Reliability HotWire

Issue 83, January 2008

Tool Tips

* How can I use Stress Profiles in ALTA PRO's Monte Carlo and SimuMatic utilities?

[Editor's Note: This article has been updated since its original publication to reflect a more recent version of the software interface.]

ALTA PRO offers the cumulative damage life-stress model, which allows you to analyze data with up to eight stress levels that vary with time. This model is used in conjunction with Stress Profiles, which describe the relationship of stress to time.

Like all life-stress models in ALTA, the cumulative damage model can also be used in the Monte Carlo utility, which allows you to generate data based on the life-stress model, distribution, parameters, and stress levels that you specify, or in the SimuMatic utility, which allows you to perform a large number of reliability analyses on data sets that have been created using Monte Carlo simulation.

To use this option in either utility, simply select a Cumulative Damage (CD) model from the Model drop-down list on the Main page, then specify a number from 1 to 8 in the Number of Stress Columns drop-down list. To apply Stress Profiles to the points that will be generated, right-click the cells in Stress columns in the spreadsheet area of the Monte Carlo or SimuMatic window and select from the available stress profiles, as shown next.

Note that the Stress Profiles field, which appears when the cumulative damage model is selected, allows you to manage the Stress Profiles available in the project without exiting the Monte Carlo or SimuMatic utility. The Stress Profile icon beside the field allows you to edit the Stress Profile currently selected in the drop-down list or, if Add New is selected, create a new Stress Profile and add it to the drop-down list.

For data generated by the Monte Carlo utility, once the Standard Folio containing the data has been created, you can also use a Stress Profile for the use stress value for any stress by clicking the Set Use Stress link on the Main page of the Standard Folio Control Panel. In the Use Stress Level window that appears, select the Profile checkbox and then select a Stress Profile from the drop-down list, as shown next.

* Why dont my results exactly match the ones in the BlockSim and RENO training guides, even when I use a "seed" for the simulation?

As you may be aware, results obtained through repeated simulations of the same BlockSim diagram or RENO flowchart will exhibit some variability. The use of a seed in simulation forces the software to use the same sequence of random numbers in each simulation, resulting in repeatability. However, you need to be aware that when you use a seed, the same stream of random numbers is utilized for all simulations that begin with the same seed, but the order of the utilizations of the stream will depend on the order in which objects were added to the diagram or flowchart. The random numbers are assigned based on an internal ID number assigned upon creation to each block, in a BlockSim diagram, or construct/definition, in a RENO flowchart. Note that you can select to display these internal IDs in the diagram/flowchart by selecting the Show Block ID option on the Appearance page of the Diagram/Flowchart Options window.

This means that two identical diagrams/flowcharts that are simulated with the same seed may return different results if the objects in both diagrams/flowcharts were not added in exactly the same order. For example, two people may have BlockSim projects containing identical diagrams using blocks A and B. If A was created before B in one diagram, A will take the first random value from the stream and B will take the second. If the other person created the same diagram but created B before A, B will take the first random value from the stream and A will take the second, thus yielding potentially different answers, even though both persons may have used the same seed in simulation. Note that this type of variability should be expected in analyses performed using simulation and that, as the number of simulations increases, this difference will be less significant.

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