Blade containment - simplified analytical approach

[FEAnalyst]

TL;DR

What is the simplified analytical approach to blade containment problem?

Hi,
one of the most interesting experimental tests performed for rotating machinery (such as gas turbines) is blade containment test - if the blade detaches from the hub, it can't break through the cover of the turbine because it could result in catastrophic damage (especially in case of airplanes). Apart from physical experiments, such tests are very often performed with FEA. However, I wonder if there's a way to perform some simplified analytical calculations (and determine whether the blade will be able to break through the cover or not) before proceeding to FEA or in order to confirm the correctness of numerical analysis. What I've found so far is approach based on the kinetic energy of the blade after detachment. The formula I've found in some scientific paper is: $$E_{K}=\frac{1}{2}m (\omega \cdot r)^{2}$$ where: $m$ - blade mass, $\omega$ - angular velocity, $r$ - radius. Is this formula correct for a situation when body suddenly switches from rotational to translation motion? What to do next? And is it possible to account for the deformation of the blade (which is significant in this case)?

Thanks in advance for your help

 

[jrmichler]

I found some good sources using search term jet engine blade containment. From one of the hits:

I had looked into a similar problem some years ago. That problem in involved orbital log saws coming apart. A chunk of steel weighing over 100 lbs coming off a 24 inch radius orbit arm running 300 RPM makes a serious dent in the steel guardhouse. In our case, I calculated that it would not penetrate a 12 gauge steel inner wall, provided that the inner wall was properly supported around the edges. The analysis involved calculating force vs dent distance, and integrating until the total work was greater than the worst case kinetic energy of the flying parts.

Brochure of the log saw mentioned above: https://catalog.bretting.com/item/saws/24-inch-orbital-log-saw/1058. I headed the team that developed that saw, the conveyor system, and the guard house.

Bottom line: No simple solution for high speed turbine blades.

 

[FEAnalyst]

Thanks for reply. Can you say more about the approach you used for log saws (some equations, sources)?

 

[anorlunda]

Building 273 at GE's Schenectady works had a turbine overspeed test stand. It had a steel inner liner, then 6 feet of railroad ties, and a steel outer liner. It did not always succeed in containing broken blades. Legend said that one engineer was killed while sitting at his desk as a blade fell from the sky and hit him.

 

[Lnewqban]

Then, we had accident of flight 1380 of SW Airlines.

Please, see:
https://en.m.wikipedia.org/wiki/Southwest_Airlines_Flight_1380

“On November 19, 2019, following the aforementioned hearing, the NTSB released the final report on the accident.[2] the probable cause reads:

The National Transportation Safety Board (NTSB) determines that the probable cause of this accident was a low-cycle fatigue crack in the dovetail of fan blade No. 13, which resulted in the fan blade separating in flight and impacting the engine fan case at a location that was critical to the structural integrity and performance of the fan cowl structure. This impact led to the in-flight separation of fan cowl components, including the inboard fan cowl aft latch keeper, which struck the fuselage near a cabin window and caused the window to depart from the airplane, the cabin to rapidly depressurize, and the passenger fatality.”