meet several critical goals, which include (1) providing advance warning of failures;
(2) minimizing unscheduled maintenance, extending maintenance cycles, and
maintaining effectiveness through timely repair actions; (3) reducing the life cycle
cost of equipment by decreasing inspection costs, downtime, and inventory; and
(4) improving qualification and assisting in the design and logistical support of fielded
and future systems (Vichare and Pecht, 2006).
The importance of PHM has also been explicitly stated in the U.S. Department of
Defense 5000.2 policy document on defence acquisition, which states that ‘program
managers shall optimize operational readiness through affordable, integrated,
embedded diagnostics and prognostics, embedded training and testing, serialized
item management, automatic identification technology, and iterative technology
refreshment’ (DoD 5000.2 Policy Document, 2004). Thus, a prognostics capability has
become a requirement for any system sold to the U.S. Department of Defense.
2. Prognostic modelling of stress and damage utilizing life cycle loads
A product can be subject to loads that arise during manufacturing, shipment, storage,
handling, operating and non-operating conditions. These life cycle loads (thermal,
mechanical, chemical, electrical, and so on), can either individually or in various
combinations, lead to performance or physical degradation of the product and reduce
its service life. The extent and rate of product degradation depends upon the
magnitude and duration of exposure to loads (usage rate, frequency and severity). In
the PoF-based prognostics approach, the life cycle loads are monitored in situ, and
used in conjunction with PoF-based damage models to assess the degradation related
to cumulative load exposures.
Ramakrishnan and Pecht (2003), and Mishra et al. (2002) used PoF-based
prognostics to assess an electronic component-board assembly placed under the
hood of an automobile and subjected to normal driving conditions. The test board
incorporated surface-mount leadless inductors soldered onto an FR-4 substrate using
eutectic tin–lead solder. Temperature and vibrations were measured in situ on the
board in the application environment. Using the monitored environmental data, stress
and damage models were successfully used to estimate consumed life.
Shetty et al. (2002) applied the PHM methodology to conduct prognostic remaining
life assessment of the End Effector Electronics Unit (EEEU) inside the robotic arm of
the space shuttle remote manipulator system (SMRS). A life cycle loading profile of
thermal and vibration loads was developed for the EEEU circuit boards. Damage
assessment was conducted using physics-based mechanical and thermo-mechanical
damage models. A prognostic estimate using a combination of damage models,
inspection and accelerated testing showed that there was little degradation in the
electronics and that their designed for life (of 20 years) could be extended.
Mathew et al. (2006, 2007) applied a similar PoF-based prognostics methodology to
conduct a remaining life assessment of circuit cards inside the space shuttle solid
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