Fillet Weld

A fusion weld, other than a butt, edge or fusion spot weld, which is approximately triangular in transverse cross section

Size of fillet welds

Unlike butt welds, fillet welds can be defined using several dimensions:

Actual throat thickness

The perpendicular distance between two lines, each parallel to a line joining the outer toes, one being a tangent at then weld face and the other being through the furthermost point of fusion penetration

Design throat thickness

The minimum dimension of throat thickness used for purposes of design. Also known as effective throat thickness, Symbolised on the drawing with ‘a’

Leg length

The distance from the actual or projected intersection of the fusion faces and the toe of a fillet weld, measured across the fusion face. Symbolised on the drawing with ‘z’

Shape of fillet welds

Mitre fillet weld

A flat face fillet weld in which the leg lengths are equal within the agreed tolerance, the cross section area of this type of weld can be considered to be a right angle isosceles triangle with a design throat thickness ‘a’ and leg length ‘z’. The relation between design throat thickness and leg length is:

           a = 0.707 × z                              (Or)                           z = 1.41 × a

Convex fillet weld

A fillet weld in which the weld face is convex, the above relation between the leg length and the design throat thickness written for mitre fillet welds is also valid for this type of weld. Since there is excess weld metal present in this case, the actual throat thickness is bigger than the design throat thickness.

Concave fillet weld

A fillet weld in which the weld face is concave, the relation between the leg length and the design throat thickness specified for mitre fillet welds is not valid for this type of weld. Also, the design throat thickness is equal to the actual throat thickness. Due to the smooth blending between the weld face and the surrounding parent material, the stress concentration effect at the toes of the weld is reduced compared with the previous type. This is why this type of weld is highly desired in case of applications subjected to cyclic loads where fatigue phenomena might be a major cause for failure.

Asymmetrical fillet weld

A fillet weld in which the vertical leg length is not equal to the horizontal leg length, the relation between the leg length and design throat thickness is no longer valid for this type of weld because the cross section is not an isosceles triangle.

Deep penetration fillet weld

A fillet weld with a deeper than normal penetration, It is produced using high heat input welding processes (i.e. SAW or MAG with spray transfer). This type of weld uses the benefits of greater arc penetration to obtain the required throat thickness whilst reducing the amount of deposited metal needed, thus leading to a reduction in residual stress level. In order to produce a consistent and constant penetration, the travel speed must be kept constant, at a high value. As a consequence, this type of weld is usually produced using mechanized or automatic welding processes. Also, the high depth-to-width ratio increases the probability of solidification centerline cracking. In order to differentiate this type of weld from the previous types, the throat thickness is symbolised with‘s’ instead of ‘a’.