Mechanism dissipating

Whenever a mechanism dissipates mechanical energy as heat there exists an inverse mechanism by which heat can generate a mechanical fluctuation. There is always a connection between fluctuation and dissipation. 

Fig 13 Critical acceleration and energy release rate curves determined from long-duration pulse experiments, as functions of shock amplitude v in PBX 9404. Energy rate shown is the net result of mechanical dissipation and exothermic chemical reaction. Following thermochemical convention, energy release rate due to exothermic reaction is denoted as a negative value of H ... 

In the low-temperature part of the /3 transition, the MGI units lead to an increase of mechanical dissipation, whatever the MGI content. In contrast, except for MGIM76 copolymers, one observes for all the other MGIMx a decrease of the dielectric dissipation. In both cases, the effect develops when increasing the MGI content, but passes through a maximum for either MGIM58 in mechanics or MGIM36 in dielectrics. The lowest effect in mechanics, even an opposite effect in dielectrics, are obtained for the MGI-richest copolymer, MGIM76. 

In the high-temperature part, opposite effects are observed in mechanics and dielectrics, except for MGIM76. Indeed, the decrease of mechanical dissipation (relatively to the MMA content response) increases with increasing temperature, but it is the opposite with dielectric dissipation. For MGIM76, in both experiments, the dissipation is larger than expected from the MMA content. 

A linear relationship exists between the toughness (integrated stress-strain curve) and the dynamic mechanical dissipation factor. The types of materials that fit this relationship include glassy polymers, elastomers, and an impregnated fabric. The existence of this relationship indicates that toughness arises from the molecular motions which give rise to the dynamic mechanical properties. 

One of these studies (7) proposed that impact strength arose from a dynamic mechanical dissipation mechanism at the temperature and frequency of the impact. This raises a question whether any one type of loss mechanism (i.e., arising from main chain local mode motions) is better in this respect than another type (i.e., arising from branch motions) as has been proposed (2), or whether the absolute magnitude of the loss at the temperature and frequency of the impact is the criterion. This assessment is the basis of the present study. 

As both referees pointed out, other reasons have been offered for energy dissipation during impact. In each case however some energy dissipating mechanism must be provided. The present work indicates that, at least for the samples used, this mechanism is provided by the dynamic mechanical dissipation factor. This in turn is attributed to the fact that at the high strain rates of impact the linear viscoelastic region is the largest, if not the only, contributor to the dissipation process. 

The entropy flow is based on a heat or material flow, the entropy generation on a heat flow in a temperature field, diffusion by mass forces and differences in the chemical potential, mechanical dissipation and chemical reactions. 

The non-negativity of the mechanical dissipation forms the basis for the construction of the material behavioral laws. Note that the equation Aj = p only "defines" the variable A ... 

Dissipation di-s9- pa-shon n. The loss modulus in a plastic part when imparted with rapid, cyclic changes (or even reversals) of stress. The product of mechanical dissipation is heat, which can raise the temperature of the part and cause it to weaken, creep rapidly, or even fail prematurely. Dissipation can also apply to electrical systems, whereby a material with small dissipation will tend to better insulate heat. This is a desirable property in electrical insulations for high-frequency applications because it minimizes the waste of electrical energy as heat. 

It can be seen that, provided that the segregation scale is large (this notion will be clarified further), stretching is the controlling process for mixing and mechanical dissipation is an interesting tool to decrease the mixing time. 

The storage modulus, E, represents the ability of the material to elastically store the absorbed mechanical energy as potential energy. The loss modulus, E", represents the ability to dissipate the absorbed energy as heat The ratio between the dissipated and stored mechanical energy is referred to as the mechanical dissipation factor. 

Figure 12.15 Polystyrene data mechanical dissipation factor versus temperature for fractions. Fractions 1, 4, 9, 29, and 34 were tested. (Reprinted with permission from Merz, E. H., L. E. Nielsen, and R. Buchdahl Influence of Molecular Weight on the Properties of Polystyrene, Ind. Eng. Chem., vol. 43, pp. 1396-1401, 1951. Cop3night 1951 American Chemical Society.)...

The dichroic dissipation factor may also be defined in a way analogous to the mechanical dissipation factor tan S (eqn [29]). 

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