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Where quantitative relationships need to be developed, their extent depends on the need and the possibility of applying the result to more than one cell. Levels of quantitative development for relationships that apply to cells depends on:
Design life.
Definition of failure, as in Step #5 of the CBDA.
Consequences of failure.
Support from monitoring and inspection available that may minimize the detail of prediction required.
Range of environmental conditions as obtained from Environmental Definition in Step #1 of the CBDA.
Insights gained from model testing.
Input from continuing field experience.
Importance of predicting early low fraction failures.
Extent of NDE sensitivity and availability.
Depending on the need for the coverage of quantitative expressions, they can be developed essentially according to the following options:
Fully developed from first principles.
Fully statistical expression that accounts for effects of physical variables on the correlation parameters.
Statistical expression that integrates the three levels of variability (inherent, heat to heat, environment).
Ordinary correlation of field data or data from accelerated test.
And there are other approaches.
Regardless, these expressions then enter the cells of the LAM. They may enter as a full expression, as times to failure, or fraction failed in some time.
Lifetime Prediction, Roger W. Staehle, Adjunct Professor, Department of Chemical Engineering and Materials Science, University of Minnesota, Staehle Consulting Co.