Fatigue test welded test bars to predict componet life 3

[2design]

Hello,

We are having some fatigue issues on welded structures. To help us predict better fatigue life, it has been proposed that we should make some fatigue test bars (coupons) and test then to come up with welded material fatigue curves. I've never heard this being done before for welds and have some hesitation going ahead with this plan. We use several differnt metal plates and several different welding wires. Has anyone had success doing something similar to help predict fatigue life?

Thanks

 

[metengr]

2design;
There are various design codes (Structural welded components) that have requirements for proper design margins regarding fatigue service. I don't believe you need or should go down this path for fatigue testing because I believe it will not be necessary.
It could be that either cyclic service loads are not correct or your weld joint design may not be optimized for fatigue service. Look around for structural design standards that account for fatigue service before pulling the trigger on expensive testing.

 

[btrueblood]

Agree with metengr, as usual. Star for him.

 

[2design]

Thanks Metengr for your input but what I am looking for was an answer to the question in the last sentence. This was my fault because I did not properly state what I was looking for.

What I want an answer to is, has anyone heard of characterizing welded joints using welded test bars in order to use these curves to predict the life of the weldment. A consultant is suggesting that we take two bars from parent material that we often use; weld them together to create a fatigue test bar. Make multiples of them and then test then to create a strain cycle fatigue curve. Use this data to predict the life of the welded joints of two parent materials of a large weldment.

I have already tried to dissuade him from this recommendation by suggesting that there are other established methods such as BS 7608. But he is still sticking with his suggestion. I think this is really a bad idea but I just wanted to know if I am wrong and other have used this method successfully. Like Metengr has stated this would be a very expensive process that will line the consultant’s pocket.

Thanks again

 

[metengr]

2design
Yes, use the BS 7608 - Code of Practice for Fatigue Design and Assessment of Steel Structures.

 

[tbuelna]

2design,

The issue with establishing metal fatigue properties comes from the nature of fatigue itself. While establishing S-N curves for wrought metals is not an exact science, establishing fatigue properties for a (fusion?) welded metal joint is even more circumspect. As metengr suggests, there are excellent published guidelines for designing weld joints to minimize fatigue issues. But as for the actual material stress limits that you should apply, this is not so clear.

You may see some published fatigue properties (S-N) for wrought metals, but you won't likely see any published data for welded materials. The reason is that metal fatigue stress limits can vary greatly depending upon variables such as grain structure, porosity, inclusions, etc., which can be controlled somewhat in wrought or forged materials, but not so easily in fusion welded materials. Published fatigue data for wrought/forged metals is based on large statistical test sample databases.

In order to establish a statistically meaningful fatigue stress value for your welded joint, you would need to test a large number of material samples produced using a controlled welding process. As other posters noted, this would be extremely expensive and time consuming.

The other alternative is to use the large and conservative weld joint stress analysis knock-down factors accepted in most industries.

Hope that helps.
Terry

 

[2design]

Terry,

This was the information I was looking for to help me move in the right direction. Star for your help.

 

[SJones]

What environment is the 'structural weld' exposed to? Is it more a case of corrosion fatigue rather than fatigue in air?

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[2design]

more rough terrain in air fatigue