proviso that analyses in which this is done be carefully justified and defended in detail.

Development of Statistical Models of Fatigue of Materials

Especially given the recent use of defense systems for longer periods of time, fatigue modeling is clearly of critical importance to DoD. Speakers pointed out that the use of fatigue modeling that derives from a statistical approach provides useful estimates in a variety of important and common DoD contexts. Special emphasis was placed on the need for understanding the science in applications involving fatigue. Presentations during the session on fatigue modeling included both illustrations of the necessary statistics/physical science partnership and descriptions of a variety of models motivated by recent research in materials science. Speakers suggested that fatigue modeling could be one of the techniques included in a revised RAM Primer.

Need for Greater Use of Physics-of-Failure Models and for Modeling Some Failure Sources Separately

Several speakers supported the greater use of physics-of-failure models (i.e., models that directly represent the physical basis for failure) whenever these approaches are applicable. Use of such models would generally provide better estimates of various characteristics of system failure as compared with models not linked to specific failure modes. Discussion of the benefits of physics-of-failure models, along with leading examples, could also be included in a revised RAM Primer.

Further, several speakers proposed dividing failures into those of mature components and those of immature components, the latter through either design or production flaws. For example, the Integrated Reliability Growth Strategy classifies design failures into type A and type B modes, i.e., associated with mature and immature design components. Further, failures in the field are typically overrepresented by poorly produced components, referred to as “bad actors.” The identification and separate modeling of failures from type B modes and from bad actors is an approach to reliability estimation that could be effective in providing better estimates of system and fleet reliability. One speaker also mentioned “special-cause” failures—those that are unpredictable and are due, for example, to changes



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