Please Assist with Homework Assignment on Fracture Mechanics!

Thank you for being honest. But friend, CR4 frowns on homework questions. What does your mentor say? What do you say about discussing among your peers/mates/seniors?

Joshi's right, we don't do homework.

Here's some background you might find helpful.

Introduction to Fracture Mechanics - MIT

Check also into hydrogen embrittlement.

Was this vessel Post Weld Heat Treated? I am (I have not read any answers yet) betting on hydrogen stress cracking caused by improper or lack of PWHT.

Didn't read the question, either, did you?

It's homework.

It should have been heat treated but no records or any other evidences are available.... So one may assume that there was no PWHT. Do you think it may be one of the causes of the failure?

This sounds like a real vessel, rather than homework. It is not a run-of-the-mill ammonia refrigeration vessel; the pressure and wall thickness are much higher. What is the full range of pressure and temperature conditions in the vessel? The Charpy requirement seems odd for 26°C. The wall thickness does suggest PWHT, as already noted.

Even if the thickness did not require PWHT the sevice would require it.

Sounds like a vessel for ammonia synthesis, Haber or similar process. Similar because pressure calculates to ~ 350 bar which is bit high for Haber, unless I'm out of date. Though according to Wiki the temperature used for ammonia synthesis is 300 - 550°C, not 26°.

But working pressure 285 MPa seems very high for a vessel containing compressed gas, even at 26°C. From memory, usual figure for low-alloy steel is ~ 170 MPa. If it was a real failure, I wonder how many people were killed?

Gents, any ideas? Or maybe it is difficult task?

There are many hints, scattered on this thread, if only you could apply your mind, you may go places.

Several suggestions have been made. Have you followed them up? What's the story behind the temp/pressure? Is it on ammonia synthesis, maybe upstream of the main reaction vessel?

It might be worth reading a book by Prof J.E.Gordon called The new science of strong materials. It's unlike any other book I've read about strength of materials. It might help and in any case it's an interesting read. ISBN 0-14-013597-9.