stress calcs for base metal of welded joint 1

[StevePAA]

a very basic question on weld calcs and I'm surprised I'm asking this, but...

I'm looking at Shigley and Mischke ME Design, 5th edition. Welded joint section, pg 399-401, the bracket for mooring small watercraft example. It's a classic example of calculating weld stress and then stress in the parent metal. Basically it's a bracket welded normal to a plate and the bkt is loaded by a force. I'm clear on the weld stress calcs performed.

The parent metal stress calc has me confused.
The bracket x sect at the weld interface is a rectangle .25w X 2.5lg with a .25 fillet all around. To calc the area of the parent metal they do: .25x2.5=.625.
This seems odd to me- it's like they're assuming the bracket is bonded to the plate- like a full penetration bevel weld and not fillet weld. Why not take the perimeter of the rectangle and multiply it by the weld size? With the perimeter method in this case you'd get a greater area- since the bkt is thin.

When they calculate the bending stress in the base metal, they use the moment of inertia of the .25 x 2.5 solid section.

For a thin bkt like this example, I guess this method makes sense, but what if you had a .75" wide x 10" lg bkt with a .38 fillet all around? Would you do the base metal area calc .75x10= 7.5, and the moment of inertia of the .75 x 10 section?

TIA!

 

[zcp]

In this example, the .25 x 2.5 refers to the parent metal above the weld. If you were looking at the parent metal below the weld, you would normally account for the bigger section. The .25 x 2.5 would be the more critical section in this case.

ZCP

http://www.phoenix-engineer.com

 

[StevePAA]

thanks. I agree, and of course that makes sense; the wording in the text wasn't clear to me as "parent metal" threw me off. They should've just said to find the stress in the bracket.

 

[chicopee]

Wait a minute guys, eventho I dont have that book, I think that they are calculating the tear strength of the parent metal below the weld. Is the parent metal 1/8" thick?

 

[StevePAA]

no, the metal below the weld is assumed thick and ignored. I believe ZCP has it right.

The book ignores allowable stresses so I think the calcs need to be performed several ways (I did for my example) calcs for stress and FS in: the parent metal, the weld metal, and the bkt xsect at the weld. For my example the metal is Sy=30ksi and allowable stress is .60Sy (18ksi); weld metal Sy=50ksi (assumed), allowable stress is .40Sy (20ksi); parent metal Sy=30ksi and allowable stress .40Sy (12ksi). I think I have that right. So, it may not be obvious which is the lowest strength item with the geometry varying and the allowable stress varying.

 

[chicopee]

If this was a real problem, you could not discount tear strength and buckling of the metal where the clip is located.

 

[StevePAA]

the shear at the hole where the rope/clip is attached was the final calc performed in the example

 

[chicopee]

Yeah, I would have done the same too in addition to tensile calculation of metal on both sides of hole.