A fillet weld is stronger than it looks
Where the throat is, and how the load actually passes through.
A fillet weld is stronger than it looks because it carries load along a specific diagonal path called the throat, where a casual glance at its shape suggests it must carry load straight through the full thickness of the plates it joins. That path, loaded mostly in shear along the entire run of the weld, turns a modest triangular bead sitting in a corner into a joint capable of carrying the great majority of structural connections without the plates ever being melted all the way through.
How load crosses a triangle in a corner
A fillet weld joins two surfaces meeting at roughly a right angle by depositing a triangular bead of weld metal into the corner between them. Beside a full-penetration groove weld, which melts and rejoins the entire thickness of both plates, a fillet looks almost incidental, a bead perched in a corner with no visible reach through the material. That impression is misleading, because the two welds do their jobs in different ways. A fillet weld transfers load across the joint along its throat, the shortest straight-line path through the weld's triangular body, and that path is loaded predominantly in shear. Shear is a mode many common structural materials resist relatively well, so concentrating load into shear along a well-chosen path is an efficient way to carry force even though the weld metal doing the carrying is a fraction of the plate's thickness.
A stitched seam holds more than the thread suggests
A line of stitching holding two pieces of fabric together has no need to match the strength of a continuous sheet of that fabric. The thread transfers load between the two pieces along a narrow row of loops, and a well-stitched seam on a heavily loaded sail routinely outlasts the fabric around it despite being a small fraction of that fabric's cross-section. A fillet weld does the same kind of work between two plates, and just as nobody judges a seam by how thick the line of stitching looks, a fillet weld's strength is poorly judged by how much metal appears to be sitting in the corner.
Pull hard enough on a well-stitched seam and it is usually the fabric that tears first, well away from the stitching. A well-proportioned fillet weld aims for the same outcome: under an extreme enough load, the surrounding plate gives way before the weld's own modest cross-section does.
Why the throat is seventy percent of the leg
For a fillet weld with equal legs, the throat runs from the corner to the middle of the sloping face, and simple geometry puts it at roughly seventy percent of the leg length (the leg multiplied by the cosine of 45 degrees). Because that throat is loaded in shear along the whole length of the run, even a modest-looking fillet over a reasonable length adds up to a large total carrying capacity. This is why fillet welds are the default for most ordinary structural steel connections: cheaper to make, needing no special edge preparation, and strong enough for the loads involved.
The same geometry shows why simply making the legs bigger is an inefficient way to add strength. Capacity grows in proportion to the throat, and so to the leg, while the cross-section of weld metal grows with the square of the leg. Doubling the leg size doubles the capacity but uses four times the weld metal, and puts correspondingly more heat and distortion into the joint.
Choosing between a fillet and a groove weld
A designer choosing between a fillet and a full-penetration groove weld is therefore very often better served by the fillet. The groove weld's extra cost, in edge preparation, in filler metal, and in the heat and distortion earlier articles in this set described, frequently buys strength the connection never needed. A fillet weld is often the correct, deliberately chosen answer for a connection, and a joint chosen because it looks more thorough deserves less trust than one chosen because the calculation called for it.
The efficiency depends on the fillet being formed correctly, with a full, evenly filled triangular cross-section. An undersized or unevenly filled bead can have a real throat far smaller than its leg length suggests, quietly giving back much of the advantage. It also depends on the load suiting a fillet, meaning shear along the weld's length. Some joints, particularly those loaded directly in tension across the full section, genuinely need the continuity of a groove weld, and no amount of throat efficiency removes that requirement. Choosing between the two on the calculation, and not on which one looks more reassuring, is the skill this article and the earlier ones in this set have each approached from a different direction.