PalmSHIELD offers continued education. Educate. Re-Educate.

PalmSHIELD fabrication engineers and supervisors recognize the importance of continued education even on the most basic principles.  Our welding techniques are all in accordance with AWS D1.2  Structural aluminum welding.  The most important weld for us is combining the plate to the post as this is a structural weld that will be tested under severe wind loading in accordance with IBC and UBC codes.  This weld is a fillet weld.

Fillet Weld Size. How long should the legs be?

If not shown on the fabrication drawings, fillet welds should be equal to the thickness of the thinner material being used to compose the joint.  If your thinner material is 3/8” thick then both legs of the fillet weld should be 3/8”.  This is a good rule of thumb but may vary in more structural applications or where the difference in material thickness between the two pieces is drastic.  In drastic differences in material thickness, the legs composing the fillet weld may vary as well using a longer leg on along the face of the thicker material.

Fillet Weld Size.  How to measure?

Fillet welds are measured by measuring the perpendicular leg on the weld.  Generally, leg lengths should be equal.

Fillet Weld Shape.   Which Profile is Best?

The best shape for a fillet weld is a miter or flat weld.  However, this is generally difficult to achieve.  Welders should strive for a miter or flat weld, leaning more to a concave appearance than a convex.  Concave welds are those that slightly dip in the middle leaving a shorter throat.

The way in which loads are carried through a fillet weld can be illustrated with stress lines. The ideal fillet weld profile is a miter fillet with weld toes that smoothly blend into the base metal. For the minimum amount of weld metal this profile provides the required effective throat. Using manual welding it can be difficult to reliably achieve a perfect miter fillet and so a welder will often aim to produce a slightly convex profile. This avoids the risk of creating a concave profile which for the same measured leg length will produce an undersized effective weld throat.

Fillet welds with a pronounced convex profile are generally not much good for anything because they create stress raisers due to the notch effect at the weld toe. The stress lines for a fillet show that the stresses pretty much take the shortest route between the two weld toes; reinforcement is of little or no benefit, it just adds to the cost.

So when would a fillet weld need to be convex? Well, apart from a corner weld where a radius edge may be required, probably never. Convex fillet welds are generally bad news for several reasons:

• The additional weld metal in the form of reinforcement adds cost with no strength benefit
• Convex welds unnecessarily add to weld distortion
• The stress raiser at each toe is highly undesirable for fatigue loading
• Convex one pass fillet welds should be viewed with suspicion as often the root of the joint will have lack of fusion.

Miter fillets and slightly concave welds are preferred where weld appearance, corrosion or clean ability is a consideration, for example for stainless steel fabrication. Apart from just looking bad, convex welds should never be used when the joint is subject to any form of fatigue loading. The profile of a fillet weld is of great importance when a good fatigue life is required – abrupt transitions at the weld toe that are characteristic of convex welds are the stress raisers that dramatically short the weld’s service life.

As well as the actual weld profile, other factors such as fatigue improvement methods must be considered to optimize the fatigue life of a weld joint. If you are involved with the design or fabrication of welded fabrications subject to fatigue loading, do not miss the Welding Centre’s upcoming seminar series Avoiding Fatigue in Welded Structures.

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