Gelation of a radiation crosslinked model polyethylene

A radiation crosslinked model linear low-density polyethylene (LLDPE) exhibits power-law relaxation,G(t) =St ^–n at its gel point (GP). The relaxation exponent has a value of about 0.46. The relaxation behavior is dominated by power laws, not only directly at GP, but in a very broad vicinity of GP and in a frequency window, which narrows with distance from the gel point. The power law exponent decreases with increasing radiation dose (increasing extent of crosslinking). Independent measurements of the gel fraction and the molecular-weight distribution of the radiated samples' soluble fraction support the rheological observations.Delivered as a Keynote Lecture at the Golden Jubilee Conference of the British Society of Rheology and Third European Rheology Conference, Edinburgh, 3–7 September, 1990.     

Rheology of rodlike polymers in the nematic phase with tumbling or shear orientation

The theory of rodlike polymers in the nematic phase is now sufficiently well developed as to allow predictions of the rheological behaviour that qualitatively compare well with the experimental observations. One of the main results of the theory is the prediction that rodlike polymers are nematics of the tumbling type at low shear rates, whereas they become shear oriented at high rates: a nonlinear effect, which is absent in low molecular weight nematics. This aspect is here reviewed in an effort to highlight the intuitive aspects of the matter. First, the low shear-rate situation is discussed in order to investigate the conditions that determine the existence of a stationary solution as opposed to a periodic one (tumbling). Then, the high shear-rate range is considered, where the shear-oriented situation prevails under all conditions. The intermediate range of shear rates is the most interesting one for its peculiar rheological behavior.Delivered as a Keynote Lecture at the Golden Jubilee Conference of the British Society of Rheology and Third European Rheology Conference, Edinburgh, 3–7 September, 1990.     

On-line measurements of the rheological properties of fermentation broth

The paper is concerned with measurements of rheological properties of fermentation broth. An on-line rheometer, Rheohelix-1, based on the application of a helical screw impeller rotating in a draught tube has been constructed. The instrument was used for measurements of the rheological parameters of fermentation broth of Aspergillus niger in a submerged fermentation process. The results of rheological and some standard measurements have been compared and proved the applicability of the instrument.Presented at the Golden Jubilee Conference of the British Society of Rheology and Third European Rheology Conference, Edinburgh, 3–7 September, 1990.     

The nonlinear strain measure of polyisobutylene melt in general biaxial flow and its comparison to the Doi-Edwards model

The nonlinear strain measure of a polyisobutylene (PIB) melt as determined by analysis of uniaxial, planar, ellipsoidal, and equibiaxial extensions is compared to the predictions of the molecular model of Doi and Edwards. It is found that the universal strain function of the Doi-Edwards model is unable to predict the nonlinear behavior of this polymer melt in general extensional flow. The qualitative agreement between predictions and experimental data for the strain dependence of shear stress and first normal stress difference in shear flow that was considered as powerful evidence for the correctness of the Doi-Edwards model seems to be accidental. The exaggerated strain dependence of the model suggests a need to reconsider the assumptions concerning the chain retraction process.Presented at the Golden Jubilee Conference of the British Society of Rheology and Third European Rheology Conference, Edinburgh, 3–7 September, 1990.Dedicated to Professor F.R. Schwarzl on the occasion of his 65th birthday     

Anisotropic conduction of heat in a flowing polymeric material

In this paper a theory is presented which relates the thermal conductivity tensor of an amorphous polymeric material to the history of deformation of the material. The basis of the theory is formed by the network theory for polymeric materials. It will be shown that the results obtained here are in good agreement with experimental results on rubber. The effect of anisotropic heat conduction on the flow of a polymeric material will be demonstrated by the simple example of viscous heating in shear flow.Presented at the Golden Jubilee Conference of the British Society of Rheology and Third European Rheology Conference, Edinburgh, 3–7 September, 1990.     

Physiology,mechanics, and rheology

Biological systems possess rather specialized mechanical properties, acquired as part and parcel of the evolutionary development of the system as a whole. Their optimization permits the system to function physiologically in the context of a biologically essential, but mechanically often widely varying environment with adequate efficiency. The system's environment is its source of food and shelter; it represents the space in which it forages or preys on other creatures and in which it has to defend itself against still others. Thus, the system has to develop an adequately pliant, rheologically matched, energy-use efficient, mechanical interface between it and its surroundings. This must be an interface that both effectively excludes, but also effectively admits, the external. Internally, as well, it has to adapt the mechanical properties of cell and connective tissue to physiological function and the efficient performance of useful work. This will be illustrated by way of examples. Blood rheology is briefly discussed and put into the context of clinical hemorheology and epithelial protection; and function, by way of a mucus coating or a mucociliary clearance system, is reviewed in some detail. The importance of all aspects of rheological matching is demonstrated.Delivered as a Keynote Lecture at the Golden Jubilee Conference of the British Society of Rheology and Third European Rheology Conference, Edinburgh, 3–7 September, 1990.     

A simple method for measuring local buckling of thin plates

A simple computer-interfaced optical system for measuring the dynamic-local-buckling deformation of thin-walled metal structural-plate elements is described in this paper with two sets of experimental results. The major advantage of this system is its simplicity and economy as well as its speedy automated process for data scanning, acquisition, and analyses by using a microcomputer.Paper was presented at the 1986 SEM Spring Conference on Experimental Mechanics held in New Orleans, LA on June 8–13.     

Extensional flow —a mathematical perspective

Extensional flow has been studied extensively for less than half the lifetime of the word rheology, although measurement of viscosity from extensional flow experiments can be traced back to the beginning of this century. In recent years extensional flow has, however, attracted a great deal of attention in conferences, workshops and special issues of journals. This has coincided with interest, both scientific and industrial, in elasticoviscous fluids, notably polymer solutions and molten polymers. The particular success of specialized, carefully targetted or focussed, workshops is highlighted.A personal reflection is offered here on some of the issues in extensional flow from the point of view of an applied mathematician. The state of the art, as far as extensional viscosity is concerned, is briefly surveyed. Both theoretical and practical aspects of the task of obtaining useful extensional flow properties are outlined, drawing heavily on a recent review by James and Walters. Particular attention is paid to spinning experiments, drawing heavily on recent theoretical investigations by the author. The merits of defining a spinning viscosity, in addition to the formally defined tensile viscosity, are advanced. The idea of general approximations for extensional flows is reviewed and some aspects of this are highlighted, particularly in connection with Metzner's idea of extensional primary field (EPF) approximations. Finally qualitative investigations of the behaviour of model fluids (in model flows) are illustrated and a new result on the boundedness of solutions for unsteady uniaxial extension of a FENS-P dumbbell model is presented.Presented as a keynote lecture at the 4the European Rheology Conference, September 4–9, 1994, Seville, Spain     

Stress concentration around several holes in structural elements

Conclusions Our analysis of specific numerical results for nonclassical problems has thus established two conclusions.1. The stresses do not increase monotonically as the holes are brought closer together (in the case of problems for shells under static loading and for plates under dynamic loading).2. For several holes in the case of problems for plates under dynamic loading, the maxima of the stress concentration factors can occur in the interior of the main region rather than at the edges of the holes, depending on the frequency and form of the applied load.These conclusions do not apply to classical problems (the planar problem under static loading) and must therefore be taken into account when stress concentrations are created.Because of space limitations, the concluding part of this article was not included in the EPMESC'92 Conference Proceedings and is therefore published here in its entirety.This is the complete text of a paper that was presented by the author at the EPMESC'92 International Conference in Talien, China, June 30-August 2, 1992, but was not published in its entirely in the Conference Proceedings.S. P. Timoshenko Institute of Mechanics, Academy of Sciences of Ukraine, Kiev. Translated from Prikladnaya Mekhanika, Vol. 30, No. 4, pp. 6–13, April, 1994.     

Vehicle and machinery design; Implements

After a general introduction the papers presented at Session IV of the 9th International ISTVS Conference concerned with “Vehicle and Machinery Design”, are discussed in groups, based on field of application. Special attention is paid to instrumentation and implementation of measurements.     

Agricultural engineering — State-of-the-art report

The 1st European Conference of ISTVS held at Rottach-Egern in 1980 is used as the starting point for a review of the state-of-the-art in the design of agricultural vehicles. The functions of these vehicles are divided into three main categories: soil processing, load carrying and self-propulsion. The limitations on design in each of these areas are defined and the relevant papers to the 7th International Conference are discussed in this context. It is concluded that there is still a requirement for the development of lighter, higher speed vehicles with low ground pressure, adequate ground clearance and good suspension. In parallel with the development of this hardware, computer simulation and modelling techniques should be developed to facilitate evaluation of new designs on the drawing board.     

Environmental aspects of machine traffic

Environmental aspects is a new topic for ISTVS and it was brought up as a keynote speech at the 11th ISTVS International Conference. Trafficability of forest soils has not received much attention by the ISTVS. It concerns mainly the forestry sector, but also military, construction works and not to forget recreational activities, and may have implications on general water quality. Over the last decade, concern regarding the preservation of forest environment has increased dramatically. At a European minister meeting in June 1993, an agreement was made on maintaining biodiversity and sustainability of the forests. The implications of these “restrictions” are increased awareness of damage on the ground and on remaining trees, site adapted operations and high demands on logging operation machinery and methods. Some of the consequences of these “restrictions” for the forestry sector are discussed in the article and possibilities for using environmentally friendly techniques and methods are assessed. It is pointed out that much more knowledge in the areas of forest-soil mechanics and techniques for handling large trees with relatively small machines will be needed to face future environmental challenges. In the last part of the article, papers submitted to the 11th ISTVS International Conference session on Environmental consideration are reviewed.     

Relaxation dynamics of selected polymer chain segments and comparison with theoretical models

Simultaneous measurement of infrared dichroism and birefringence is used to study selected polymer segment dynamics in isotopically labeled block copolymers. Two different polymers were studied: polybutadiene and poly (ethylene propylene). The first type consisted of a triblock with a short middle block labeled and a diblock with a short end block labeled, while the second type consisted of a triblock with three equal blocks and the end blocks labeled. Results of step strain experiments at –10°C for polybutadiene and at room temperature for poly(ethylene propylene) indicated that segments located at chain ends relax faster than segments located at chain centers. These experimental data were compared to the predictions of two molecular models: the bead-spring model of Rouse and the tube model of Doi and Edwards, and it was found that both models correctly predict the qualitative features of segmental relaxation. However, the tube-model predictions were closer to the experimental results. In addition, when the effects of orientational coupling interactions between segments in the melt were incorporated into this model, its predictions quantitatively agreed with the experimental results. The orientational coupling coefficient for poly(ethylene propylene) was 0.45 as measured from previous work, and for polybutadiene it was found to be 0.4.Delivered as a Keynote Lecture at the Golden Jubilee Conference of the British Society of Rheology and Third European Rheology Conference, Edinburgh, 3–7 September, 1990.Dedicated to Prof. R.S. Stein, University of Massachussets at Amherst, USA, on the occasion of his 65th birthday.     

Challenges in process rheometry

Process rheometers are useful as process sensors for on-line quality control as well as for process control. Unlike passive sensors for equilibrium properties, such as temperature and pressure, a rheometer must subject the material of interest to a controlled deformation while monitoring the resulting stress. To accomplish this in a compact, robust device suitable for use in a manufacturing environment is not straightforward. Problems that arise in processing applications include the need for frequent sample renewal without interference from the measurement or the process itself and the fact that the process stream is usually under pressure. Considerable ingenuity has been applied to developing rheometers to meet these requirements.However, there remain many potential applications for which presently available units are not suitable. In particular, special problems arise when a very fast response is required, when the fluid being processed is non-Newtonian or has a very high viscosity, and when it is a multiphase system, as for example in the case of mineral slurries. In addition, in food processing and fermentation applications, all the exposed surfaces of the rheometer must be sterilizable. Possible solutions to these problems are described.Delivered as a Keynote Lecture at the Golden Jubilee Conference of the British Society of Rheology and Third European Rheology Conference, Edinburgh, 3–7 September, 1990.     

A short account of some women in mathematics,engineering and astronomy

Editor's Note: This paper may serve as a greeting to the United Nations Fourth World Conference on Women to be held in Beijing in September, 1995. It is also the first paper contributed to the newcolumn “The History of Mechanics” in our journal, which was set up also at the suggestion of Dr. Johnson. We will welcome further contributions on that line.     

Problems of hydroelasticity for compressible viscous fluids

The complete text of a paper at the International Conference on Applied Mechanics (Peking, August 21–25, 1989), with a brief content published in the conference proceedings.     

Simulation of three-point bending at large deflections by an elastoplastic contact analysis

Three-point bending is simulated by an elaborate numerical procedure based on an elastoplastic, large deflection, contact analysis. A minimization formulation is used, which is equivalent to the incremental form posed as partial differential equations with inequalities. A sequential quadratic programming approach based on the finite-element technique is adopted as a method of solution. To examine the validity of the simulation method, experiments are carried out for specimens that have various widths.Paper was presented at the 1988 SEM Spring Conference on Experimental Mechanics held in Portland, OR on June 5–10.     

Molecular motions in macromolecules under mechanic and electric force fields

General aspects of the dynamics of macromolecules are reviewed on the basis of a parallel analysis of the material response under mechanical and electric force fields.Long-range (normal and segmental dynamics) as well as short-range motions are studied on the basis of experimental data, correlation function, and memory equation techniques.Accordingly, several methods to describe the relaxational behavior of macromolecules and their pendant molecular groups are proposed.Presented as a keynote lecture at the 4th European Rheology Conference, September 4–9, 1994, Seville, Spain     

Processing,characterisation and rheology of transparent concentrated fibre-bundle suspensions

Highly concentrated planar fibre-bundle suspensions with a transparent PMMA matrix were processed with various initial bundle contents and orientations. They were submitted to simple compression and plane strain compression deformation modes. First rheological measurements are presented. They highlight the role of the bundle content and orientation on recorded stress levels. The transparent matrix allows the observation of fibrous microstructures before and after compressions: The in-plane deformation of bundles (flattening and bending) as well as the evolution of their orientation are analysed and discussed. This paper was presented at the 3rd Annual European Rheology Conference (AERC) 2006, held in Hersonisos, Crete, Greece, April 27–29, 2006.     

Investigation of high frequency noise generation in the near-nozzle region of a jet using large eddy simulation

This paper reports on the simulation of the near-nozzle region of an isothermal Mach 0.6 jet at a Reynolds number of 100,000 exhausting from a round nozzle geometry. The flow inside the nozzle and the free jet outside the nozzle are computed simultaneously by a high-order accurate, multi-block, large eddy simulation (LES) code with overset grid capability. The total number of grid points at which the governing equations are solved is about 50 million. The main emphasis of the simulation is to capture the high frequency noise generation that takes place in the shear layers of the jet within the first few diameters downstream of the nozzle exit. Although we have attempted to generate fully turbulent boundary layers inside the nozzle by means of a special turbulent inflow generation procedure, an analysis of the simulation results supports the fact that the state of the nozzle exit boundary layer should be characterized as transitional rather than fully turbulent. This is believed to be most likely due to imperfections in the inflow generation method. Details of the computational methodology are presented together with an analysis of the simulation results. A comparison of the far field noise spectrum in the sideline direction with experimental data at similar flow conditions is also carried out. Additional noise generation due to vortex pairing in the region immediately downstream of the nozzle exit is also observed. In a second simulation, the effect of the nozzle exit boundary layer thickness on the vortex pairing Strouhal frequency (based on nozzle diameter) and its harmonics is demonstrated. The limitations and deficiencies of the present study are identified and discussed. We hope that the lessons learned in this study will help guide future research activities towards resolving the pending issues identified in this work.

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Presented as AIAA Paper 2006-2499 at 12th AIAA/CEAS Aeroacoustics Conference, 8–10 May 2006, Cambridge, MA, USA.