F.E.A resuts

[Lazer]

Hi all, we paid £2000 for an stress analysis on a seat frame, the frame was made of HR4 mild steel material, a load of 200kg was applied and the report came back a with a failed result, the welding would rip away parent matal and the frame would buckle.

 

Thinking this report was rubbish we carried out our own test, we built the frame and bolted it to a steel frame the put 200kg of weights of it, guess what, it did not break, so we upped the weights to 320kg still did not break.

 

We phoned the F.E.A people and asked them to explain why the test said it failed and they said they had to test to the min yield strength of HR4 mild steel that is 170mp and they said at 170mp it would fail but the reason our frame passed was because we must have used steel with over 250mp.

 

I bought HR4 and tested it and it passed, so what he is saying is if I buy it another day it may fail? If thats the case then all tests are worthless?

[PS_Port]

hey Lazer,

 

250MPa grade steel is the ultimate strength of the steel, commonly used however these days you can get 300, 350.

 

When designing steel members (Using limit state) The yield limit of the steel is basically 0.66 of the ultimate strength, here in OZ for 250MPa we can use 165MPa, this is the elastic yield limit of the steel, not the ultimate of the plastic range. Note that limit state design has been replaced with Ultimate state, where the loads are factored up, and you use the ultimate 250MPa.

 

So instead of failing maybe they ment to say you design would exceed the 'designs allowable stress'.