Jan-Fredrik Jansson

Some phenomenological relaxation rate equations based on Bose–Einstein similar kinetics

The possibility of describing transient phenomena associated with flow and consolidation of solids, such as stress relaxation or physical aging, in terms of a kinetic mechanism comprising spontaneous and induced events is discussed. The starting point is the differential equation dn/dt=−an[1−(b/a)n], with n denoting the number of relaxed entities and n=dn/dt (a,b are constants, t is time), yielding an n(t) function reminiscent of a Bose–Einstein distribution. The corresponding n(t) relation describes the linear variation of n with log t, and the exponential dependence of n on n, as often found experimentally. Replacing n in the starting equation by the relative rate n/n yields a power‐law‐type n(n) dependence. A further modification, where the induction term n/n is not linear but raised to a power ≳1, finally produces a generalized version of the stretched exponential. When interpreted formally in terms of a spectrum of relaxation times τ, all three equations produce response functions with disc...

A new method for the detection of thermal oxidation in polyethylene pipes

Abstract Dumb-bell shaped specimens containing the inner wall surface of the pipe cut from 12 polyethylene pipes of different origin were subjected to constant uniaxial tensile loads at 313 K in air. The brittleness of the inner wall surface layer of the thermally oxidized pipes manifested itself in a shift of the unstable/stable necking transition to lower stresses and longer times. With the aid of optical and scanning electron microscopy, the brittleness of the inner wall surface layer of the oxidized pipes was demonstrated in distinctive surface cracks, which were the dominant feature of the drawn samples of the oxidized pipes. No such surface cracks were observed in the non-oxidized samples. The surface crack patterns were characterized in terms of crack frequency (longitudinal and transverse), width of fragments of cracked top layer (lB), plastic strain of the fragments of the cracked top layer (epl) and thickness of the cracked top layer (H). Correlations were found that can be interpreted according to basic principles of fracture mechanics. Through knowledge of, for example, epl and lB the values of the other variables can be predicted. The thickness of the cracked top layer corrected for the reduction in thickness due to plastic deformation is approximately equal to the thickness of the oxidized layer as determined by polarized microscopy. Drawing in an Instron Tensile Testing Machine at an elongation rate of 10 mm/min at 298 K also revealed the distinctive surface cracks in the oxidized samples. On the basis of these results, a new method for the detection of thermal oxidation in polyethylene pipes is proposed.

Influence of microtomy on measurements of orientation

Abstract The correct measurement of orientation and crystallinity on thin sections microtomed from larger specimens requires a knowledge of the influence of sectioning on these properties. In this work, the measuring technique used is FTIR, using dichroism to determine the orientation and regular FTIR to determine the degree of crystallinity. This paper shows that the sectioning influences the measured orientation of the sections in the case of both amorphous polycarbonate and crystalline polypropylene, while the crystallinity seems to be unaffected in the case of the unoriented samples.

A survey of methods for the detection of thermal oxidation in high-density polyethylene pipes

Abstract Seven methods for the detection of thermal oxidation of the inner wall surface of high-density polyethylene (HDPE) pipes are presented. The methods presented include infrared spectroscopy, polarized light microscopy, differential scanning calorimetry, scanning electron microscopy, gloss measurements and uniaxial creep tests. These tests have been developed on the basis of earlier reported data for a large number of PE pipes. The tests are compared with each other and with the internal pressurizing test with respect to reliability of results, the time taken to determine whether or not a pipe is oxidized, the experimental difficulties, costs, etc.

Applications of LCP Materials

The industrial development of thermotropic liquid crystal polymer (LCP) materials can be traced from its theoretical origins, through the identification of useful compositions, to full commercialization. The future industrial challenge will be to define and develop applications which take advantage of the unique properties of these materials.

Effects of Processing on the Structure and Properties of PVC

This work includes an extensive study of the effect of the gelation or fusion stage of processing on the ultimate mechanical properties of PVC pipes by variation of the mass temperature at extrusion. Moreover, the influence of gelation on the supermolecular structure and on some physical properties was studied.

Creep and Fracture Initiation in Fibre Reinforced Plastics

While most fibres used as a reinforcing constituent in polymer-based composites have reasonably simple deformation and fracture properties, the polymer matrices are anelastic and even non-linear viscoelastic with fracture properties which are strongly dependent on temperature, deformation rate, loading time etc. Therefore the mechanical behaviour is usually characterized as being either fibre or matrix dominated, i.e. more or less determined by only one of the constituents.

Structure and Fracture of Thermally Oxidized Pipes of High-Density Polyethylene

The structure and fracture properties of high-density poly ethylene pipes, thermally oxidized during processing, were studied.

The Appearance of Nonlinear Viscoelasticity in Glassy Polymers

Glassy polymers show linear or approximately linear viscoelastic behaviour only for small uniaxial tensile stress-strains as can be observed by the deviation from linearity in the isochronous stress/strain creep diagram.

The Influence of Cooling Rate on the Transition to Marked Nonlinear Viscoelasticity in Poly(Methylmethacrylate)

For amorphous glassy polymers a transition exists from approximately linear to marked nonlinear behaviour. The transition has been suggested to be due to stress activated changes in the deformation mechanisms1,2,3. In poly(methylmethacrylate),(PMMA), as well as in other polymers both the α- and β-mechanisms have been shown to be essential for the appearance of the nonlinearity1,2. The transition is accompanied by an accelerated stress dilatation and is supposed to be influenced by the available free volume in the material4.