Thermal aspects stress

Another reason that a low operating temperature is an inherent advantage is that the influence of thermal cycles is reduced for smaller temperature ranges. Thermal cycling is an inevitable aspect of discontinuous operation, and small differences between operating and ambient temperatures help minimize thermally induced stresses. The effects of thermal cycles on the membrane, as well as related changes in relative humidity, are discussed in Sect. 4. In addition, the durability of seals depends upon the number and magnitude of thermal cycles this issue is not discussed further in this chapter. 

The evolution of T, is just an exercise in mesoscale thermodynamics [13]. These expressions, in combination with (7.54), incorporate concepts of heterogeneous deformation into a eonsistent mierostruetural model. Aspects of local material response under extremely rapid heating and cooling rates are still open to question. An important contribution to the micromechanical basis for heterogeneous deformation would certainly be to establish appropriate laws of flow-stress evolution due to rapid thermal cycling that would provide a physical basis for (7.54). 

Another aspect of epoxy resin mortar floorings which needs careful attention is that their coefficients of thermal expansion are approximately three times that of concrete. This, coupled with the relative low thermal conductivity of epoxy mortar, can cause stresses to be induced at the resin mortar/concrete interface under conditions of thermal shock (e.g. thermal cleaning), resulting in break-up of the flooring due to initial failure in the concrete. Two approaches have been tried to overcome this problem ... 

The most general and specific aspect of thermal aging of polymer networks is the existence of a postcuring effect, often predominant in the early time of exposure (Fig. 14.19). The curves exhibit a maximum (for the ultimate stress of unsaturated polyesters, P can increase by more... 

Powell, K.L., Smith, P.A., Yeomans, J.A. (1993), Aspects of residual thermal stresses in continuous-fibre-reinforced ceramic matrix composites , Comp. Sci. Tech., 47, 359-367. 

The expansive internal stress in a plasma polymer is a characteristic property that should be considered in general plasma polymers and is not found in most conventional polymers. It is important to recognize that the internal stress in a plasma polymer layer exists in as-deposited plasma polymer layer, i.e., the internal stress does not develop when the coated film is exposed to ambient conditions. Because of the vast differences in many characteristics (e.g., modulus and thermal expansion coefficient of two layers of materials), the coated composite materials behave like a bimetal. Of course, the extent of this behavior is largely dependent on the nature of the substrate, particularly its thickness and shape, and also on the thickness of the plasma polymer layer. This aspect may be a crucial factor in some applications of plasma polymers. It is anticipated that the same plasma coating applied on the concave surface has the lower threshold thickness than that applied on a convex surface, and its extent depends on the radius of curvature. 

Figure 21 Effect of aspect ratio on thermal stresses. (From Ref. 48, courtesy of SAE.)...

Some most impressive computer simulations have been made in efforts to model the structure of liquid water. Yet, because these calculations usually are based on pair additivity of the potentials for the H-bonded water molecules, the possibility exists that subtle effects may escape the theoretician, as no means are provided to incorporate the possibility of extensive cooperativity—an aspect that Henry Frank (1972) has so eloquently stressed. Very likely, this is the crux of the problem of interfacially modified water if nothing else, the thermal anomalies (discussed below) in the properties of vicinal water strongly implicate cooperativity on a large scale—a collective behavior of water molecules that no existing potential function is able to reproduce. The cooperativity reflects nonpair additivity, and it does not seem plausible that effective potential energy functions can be devised that will remedy the specific lack of a detailed understanding of many-body interactions in water. Attempts to allow for cooperativity have been made by Finney, Barnes, and co-workers, notably Quinn and Nicholas (see Barnes et al., 1979). 

---