Deformation macro

One of the prominent features of polymeric liquids is the property to recover partially the pre-deformation state. Such behaviour is analogous to a rubber band snapping back when released after stretching. This is a consequence of the relaxation of macromolecular coils in the system every deformed macro-molecular coil tends to recover its pre-deformed equilibrium form. In the considered theory, the form and dimensions of the deformed macromolecular coil are connected with the internal variables which have to be considered when the tensor of recoverable strain is to be calculated. Further on, we shall consider the simplest case, when the form and dimensions of macromolecular coils are determined by the only internal tensor. In this case, the behaviour of the polymer liquid is considered to describe by one of the constitutive equations (9.48)-(9.49) or (9.58). 

Casey, J. and Naghdi, P.M., Constitutive Results for Finitely Deforming Elastic-Plastic Materials, in Constitutive Equations Macro and Computational Aspects (edited by K.J. Wiliam), ASME, 1984, pp. 53-71. 

It is somewhat difficult conceptually to explain the recoverable high elasticity of these materials in terms of flexible polymer chains cross-linked into an open network structure as commonly envisaged for conventionally vulcanised rubbers. It is probably better to consider the deformation behaviour on a macro, rather than molecular, scale. One such model would envisage a three-dimensional mesh of polypropylene with elastomeric domains embedded within. On application of a stress both the open network of the hard phase and the elastomeric domains will be capable of deformation. On release of the stress, the cross-linked rubbery domains will try to recover their original shape and hence result in recovery from deformation of the blended object. 

Far below the glass transition temperature (T see Sect. 2.3.4.3) the macro-Brownian motions are frozen in completely, and most of the micro-Brownian motions are frozen in as well ( glassy state ). Near Tg, the micro-Brownian motions set in and become stronger with increasing temperature. The material softens. Finally, upon further raise of temperature, the macro-Brownian motions set in as well, and the polymer can be deformed by applying an external force. 

The rate of all these processes, of course, depends strongly on the temperature in the vicinity of Tg the polymer chains are still relatively inflexible. Thus deformation requires considerable forces, and recovery occurs very slowly. Well above Tg the melt deforms more easily, but the tendency to flow as a result of increased macro-Brownian motion is still outweighed by the elastic recovery. The temperature range for pronounced elastic behavior of the polymer melt depends... 

In this review we are mainly concerned with thermotropic materials, i.e. with liquid crystals and LC-glasses which do not contain a solvent. The transitions of the macro-molecular, thermotropic liquid crystals are governed then by temperature, pressure and deformation. In lyotropic liquid crystals and LC-glasses a solvent or dispersing agent is present in addition. The transitions then also become concentration dependent. 

In the simplest cases it can be reduced to the tensor of deformation of macro-molecular coils... 

In the study of food texture, attention is given to two interdependent areas the flow and deformation properties and the macro-and microstructure. The study of food texture is important for three reasons ... 

This model is based on quasimolecular dynamics, in which the medium is assumed to be composed of an assembly of meso-scale discrete particles (i.e., finite elements). Tlie movement and deformation of the material system and its evolution are described by the aggregate movements of these elements. Two types of basic characteristics, geometrical and physical, are considered. In tlie geometrical aspect, sliapes and sizes of elements and tlie manner of their initial aggregation and arrangement are the important factors. In the physical aspect, mechanical, physical, and chemical characteristics, such as the interaction potential, phase transition, and chemical reactivity may be tlie important ones. To construct this model, many physical factors, including interaction potential, friction of particles, shear resistance force, energy dissipation and temperature increase, stress and strain at the meso- and macro-levels, phase transition, and chemical reaction are considered. In fact, simulation of chemical reactions is one of the most difficult tasks, but it is the most important aspect in shock-wave chemistiy. 

When a specimen is stretched plastically a few percent and then unloaded, x-ray measurements show a line shift indicating residual compressive macrostress in the direction of prestrain. The effect is symmetrical after plastic compression, x-rays indicate residual tensile stress. It is not a surface effect, because x-ray measurements made after successive removal of surface layers show that the stress persists throughout the specimen. On the other hand, dissection measurements show that a true macrostress does not exist, and, in fact, none would be expected after uniform deformation. The stress indicated by x-rays is called pseudo-macrostress, pseudo because it is not a true macrostress causing strain on dissection and macro because it causes an x-ray line shift. Pseudo-macrostress is actually an unusual kind of microstress, in which the portions of the material that are in tension and in compression are unequal in volume. It has been discussed in various reviews [16.26-16.28]. 

It seems possible to amplify the photostimulated conformational changes in solution at the molecular level into shape changes of polymer gels or solids at the visible macro level. The first proposal to use the structural changes at the molecular level for direct conversion of photon energy into mechanical work has been made by Merian (13.) in 1966. Since then, many materials, most of which contained azobenzene chromophores, have been reported to show photostimulated deformation(JM). Till now, however, the reported deformations were limited to less than 10%. In addition, Matejka et. al. have pointed out that in many cases photo-heating effect instead of photochemical reaction plays a dominant role in the deformation(15,16). 

Investigation of the deformation relief occurring on the surface of samples additionally subjected to by 15% strain after different number of compression steps have shown that plateau on the initial portion of strain curves is result of strain localization (Fig. 2a) in macro shear bands (MSB). Its appearance is result of scattering some dislocation boundaries onto individual dislocations (Baushinger effect) and formation of avalanche of mobile dislocations (Fig. 2b). So, in this case yield of titanium is controlled by substructure that, probably, leads to weak dependence of yield stress on strain. Macrobands formed at the beginning of the cycle of loading remain until the end of loading. So, plastic flow of titanium is localized. 

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