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Degradation analysis is a technique that uses the performance measurements (degradation measurements) of a product over time to predict the point at which each unit in the sample is expected to fail. This analysis is useful for tests performed on products with very high reliability, where it is not possible to test the products to failure under normal operating conditions.
The Weibull++ degradation analysis folio extrapolates the times at which the amount or level of degradation is serious enough that the unit is considered to be failed. In addition, the folio automatically performs life data analysis on the extrapolated failure times. This allows you to obtain the life characteristics of the product.
The ReliaWiki resources portal has more information on degradation analysis at: http://www.ReliaWiki.org/index.php/Degradation_Data_Analysis.
This topic discusses the following:
For an example of an analysis, see Example of a Weibull++ Degradation Analysis.
To add a Weibull++ degradation analysis folio to a project, choose Insert > Folios > Weibull++ Degradation or right-click the Specialized Folios folder in the current project explorer and choose Add Weibull++ Degradation on the shortcut menu.
The following example shows a degradation analysis of two automobile tires, A and B.
The data sheet contains the following information:
The Inspection Time column indicates the time at which the degradation of the unit was measured. Time may also be measured as the amount of usage the unit accumulates or as the factor that degrades the unit. For example, in the data sheet, the inspection time is in mileage because of its direct relation to the wear on the tires. The level of degradation may be measured either continuously or at predetermined intervals of time.
The Degradation column indicates the degradation measurement of a unit. In this example, the degradation is obtained by directly measuring the depth of the tire tread. In some cases, measuring the level of degradation is not possible without invasive techniques or destroying the specimen. In those situations, the degradation may be measured through a performance indicator, such as the luminosity of a bulb or resistance of a dielectric.
The Unit ID column indicates the unit being measured. The software extrapolates the point of failure for each unique unit based on its degradation measurements.
After you have entered the degradation data in the data sheet, the next step is to go to the control panel to select the settings for the analysis.
The degradation analysis folio control panel is arranged into sections of related tasks. Each group has its own page on the control panel.
Tip: There is a horizontal splitter bar directly above the Main button. If you drag it as far up as it will go, all of the pages of the panel will be accessed by large buttons. If you drag it all the way down, all of the pages will be accessed by small icons. Positions in between allow you to use some large buttons and some small icons. See Control Panels.
The Main page is probably where you will spend most of your time because it contains most of the tools you would need to analyze a data set:
The Degradation Model area allows you select the mathematical model to use for extrapolating the times at which the critical degradation level would be reached for each unique unit ID in the data sheet. The following is a list of all available models and their corresponding equations. The variable x represents the failure time, y represents the critical degradation, and variables a, b and c are the model parameters to be solved for in the extrapolation.
Linear: y = a*x+b
Exponential: y = b*exp(a*x)
Power: y = b*(x^a)
Logarithmic: y = a*ln(x)+b
Gompertz: y = a*b^(c^x)
Lloyd-Lipow: y = a-b/x
Tip: If you are not certain which degradation model would provide the best fit for your data, you can use the Degradation Model Wizard to guide you through the selection process.
The Critical Degradation field is for defining the level of degradation that is considered a failure. For example, you might wish to define failure as the time when the depth of the tire tread is less than 2 millimeters.
The Suspend After option allows you to define a censoring time past which no failure times are extrapolated. In practice, there is usually a rather narrow band in which the censoring time has any practical meaning. If the censoring time is relatively low, no failure times will be extrapolated, which defeats the purpose of the degradation analysis. On the other hand, a relatively high censoring time would occur after all of the theoretical failure times, thus rendering the censored data meaningless. Nevertheless, certain situations may arise in which it is beneficial to censor the degradation data. For example, if your product has a maximum warranty life of 10,000 hours, then you may want to treat all failures beyond that time as suspensions.
The Life Data Model area contains the settings for performing life data analysis on the failure times that were extrapolated via the degradation analysis. These settings have no effect on the degradation analysis calculations; they affect only the life data analysis calculations. The settings in this area are identical to the settings on the control panel of a Weibull++ standard folio, except for the following option:
The Use extrapolated intervals option allows you to extrapolate the failure/suspension times as interval censored data. The intervals are calculated as bounds around the extrapolated data points.
What's Changed? In previous versions of Weibull++, you used the Associate Specialized Folio tool to transfer the extrapolated data points to a standard folio data sheet in order to perform life data analysis. In Version 8, you can conveniently perform life data analysis calculations within the degradation analysis folio.
When you click the Calculate icon on the control panel, the software extrapolates the points at which the degradation measurements will reach the point of critical degradation. It then performs a separate life data analysis on the extrapolated points based on the settings you have specified in the Life Data Model area.
The Degradation Results area allows you to display the parameters of the chosen degradation model and the extrapolated failure/suspension times. Click the Degradation Results (...) button to open the Results window, which allows you to view, edit and print the calculations.
The Life Data Results area allows you to display the parameters of the life data model. Click the Life Data Results (...) button to open the Results window.
Note: If the distribution and parameters are associated with a published model, then the model's name will appear as a link at the bottom of the Analysis Summary area. Click the link to view the model's properties. For details on how to publish the results as a model, see Control Panel Publishing page.
The folio tools are arranged on the left side of the panel. Use these tools to manage data and experiment with the results of your analysis.
Calculate extrapolates
the points at which the degradation measurements will reach the point
of critical degradation (i.e., the times at which the degradation is serious
enough that the unit is considered to be failed). The software then performs
life data analysis calculations on the extrapolated data points. This
tool is also available by choosing Degradation
> Analysis > Calculate.
Plot
creates a new sheet in the folio that provides a choice of applicable
plot types. This includes the degradation vs. time plot in addition
to the life data analysis plots.
This tool is also available by choosing Degradation
> Analysis > Plot.
QCP
opens the Quick
Calculation Pad, which allows you to calculate life data analysis
metrics, identical to the calculations you can perform in a Weibull++
standard folio. This tool is also available by choosing Degradation
> Analysis > QCP.
Degradation Model Wizard
opens the Degradation Model Wizard,
which helps you select the degradation model that best fits your data.
This tool is also available by choosing Degradation
> Analysis > Model Wizard.
Distribution Wizard
opens the Distribution Wizard,
which helps you select the distribution that best fits the data set based
on the chosen parameter estimation method (i.e., RRX or RRY). In the degradation
folio, the ranking in the Distribution Wizard is based on the life data
model. This tool is also available by choosing Degradation
> Analysis > Distribution Wizard.
Change Units opens
the Change Units window, which allows you
to change the time units of an existing data sheet.
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