one of two main types of weld joints, the other being the groove weld. Fillet welds are the most common type of weld and are usually the most economical to perform. Fillet welds are typically used to join two pieces of metal whose surfaces meet at a 90 degree (or similar) angle, forming a tee joint.
One might imagine two plates either being lap-spliced or meeting at a tee as shown below.
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| fillet weld joint geometry |
For 90 degree tee joints, the cross-section of the fillet weld is usually an equal-legged right (45-45-90) triangle. The portion of the weld at the right angle is called the root and the portions at the two other angles are called the toes. The size of a fillet weld matches the leg dimension of the triangle. The face of a fillet weld corresponds to the triangle's hypotenuse and may be either flush, convex, or concave. The strength of a fillet weld is proportional to the throat, which is the perpendicular distance between the root and the face. "Theoretical throat," synonymous with "effective throat," is based on the idealized triangle and does not account for convexity or concavity. Theoretical/effective throat is what structural engineers use in calculations during design. "Actual throat" is larger for convex fillet welds and smaller for concave fillet welds. Therefore, hopefully the face as welded is not concave!
