Impact Test Exemptions, VIII/1

[wsmith22]

Per UCS-66, why doesn't doesn't the actual part thickness matter when exemption materials? For example a 3/4" shell welded to a 2" flat head. To exempt the head, the governing thickness is the thinner of the 2 parts.

 

[CodeRef]

Your example is for determining the governing thickness when joining two parts, or as stated in the Code, " the governing thickness of a welded part " The thinner of the two parts would have a thickness equal to the weld, whereas the thicker part would not. Ref. UCS-66(a)(1)

If you want to apply to a non welded part, you need to apply the rules for governing thickness of non welded parts or materials as you put it, which are different.

Read UCS-66(a)(1), (2), and (3) because there are different criteria for each and each shall be treated as separate components.




There are three kinds of people in this world; those who can count and those who can't.

 

[CodeRef]

I'll add this.....to address your example of a 3/4" shell welded to a flat head, and applying UCS-66(a); the shell and the flat head are separate components and are to be evaluated individually. See Appendix L Ref. L-9.3 Governing Thickness for Corner Joints /Lap Welds.

When the vessel design is complete, a review of all of the impact test evaluation results from every componenet will reveal the warmest value for all governing thicknesses. This will be your vessel MDMT.

As to why the governing thicknesses for welded and non welded parts are what they are? I don't know, I only enforce the requirements. I hope some engineer can shed light on the answer you need.

There are three kinds of people in this world; those who can count and those who can't.

 

[PVinspector]

Also, a flat head is figured with primary bending stress instead of primary membrane stress and the Code usually allows higher bending stresses therefore a thicker plate would be exempt from impact.

 

[eliebl]

The governing thickness of a component in bending (flat head for example) is the nominal thickness divided by 4.

I believe that the concern is more the weld and the weld HAZ as opposed to the base material. The thinking is that the base material will have a MDMT given by one of the curves in Fig UCS 66. The thinner of the two parts will be the more highly stressed, and if a thick shell is attached to a thin shell, the high stress will be in the thin HAZ of the weld. As the weld tapers to the thicker section the stress is reduced.

EJL

 

[SnTMan]

I don't have any proof of this, but my thinking is that a brittle fracture will propagate thru the thinner member more readily than the thicker.

Regards,

Mike