A factor is included in 217Plus™ to account for the environmental effects of vibration and temperature cycling at the system level. At the component level, the and factors in 217Plus™ account for temperature cycling, vibration, and relative humidity when
the component models are used.
At the system level, if the specific environmental stresses to which the system will be exposed in field use are known, then the environmental correction factor is calculated using the formula:
If the actual values of these variables are unknown, the default values that should be used are PTC = 0.80 and PRV = 0.20. The SS value is the screening strength and has been derived from MIL-HDBK-344. It is an estimate of the probability of both precipitating a defect to failure and detecting it once precipitated by the test. Whenever possible, the actual values of delta T (T) and vibration (Grms) should be used for the intended application environment. The 217Plus™ software tool includes default values for these values that are a function of the generic environment that can be used when the model user does not know the specific environmental stresses to which the system will be exposed.
- PTC = percentage of failures resulting from temperature cycling stresses
- PRV = percentage of failures resulting from random vibration stresses
- SS = screening strength applicable to the application environmental values
Default values for environmental stresses can be found in the "Handbook of 217Plus™ Reliability Models" which comes for free with the 217Plus™ software. Additionally, as with the operational profiles, custom profiles can be added to the system. For example, say that a ground communications system is operated in an environment with an operating temperature of 30°C, dormant temperature of 10°C, relative humidity of 50%, and a vibration of 0.5 Grms. To use this profile in related systems and system reassessments, it is recommended that a custom profile be created. To add custom environmental profile or to review the default operational profiles, select "Environments" from the Libraries menu.
For more information on 217Plus™, feel free to contact the 217Plus™ team by phone (315-351-4202) or by E-mail (inquiry@theRIAC.org). To obtain additional information including a demo version of the software, go to (http://theRIAC.org/ProductsAndServices/Products/217Plus/index.asp).