Mechanical anisotropy in oriented polychlorotrifluoroethylene

The dynamic tensile modulus (E) and loss (tan δ) of various oriented samples of polychlorotrifluoroethylene have been measured at 0°, 45° and 90° to the draw direction over the range of ? 100 to 160°C. For the cold-drawn samples the mechanical anisotropy at the lowest temperature is determined by the overall chain orientation resulting in E₀ >E₄₅ >E₉₀. Above the γ relaxation (20°C) a shear process is activated leading to a change in the anisotropy pattern to E₀ >E₉₀ >E₄₅. Cold-drawing followed by annealing gives rise to further changes so that E₉₀ >E₀ >E₄₅ above the β relaxation (120°C). The effect of annealing has been attributed to the relaxation of the amorphous regions and the development of lamellar texture, and the anisotropy in tan $\text{δ}\text{E}$ at the β relaxation is found to be consistent with the interlamellar shear model.     

Correlation between uniaxial and shear creep of thermoplastics

Creep data in uniaxial tension, compression and shear on PVC, PMMA, and PP sheet were used to test analytical correlating procedures. It was shown that shear and uniaxial creep could be closely related using the concept of shear stress and shear strain on octahedral planes. This correlating procedure was only effective if due allowance was made for the different creep response in simple tension and compression for each of these materials. It was also shown that the relationship between the viscoelastic creep moduli and strain ratio, which is stress and time dependent, has the same form as the analogous relationship between the linear elastic constants.     

Shear and tensile creep of thermoplastics at finite strains

The shear and tensile creep behaviour of polypropylene and low-density polyethylene has been determined for a range of stress levels and creep times from 5 to 10⁵ seconds. The data at finite strains have been correlated using the concept of shear on octahedral planes. When the experimental 100 second shear strains were matched to those predicted from the tensile tests, the experimental and calculated shear creep curves remained coincident, even in tests where the shear strains reached 8% and the materials were highly non-linear viscoelastic. Manipulation of data in the literature for two amorphous polymers suggests a different pattern of behaviour and this is related to differences in the affect of the hydrostatic component of stress in the tensile tests on the creep response of the four polymers.     

The injection molding behavior of thermoplastics in thin rectangular cavities

Two models are presented for simulating the injection molding of thermoplastics in thin, rectangular cavities. One is a much simplified bounded-radial-flow model that utilizes an existing numerical model for semi-circular cavities to adjust for the non-radial flow region bounded by the lateral walls. The other is a two-dimensional analysis which assumes that both viscosity and temperature change strongly across the narrow gap but vary weakly in the directions of flow. The latter analysis allows application of the potential theory and the determination of streamlines and progressing front shapes. Both models deal with a non-Newtonian viscosity with temperature variation. Comparisons between experimental and computational results are shown.     

Effect of molecular weight and branch content on the creep behavior of oriented polyethylene

Creep studies were carried out on a range of homopolymers and copolymers of polyethylene with well‐defined molecular weight and branch content. The creep data were analyzed in terms of two thermally activated processes acting in parallel and the effects of molecular weight and branch content are discussed. It is shown that increasing either the number‐average molecular weight or the weight‐average molecular weight gives improved creep behavior at all stress levels. The introduction of butyl branches leads to lower creep at low‐stress levels but can give rise to higher creep at high stress. Plots of the equilibrium log₁₀(strain rate) versus stress at fixed draw ratio (strain) can be used to define sections through a unique true stress/true strain/strain rate surface for each material. These creep results have an additional value in terms of the link between slow crack propagation (SCG) in polyethylene and fibril creep, confirming the proposal made elsewhere that SCG can be quantified in terms of creep to failure across the true stress/true strain/strain rate surface. © 2003 Wiley Periodicals, J Appl Polym Sci 89: 1663–1670, 2003     

Fatigue behavior of glass-fiber fortified thermoplastics

This investigation deals with the fatigue behavior of a group of thermoplastics fortified with discontinuous glass fibers dispersed by an injection molding process. The thermoplastics included nylon, polystyrene and polyethylene reinforced with short (1/8 in.) and long (1/2 in.) glass fibers. Several aspects of the fatigue behavior are included in the study. First, classical S–N curves were generated under fluctuating tension with R = 0.05 to show the loss of strength due to cyclic load application. Next, the extent of progressive fatigue damage was established by measuring the residual strength after cyclic loading. Finally, hypotheses pertaining to the fatigue mechanisms operative in all four materials were made based on microscopic examinations of sections removed from fatiguedamaged specimens.     

Large deformation behavior of some thermoplastics

In this experimental study the deformation behavior of two thermoplastics under uniaxial and biaxial loadings has been investigated. The experiments were conducted on thin walled tubular specimens of nylon-6, a semicrystalline polymer, and polymethyimethacrylate (PMMA),:an amorphous polymer. These specimens were subjected to tension, torsion and combined tension-torsion loadings until very large deformations were produced or specimen failure took place. The results are presented in the form of generalized stress-generalized strain curves which are applicable to all types of loadings, provided the generalized stress is monotonically increasing. In addition, the results show that the generalized stress-generalized strain relationship for nylon-6 is parabolic in nature, whereas that for PMM A can be represented by a bilinear curve. This characterization of the response of the thermoplastics under a biaxial stress field can be used to obtain a more realistic stress interaction between fiber and matrix in composite materials, since until now all theoretical studies on this aspect have assumed an elastic or elastic-perfectly plastic behavior of the matrix.     

Prediction of creep of oriented polycaproamide fibres

A method of predicting permanent creep with dynamometric curves was proposed for oriented PA-6 monofilament. The curves of the changes in the modulus of elasticity in different stretching modes were used to estimate the effect of the change in the modulus of elasticity on the deformability of the monofilament in the creep mode.Blagoveshchensk Institute of Technology. Institute of Textile and Light Industry, St. Petersburg. Translated from Khimicheskie Volokna, No. 6, pp. 38–41, November–December, 1992.     

Fracture behavior of rubber-modified thermoplastics after aging

Experiments show that the effects of outdoor aging on rubber-modified thermoplastics can be reproduced by laminating a layer of a glassy polymer onto the surface of unaged specimens. This technique is used to study the effects of fracture temperature, specimen geometry, and polymer composition on the impact strength of aged HIPS and ABS. Aging reduces the energy of crack initiation, so that the impact strength is determined by the crack-propagation energy, which is in turn governed by the nature and concentration of the rubber and by the fracture temperature.     

The mechanical properties of oriented discontinuous fiber-reinforced thermoplastics. I. Unidirectional fiber orientation

The mechanical properties of a series of thermoplastics reinforced with unidirectionally oriented short fibers are reported. Both organic and inorganic fiber reinforcements were used in fiber volume fractions of 0.10 to 0.50. A number of these composites were found to have excellent strength and stiffness properties combined with good toughness and low density. The dependence of composite properties on the properties of the constituent materials is discussed. Fiber efficiency factors for strength and modulus are presented and models for predicting composite mechanical behavior are reviewed.     

The influence of hydrogen bonding on mechanical anisotropy in oriented nylon-12

The anisotropy of dynamic tensile modulus in uniaxially and uniplanar-axially oriented nylon-12 sheets has been measured over the temperature range encompassing the α′-relaxation process. The influence of hydrogen bonding on the mechanical behaviour has been demonstrated by comparing these anisotropy measurements for specimens which have been subjected to different thermal treatments.     

Melt viscosity behavior of some engineering thermoplastics

Frequently, the enhanced elevated-temperature rigidity of engineering thermoplastics (ETPs) is a consequence of high glass-transition temperature, and many ETPs contain aromatic ring structures in the backbone chain. These factors can lead to difficulty in melt processing, or fabrication, of parts. Thus, the definition of the melt rheology of such systems is of considerable technological, as well as scientific importance. The investigation reported here first defines the viscosity-temperature dependence of five ETPs over a relatively narrow range of temperatures appropriate to melt processing in terms of superposition methodology. The five ETPs studied were bisphenol A polycarbonate, polysulfone, the condensation polymer of bisphenol A and mixed iso- and terephthalic acids, and two experimental condensation polymers: bisphenol A/isophthalic acid and 1,2 bis(4,4′-hydroxy phenyl) ethane/isophthalic acid. Viscous flow energies of activation are examined in terms of polymer chain structure. In the second portion of the investigation it is shown that, for the latter two condensation polymers, the molecular weight, temperature, and shear rate dependence of the viscosity may be expressed in terms of a modified Carreau model. The Newtonian limiting low-shear viscosity dependence on molecular weight and that of the shear rate shift factor (relaxation time) are found to be somewhat greater than that normally observed.     

The creep behavior of solid-filled rubber composites

This paper is using an experimental method to study the creep behavior of solid-filled silicone-rubber composites (GFSC). The specimens made of silcone-rubber as the matrix and 0–40% volume fraction (V_f) of glassbeads, have been through uniazial tensile tests and constant-loading creep tests. The results show that the material behavior is ovviously nonlinear and the Eyring's reaction rate principle is suitable to describe the viscoelastic behavior of this rubber composite. Thus, to solve the creep problems of rubber composites, the Boltzmann's superposition technique can not be applied. Although Findley et. al (1976), have proposed the mutiple integrals form, but solving the multiple integrals form is quite complicated and does not be guarantee convergence. We have employed SoChen's form of the nonlinear stress-strain relationship of rubber elasticity. This can avoid the above mentioned problems and the analytic solution is obtained. The resulting solution is in very good agreement with the experimental results.     

Changes in mechanical behavior during fatigue of semicrystalline thermoplastics

The tensile fatigue behavior of two engineering thermoplastics (polyacetal and nylon_6,6) were studied by measuring changes in the dynamic viscoelastic response together with changes in potential energy density, strain energy density, and irreversible work. The results show that both stress softening and hardening can occur in controlled load cyclic conditions. At high stress levels and/or frequencies, both the polyacetal and nylon_6,6 show evidence of thermal softening as characterized by changes in their dynamic viscoelastic properties and decrease in storage modulus with corresponding increases in loss modulus and loss tangent. This effect is supported by observed decreases in the overall crystallinity as measured in DSC experiments. At lower stress levels (the mechanically dominated region), all results indicate that, although fatigue crack propagation (FCP) is one of the mechanisms governing the fatigue life, its contribution is minor and crack initiation time constitutes the majority of the fatigue life. Also, during the initiation stage, both materials become less viscoelastic and more elastic. This phenomenon is evidenced by overall reductions in the loss modulus, loss tangent, and irreversible work densities while the storage modulus is maintained. © 1995 John Wiley & Sons, Inc.     

Mechanical anisotropy in oriented linear polyethylene of various crystallinities

We have studied the mechanical moduli of oriented linear polyethylene with crystallinities X varying from 0.44 to 0.63 and draw ratios λ = 1–9 by using a dynamic tensile method at 10 Hz and an ultrasonic technique at 10 MHz. Wide-angle X-ray diffraction and birefringence measurements reveal that the chains in the crystalline regions are fully aligned at λ > 4, but the degree of amorphous orientation increases steadily up to the highest draw ratio. From −180°C to the β relaxation region (near 0°C at 10 Hz) the mechanical behavior at all crystallinities is controlled by three factors: molecular orientation, weak c-shear deformation and stiffening effect of taut tie molecules. At low temperature the chain alignment in an oriented sample gives rise to an axial Young's modulus E₀ which is much larger than the transverse Young's modulus E₉₀, with the modulus for the undrawn material lying in-between. However, the results that E₄₅ < E₉₀ and C₄₄ (axial shear modulus) < C₆₆ (transverse shear modulus) imply that a weak c-shear process occurs even at low temperature. At the β relaxation where the amorphous regions are rubbery, the stiffening effect of taut tie molecules becomes prominent and leads to increases in all moduli upon drawing. For the polyethylene with the lowest cyrstallinity a strong c-shear process is activated at the α relaxation (about 50°C at 10 Hz), which gives rise to very low values of C₄₄ and E₄₅. This effect becomes weaker with increasing crystallinity and is hardly observable at X > 0.6.     

Strain ratio and volume change during tension and compression creep of thermoplastics

Creep tests up to 10⁶ seconds in tension and compression were conducted on polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), and polypropylene (PP). Measurements of longitudinal and lateral strain provided values for tension and compression strain ratio. These increased with time and stress for PMMA and PVC from about 0.38 to 0.44. The results for PP increased from 0.44 to above 0.5 in. tension and showed a decreasing ratio in compression, which suggested a time and stress dependent structural change. Volumetric strain was computed from the linear strains and plotted against stress and axial strain. Except at 10² seconds, the relationships were non-linear up to volumetric strains of about 0.3% and in the cases of PMMA and PVC there were greater changes of volume in tension than compression.     

A comparison of constant and complex creep loading programs for several thermoplastics

Polymethyl methacrylate, polyacetal and polypropylene samples were subjected to constant-load uniaxial creep in tension and compression up to 3% strain in times up to 10⁵ sec. A higher creep resistance was obtained in compression compared with tension due to the influence of free volume on mobility. Square wave cyclic creep tests alternately in tension and equal compression for dwell times of 10, 100, 1000 sec were also conducted on each material up to a total time of 10⁵ sec. Under low cyclic creep stresses tension creep cancelled out compression creep in each successive half cycle so that there was no overall increase in creep strain as cycling proceeded. The unidirectional creep data was used successfully to predict this behavior. At higher cyclic stress levels creep strain range increased steadily throughout the test indicating a softening process. A few tests were conducted in unidirectional and cyclic stress relaxation on polymethyl methacrylate.     

The stress-strain behavior of materials exhibiting andrade creep

The stress-strain behavior of a material exhibiting Andrade creep (for which the creep compliance is linear in the cube-root of time) has been calculated for loading at constant strain rate \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm \varepsilon}\limits^{\rm .} $\end{document} and at constant stress rate \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \sigma \limits^. $\end{document} for the limiting case of linear viscoelastic behavior and at constant \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop \sigma \limits^. $\end{document} for one type of nonlinear viscoelastic response. The recoverable strain after the stress has been removed has also been calculated for these three cases. The results of the calculations are compared with experiment.