How to calculate fatigue life in the creep realm?

[wallymct]

I am looking for some information on how to calculate fatigue life for materials at temperatures elevated into the creep realm. ASME Sec 8 Div 2 describes how to do this for carbon steel below 700F and stainless below 800F, but I am interested in temperatures > 1000F. I am performing an FEA and I have some high peak stresses (hot spots) that I do not how to deal with. Thanks.

 

[Pkruse]

What is the application?

 

[wallymct]

Pressure Vessel but the peak stresses are not on the vessel boundary wall. The peak stresses are from discontinuities on stiffener plates.

 

[Pkruse]

I've never gotten into fatigue or creep on a boiler code application. Mine are generally gas turbine and jet engine applications. We have a whole group of analysis folks who specialize in that, but we mechanical design engineers must get into the ball park before we can produce a design for them to analyze. We use a massive amount of proprietary material test data that was gathered at great cost over the last several decades. I'd love to see someone more knowledgeable than me speak up concerning boiler code applications.

 

[wallymct]

I see these very localized peek stresses "waved off" a lot (wouldn't be good practice for jet engines I imagine). Maybe the data to do the fatigue analysis is not generally available.

 

[Pkruse]

For decades, it was assumed that so long as we did not exceed yield at any K_T location, we did not have a fatigue limit. But I've heard that some of the larger engine manufacturers have now spent enough time and money collecting enough data to know that is not entirely true; but I've never had access to this data.

But the fact of the matter is that every engine flying has places on critical components operating at or above yield. (Naturally, "above yield" is not entirely true because the metal will yield and redistribute the loads so as to not exceed yield.) So every engine flying will have a fatique life and a creep life, both of which can be seriously shortened depending on the actual load cycle the pilots apply to the engines. A single 50 degree F over temp event can cut these numbers by half or more. (Keep in mind that max turbine temps are typically several hundred degrees higher than the melting temperatures of the alloys they make the engine out of.) Required overhaul periods are planned around these limits, and actual cycles are tracked carefully. They also do periodic bore scope inspections to determine that no cracks exceed their critial acceptable length.

A really exciting new technology will monitor an engine full time the entire time the engine is operating. It knows the condition of every blade during every revolution of the engine, and can sound alarms when micro cracks so the engine can be shut down for an overhaul before it explodes. So far, these are only being put on industrial gas turbines because the fly boys don't like the extra weight. But my opinion is that it will soon find itself into the aviation market. So if the calculations say to overhaul the engine at 5000 hours, real time condition monitoring might let you get much more time between expensive overhauls.

 

[wallymct]

It is amazing that there is technology that can detect micro fractures and conditions in real time. In the pressure vessel world it seems that we build with multiple levels of safety factors and end up with some very over designed systems (of course weight is no an issue for us). I would think that with the advancements of FEA tools, we would start to build with a little less conservatism, but that does not seem to be the trend. Maybe if the cost of materials increases this will change. I know that there are some methods for determining creep fatigue, but they are nonlinear and are strain based.

 

[Pkruse]

Adding weight to something that gets very hot and spins at 10,000 rpm is never a good thing. We try to avoid doing any more of that then we absolutely have to.

 

[AlephZero]

It is amazing that there is technology that can detect micro fractures and conditions in real time.

It doesn't take any significant extra weight to use a computer to "listen" to the vibration noise you are collecting anyway, if only to drive the cockpit vibration level indicators. The clever bit is figuring out what to listen for.

This technology is already flying on commercial aircraft in service.

I would think that with the advancements of FEA tools, we would start to build with a little less conservatism, but that does not seem to be the trend.

Less conservatism $\ne$ more risk - provided you understand the sitation properly and you can control the relevant variables. That's one motivation for using advanced materials technology like turbine blades made from a single metallic crystal - everything you thought you knew about cracks propagating from grain boundaries doesn't apply when there are no grain boundaries any more.