Impact Design of Fillet Welds 2

[Ussuri]

I have to design a fillet welded connection that will subject to a 10kJ impact force. I am concerned that the limited ductility of the weld will affect the available strength of the weld not allowing it to reach its full potential.

Does anyone know of any design guidance that will help me calculate the actual weld capacity under impact?

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[desertfox]

Hi Ussuri

I think you need to find the actual force that your imparting onto the weld and not just the energy of the impact.Have a look at this site for weld calculations anyway it might help:-

http://www.roymech.co.uk/Useful_Tables/Form/Weld_strength.html

regards

desertfox

 

[metengr]

You state an impact force in terms of energy units (J). Can you determine the peak impact force and its direction(s) on the component? Is this a repeated or cyclic peak force on the component or a one time event? Two main concerns you need to deal with regarding design of welded components - toughness at temperature and weld type/size for static or dynamic load-carrying capacity. If this is a welded connection to an object, in my opinion this is really no different in comparing a welded nozzle on a vessel which is subjected to horizontal and/or vertical moments in service. These moments can be used to determine bending stresses at the welded connection.

There is a significant amount of data on weld metal toughness (impact energy, J or ft-lbs, as a function of service temperature) to assure adequate fracture toughness. The load-carrying capacity can be calculated for fillet welds and groove welds (I would refer you to Design of Weldments, by Omer W. Blodgett for additional reading on this subject). This is the reason why I was asking the second question - will this be repeated peak loads or a one time peak force.

 

[Ussuri]

I know what I expect the magnitude of my impact force to be (50kN vertical 25kN lateral) and I have sized the welds based on these forces under a static loading condition (moment shear and torsion). There will be no cyclic load as the impact is a one-off event.

I am concerned that I will get a fracture at the weld because of its lack of ability to deform with the load. To my way of thinking this will lead to a higher instantaneous force in the weld when compared to the static load case. I guess I am thinking in terms of impulse.

 

[stanweld]

Does the impact load produce highest tensile loads transverse to the root of the fillet weld? If so your static loading analysis will prove inadequate.

 

[Ussuri]

If I understand your meaning of transverse as perpendicular to the weld run then that is the direction the 25kN is applied, the 50kN is applied parallel to the weld run.

 

[desertfox]

Hi Ussuri

I might be missing something here but if you have stressed your welds statically with a good margin of safety I do not understand how you can be concerned about it failing.
what are your margins of safety in the welds?

regards

desertfox

 

[JoeTank]

Ussuri,
Why are you assuming that the weld will not deform with the rest of the structure? That seems counter-intuitive. Part of the qualification of a welding procedure is to demonstrate toughness (Charpy), strength (tensile strength) and ductility (bend tests).

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[metengr]

I just looked thru my handy copy of "Design of Weldments" by Omer Blodgett, and as I expected he has a complete section (3.1) devoted to Designing for Impact Loads. You really need to review this information.

Blodgett provides the necessary information regarding what is an impact force, properties of sections with equations and most importantly impact properties of common design materials. An excellent resource book.

 

[Ussuri]

metengr, Thanks. I will review the information. My only bug with Blodgett is that it is in American units. It would be nice if they published a metric version.

Steve, the main reason I was concerned comes from speaking with colleagues. The example cited was if you are on site and wish to remove a piece of fillet welded steel you just hit it with a 14lb sledge. Because of limited ductility of the weld the section will not deflect but just snap off. I spoke with our materials department and the told me the Weld Qualification Procedure Tests do not usually cover ductility testing for fillets only butt welds.

 

[stanweld]

Ussuri,
The fillet weld does not necessarily have "limited ductility". The root of the fillet acts as a notch. Flaws in the root, often undetectable, exacerbate the effect of the notch. If the joint is designed appropriately, and the weld is made to preclude flaws in the root, failure at the applied dynamic load can be negated.