Related Topics:

Criticality Analysis

Quantitative Criticality Analysis

This topic describes how to perform the new methodology for quantitative criticality analysis that was introduced in the Synthesis version of Xfmea. This method is patterned after the concepts in MIL-STD-1629A, but has been modified to use a more general (and more appropriate) approach that overcomes several inherent limitations and simplifications present in MIL-STD-1629A, including the assumption of a constant failure rate. Furthermore, and instead of dealing with an arbitrary number, it presents the computed criticality number in terms of the probability "that a failure will be caused by the current mode," which is significantly more meaningful and powerful. For specific details on this approach, see http://www.ReliaWiki.org/index.php/Criticality_Analysis.

This analysis is performed directly in the FMRA with report output generated from the Reports window. If your organization is obligated to use the original method instead, see MIL-1629A Criticality Analysis.

To perform a quantitative criticality analysis in the FMRA, do the following:

The software will automatically update the model when you change the rating. For more information on how the quantitative model is calculated, see Using Rating Scales to Set the Reliability Policy.

      • Define at this level: If you know the probability for this failure mode, you can either create a new model or select an existing model to define it here.

      • Inherit: If you have defined the probabilities for all of the associated causes, the software can automatically calculate the probability that the failure mode will occur. For details, refer to Setting the Reliability Policies.

To set the Operation properties, select the top-level item in the FMRA hierarchy to display its properties in the Analysis panel.

The Operating Time field must be entered by the user.

Note: The operating time can be defined only for top-level (system) items in the hierarchy, and this time applies to all of the system's dependents. Thus, if your criticality analysis considers the failure modes associated with only one system, and if you want to compare the failure modes, then you only need to define the operating time for that system. However, if your criticality analysis considers multiple systems that you want to compare, then, to make a valid comparison, you need to enter the same operating time for each system.

By default, Xfmea assumes that the item was new at the beginning of the mission (Current Age = 0) and it will operate for 100% of the operating time (Duty Cycle = 1), but you can adjust these factors if desired.

The Calculation Options window provides the following options:

When you click OK, the software will calculate the probability of failure for each failure mode and automatically determine the Mode Ratios based on all of the failure modes that have been defined for each item. (To give a very simple example, if the item has two failure modes that both have the same probability of failure, then they will both contribute equally to the item’s probability of failure, and therefore they both have Mode Ratio = 0.5.)

Then the software will calculate the mode criticality and item criticality, and the results will be displayed in the Criticality column of the FMRA hierarchy.

Tip: To specify the displayed math precision for the calculated values, choose File > Application Setup then define your preferences on the Settings page.

The following reports are applicable for quantitative criticality analysis:

Reports (4) and (5) are not applicable here because they're used with qualitative criticality analysis.

 

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