The molecular interaction between polyphenylene-ether (PPE; poly (oxy-2, 6-dimethyl-1, 4-phenylene)) and organic phosphates has been studied. When a small amount of organic phosphates such as aromatic bis-phosphate was added to PPE, the flexual modulus increased. The modulus was increased with increasing content of aromatic biphosphate. The modulus of PPE with 20% phosphate was 2.5-2.7GPa comparing to 2.45GPa for PPE without phosphate. The higher the molecular weight of phosphate was, the more increase of the modulus was observed. The results of viscoelastic measurements below room temperature suggests that the increase of the modulus is related to a restriction of local motion of the chains.
Complexity of strain energy stored in largely deformed polymethyl methacrylate (PMMA) was studied through the tests of static strength and deformation recovery by heating the deformed samples. A relatively lower strain rate and higher temperature below the glass transition temperature at stretching of the PMMA samples to a strain of 0.10 lessened the ductility of the stretched samples. The long time stress relaxation at the fixed strain of 0.10 also lessened the ductility. On heating of the less ductile samples, their deformation recovery stimulated by internal strain energy stored in the samples commenced at temperatures much higher than those for the ductile samples. Thus, the ductility of the deformed PMMA samples was closely correlated with their deformation recovery behavior at heating. This correlation is probably associated with the inhomogeneous distribution of internal strain energy stored in the deformed polymer solid.
Static strength of polymethyl methacrylate pre-loaded with cyclic high stress was studied. The cyclic pre-loading was carried out using triangle waves of high tensile stress at very low frequencies to avoid the temperature rise in specimens. Fracture in most of the specimens without the cyclic pre-loading started at their surfaces, whereas for a great majority of the specimens subjected to the cyclic pre-loading the fracture started from the inside of the specimens, although a number of visible crazes had been formed on the specimen surface as a result of the cyclic loading. Hence, the cyclic pre-loading of high stress was likely to lessen the role of craze as a stress-raiser or a pre-crack. Moreover, it was deduced from the data of static strength for the specimens pre-loaded with a variation of stress amplitude and frequency that the cyclic pre-loading probably introduced some kind of fracture nuclei inside the specimen. As a possible origin of the inside fracture, development of inhomogeneous distribution of strain energy stored in the specimen during the cyclic loading was proposed.
Dynamic viscoelasticity of ethylene-co-methacrylic acid (EMAA) based ionomers containing Zn or Na as well as EMAA was investigated. The film samples used for viscoelasticity measurements were molded at 195°C. Two kinds of specimens, quenched and slowly cooled samples after molding, were prepared for each polymer specimen. For the quenched samples, the effect of aging on dynamic viscoelasticity was also examined. The temperature dispersion curves of dynamic storage modulus (E') of the ionomoers obtained by quenching showed a large decrease around 35°C due to the glass transition of EMAA ionomers. The ionomers prepared by slow cooling showed a high value of E' in the high temperature region, compared with the quenched samples. This may be due to the difference in the degree of ionic cluster formation in the ionomers. The values of E' at low temperatures increased with aging time for the quenched samples, which originates from the equilibration of the glassy state by aging.
Mixtures of Poly (metyl methacrylate) (PMMA) and poly (α-methylstyrene-co-acrilonitrile) (αMSAN) exhibit a LCST type of phase behavior. Dynamic rheological properties of PMMA/αMSAN blends of various compositions were measured at temperatures below, near, and above the phase-separation temperature. The time-temperature superposition was tried to obtain the master curves of G' and G" at 160°C. The time-temperature superposition principle failed above a certain temperature due to a phase separation. The “breakdown” temperature is close to the cloud temperature Tc of the blend samples. Comparison of logG' vs. logG" relation between the two components and the blends was made. The results show that the slopes of logG' vs. logG" plots in the terminal region are dependent on the composition of the blends. The rheological measurements have been found to be useful for characterizing the phase-separation of PMMA/αMSAN blends.
Tribological and morphological properties, and crystalline structure of polyamide 6 (PA6) in polyamide 6/maleic anhydride grafted high density and low density polyethylene (MAH-g-HDPE, LDPE) blends were investigated. Introduction of MAH into HDPE and LDPE greatly decreased the PE particle size in PA6 matrix and converted the crystalline structure of PA6 from α to γ form. Pure LDPE showed highest and HDPE showed lowest coefficient of friction and frictional wear. PA6 showed intermediate values. Although both the coefficient of friction and the frictional wear of HDPE tended to increase with MAH grafting ratio, those of PA6/HDPE blends decreased with MAH grafting ratio and reached to those of HDPE. PA6/LDPE blends showed an opposite tribological behavior, increasing coefficient of friction and wear with MAH content. When α form of PA6 was dominant in PA6/HDPE blends, the coefficient of friction and the frictional wear were almost constant, and these decreased with γ content. Although the coefficient of friction of PA6/LDPE blends had almost constant value irrespective of γ/α ratio, the frictional wear increased with γ/α ratio. These differences are due to the different way of formation of the transferred film onto the counter metal surfaces.
A new method has been proposed to determine directly the temperature dependence of the nucleus number N and the linear growth rate G of spherulite under high pressure directly from PvT data. The PvT measurements of polypropylene were carried out for isothermal crystallization at 170°C, 165°C and 160°C under 100MPa, and the temporal variation of the crystallinity xc(t) was obtained from the PvT data. The crystallization parameters N and G were determined so that the xc(t) calculated from the parameters agreed most well with that determined from the PvT data. The simulation of isothermal crystallization by using the determined parameters agreed very well with the experimental PvT data. PvT measurements for non-isothermal crystallization were also carried out at constant cooling rates of -1 and -0.25°C/min under 100MPa. The simulation of this non-isothermal crystallization also agreed well with the experiment, except for slight overstimate of the crystallization temperature which may be due to the inhomogeneity of temperature within the sample.
Polypropylene (PP) has a wide range of application because of its high stiffness and high heat resistance. PP is, however, not suitable for extrusion foaming process, because the cells of PP have low resistance to rupture during expansion. In order to give good processability to PP under extrusion foaming, PP is blended with polytetrafluoroethylene (PTFE). PTFE turns into many thin fibers during the mixing process, and these fibers may increase the resistance to rupture. Various samples of the blends were prepared by using three PP's with different viscosity values (melt index=0.4, 7.0 and 20) and various amounts of PTFE. As the indicators of the processability, the melt tension and the crystallization temperature of the blends were measured. The melt tension of the blends increased with PTFE content. The crystallization temperature also increased with PTFE content, especially when the PP with high viscosity was used. The extrusion foaming process of these blends was studied by using a tandem extruder. It was found that the blends of low viscosity PP can be successfully made into foamed products. The expansion ratio of the products increased with PTFE content. However, the size of the cells in the foamed products had a wide distribution, and the volume fraction of the closed cells decreased with PTFE content. These problems of wide cell size distribution and low content of closed cell may be caused by the inhomogeneous distribution of PTFE fibers in the PP matrix.
In this paper, a cost effective design procedure for laminated composite structure using GA (Genetic Algorithms) is proposed. Design variables considered to enhance the rigidity of the laminated composite structure are the number of laminae, the fiber orientation and the stacking sequence. The fiber orientations are chosen among some prepared angles. In the proposed design procedure, a rigidity function is defined to determine which angle to reinforce by means of weighing coefficients and transformed elastic constants in the prepared angles. The weighing coefficients are defined to represent the distribution of principal stress in the structure. Then, the orientation of reinforcements can be found according to the weighing coefficients such that the defined rigidity function takes maximum. Futhermore, to find a global optimum solution, the weighing coefficients are adjusted. In this paper, the detail of our developed procedure and some applications are shown.
In semiconductor devices, stress in silicon substrates sometimes generates dislocations during the fabricating process at high temperatures. Although most of the dislocations are generated at the stress singularity fields, dislocation generation has been discussed without considering stress singularity problems. This paper shows that dislocation generation can be predicted by using stress singularity parameters. In the experiment, the specimens were silicon substrates with stressed thin film bands, at whose edges the stress singularity fields were formed. The strength of the singularity was controlled in order to change the bandwidth. Whether or not dislocations at the film edges appeared was compared with the value of the singularity parameter. This comparison was performed for two structures of thin films and at three temperatures, and the results show that the singularity parameter can be used to predict the generation of dislocations.
Using hot-rolled 304 stainless plate (60mm thick) and 308 filler wire, a narrow-gap butt welded joint was prepared by GTAW (gas tungsten arc welding) process. In addition to the conventional round-bar specimens of the base metal and weld metal, the full-thickness joint specimens were prepared and subjected to creep tests at 550°C, from 196 to 294MPa. The creep strain distribution on the surface of each large joint specimen was measured by moiré interferrometry. The hardness of the weld metal at the surface is less than that of the inside which is significantly affected by thermal cycling during welding. This means that the heart of the weld metal is overmatched, but the layers near the front and back surfaces are undermatched. The creep-rupture strength of the large joint is about 85% of that of the overmatched weld metal. The initiation of cracks occurs in the beads at closest to the front and back sides. On the basis of the above observations, it is recommended to have the data of large joint specimens in order to interpret the creep life of the multi-pass welded joints.
This paper deals with the numerical solutions of singular integral equations of the body force method in an interaction problem of arbitrarily distributed elliptical inclusions under general loading conditions. The problem is formulated as a system of singular integral equations with Cauchy-type or logarithmic-type singularities, where the densities of body forces distributed in the x- and y-directions of infinite plates having the same elastic constants as those of the matrix and the inclutions are unknown functions. In order to satisfy the boundary conditions along the inclusions, eight kinds of fundamental density functions proposed in our previous paper are used. Then the body force densities are approximated by a linear combination of the fundamental density functions and polynomials. The calculations are carried out for several arrangement of the inclusions, and it is found that the present method yields rapidly conversing numerical results for arbitrarily distributed elliptical inclusions. The numerical results of weight functions and stress distributions along the boundaries are shown in figures to demonstrate the present solution.
This paper provides an improved directional simulation method to estimate the structural failure probability. A method is proposed to construct a directional importance sampling density, which generates more samples of direction vectors in the direction where the limit state surfaces are closer to the origin. The proposed method is composed of two-stage procedures. In the first stage, a preliminary simulation is executed to search the distances from the origin to the limit state surfaces in the directions normal to finite element meshes allocated on the unit sphere. A directional importance sampling density is constructed so as to be proportional to the upper probability of chi-square distribution corresponding to each distance. In the second stage, a directional importance sampling simulation is executed to estimate the structural failure probability. All samples are effectively generated to estimate the structural failure probability and the proposed method is effective for the reliability assessment of the structural systems. Numerical examples are provided to show the validity of the proposed method.