Life Prediction Methodology Allison

Life Prediction Methodology - D. L. Vaccari P, K. Khandelwal (Allison Gas Turbine Division of General Motors Corporation) 

Oxidation - Oxidation of a ceramic Is taken to be a surface effect governed by the parabolic diffusion law and a thermal activation process of constant activation energy, with an Arrhenius type of temperature dependence. By the assuming the Griffith flaw size to be proportional to the surface oxide thickness, we can structure the 

As currently planned, a number of specimens W M be oxidized at a selected reference temperature and time. Room temperature, four point bend tests allow the development of the two Weibull PIA surface constants within CARES. The material constant related to activation energy is obtained by the regression analysis of room temperature fracture tests of specimens exposed to a variety of temperatures and times. In this regression, data is examined by CARES to determine if the data contains outliers (i.e. data whose normalized residual exceeds the critical value at the luX significance level). The outliers are deleted from the data set prior to computing the activation energy constant. 

One additional piece of material data required in this model is a table of critical stress intensity factors at room temperature and the component temperatures. This is currently used to determine the Neibull scale factor at temperature from room temperature data. Once the material constants are available, a component survivability may be determined at as many as six mission times. 

Creep - The creep model used in CARES combines the classical Norton isothermal power law with a thermal process of constant activation energy showing the Arrhenius type of temperature dependence. Material creep data is determined by a log linear regression analysis of specimen steady state creep data determined at a variety of temperatures and stress levels. Similar to the oxidation analysis, data is examined by CARES to determine if it contains outliers. Any outliers are deleted prior to the final determination of the three material constants. 

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