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composite stress analysis, pt2 1

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rb1957

Aerospace
Apr 15, 2005
15,672
We're (well a supplier is) using Tsai-Hill to analyze their structure. So FEA gives them a FI (= applied stress/allowable).

To calculate MS I've used RF = allow/applied = 1/FI, MS = RF-1 = 1/FI-1

Supplier is saying it is MS = 1/sqrt(FI) -1 because of the nonlinearities of Tsai-Hill. If this is the case, then it's something like a bolt analysis (where tension and shear interact with different powers). Then Rt = applied/allowable is "like" an FI ... but I understood that FI was for all stress components ?

I understand that Tsai-Hill is probably not the favoured analysis for composites these days. Is it 'Rong (how I emphasise Wrong), or just not current ?
Is there a better analysis ? the structure is not primary structure, a radome

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.
 
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Tsai-Hill is rubbish, though not as bad as Tsai-Wu criteria. A lot depends on what lamina allowables are being input. And use with fabric material is generally less bad than with uni tape materials. See previous posts of mine on eng-tips, and the papers by Hinton et al 20+ years ago.

Yes, as I recall the supplier is correct, as TH uses stress^2 terms. Will have to check text books when I get back home in a week or so.
 
supplier says RF = 1/sqrt(FI) ... because of the squared terms. Ok, the interaction is complicated ... but goal seek the equation for RF ... why not ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.
 

rb1957's suggestion about goal seek is a good one, which is useful for any interaction equation, and in some cases is the only real option.

To explain further, if the applied stresses are sx, sy, sxy, multiply them by a common factor, call it k, and insert the products k*sx, k*sy, k*sxy into the interaction equation. Iterate k until the interaction equation = 1. Then MS = k - 1.

However, for some failure criteria, you can get an analytical solution for the MS by writing out the equations. In the specific case of the Tsai-Hill criteria, since all the terms are squared, you will find that a common term of k^2 factors out. You do not need to iterate in this case. Failure will be when k^2*FI = 1 (where FI is based on the original, unscaled stresses). Then the k value for failure will be 1/sqrt(FI), and therefore MS = k - 1 = 1/sqrt(FI) - 1.





 
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