ASME STP-PT-080:2016 pdf free download

ASME STP-PT-080:2016 pdf free download

1.7 Limitations of the Non-Linear Combination Model (NLCM) for Creep
It is the opinion of the authors that the question about which model provides the best lit to the data is beyond the expertise of any individual contributor, here we present the data and methods, and remain open to criticism. Consideration should not only be given to the quality of the lit for thc NIMS data but the qua1iE of fit of other data sets, the robustness of the data (when it comes to extrapolation), the simplicity of expression, and, most importantly, the appropriateness of the model for its intended application. This requires the free and open exchange of ideas and the participation of engineers with diflrcnt skills and backgrounds which wc welcome.
One key limitation of the model comes in the description of the tertiary parameters. Outside the range of the data, the extrapolations appear to be suspect. This applicable stress range is roughly 30 MPa to 400 MPa (4,5 ksi to 60 ksi). A second limitation is that the parameters fbr the primary creep description are built entirely upon the NIMS data set. The results were compared against the Ellis data, but we did not attempt to find the best expression to fit both Ellis and NIMS data. Third. this data were digitized from scans, and though the accuracy of the digitization process is good, it does introduce small errors.
Though not limitations of the model per se other points deserve more consideration.
It is recognized that the NLCM has quite a bit of added complexity. It is hoped that the explanation in this report of the F-parameter provides a sufficient explanation as to why this form (equation 1.35) was chosen. Whether the Larson-Miller expression is ‘best” for the various parameters is subject to debate. Some small effort was made to explore the use of alternative parametric forms and stress ftinctions beyond what has been presented here, but this avenue has not been fully explored.
Finally, the approach presented here was to build the method on the tertiary creep parameters and adjust the primary creep parameters to produce as accurate of rupture times as possible. This was done because the primary creep parameters appeared to be more problematic. However, in comparing the model with the I3PV Ill-Nil isochronous curves it appears that the NI-I curves have more strain at high temperatures and low stresses. Starling with the primary creep parameters and then adjusting the tertiary creep parameters may provide a different fit in this important region of the curve. Further investigation is recommended.