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Accelerated life testing

In the quest for developing products that interact effectively to the design intent, with other components, sub systems and systems, delivering reliability and durability, we go to great lengths to assess product viability.

A typical design life curve depicts early life failure, reliability and durability zones, with a clear target and expectation of reaching wear-out distributions during the durability period.

We constantly try to find methods of assessing capability and risk, so the purpose of testing, computer modeling such as finite element analysis, multi physics engineering analysis, probability studies, Failure Modes Affects Analysis (FMEA), Fault tree analysis, (FTA) are to generate and evaluate information on that designed capability, so that data informed decisions can be made.

The problem with testing is that the time duration of normal life stress exposure testing over extended time is a luxury that can be rarely afforded, as businesses strive to be first to market, bringing innovation which provides that competitive edge and attracts customers to the product.

In terms of testing of components, there are literally endless opportunities to develop assessment of features, properties and interactions. However, multiple feature interactions rarely exhibit linearity, especially when considering complex materials and compounds often used in electrical machine design. The ability to isolate one feature, property or interaction, apply the appropriate stress, and develop a solution that is both predictable and repeatable, whilst maintaining all other features, is sometime difficult (and expensive) to manage.

If we consider designing a product for 40,000 hours life expectancy, to prove reliability we would need to test 24/7 for approximately 4.5 years. As you can appreciate this is impractical and cost prohibitive.

In an effort to reduce longevity of normal life stress testing and speed strong data based decision making, accelerated life test methods have been developed for component life tests with components operated at high or extended stresses and failure data observed.

While high stress testing can be performed for the sole purpose of seeing where and how failures occur and using that information to improve component designs, such as selection of alternate material strength properties or to make better component feature selections, accelerated life testing can be utilised for the following two purposes:

' To study how failure is accelerated by stress and fit an acceleration model to data from multiple stress cells running in parallel.

' To obtain enough failure data at high stress to accurately project (extrapolate) what the component life at use will be.

' Both provide models on which model or range assessment can be reliably made, without the need to repeat testing infinitely!

Of course insulation systems are important part of the validation requirements for rotating electrical machines, and Cummins Generator Technologies carry out a range of accelerated life evaluation on other systems, sub systems and components, for example in considering our design application of bearing assemblies.

Test planning and operation for a (multiple) stress cell life test experiment usually consists of the following:

' Picking several combinations of the relevant stresses (the stresses that accelerate the failure mechanism under investigation). Each combination is a "stress cell". Note that we plan for only one mechanism of failure at a time. Failures on test due to any other mechanism will be considered censored run times.

' Making sure stress levels used are not too high - to the point where new failure mechanisms that would never occur at use stress are introduced. Picking a maximum allowable stress level requires experience and/or good engineering judgment.

' Placing random samples of components in each stress cell and running the components in each cell for fixed (but possibly different) lengths of time.

' Gathering the failure data from each cell and using the data to fit an acceleration model and a life distribution model and using these models to project reliability at use stress conditions.

A word of warning, when you test components in a stress cell for a fixed length test, it is typical that some (or possibly many) of the components end the test without failing. This is the censoring problem, and it greatly complicates experimental design, which can be tough to manage in statistical science.

Where do we use accelerated life testing'
Commonly, electrical machines, which rely upon the properties of insulating materials to manage electrical charge stress, use Accelerated life testing, to predict insulation components or system life.

If we think about insulation materials, there are a number of factors which might be considered as accelerators in degrading the electrical insulation properties.

Let's take for example, coil winding insulation system components, such as Nomex' insulation paper, manufactured by DuPont, or copper wire coatings, for example. When combined into sub systems, or systems, evaluation of the combination of materials through accelerated life testing helps us understand material interactions to operational stress, for example environmental stress, or mechanical stress. Ultimately, the accelerated life evaluation of the system enables life predictions to support design life evaluation and capability confirmation.