Reliability HotWire: eMagazine for the Reliability Professional
Reliability HotWire

Issue 34, December 2003

Tool Tips

* Is there a quick way to assign stress profiles to data points in ALTA 6 PRO? 

Once you have attached the profile library to the current ALTA 6 PRO Data Folio, you can assign any of the stress profiles in the library to the data points in the current Data Entry Spreadsheet. First, select Cumulative Damage as the life-stress relationship. On the Set Analysis page of the Data Folio Control Panel, select Power or Exponential as the Cumulative Damage Relation.

In the Data Entry Spreadsheet, select which stress column you want to assign the time-varying stress profile to. Right-click one of the cells in the column. A list of the stress profiles from the attached stress profile library will appear, as shown next.

Assigning stress profiles

Select the stress profile you want to assign to the cell and click the Add button. You can also activate the Stress Profile Explorer by clicking the Stress Profile Explorer icon in the bottom right corner of the list.

Once a stress profile has been assigned, the stress profile name will appear in the selected cell in the stress column. Repeat this process to continue assigning stress profiles into the stress column. You can also copy one cell or drag the cell if all values in the column use the same profile. The assigned stress profiles will be analyzed when you calculate the parameters using the cumulative damage life-stress relationship.
 

* What do the parameters B and C represent in ALTA?

The parameters B and C are used by ALTA for the Arrhenius and temperature non-thermal life-stress relationships. The application returns values for these variables, along with the shape parameter for the selected distribution (beta for the Weibull distribution or standard deviation for the lognormal distribution). However, it is much more difficult to glean information about the test items from the B and C parameters than from the shape parameter. Essentially, B and C are just variables for which ALTA solves in order to determine the relationship between stress and life. Therefore, it is difficult to draw any conclusions about the behavior of the products based on the values of these parameters. However, you can get an indication of the nature of the relationship between life and stress from the parameter B. The sign of the B parameter indicates the direction in which the life-stress relationship lies. If B is positive, the life of the product decreases with increasing stress. If B is negative, the life of the product increases with increasing stress (an unusual, but by no means impossible, situation). Similarly, the magnitude of the B parameter can indicate how quickly the life of the product changes with stress. Values with larger magnitudes indicate that the life of the product is more sensitive to the stress. However, all of this is relative and one cannot draw any conclusions based on the value of B alone. The parameter C does not impart any useful information at all, it is merely a variable that ALTA solves for in order to complete the analysis.

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