Expansion stresses

Because oxides are usually quite brittle at the temperatures encountered on a turbine blade surface, they can crack, especially when the temperature of the blade changes and differential thermal contraction and expansion stresses are set up between alloy and oxide. These can act as ideal nucleation centres for thermal fatigue cracks and, because oxide layers in nickel alloys are stuck well to the underlying alloy (they would be useless if they were not), the crack can spread into the alloy itself (Fig. 22.3). The properties of the oxide film are thus very important in affecting the fatigue properties of the whole component. 

As mentioned above, when the oxygen partial pressure is reduced, the lattice expands as CC+ is oxidized to Cr3+. If this expansion is too large, just as with thermal expansion, stresses can be generated and lead to cracking. Increasing the dopant... 

Many properties of adhesive bonds are influenced by the Tg or mobility of the molecular chain structure, as shown in Fig. 3.16. When chain segments can move easily, such as when the temperature exceeds the Tg, they can deform under impact or assume new alignments under mechanical or thermal expansion stresses. This movement spreads the applied energy over a greater number of atoms and thus gives the bond a better chance to resist stress. Brittleness is, therefore, reduced and flexibility is increased. 

There is reduced risk of solid electrolyte mechanical failure since the lower operating temperature places reduced thermal coefficient of expansion stresses on the ceramic. 

The gel is more thinly distributed, and Ca ions are therefore more readily available to it, accelerating its conversion into the virtually non-expansive C-S-H (P54). The thinner distribution of the gel might also in itself decrease the expansive stresses. 

In a fire-tube boiler, the inlet tube sheet and the tube sheet welds are exposed to the extreme temperature of the reformed gas, which creates rather large temperature gradients and therefore high expansive stress. A positive feature of the fire-tube design, however, is that debris in feed water (mainly magnetite particles spalling from the water-side surface of the tubes) can collect at the bottom of the horizontally-mounted... 

This curling can be attributed to an internal stress arising in the plasma polymer during polymer deposition. It is important to recognize that the internal stress in the plasma polymer (in the as-polymerized state) is an expansive stress and that this is in marked contrast to what would be expected if the adsorbed monomer were polymerized at the surface of the substrate, which would create, with very few exceptions, contractive stress due to the contraction of volume on the polymerization of a monomer. 

Expansion stress, must be less than or equal to the allowable stress. ... 

The product was developed in Japan in the 1970s [32.37]. Initially, mixes containing hard-burned quicklime were used. However, the expansive stress, which can exceed 5000 t/m after 48 hours was sometimes relieved by blow-outs (i.e. ejection of the lime from the hole), rather than causing splitting. 

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