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  1. sushi75

    Fatigue and crack propagation life

    Hello, Looks like it becomes a hot topic! So just to make I got the point, let me recap your answers: -SN will predict life to failure at a given stress, without an initial flaw. As it's based on tests, a scatter factor shall be applied at the end to account for variations. If required, I...
  2. sushi75

    Fatigue and crack propagation life

    Ok so we have also da/dN curves which predict crack growth rate, for different samples, and mode I II III. But, if the SN curve predicts life up to critical flaw size, then the crack becomes unstable and leads immediately to failure, right? I'm still confused sorry! I believe the best answer...
  3. sushi75

    Fatigue and crack propagation life

    Thanks for your reply, it helps! So from your explanation, the SN curve provides the life from "a 0mm crack" to failure, found from test. However, what we do in crack prop analysis is to assume an existing flaw from NDI capability, and then find how ,any cycles it will take to failure. This...
  4. sushi75

    Fatigue and crack propagation life

    Hello! I'm a bit puzzled about fatigue and crack propagation. Given a SN curve, this one will predict for a specific level of stress what number of cycles are required to FAILURE, let's say 10e6 cycles. Assuming an initial flaw of 1mm, and using Paris' law, a crack propagation analysis by...
  5. sushi75

    Stiffness of shell vs plate

    Thanks all!! So BAretired, when you mention that shell resist applied load primarily by membrane stresses, can you elaborate the mechanism occuring? this is probably the point I did not manage to understand :-) Cheers
  6. sushi75

    Yield Criteria

    Thanks for your reply, it's much clearer now! Just another quick question, in case I have high compressive stresses for the max principal, should it considered also critical? Is Yield Strength in compression and traction the same? Cheer!
  7. sushi75

    Stiffness of shell vs plate

    Hi, thanks for your quick reply. Actually the question is not necessarily related to FE, it's more a matter of structural mechanics and try to understand and be able to explain with word, why physically a shell surface will be stiffer than the same flat structure. I can sort of picture it from...
  8. sushi75

    Stiffness of shell vs plate

    Hello! I try to understand why a shell is stiffer than a plate. I've read a note on the internet explaining that moving a point out of the surface of a thin shell like an egg takes more energy as the adjacent fibers have to be stretched, unlike a plate. But it does not make much sense, and I...
  9. sushi75

    Sine Environment

    Thanks a lot for this first reply, it's helpful already! I dealing with spacecrafts, so yes it makes sense now to avoid the sine environment if it's related to the launch vehicle frequencies. As you know, vibrations and dynamics is essential for this industry, and I'm just starting, but it's a...
  10. sushi75

    Buckling Imperfection Factor

    Hi TSG4, Thanks a lot for these guidance. So I've followed your advice, run t a buckling analysis for a spehrical cap under pressure. I've got a BLF of 10 and the shape of the first mode. it's not really the one I expected at it shows a direction opposite to the load. So from that, I will...
  11. sushi75

    Ti-6Al-4V Heat Treatment

    Hello! I'm currently working on a structure made of Titanium, Ti-6Al-4V, and it is heat treated to improve mechanical properties. So I''ve been through a lot of books, documents, internet to get a better understanding of the heat treatments, and now I'm a bit confused. So I need some help to...
  12. sushi75

    Buckling Imperfection Factor

    Hi TGS4, I had a look at the ASME design rule but i'm still a bit puzzled about the rationale I should take. I'm using the NASA handbook 8032 for doubly curved shell. So what I've got assess is the hemispherical cap under pressure (imagine a bottle with the cylinder ending with a curved...
  13. sushi75

    Yield Criteria

    Hello, Thanks all for your replies! The material is ductile so the VM criterion makes sense. it's still a bit confusing to see that in this analysis (pressure vessel), there are peak hoop stress slightly higher than yield, but that we consider the VM for compliance. metengr, I don't fully...
  14. sushi75

    Modelling a weld

    Hello! I would like to assess the stress level at the location of a weld. Basically this is a tube into a solid; so the interface will be the surface contact which is a cylinder in that case. I'm aware that there is a connector menu in femap which has the option of weld, but i don't think I...
  15. sushi75

    Buckling Imperfection Factor

    Thanks TGS4, I was not aware of ASME design rules, so I'll have a look I guess there will be relevant information for this buckling assessment!! From your experience, what range of KDF do you expect? 2-3 ish?
  16. sushi75

    Yield Criteria

    Hello! I'm running a FE stress analysis, and checking the maximum Von Mises stresses, the values remain under the yield strength. However, looking at the maximum principal stress, there are values above the Yield Strength. So I'm a bit puzzled, according to VM there will be no yielding but...
  17. sushi75

    Buckling Imperfection Factor

    Hello, I've started a thread in another forum, but following advice of one of the member I post it here as my analysis is applicable to a pressure vessel. So here is the summary of the problem, hope someone can give me guidances and good information to progress!! Thanks! I'm currently going...
  18. sushi75

    Sine Environment

    Hi all, I'm going through dynamic analysis, and I've got a simple question: why is it so important to have the structure outside the sine environment?? If the first natural frequency is outside this range, let's say above 100 Hz, it doesn't mean that it will not be met. Hope someone can...
  19. sushi75

    Buckling Imperfection Factor

    Hello, Thanks for your quick feedback, and the information provided. TGS4, the structure is a pressure vessel, so I believe it's worth following your strategy. I'm not aware of the 0.55*Sy as a limit, what is it based on? So based on the mode shape, How do you perform the non linear...
  20. sushi75

    Buckling Imperfection Factor

    Hello! I'm currently going through buckling analysis, for a hemispheric shell. So it a basically a portion of a sphere. It is subjected to a pressure load on the external surface, therefore it can buckle. From handbooks I could calculate the thickness, radius and angle using handcalcs. But I...

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