A system level partitioning approach for analyzing the origins of variability in life prediction of tungsten filaments for incandescent lamps
✍ Scribed by R. Raj; M.P. Enright; D.M. Frangopol
- Publisher
- Elsevier Science
- Year
- 1999
- Weight
- 249 KB
- Volume
- 21
- Category
- Article
- ISSN
- 0261-3069
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✦ Synopsis
The assessment of the origins of variability in the life expectancy has important ramifications in optimizing the cost᎐benefit ratio for the design and manufacture of a 'system'. In this paper we study a model example, the tungsten filament, to develop a methodology for this process. The lifetime of tungsten filaments, in service, can vary for several reasons. The microstructure of tungsten may vary, the geometrical parameters of the filament coil or the radius of the tungsten wire may change, or the operating voltage for the filament may fluctuate. Additional uncertainty may arise from the fundamental material parameters, such as the activation energies for self-diffusion. Using a system level partitioning of system variables' approach permits a quantitative examination of each of these parameters on the total lifetime of the filament. Important and surprising results emerge when the partitioning approach is utilized, notably, the uncertainty arising from fluctuation in temperature, which is related to fluctuation in the operating voltage, produces a variability that is comparable to the variability expected from the Ž . distributed nature of the microstructure. Furthermore, we find that a relatively narrow normal Gaussian distribution in the operating voltage transforms into a broad and log-normal distribution for the lifetime because of the thermally activated mechanisms that control the rate of deformation in tungsten. The results presented here may have a significant influence on cost analysis; they suggests that gains in reliability emanating from better microstructural control must be weighed against variability that is inherent in the system's operating environment.