Peening, before or after stress relief ?

Date 06-25-2002 17:21

D1.1:2002 section 5.27 provides some direction.
One note, however, the reference to the exception in section 2.36.6.6(3) appears to be incorrect. It is the same reference in the D1.1:2000, yet the D1.1:2002 has changed section 2 dramatically. Check Annex P of the D1.1:2002 for the corresponding section. Should be 2.20.6.6(3).

By steveho (*)

Date 06-25-2002 18:47

Appreciate the comments. Yes, it's 2.20.6.6 now, but not certain what you may have been leading to. In this case, as the Engr' won't even identify the project's primary members, let alone the loading details beyond "cyclic", and table 2.7 appears to not help defining if there may be problems in peening after stress relief for both profiled and flat topped welds. If nobody sees any potential here for problems, has anyone even heard of these being done this way ?

By Niekie3 (***)

Date 06-25-2002 18:47

I am not a D1.1 expert, so my answer will not necessarily be in line with the code requirements. I will however try to give direction based on "first principles".

Fatigue cracking occurrs when a component experiences repeated tensile stresses. The fatigue rate is a function of the stress amplitude rather than only the maximum stress experienced. To increase a component's fatigue resistance then, the aim is to reduce both the stress amplitude and maximum stress. This is done (or attempted) in a number of different ways. In addition, a fatigue crack will initiate at the area with the highest localized stress. This is usually experienced at stress raisers. (which can be either on a macroscopic or microscopic scale) Such stress raisers and subsequent crack initiation are mostly found on the surface of the component.

With peening, the intention is to place a compressive residual stress in the surface of the material. As fatigue cracking only really takes place under tensile loads, this tends to reduce the "tensile" stress amplitude experienced by the surface of the material.

With stress relieving, the residual stresses are reduced which means that the maximum tensile stress will be reduced.

If you perform stress relieving after peening, you will also reduce the compressive residual stresses that are the main intention of the peening opperation. As such, it only makes sense to perform the peening after PWHT.

If you want my personal oppinion, I can only state that I am not a strong believer in peening. It is generally such an uncontrolled operation that it can cause just as much dammage as benefit.

In addition, there are differences of opinion regarding the effectivity of a stress relieve treatment on the fatigue resistance of structural components. In fact, there are "fatigue codes" that gives one no credit for a stress relieve treatment.

What is much more important is to reduce the presence of stress raisers, and in particular to make the design such that the welds are not in the areas experiencing the highest cyclic stresses.

Hope this helps

Regards
Niekie Jooste

By steveho (*)

Date 06-25-2002 19:10

Appreciate the basic info. Under the circumstances, that's just what I needed. The simplisity of the practicalities of peening (after PWHT) seemed obvious, but despite the temptation to play Engr', this and most other Inspectors generally are wise to not go there and instead seek advise of those having been there already.

By Michael Sherman (***)

Date 06-25-2002 21:05

I would like to address the "flat topping of welds" statement. I have always believed that flat topping a weld causes a stress riser. So much so that I don't allow flat top welds on my weldments. Any ground off weld must be flush or rounded. Would anyone like to address this? I am always interested in learning when I am wrong.

Respectfully,
Mike Sherman
Shermans Welding