  An optimization model has been formulated and solved to determine
on-condition failure thresholds and inspection intervals for multi-component
systems with each component experiencing multiple failure processes due to
simultaneous exposure to degradation and shock loads. In this new model, we
consider on-condition maintenance optimization for systems of degrading
components, which offers cost benefits over time-based preventive maintenance
or replace-on-failure policies. For systems of degrading components, this can
be a particularly difficult problem because of the dependent degradation and
dependent failure times. In previous research, preventive maintenance and
periodic inspection models have been considered; however, for systems whose
costs due to failure are high, it is prudent to avoid the event of failure,
i.e., we should repair or replace the components or system before the failure
happens. The determination of optimal on-condition thresholds for all
components is effective to avoid failure and to minimize cost. Low on-condition
thresholds can be inefficient because they waste components life, and high
on-condition thresholds are risky because the components are prone to costly
failure. In this paper, we formulated and solved a new optimization model to
determine optimal on-condition thresholds and inspection intervals. In our
model, when the system is inspected, all components are inspected at that time.
An inspection interval may be optimal for one component, but might be
undesirable for another component, so the optimization requires a compromise.
The on-condition maintenance optimization model is demonstrated on several
examples.
