Why Does MDMT Increase with Nominal Wall Thickness? 1

[RefineryMechEng]

In UCS-66 of the ASME BPVC SEC VIII DIV I, the figure used to determine the lowest MDMT without impact testing says that the MDMT without impact testing increases with increasing nominal wall thickness. What is the physics behind this? Why would a thinner wall pressure vessel be less susceptible to brittle fracture at low temperature than a thicker one of the same material?

 

[gr2vessels]

Because the thin wall is more flexible.
cheers,
gr2vessels

 

[SJones]

'Flexibility' is not quite the term. What gr2 should have said is "because thickness and constraint are directly related with increased constraint leading to a lower fracture toughness as a result of the reduced ability to plastically deform material at a crack tip". The code then attempts to counter this phenomenon by using an increased minimum temperature limit.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdoweb/

 

[metengr]

More simply, it is a matter of plane stress versus plane strain conditions in service. If a component thickness is reached where plane strain dominates, this stress condition results in enough lateral restraint to increase susceptibility to brittle fracture. That is it.

 

[RefineryMechEng]

Thanks for the replies!