Failure Mode Control Variables

Most failure modes are defined relative to a failure mode control variable (FMCV). This is the variable that is referenced by the failure mode to determine when failure occurs. (For those failure modes that are defined as distributions, the control variable represents the x-axis of a failure distribution plot.)

Note: Demand>Capacity, Event-triggered failure, and Unreliable failure modes are the only failure modes that are not defined relative to a failure mode control variable (i.e., they are not FMCV-based).

For FMCV-based failure modes, the failure mode calculates the FMCV “age” that will result in its next failure at the start of the realization, after each time the mode is repaired, and after each time the component is replaced. When the FMCV exceeds this value, the failure occurs.

Each (FMCV-based) failure mode has its own FMCV, which is defined in terms of a specified “base variable” (e.g., total simulation time, operating time, mileage). The FMCV for a particular failure mode and the base variable are related as follows:

FMCV(t) represents the value for the FMCV at time t. The sum is made over all the timesteps from the beginning of the realization (or the time the FMCV was reset) to time t. The Initial Value and Acceleration Factor are user inputs for each failure mode. The FMCV calculation can be reset by certain events such as replacement of the component or repair of the failure mode. This is done by resetting the Initial Value (to a specified FMCV value) and restarting the sum.

For example, if you were simulating the wear-out failure mode for a brake system in a car, the Base Variable would be mileage, and the Acceleration factor could change dynamically (from 1) depending on whether the car was being used in the city or on the highway. The FMCV calculation would be restarted (by resetting the sum the Initial Value to 0) whenever the brakes were replaced.

Each failure mode’s FMCV properties can be accessed from within the Failure Modes tab by selecting the failure mode and pressing the Settings… button. This displays the following dialog: