loss peak, Δmax, is maximum for the extrudate with R=50%. The results of tan δ, damping, and the intensity of the β-loss peak indicate that the mechanism of molecular chain deformation is divided into two stages, below and above R=50%. The results are due to spherulitic changes, i.e. the shape of spherulites changed from spherical to elliptical in the extrudates above R=50% and the spherulite with R=50% changed from coarse structure to a finer one by the imposition of hydrostatic pressure.

Spherulite deformation at low strains in uniaxially extended polypropylene

The microscopic structure of uniaxially extended polypropylene (PP) at comparatively low strains was investigated by use of thermophotometry (TP) tests and polarizing microscopy. The spherulites for the uniaxially extended samples were deformed to an elliptic character with an increase in strains up to three, without the formation of the finer spherulites. The process of the spherulite deformation in the extended sample is different from that in hydrostatically extruded samples. The shape of TP curve in the extruded sample with a reduction cross-sectional area of 50% is fairly similar to that in the extended film with the strain of 1.0. The bulk strain was larger than the spherulite strain below the strain of 1.5. The results are explained in terms of the extension regions between the spherulites.

Physical Vapor Transport Process in Closed Tubes

In some closed-tube systems, the growth rates (R) depend on the temperature of the growth site (Tc), but rarely on the temperature of the vapor source site (Tv). While this observation cannot be explained by a diffusion model, we give our interpretation in a heat transport theory. In general, R is proportional to both supersaturation σ and equilibrium vapor pressure P0(Tc) at the growth site. Both experimental and theoretical results show that σ in the closed tubes is proportional to the temperature gradient, but not to the differenee of the vapor pressure: P0(Tv)-P0(Tc). This shows that σ is controlled by the flux of transported heat. However, as the variation of σ is smaller than that of P0(Tc), the growth rate is highly dependent on Tc or P0 (Tc).

Effects of Hydrostatic Extrusion on the Thermal Properties of Amorphous Polymers

The feasibility of hydrostatic extrusion has been studied for various polymers: including polyimide, polysulfone, high-density polyethylene, low-density polyethylene, and polystyrene [1,2]. Recently, the hydrostatic extrusion of brittle amorphous polymers, poly(methyl methacrylate) (PMMA) and high-impact polystyrene (HIPS), was successfully performed by sheathing these polymers with rubber at ambient temperature, without back pressure [3,4]. Nakayama and Inoue [3] reported that the elongation of the PMMA as well as HIPS extrudates, when tested in tension under atmospheric pressure, exhibited a marked increase, and that this behavior was due to the rearrangement of the molecular chains in the direction of extrusion.