Energy to Break Apart a Weld

[aueng06]

I'm a structural engineer working for the department of defense. I am currently trying to predict the throw distance for pieces of a structure following an internal detonation of a munition.

We have a 3 sided box with a ceiling. The box is made of steel panels which are welded to a steel channel frame. We know from testing that when a detonation occurs the steel panels are torn off the frame and thrown away from the center of the blast. We have establlished methodology for determining the pressure and impulse on the panels from the blast. We can then take that pressure and impulse and approximate a conservative velocity that will be imparted on the panels. From there we can determine the initial kinetic energy and make some reasonable assumptions about the launch angle in order to use physics to determine the final location of the panels.

However, our inital KE is too conservative because of the energy disappated in breaking apart the weld.

My questions...

What energy is used up in breaking apart the weld and how do I determine that energy?

I can determine the maximum force resisted by the weld. Would it be reasonable to subtract the weld resistance from the total force imparted per linear inch then back-calculate a new (reduced) pressure to the plates?

Any other thoughts on additional methods?

 

[metengr]

What energy is used up in breaking apart the weld and how do I determine that energy?

Well, as a first approximation based on very high strain rate conditions, I would look at obtaining Charpy V-notch impact data (ft-lbs) for the welds/base metal. The impact energy absorbed by the steel and weld needs to be taken into account.

Would it be reasonable to subtract the weld resistance from the total force imparted per linear inch then back-calculate a new (reduced) pressure to the plates?

Most likely this would be a small contribution because the weld strength may be matched or higher in comparison to the base metal. Try it and see what happens.

 

[paddingtongreen]

Weld metal will unzip if the separation is started; often started at the point where the weld rod was lifted. What I'm saying is that weld metal has a grain. Even though the weld metal is of higher yield than the base metal, the tear will run through the weld, not the base metal. This could happen in your scenario.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Guest102023]

paddingtongreen, there is no rule about where or how a weld will fracture, or if it will at all. 'Unzip' is not a technical term known to me.

 

[unclesyd]

Assuming fillet welds are the primary welds in question here two papers covering fillet welds under high dynamic loadings.
The consideration of full penetration v-groove welds under your loading conditions would be a bear, probably two.

My personal experience with two defalcations that completely destroyed 2 vessels designed to ASME Sec Viii Div 1. In both cases all failures were the base metal tearing, there were no weld failures.

http://dspace.mit.edu/bitstream/handle/1721.1/40240/36058584.pdf?sequence=1

http://www.shipstructure.org/pdf/81symp14.pdf

braimstoner,
Unzipping is a term quite often used in the description of weld failures especially Aluminum. There are several technical papers with unzipping in the title.

 

[paddingtongreen]

brimstoner, I used the term because it is a simple way to describe the propagation of a tear through a weld, this can happen even though it is a "matching material" and has a higher yield point than the base material. unclesyd is correct, do a web search for "weld unzipping".

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[ornerynorsk]

Would a more conventional blow-out panel be better suited and more predictable than a welded assembly? Just asking.

 

[unclesyd]

I think something is restricting the release of papers on structural damage by explosions.

I haven't tried to find this article online.

http://www.iop.org/EJ/article/0305-624/8/3/I01/ptv8i3p94.pdf

Old paper but may have some leads to others.

http://www.shipstructure.org/pdf/004.pdf

http://dspace.dsto.defence.gov.au/dspace/bitstream/1947/3937/1/DSTO-TR-0383 PR.pdf

Here is the index to information that may be of value at ASTM.

http://www.astm.org/DIGITAL_LIBRARY/STP/SOURCE_PAGES/STP40038S_index.pdf

I read one report on the strength of welds during a catastrophic event in either the this FEMA Report or one of the parallel investigations.

http://www.fema.gov/rebuild/mat/wtcstudy.shtm

 

[unclesyd]

Here is a link to the Blast Resistant Steel Structures paper from Modern Steel Magazine. It has a lot of references that you maybe able to use.

http://www.modernsteel.com/Uploads/Issues/October_2003/30722_qa.pdf