Fatigue thermal

Thermal fatigue characteristically results from temperature cycles in service. Even if an alloy is con ectly selected and operated within normal design limits for creep strength and hot-gas corrosion resistance, it can fail from thermal fatigue. 

Thermal fatigue damage is not confined to complex structures or assemblies. It can occur at the surface of quite simple shapes and appear as a network of cracks. 

A metal tumbler would not crack because its elastic limit must be exceeded considerably before it fails. However, repetitions of thermal stress, when some plastic flow occurs on both heating and cooling cycles, can result in either cracking or so much deformation that a part becomes unserviceable. 

Thermal Stress. The formula for thermal stress can be rearranged to calculate the tolerable temperature gradient for keeping defonnation within arbitrary limits. 

M = Modulus of elasticity (Young s Modulus) in psi, or modulus of plasticity. 

Note When the elastic modulus is used, the elastic limit or proportional limit should be used with it in the formula. When the plastic modulus or Secant Modulus is used, it should be used with the corresponding yield strength. 

The introduction of compressive stress and surface hardening are effective methods for preventing thermal fatigue. It is likely that their inhibiting effects are associated with both delayed-crack nucleation and crack growth, if cracks or pores are already present in the material (which is the actual case in most ceramics). Several methods for surface modihcation have been mentioned in the previous section, among them SP is a technique used to improve the fatigue properties by 

One of the reasons for the improved strengfli properties of the functionally-graded ceramics is due to the surface formation of compressive-residual stresses, which counteract some of the tensile stress generated during the thermal shock process. 

The crack-propagation rate (i.e., delamination da/dN) under laser-thermal cyclic loads may generally be expressed in terms of the Paris law, given below as  

Themial cycle fatigue O 1 h hold at 1000 10hholdat1000 C d 1 hhohJatSSOK  

AK is the stress-intensity factor da/dN is the crack-propagation rate m is an appropriate exponent and C is a constant  

Polymers deform viscoelastically. Under cyclic loads, the stress-strain curve upon unloading is not the same as upon loading. Therefore, there is a hysteresis between stress and strain, causing energy dissipation during the deformation, thus producing heat. This hysteresis is discussed in detail in exercise 26. 

Thermoplastic polymers fail by plastic yielding under thermal fatigue because the yield strength decreases with increasing temperature. Elastomers and duromers can also fail by thermal fatigue due to the reduction of Young s modulus with temperature which causes a continuously growing deformation. 

Thermal fatigue is observed mainly in stress-controlled loading because the strain amplitude increases in this case due to the reduction of stiffness with increasing temperature. The heat generated per cycle thus increases with time. If the loading is strain-controlled, thermal fatigue is usually not problematic because the stress decreases in this case. 

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Extend the safe useful operation life of major HT/HP power plant items, subject to time-dependent creep and thermal fatigue damage, with benefits in terms of delayed costs for component replacement. 

Tips of platinum, platinum—nickel alloy, or iridium can be resistance-welded to spark-plug electrodes for improved reHabiHty and increased lifetime. These electrodes are exposed to extremely hostile environments involving spark erosion, high temperature corrosion, thermal shock, and thermal fatigue. 

Thermal Fatigue. Cemented carbide tools sometimes exhibit a series of cracks perpendicular to the tool edge when appHed in intermpted cutting conditions such as milling. These thermal cracks are caused by the alternating expansion and contraction of the tool surface as it heats while cutting... 

Highly ductile materials tend to be more resistant to thermal fatigue and also seem more resistant to crack initiation and propagation. 

The operating schedule of a gas turbine produces low-frequency thermal fatigue. The number of starts per hours of operating time directly affects the hfe of the hot sections (combustor, turbine nozzles, and blades). The life reduction effect of the number of starts on a combustor liner could be as high as 230 hours/start and on the turbine nozzles as high as 180 hours/start. The effect of full load trips can be nearly 2-3 times as great ... 

The insulation around the central electrode is an example of a non-metallic material - in this case, alumina, a ceramic. This is chosen because of its electrical insulating properties and because it also has good thermal fatigue resistance and resistance to corrosion and oxidation (it is an oxide already). 

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. 

But they all oxidise very rapidly indeed (see Table 21.2), and are utterly useless without coatings. The problem with coated refractory metals is, that if a break occurs in the coating (e.g. by thermal fatigue, or erosion by dust particles, etc.), catastrophic oxidation of the underlying metal will take place, leading to rapid failure. The unsafeness of this situation is a major problem that has to be solved before we can use these on-other-counts potentially excellent materials. 

The operating schedule of a gas turbine produces a low-frequency thermal fatigue. The number of starts per hours of operating time directly affects the blade life. Table 11-1 shows fewer starts per operating time increases turbine life. 

Composition of the coating that includes environmental and mechanical properties such as thermal fatigue. 

Coating Thickness that provides a greater protective reservoir if thicker. However, thicker coatings may have lower thermal fatigue resistance. 

Environments. Among the environmental factors that can shorten life under thermal fatigue conditions are surface decarburization, oxidation, and carburization. The last can be detrimental because it is likely to reduce both hot strength and ductility at the same time. The usual failure mechanism of heat-resistant alloy fixtures in carburizing furnaces is by thermal fatigue damage, evidenced by a prominent network of deep cracks. 

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