Ikuo Narisawa

Notch brittleness of ductile glassy polymers under plane strain

The brittle fracture behaviour of ductile glassy polycarbonate, poly(vinyl chloride) and poly(methyl methacrylate) notched bars subjected to plane strain tension has been studied at varying strain rates. Morphological observations of thin sections and fracture surfaces revealed that a disc-type craze was nucleated at the tip of the plastic zone which spread from the notch root. A slip-line theory modified so that the yield criterion is influenced by a hydrostatic stress component allows the calculation of the stress components at the elastic-plastic boundary, where the hydrostatic stress is highest, from the knowledge of the location of the fracture origin. An analysis of the data resulted in the conclusion that Orowans analysis for notch brittleness is appropriate.

Fatigue characteristics of a glass-fiber-reinforced polyamide

This paper deals with prediction of the temperature rise in the stress-controlled fatigue process of a glass-fiber-reinforced polyamide and the application of a temperature and frequency superposition procedure to the S-N curve. An experimental equation was derived to predict the temperature rise from calculations based on the fatigue test conditions and viscoelastic properties of the material. The temperature rise (ΔT) can be expressed as a product of a coefficient term Φ(L, κ) concerning heat radiation and the test-specimen shape and a function term Pfat concerning the viscoelastic properties and fatigue test conditions. Φ(L, κ) was found experimentally to derive the equation for predicting the temperature rise blow or above the glass transition temperature (Tg) of the material. The equation σR = −STfA log NfR + STfB was obtained as a procedure for applying temperature and frequency superposition to S-N curves in consideration of ΔT. This procedure was obtained by combining both temperature- and frequency-superposition techniques. Here, σR and log NfR represents the stress and the fatigue lifetime calculated at a given temperature and frequency, A and B denote the slope and intercept of any arbitrarily chosen S-N curve, and STf is a shift factor for temperature and frequency superposition.

Brittle fracture in glassy polymers

Abstract The brittle fracture behavior of round-notched poly(methyl methacrylate), poly(vinyl chloride), and polystyrene bars has been studied at varying temperatures and bending rates. Brittle fracture originating from internal crazes occurs in a restricted range of temperature. When the plastic deformation zone at the tip of the notch reached a certain size, which decreased with decreasing temperature or increasing bending rate, the internal crazes nucleated at the elastic-plastic boundary. The extrapolation of the position of internal craze nucleation versus temperature plots showed that the nucleation of well defined internal crazes developed above the temperature of the 3 loss peak in each glassy polymer. The mechanism of the ductile-brittle fracture transition at the β-transition temperature in glassy polymers can be understood in terms of variation of the position of internal craze nucleation with temperature and/or strain rate.

The effect of heat treatment on plane strain fracture of glassy polymers

The effect of annealing on the plane strain fracture of round-notched polycarbonate and poly(methyl methacrylate) bars has been investigated. Morphological observations of thin sections and fracture surfaces revealed that the fracture initiated from internal crazes which were nucleated at the tip of a local plastic zone. The critical hydrostatic stresses for internal craze nucleation were nearly constant regardless of annealing, while the shear yield stress increased with increasing annealing time. The reduction in toughness by annealing can be ascribed to the decrease of the maximum extent of the plastic zone which gives the critical hydrostatic stress for craze nucleation.

Internal fracture of notched epoxy resins

Abstract The brittle fracture of round-notched epoxy resin bars subjected to plane strain bending has been studied at varying strain rates. Observations of fracture processes and surface morphologies revealed that the internal crack was nucleated at the plastic-elastic boundary when the plastic deformation zone at the notch root reached a certain size. A slip-line field theory allows calculation of the stress components at the plastic-elastic boundary from a knowledge of the location of the internal crack. An analysis of the data concluded that the triaxial stress level ahead of the plastic zone was raised by plastic constraints to an ideal fracture stress which is considerably larger than that of glassy thermoplastics.

Fracture of notched polycarbonate under hydrostatic pressure

The brittle fracture behaviour and plastic deformation of round-notched polycarbonate bars subjected to three-point bending under hydrostatic pressure have been studied. Below a certain critical pressure, the brittle fracture initiated from an internal craze nucleated at the tip of the local plastic zone ahead of the notch rooT. The position of the nucleation of the craze receded from the tip of the notch with increasing applied pressure. When the pressure was increased over a critical value, general yielding occurred by passage of the plastic zone across the notched cross-section, that is, the brittle to ductile transition took place. A qualitative analysis of the stress distribution within the plastic zone explains that the brittle to ductile transition under hydrostatic pressure occurs when the general yield takes place before a critical stress for brittle crack propagation is reached.

Degradation of crosslinked polyethylene in water by gamma-irradiation

Abstract The degradation of crosslinked polyethylene by gamma-irradiation in water was studied. Change in the physical properties and the growth of carbonyl group after irradiation showed a good correlation. The degradation observed at 80°C is the least, and that at 60°C is the severest. The distribution of the oxidized layer in the sample was measured and was also calculated from the diffusion model using the observed parameters. Comparison of both results suggest that some products caused by gamma-irradiation of water supress the degradation of XLPE at 80°C.

Synthesis of polysiloxane‐grafted fluoropolymers and their hydrophobic properties

A polydimethylsiloxane with an epoxide end group was grafted onto a fluoropolymer that had hydroxyl side chains by using protonic or Lewis acids as catalysts. Strong acids such as trifluoromethanesulfonic acid were found to be effective for the grafting of the polydimethylsiloxane of relatively large molecular weight (Mn 4440). A coating prepared from the graft polymer was so hydrophobic that a water droplet of 10 μL slid down on its inclined plane surface (30° with respect to the horizontal). ESCA analysis revealed that the siloxane branches are preferentially present on the surface.

High-Speed Tribology of Polymer Composites

Abstract Phenolic-based composites for automobile friction materials contain a variety of inorganic fillers and modifiers in addition to the base resin and fibres (mostly asbestos). The friction behavior at low speeds does not provide a sufficient foundation for the prediction of the friction and wear behavior at high speed, since the temperature rise at high speeds causes chemical changes in some of the friction constituents of the composite system. The purpose of this chapter is to provide a clear interpretation of the frictional performance of phenolic-based composites at high speeds. The friction behavior of commercial brake material at high speeds involves changes in the friction coefficient with sliding distance: an initial decrease, then an increase and finally a constant value with frequent fluctuations, with increasing distance. Finally, the friction tends to have a rather constant value although sporadic fluctuations still remain. The final constant friction behavior is closely related to decomposition of the resin due to the temperature rise at the contacting surface. The critical temperature to reach constant friction depends on the sliding speed and normal load. At high-speed friction of composites simply consisting of a phenolic base resin and carbon, glass, or aramid fibres, the amount of fibres affects the friction behavior. Generally, incorporation of fibres in the resin reduces the friction coefficient. Glass fibre is most effective in decreasing the friction coefficient, and relatively higher amounts of fibres are needed for carbon and aramid fibres to minimize the friction coefficient. The fluctuations in the friction disappear when putting fibres into the resin. This result suggests that reduction of the deformation at high temperatures and products of fibre wear during sliding are responsible for the low friction coefficient of the composites at high speeds.

Evaluation of impact properties of butt-fusion-jointed medium-density polyethylene pipes for gas distribution

Abstract High-speed tensile and Charpy impact tests were conducted on fusion-joined specimens to clarify the impact properties of medium-density polyethylene pipes for gas distribution. Comparison of the impact energy of substrates with those of butt fusion joints reveals that the impact energy differs with resin grades, and that, in all cases, the impact energy of the fusion joints is lower than that of the substrates. The effects of fusion conditions and combination of different grades of resins on fusion strength are also discussed.