Polymer Science

2019

The mechanical properties of polymers are characterized by the way in which these materials respond to applied mechanical stresses, the latter being of the stress or strain type. The nature of this response depends on the chemical structure, temperature, time, and morphology defined during polymer processing. The molecular structure of the polymers provides a viscous behavior, such as liquids, superimposed with an elastic behavior, such as Hookean solids. This phenomenon is called viscoelasticity and occurs for both plastics and fibers. The elastomers have a unique behavior known as rubber elasticity. This type of elasticity is very particular because it imposes great deformations in the rubbery chains that are amorphous, cross-linked, and very flexible. Another parameter to consider is the timescale in which the polymer is stressed. The mechanical testing can be carried out quickly, called a short duration, or slowly, called a long duration. Testing under impact is classified as a very short duration, and the polymer is requested only for a few milliseconds. The creep and stress relaxation testing, in turn, characterize the mechanical behavior of the polymer over a much longer timescale, reaching many years. The importance of the duration of the applied mechanical demand is related to the time the polymer needs to relax during this period of time. The evaluation of the mechanical properties can be performed in a static or dynamic way. In addition, the characterization of the mechanical behavior can be done by reaching or not reaching the breaking of the material. For example: elastic moduli, yield strain and stress, maximum stress, etc., are parameters to be characterized without reaching the rupture of the polymer. On the other hand, tensile and deformation at rupture, impact strength, number of life cycles under fatigue, etc., are mechanical properties determined upon the rupture of the polymer. The mobility of a polymer chain determines the physical characteristics of the product whether it is a hard and brittle material, rubbery and tough plastic, or a 9 238

Chemical …, 2009

Polymeric materials exhibit an extraordinary range of mechanical responses ), which depend on the chemical and physical structure of the polymer chains. 3 Polymers are broadly categorized as thermoplastic, thermoset, or elastomer. Thermoplastic polymers consist of linear or branched chains and can be amorphous or semicrystalline. The mechanical response of thermoplastic polymers is highly influenced by the molecular mass, chain entanglements, chain alignment, and degree of crystallinity. Thermosetting polymers consist of highly cross-linked three-dimensional networks. The mechanical properties of these amorphous polymers depend on the molecular mass and the cross-link density. Elastomers (e.g., rubber) are highly deformable elastic networks that are lightly cross-linked by chemical or physical junctions.

Materials Letters, 2015

Materials are often characterized in terms of their toughness, though more than one definition of toughness exists. Likely the most widely recognized means of defining material toughness, denoted here as τ, is by the area under the stress strain curve from a tensile test. Another important feature describing the nature of materials is that property known as brittleness, which has for a long time been much less quantitatively understood. Using a quantitative definition of brittleness provided in 2006, we demonstrate the existence of a quantitative relationship between τ and brittleness B, valid for polymers with a very wide range of chemical structures and properties, for some polymer-based composites, and also for steel and aluminum. We provide an equation relating toughness to brittleness, while for polymers we mark the determining influence of chemical structures on the properties B and τ.

Encyclopedia of Continuum Mechanics

Effect of pressure on material properties of polymers Definitions Polymer is a substance consisting of macromolecules of different lengths, which have a long sequence of one or more groups of monomeric units (shorter molecules) linked together with a primary or covalent chemical bond. Time-dependent mechanical property is the characteristic of a polymer (or any other viscoelastic material) that interrelates the cause in the form of stress or strain and the timedependent response (in the form of increasing strain or decaying stress). Time-pressure superposition principle claims that material function can be generated by shifting isobaric segments of time-dependent mechanical properties measured at different pressures along the logarithmic time scale in respect to the segment selected at reference pressure. Compressibility is a measure of relative change of the volume of polymer exposed to the pressure change.

Springer handbooks, 2008

In many cases residual stresses are one of the main factors determining the engineering properties of parts and structural components. This factor plays a significant role, for example, in fatigue of welded elements. The influence of residual stresses on the multicycle fatigue life of butt and fillet welds can be compared with the effects of stress concentration. The main stages of residual stress management are considered in this chapter with the emphasis on practical application of various destructive and nondestructive techniques for residual stress measurement in materials, parts, and welded elements. Some results of testing showing the role of residual stresses in fatigue processes as well as aspects and examples of ultrasonic stress-relieving are also considered in this chapter. The presented data on residual stresses are complimentary to the detailed review of various methods of residual stress analysis considered in two handbooks on

Progress in Polymer Science, 2005

A direct relation between molecular characteristics and macroscopic mechanical properties of polymeric materials was subject of a vast number academic and industrial research studies in the past. Motivation was that an answer to this question could, in the end, result in guidelines how to construct tailored materials, either on the molecular level or, in heterogeneous materials, on the micro-scale, that could serve our needs of improved materials without the need of extensive trial and error work. Despite all attempts, no real universally applicable success was reported, and it was only after the introduction of the concept of the polymer's intrinsic deformation behavior that some remarkable progress could be recognized. Thus, it is first important to understand where intrinsic deformation behavior of polymeric materials stands for. Second, it is interesting to understand why this intermediate step is relevant and how it relates to the molecular structure of polymers. Third and, in the end, the most computational-modeling-based question to be answered is how intrinsic behavior relates to the macroscopic response of polymeric materials. This is a multi-scale problem like encountered in many of our present research problems. q