<p>As pressure ratios and firing temperatures continue to rise, creep becomes of greater concern everywhere within a gas turbine engine.&nbsp; As a rule of thumb, just a 14C increase in metal temperature can halve the expected rupture life of a part.&nbsp; In the past, companies might get by with conservative estimates based on Larson-Miller-Parameter curves and 1D calculations. But now companies need functional implicit-creep models with finite element analysis for an ever-increasing number of materials.&nbsp; Obtaining adequate test data to create a good creep prediction model is an expensive and time-consuming proposition.&nbsp; Test costs depend on temperature, material and location, but a single, 10,000hr, rupture test may reasonably be expected to cost &gt;$20,000.&nbsp; Other than large OEMs, most companies and individuals lack the resources to create creep models from their own data.&nbsp; This paper will lead the reader through the creation of a modified theta projection creep model of Haynes 282, a high-temperature, combustion alloy, using only literature data.&nbsp; First, literature data is collected and reviewed.&nbsp; Data consists of very few complete curves, estimated stresses for rupture and 1% strain, and discrete times to individual strains for individual tests.&nbsp; When adequate data exists, individual tests are fit to theta projection model curves.&nbsp; These “local” theta fit of different test conditions are used as input for the "global" model.&nbsp; Global fits of theta parameters, as a function of stress and temperature, are made of the full data set.&nbsp; As the global creep model is improved, correction factors are used to account for true stress and strain effects.&nbsp; A statistical analysis is made of actual rupture time versus predicted onset of failure, theta5=1.&nbsp; A scatter factor is generated to determine the required temperature margin required to reduce risk.&nbsp; After the creep model was completed, Haynes International, the material inventor, provided specific test conditions (stress and temperature) of tests that had already been run.&nbsp; Creep predictions were generated for these test conditions, before viewing the actual results.&nbsp; The creep model predicted strain curves matched actual tests very well, both in shape and time to rupture.&nbsp; As more data is available, the model may be refined again.</p><p>&nbsp;</p>

Developing an Implicit Creep Model From Open Literature Data

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2021/volume/84898

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