Mechanical behaviour viscoelasticity

Polymers have found widespread applications because of their mechanical behaviour. They combine the mechanical properties of elastic solids and viscous fluids. Therefore, they are regarded as viscoelastic materials. Viscoelastic... 

This document gives definitions of terms related to the non-ultimate mechanical behaviour or mechanical behaviour prior to failure of polymeric materials, in particular of bulk polymers and concentrated solutions and their elastic and viscoelastic properties. 

The most straightforward rheological behaviour is exhibited on the one hand by Newtonian viscous fluids and on the other by Hookean elastic solids. However, most materials, particularly those of a colloidal nature, exhibit mechanical behaviour which is intermediate between these two extremes, with both viscous and elastic characteristics in evidence. Such materials are termed viscoelastic. 

Mechanical and viscoelastic behaviour of materials can be determined by different kind of instrumental techniques. Broadband viscoelastic spectroscopy (BVS) and resonant ultrasound spectroscopy (RUS) are more commonly used to test viscoelastic behavior because they can be used above and below room temperatures and are more specific to testing viscoelasticity. These two instruments employ a damping mechanism at various frequencies and time ranges with no appeal to time-temperature superposition. Using BVS and RUS to study the mechanical properties of materials is important to understanding how a material exhibiting viscoelasticity will perform. 

Aklonis JJ, MacKnight WJ and Shen M, "Introduction to Polymer Viscoelasticity", Wiley, Inc, New York, 1972. Alfrey T, "Mechanical Behaviour of High Polymers", Interscience, New York, 1948. 

An overview of the physics of glassy polymers and the relationships between molecular mechanisms and macroscopic physical, mechanical and transport properties of polymer glasses is presented. The importance of local translational and/or rotational motions of molecular segments in the glass is discussed in terms of the implications for thermodynamic descriptions of the glass (configurational states and energy surfaces) as well as history dependent properties such as expansivity, refractive index, gas permeability, and viscoelastic mechanical behaviour. 

Creep and stress relaxation accelerate once the temperature exceeds the Tg of the matrix. Viscoelastic behaviour is obviously relevant to durabiUty, but fortunately the addition of suitably oriented fibre reinforcement can dramatically decrease or suppress viscoelastic behaviour, to an extent that depends on fibre direction, fibre volume fraction, and so on. This is an important reason for using fibrous reinforcement, even when it seems unnecessary from a consideration of short term mechanical behaviour. 

The knowledge of the viscoelastic behaviour and in general the mechanical behaviour of bitumen is of significant importance to the engineer. 

Asphalts contain two materials with different mechanical behaviours bitumen, with viscoelastic behaviour under normal loading conditions, and aggregates, with almost elastic... 

Non-linear viscoelastic mechanical behaviour of a crosslinked sealant was interpreted as due to a Mullins effect. The Mullins effect was observed for a series of sealants under tensile and compression tests. The Mullins effect was partially removed after a mechanical test, when a long relaxation time was allowed, that is the modulus increased over time. Non-linear stress relaxation was observed for pre-strained filler sealants. Time-strain superposition was used to derive a model for the filled sealants. Relaxation over long periods demonstrates that the Mullins effect is caused by non-equilibrium with experimental conditions being faster than return to the initial state. If experiments were conducted over times of the order of a day there may be no Mullins effect. If a filled elastomer were only required to perform its function once per day then each response might be linear viscoelastic. 

Chazeau L, Cavaille JY, Canova G et al (1999a) Viscoelastic properties of plasticized PVC reinforced with cellulose whiskers. J Appl Polym Sci 71 1797-1808 Chazeau L, Cavaille JY, Terech P (1999b) Mechanical behaviour above Tg of a plasticised PVC reinforced with cellulose whiskers a SANS structural study. Polymer 40 5333-5344 Chazeau L, Paillet M, Cavaille JY (1999c) Plasticized PVC reinforced with cellulose whiskers. I. Linear viscoelastic behavior analyzed through the quasi-point defect theory. J Polym Sci Part B Polym Phys 37 2151-2164... 

Mechanical property characterisation of artificial polymers (fibrous and non-fibrous) is often preceded by a mechanical conditioning treatment (Ward and Hadley, 1993) if the material is vi.scoelastic. This treatment is designed to provide a standard, reproducible microstructural state, so that results from different experiments, materials and laboratories can be compared easily. The conditioning treatment is deemed necessary because the mechanical properties of viscoelastic materials are affected by their entire previous mechanical history, as articulated in the Boltzmann superposition principle (Ward and Hadley, 1993). To predict mechanical behaviour accurately, one ought in theory to know the entire loading history of specimens since their manufacture Under practical conditions, only comparatively recent history is relevant, so specimens can be... 

Continued cross-linking leads to an increase in both resin equilibrium modulus and viscoelastic relaxation times. Thus, material behaviour becomes increasingly elastic and generated stresses begin to decay more slowly. Finally, when the resin instantaneous glass transition temperature (Tg) exceeds its local temperature (i.e., vitrification occurs), relaxation times jump sharply and mechanical behaviour becomes highly elastic. 

The viscoelastic and viscoplastic nature of polymeric materials that constitutes the composite matrix makes their mechanical behaviour time-or rate-dependent. Polymers are also temperature- and moisture-dependent displaying, in certain cases, large stiffness variations. Physical ageing is also an important issue that is often ignored for simplification purposes. A recent overview concerning these important matters is given elsewhere [9]. 

The mechanical and viscoelastic behaviours of natural rubber based blends and interpenetrating polymer networks (IPNs) are fimctions of their structures or morphologies. These properties of blended materials are generally not constant and depend on the chemical nature and type of the polymer blends, and also enviromnental faetors involved with any measurements. Preparations of natural rubber blends and IPNs are well known as effeetive modifieation methods used to improve the original meehanieal and viscoelastie properties of one or both of the eomponents, or to obtain new natural rubber blended materials that exhibit widely variable properties. The most common consideration for their mechanical properties include strength, duetility, hardness, impact resistance and fracture toughness, each of which can be deformed by tension, compression, shear, flexure, torsion and impaet methods, or a eombination of two or more methods. Moreover, the viseoelastieity theory is a way to predict the behaviours of deformation of natural rubber blends and IPNs. The time and... 

Natural rubber based-blends and IPNs have been developed to improve the physical and chemical properties of conventional natural rubber for applications in many industrial products. They can provide different materials that express various improved properties by blending with several types of polymer such as thermoplastics, thermosets, synthetic rubbers, and biopolymers, and may also adding some compatibilizers. However, the level of these blends also directly affects their mechanical and viscoelastic properties. The mechanical properties of these polymer blended materials can be determined by several mechanical instruments such as tensile machine and Shore durometer. In addition, the viscoelastic properties can mostly be determined by some thermal analyser such as dynamic mechanical thermal analysis and dynamic mechanical analysis to provide the glass transition temperature values of polymer blends. For most of these natural rubber blends and IPNs, increasing the level of polymer and compatibilizer blends resulted in an increase of the mechanical properties until reached an optimum level, and then their values decreased. On the other hand, the viscoelastic behaviours mainly depended on the intermolecular forces of each material blend that can be used to investigate the miscibility of them. Therefore, the natural rubber blends and IPNs with different components should be specifically investigated in their mechanical and viscoelastic properties to obtain the optimum blended materials for use in several applications. 

Mechanical behaviour of asphalts is basically viscoelastic. Under high loading rates they are elastic and brittle. When the load is imposed over a long time their deformability is similar to viscous materials. With respect to temperature variations, tars behave as thermoplastics this means that with increasing temperature they are transformed gradually from brittleness to fluidity, with simultaneous decrease of material adhesion and cohesion when softening temperature is attained. That transformation is entirely reversible within a certain range of temperature. 

An oriented polymer is in the strictest terms an anisotropic non-linearly viscoelastic material. A comprehensive understanding of anisotropic mechanical behaviour is therefore a very considerable task. In this chapter, we will restrict the discussion to cases where the... 

As clearly discussed, the mechanical behaviour of polymers changes rapidly as the temperature is reduced or the strain rate increased. Brooks et al. have shown that for polyethylene there is a sudden transition in the yield strain at temperatures below ambient, the exact temperature depending on the sample morphology. Figure 12.31 shows results for linear PE. It was also found that this temperature marks the change from classical elastic-plastic behaviour to time-dependent viscoelastic behaviour where the yielded samples show... 

Since the publication of the second edition in 1983, the subject has advanced considerably in many respects, especially with regard to non-linear viscoelasticity, yield and fracture. We have altered some chapters very little, notably those dealing with viscoelastic behaviour and the earlier research on anisotropic mechanical behaviour and rubber elasticity, only adding sections to deal with the latest developments. 

COMPOUND VISCOELASTIC PROPERTIES AND FRACTURE MECHANICAL BEHAVIOUR IN TRUCK TIRE DURABILITY PREDICTION... 

Compressive tests demonstrated anisotropy in mechanical behaviour, with an axial modulus up to 1.5 times greater than the transverse modulus. Composite scaffolds also showed viscoelastic behaviour with increased modulus for higher... 

Ilavsky M, Somvdrsky J, Bouchal K and Dugek K (1993) Structure, Equilibrium and Viscoelastic Mechanical Behaviour of Polym ethane Networks Based on TYiisocyanate and Poly(oxypropylene)Diols, Polymer Gels and Networks 1 159-184. 

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