Focused Practice

I. Answer the following questions:

1. What requirements must the electrolyte satisfy from an electrochemical point of view?

2. What must the electrolyte have sufficient ionic conductivity for?

3. Where should the electrolyte be electrochemically stable?

4. Why did polymer chemists begin to design polymer hosts?

5. What tendency do alternative host polymers have?

6. What practical application will alternative polymer hosts synthesized have?

II. Analyse the grammar structures underlined in the above text.

III. Speak on: Thermal and mechanical stability of electrolytes.

SECTION II

Electrical engineering and electromechanics

Unit 17

Grammar: The Noun as an Attribute

Word List:

1. ductility	пластичность
2. service life	эксплуатационный ресурс, срок службы
3. fatigue cracks	трещины, вызванные усталостью материала
4. crack path	ход трещин
5. stress	напряжение
6. creep-fatigue	крип-усталость; ползучесть в сочетании с усталостью
7. grain boundary	граница между гранулами, волокнами
8. corroboration	подтверждение теории
9. finite-element	конечный элемент
10. integrity	целостность
11. tensile stress	напряжение при растяжении
12. fretting fatigue to fret	Фреттинг-усталость изнашивать, разъедать, вызывать коррозию
13.initiation site	место зарождения трещины
14. a series of straight extensions	серия прямолинейных удлинений трещин в ходе эксперимента
15. infinitesimal	бесконечно малая (величина)
16. curvilinear	криволинейный
17. incremental changes	увеличение роста трещины

Fatigue Cracks in Turbine Discs

The structural integrity of turbine disc components is dependent upon the initiation and growth of fatigue cracks. Countermeasures such as careful material selection aim to minimize crack growth but the probability of fatigue failure remains whilst materials such as nickel-based superalloys used to manufacture turbine discs have some ductility. To calculate the service life of each component of the turbine disc requires knowledge of the probable crack path(s) and the stress intensity factors associated with them. The use of techniques that provide engineers with this information at the design stage will encourage the development of components with higher structural integrity.

One of the principal experimental methods of stress analysis available to the design engineer is photoelasticity. This can be used to perform independent stress analyses or for corroboration of finite-element results. The determination of stress intensity factors using two-dimensional photoelasticity has almost been fully optimized. Techniques that predict the directions of crack growth have also been developed in line with the need to assess the likely mode of failure, particularly in aircraft structures.

Fatigue crack growth at the high temperatures existing in turbines is a complex interaction between the mechanisms of creep-fatigue, grain boundary microstructure and the operating environment. To enable predictions to be made of fatigue crack paths in any component requires information regarding initiation sites and the mechanism by which the crack propagates which are influenced by the state of stress at the crack tip and the associated behaviour of the material.

Although crack initiation has been reported to involve a number of complex processes, only the initiation site is required to determine the crack path. Typically, initiation occurs at locations of highest tensile stress at a boundary or within regions of contact where the crack is developed through the process of fretting fatigue.

The photoelastic prediction of the fatigue crack path is constructed from a series of straight extensions made to the experimental crack whereas the real fatigue crack grows in infinitesimal extensions following a curvilinear path. The error incurred here is minimized as the incremental changes in direction of the predicted crack path are restricted to be less than 5-10°.

In modelling a turbine disc the degree of accuracy of test results depends on how well the prototype material compares with the photoelastic material.