Older One
Unlimited safety is the oldest criterion. It requires local stresses or strains to be essantially elastic and safely below the pertinent fatique limit. For parts subjectedto million cycles, like engine valve springs, this is still a good design criterion. However, most parts experience significant variable amplitude loading and the pertinent fatigue limit is difficult to define or obtain. In addition, this criterion may not be economical or practical in many design situations. Expamles include excessive weight of aircraft for impracticality and global competitiveness for cost effectiness.
(Ralph I. Stephens, Metal Fatigue in Engineering, 2nd Edition; Page: 23)
New One
Criteria for fatigue design have evolved from infinite life to damage tolerance. Unlimited safety is the oldest criterion. It requires local stresses or strains to be essentially elastic and safely below the fatigue limit. For parts subjected to many millions of cycles, like engine valve springs, this is still a good design . This criterion may not be economical (i.e. global competitiveness) or practical (i.e. excessive weight of aircraft) in many design situations criterion.
(Fatemi A, Fatigue Design Methods, pages: 7-8 )
2-)Safe Life Design - METHOD
Older One
The safe life must include a margin for the seatter of fatgue results and for other unknown factors. The calculations may be based on stress-life, strain-life or crack growth relations. Safe life design may be based solely or partially on field and simulated testing. The margin for safety in safe life design may be taken in terms of life, in terms of load or by specifying that both margins must be satisfied.
(Ralph I. Stephens, Metal Fatigue in Engineering, 2nd Edition; Page: 23,24)
New One
The practice of designing for a finite life is known as "safe-life" design. It is used in many industries, for instance automotive industry, in pressure vessel design, and in jet engine design. The calculations may be based on stress-life, strain-life, or crack growth relations. Ball bearings and roller bearings are examples of safe-life design. The safe life must include a margin for the scatter of fatigue results and for other unknown factors. The margin for safety in safe-life design may be taken in terms of life, in terms of load, or by specifying that both margins must be satisfied, as in the ASME Boiler and Pressure Vessel Code.
(Fatemi A, Fatigue Design Methods, page: 9 )
3-) Fail - Safe Design - METHOD
Older One
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)
New One
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 failsafe design.(Fatemi A, Fatigue Design Methods, page: 10 )
4-) Damage Tolerant Design - METHOD
Older One
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)
New One
This philosophy is a refinement of the fail-safe
philosophy. It assumes that cracks will exist, caused either by
processing or by fatigue, and uses fracture mechanics analyses and tests to check whether such cracks will grow large enough to produce failures before they are detected by periodic inspection. Three key items are needed for successful damagetolerant design:- Residual strength,
- Fatigue crack growth behavior, and
- Crack detection involving nondestructive inspection.
(Fatemi A, Fatigue Design Methods, page: 11 )
5-) Computer Aided Maintenance - SYSTEM
Older One
Computer aided maintenance depends on sensory systems to provide inputs for conditioning monitoring.Sensors can measure equipment performance and act like watchdogs to detect system faults and degradation.
(Computer aided maintenance methodologies and practises edited by Jay lee and Ben Wang,p.19)
New One
Computer Aided Maintenance management system is one of the important computer - aided tools in the effective management of mantenance functions. In the recent past almost all major process industries like rafineries, steel plants, petrochemicals have gone in for such facility. Basically, the capability to store and retrieve asset information, generation of purchase orders, history of failure data and even Computer Aided Drawings overweigh the utility of such tool.
Main applications of Computer Aided Maintenance Systems are;
- To maintain data as well as to display information on preventive, predictive maintenance schedules.
- To prepare and display management information system reports.
- Inventory management for spares.
- Shutdown planning and to carry out the project management.
- Fault diagnosis/expert systems with the Computer Aided Maintenance having machine interface.
(Vebkataraman K., Maintenance Engineering and Management,p.146)
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