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ALTA Standard Folio

ALTA Standard Folio Control Panel

Stress Profile Example

Time-Dependent Stress Profiles

Stress levels under test conditions and/or during normal use may vary with time. For example, an engineer may decide that each unit in a sample will be tested at 310 K for the first 10 hours of a test, 320 K for the next 10 hours, and 330 K for the remainder of the test. ALTA PRO allows you to define and save stress profiles that describe how stress levels will vary with time. When you select a cumulative damage model in the ALTA standard folio, all of the stress profiles validated in your project become available to describe use level stress conditions and stress conditions during testing.

Tip: The stress profile functionality is available only in ALTA PRO. If you are an ALTA Standard user who needs to use time-dependent stress profiles in your analysis, please contact ReliaSoft for information about upgrading to ALTA PRO.

This topic explains several tasks associated with time-dependent stress profiles, including:

Creating Stress Profiles

A stress profile is a representation of how stress levels will vary with time. It consists of a basic pattern that either repeats as a cycle or occurs once and then continues from its last defined stress setting. To create and save a stress profile, follow the steps below.

These segments are defined in the data sheet, as shown next.

Tip: Because stress units (e.g., volts) and time units (e.g., hours) are not defined in the stress profile, it is important to apply stress profiles only to folios that are intended to use the same stress and time units. In the Stress Profile window, you may want to rename column headers (by double-clicking inside the header) or use the Comments page as a reminder of which units are applicable to that profile. You can also create sub-folders under the Tools heading in the current project explorer to organize your profiles in a way that will make them easier to manage for future use.

For example, suppose the stress unit is psi and the time unit is seconds. Now suppose you wanted to define the following 120-second pattern: a stress value of 30 psi for 60 seconds, followed by a stress value of 50 psi for 30 seconds, followed by a stress function that begins at 50 psi and gradually decreases at a rate of 1 psi per second for the remainder of the pattern. For this pattern, you could fill out the data sheet as follows:

Notice that the last segment of this profile uses a stress function. Since t is the test time (or, if the profile is cyclical, the time since the pattern was last restarted), t = 90 when the last segment begins. So the last segment’s stress level starts at 50 - (90 - 90) = 50. Then, at 91 seconds, the stress level will have dropped to 50 - (91 - 90) = 49. After another second it will have dropped to 50 - (92 - 90) = 48, and so on.

If you select Continue from last stress, all times after the last segment will use the stress value/function defined in the last segment.

If you select Repeat cycle, the entire pattern of segments will be treated as a repeating cycle.

For example, suppose you defined a pattern that is made up of five segments, each an hour long and increasing stepwise from the segment before. The graphs below illustrate the difference between continuing from the last stress (left) and selecting to repeat (right). In this example, the test has a duration of 20 hours, and the graphs explain what would happen through the entire duration of the test.

Note: Clicking the Validate Stress Profile icon will not automatically update your stress vs. time plot. To make sure your plot reflects the most recent profile information, click the Plot icon.

Using Stress Profiles in the ALTA Standard Folio

There are two ways stress profiles can be used in the ALTA standard folio. First, they can be used to describe the time-varying stress conditions that sample units will experience during testing (as shown in the data sheet below). Second, they can be used to describe the time-varying stress conditions that the product will experience during normal use (as shown in the Use Stress Level window below).

Note: Some plots (e.g., the use level probability plot) are not available when you use a stress profile to describe the product's use level conditions.

Note: If you update a stress profile that is associated with an ALTA standard folio, the calculations will not be updated automatically. To update the ALTA standard folio calculations, click the Calculate icon on the control panel.

 

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