This philosophy is a refinement of the fail-safe philosophy. It resumes that cracks will exist, caused either by processing or by fatigue, and uses fracture mechanics analyses and tests to determine whether such cracks will grow large enough to produce failures before they are detected by periodic inspection. Three key items are needed for successfull damage-tolerant design; residual strenght, fatigue crack growth behavior and crack detection involving nondestructive inspection.
(Fuchs H.O., Stephens R.I., Metal Fatigue in Engineering, pg.24, Kayra Ermutlu)
Fail Safe Design
When a component, structure or vehicle reaches its allowable safe life, it must be retired from service. This can be inadequate since all the fleet must be retired before the average calculated life or test life is attained. This practice is very costly and wasteful. Also testing and analysis cannot predict all service failures. Thus fail-safe fatigue design criteria were developed by aircraft engineers.
Fail safe design requires that if one part fails, the system does not fail. Fail safe design recognizes that fatigue cracks may occur and structures are arranged so that cracks will not lead to failure of the structure before they are detected and repaired. Multiple load paths, load transfer between members, crack stoppers built at intervals into the structure and inspection are some of the means used to achieve fail safe design.
(Fuchs H.O., Stephens R.I., Metal Fatigue in Engineering, pg.24, Kayra Ermutlu)
Cost Accounting
Cost accounting is the process of accounting for costs, which begins with recording of income and expenditure or the bases on which they are calculated and ends with the preparation of statistical data. Cost accounting is thus the formal mechanism by means of which costs of products or services are ascertained and controlled.
(Bhar B.K., Cost Accounting, pg.12, Kayra Ermutlu)
Surface Roughness
Surface roughness is a measurable characteristic based on the roughness deviations as waviness, lay and flaws.
The most commonly used measure of surface texture is surface roughness. Surface roughness can be defined as the average of the vertical deviations from the nominal surface over a specified surface length. An arithmetic average (AA) is generally used, besed on the absolute values of the deviations and this roughness value is referred to by the name average roughness. Arithmetic Average (AA) method is the most widely used averaging method for surface roughness today.
(Groover M.P., Fundamentals of modern Manufacturing:Materials, Processes and Systems, pg.89, Kayra Ermutlu)
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