Brittle Fracture

[meca]

I have a question with regard to brittle fracture and design of steel stacks. In pressure vessels we must consider the minimum design metal temperature and determine if the material selected is suitable for this temperature so that brittle fracture will not occur. However, my understanding of brittle fracture is that it only occurs as a result of primary membrane stress. Since the stresses on stacks are usually secondary bending stresses, is brittle fracture a concern?

 

The key consideration is whether the component is in tension, if it is then brittle fracture is a concern.

 

[CoryPad]

Brittle fracture is a concern with bending stresses - the tensile stress induced by the moment can cause brittle fracture if certain conditions exist (e.g. temperature).

 

[meca]

CoryPad,

Can brittle Fracture Occur due to tensile stress that is a result of bending? My understanding was that you must have primary tensile stress, such as hoop stress created by internal pressure.

Thanks,

Chris

 

[CoryPad]

Chris,

Yes, you can have brittle fracture occur essentially anywhere there is a tensile stress, including the tension side of a beam in bending.

 

[meca]

CoryPad,

Thanks for the help.

What code or standard do you use to determine if the material is acceptable for a given temperature? I have been using Fig UCS-66 in ASME Sec. VIII, Div. 1 to establish an MDMT. However, in Fig UCS-66.1 you can further reduce the MDMT if your design stress is less than the allowable. Do you believe that this is can be applied to structural steel applications, or do you know of a better method?

Thanks again,

Chris

 

[Mechineer]

Look in a materials handbook to see if it gives a ductile to brittle transition temperature for the material you are using.

 

[Biggadike]

I would suggest Linear Elastic Fracture Mechanics as a method of determining the mode of failure for your stacks.
You will need the stress intensity factor (K) for the steel you are using.
This method will allow you to determine a number of useful things:

1) The largest flaw your vessle can contain without failure at its point of maximum tensile stress.
2) The mode of failure (Brittle fracture of ductile collapse)

You need to be very concerned with the state of your steel after welding (if relavent)

In answer to your question, large sections of steel are more likely to fail in a brittle mode as they undergo plane strain in their centres under tension. Bending moments also give rise to tension on one surface (obviously) and will fail in a brittle but more complicated way if the section is too large or the material is too brittle.
Also, large sections are more likely to contain flaws and residual stresses which will superimpose on your working stresses.