“Failure Mode and Effects Analysis (FMEA) and Failure Modes, Effects and Criticality Analysis (FMECA) are methodologies designed to identify potential failure modes for a product or process, to assess the risk associated with those failure modes, to rank the issues in terms of importance and to identify and carry out corrective actions to address the most serious concerns.”
FMECA is a critical analysis required for ensuring viability of a system during operations and support phase of the lifecycle. A major part of FMECA is understanding the failure process and its impact on the operations of the system. The figure below shows an example of how to model a process to include the potential of failure. Duration attributes, Input/Output, Cost and Resource entities can be added to this model and simulated to begin estimating metrics. You can use this with real data to understand the values of existing systems or derive the needs of the system (thresholds and objectives) by including this kind of analysis in the overall system modeling.
Step one is to build this Action Diagram (for details on how to do this please reference the Guide to Model-Based Systems Engineering. Add a loop to periodically enable the decision on whether or not a failure occurs. The time between these decisions can be adjusted by the number of iteration of the loop and the duration of the “F.11 Continue Normal Operations” action.
Adjust the number of iterations by selecting the loop action (“F.1 Continue to operate vehicle?”) and press the </>Script button (see below). A dialog appears asking you to edit the action’s script. You can use the pull-down menu to select Loop Iterations, Custom Script, Probability (Loop), and Resource (Loop). In this case, select “Loop Iterations.” The type in the number (choose 100) as see in the figure below.
Next change the duration of this action and the F.11. Since the loop decision is not a factor in this model, you can give it a nominally small time (1 minute as shown). For the “F.11 Continue Normal Operations” choose 100 hours. When combined with the branch percentage of this path of 90%, means that we have roughly 900 operating hours between failures, which is not unusual for a vehicle in a suburban environment. We could provide a more accurate estimate, including using a distribution for the normal operating hours.
The 90% branch probability comes from the script for the OR action (“F.2 Failure?”). That selection results in the dialog box below.
Now if you assume a failure occurs approximately 10% of the time you can then determine the failure modes are probabilistic in nature, the paths need to be selected based on those probabilities. The second OR action (“F.3 Failure Mode?) shows three possible failure modes. You can add more by selecting F.3 and using the “+Add Branch” button. You can use this to add more branches to represent other failure modes, such as “Driver failure,” “Hit obstacle,” “Guidance System Loss,” etc.
Note to change the default names (Yes, No, Option) to the names of the failure modes, just double click on the name and a dialog will pop-up (as on right). Just type in the name you prefer.
To finish off this model add durations to the various other actions that may result from the individual failures. The collective times represent the impact of the failure on the driver’s time. Since you do not have any data at this time for how long each of these steps would take, just estimate them by using Triangular distributions of time (see sidebar below).
This shows an estimate from a minimum of ½ hour to a maximum of 1 hour, with the mean being ¾ hour. If you do this for the other actions, you can now execute the model to determine the impacts on time.
Note, you could also accumulate costs by adding a related Cost entity to each of the actions. Simply create an overall cost entity (e.g., “Failure Costs” and then decompose it by the various costs of the repairs. Then you can assign the costs to the actions by using a Hierarchical Comparison matrix. Select the parent process action (“F Vehicle Failure Process”) and use the Open menu to select the comparison matrix (at bottom of the menu). Then you will see a sidebar that asks for the “Target Entity,” which is the “Failure Costs” you just created. Then select the “Target Relationship,” which is only one “incurs” between costs and actions, then push the blue “Generate” button to obtain the matrix. Select the intersections of the between the process steps and the costs. This creates the relationships in between the actions and the costs. The result is shown below.
If you have not already added the values of the costs, you can do it from this matrix. Just select one of the cost entities and its attributes show up on the sidebar (see below).
Note how you can add distributions here as well.
Finally, you want to see the results of the model. Execute the model using the discrete event and Monte Carlo Simulators. To access these simulators, just select “Simulate” from the Action Diagram for the main process (“F Vehicle Failure Process). You can see the results of a single discrete event simulation below. Note that the gray boxes mean that those actions were never executed. They represent the rarer failure mode of an engine failure (assume that you change your oil regularly or this would occur much more often).
To see the impact of many executions by using the Monte Carlo simulator. The results of this simulation for 1000 runs is shown below.
As a result, you can see that for about a year in operation, the owner of this vehicle can expect to spend an average of over $1560. However, you could spend as much as over $3750 in a bad year!
For more detailed analysis, you can use the “CSV Reports” to obtain the details of these runs.
 From http://www.weibull.com/hotwire/issue46/relbasics46.htm accessed 1/18/2017