Test Design Folio

In a zero-failure reliability demonstration test (RDT), the engineer aims to demonstrate a specified target metric (e.g., an MTBF of at least 500 hours with 90% confidence) by testing a specified number of systems for a predetermined time. If no failures occur, then the target metric is demonstrated. This method has been adapted for scenarios where the target metric can be demonstrated even if some failures occur, so long as a specified number of allowable failures is not exceeded. For example, in a demonstration test where the number of allowable failures is 2, the target metric is demonstrated if no more than 2 failures occur during the test.

The test design folio can assist you in using the NHPP model to design a demonstration test for repairable systems by solving for either the test time required per system or the number of systems that must be tested. To create a test design folio, choose Home > Insert > Test Design.

IMPORTANT: The test design folio in RGA (known as the "Repairable Systems - Design of Reliability Tests" tool in prior versions of the software) is based on the non-homogeneous Poisson process (NHPP), so it is suitable only for designing tests involving repairable systems. For tests involving non-repairable items, use the RDT utility in Weibull++, which provides four test design methods that are suitable for non-repairable systems: parametric binomial, non-parametric binomial, exponential chi-squared and non-parametric Bayesian.

Follow the steps outlined next to use the utility:

In the Calculation Options area on the control panel, specify the Units of measurement for the calculation. Note that "Assumes Beta = 1" is displayed in this area, which means that the utility assumes the failure intensity is constant during the test.
Select the Display systems as integers option on the control panel if you will be solving for the number of systems to test and want to round the calculation up to the nearest whole number (e.g., any number greater than 4 would both be displayed as 5).
In the What metric would you like to demonstrate? area on the RDT sheet, specify the metric that you intend to demonstrate with the test you are designing.
- In the Metric drop-down list, specify whether you will demonstrate the system's MTBF or failure intensity.
- The Demonstrate this [metric] field allows you specify the value of the metric you wish to demonstrate. For example, if you wish to demonstrate an MTBF of 1,000 hours, you would enter 1,000 in this field.
- The With this confidence level (%) field allows you to specify, as a percentage, the confidence level at which the target metric will be demonstrated. For example, if you wish to demonstrate with 90% confidence that the MTBF is at least 1,000 hours, enter 90 in this field.
In the Solve for this value area, specify what value you are using the RDT utility to solve for. The Value drop-down list allows you to choose whether you are solving for the test time (per system) that will be required to demonstrate the target metric, or the number of systems that you'll need to test.
- If you select Required test time, you'll enter the number of systems that will be tested in the next input field.
- If you select Required number of systems, you'll enter the test time in the next input field.

In the last input field, enter the maximum number of failures that you will allow to occur during the test. For example, if you want to demonstrate the target metric with 2 failures or less (e.g. two systems fail once, but no other failures occur), then you would enter 2 in this field.

After you have made all of the required inputs, click the Calculate icon to solve for the specified value. To examine other possible test scenarios, you can also create a table and plot based on the inputs.

See Repairable Systems RDT Example.

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