Basics of Fillet Weld: A Step-by-Step Guide for Beginners

Basic Fundamental of fillet weld:

When two members overlap, weld is done to connect both members. Generally, these welds are convex or concave-shaped welds depending on the situation. From an appearance point of view, The concave Fillet weld appears to be larger in length than the convex Fillet weld.  

A concave Fillet weld has less penetration and smaller throat thickness as compared to a convex Fillet weld so that convex plate weld is stronger than a concave Fillet weld. 

When a convex Fillet weld is cooling down welding material shrinks and compressive stress develops on the outer surface whereas concave fillet weld is cooling tensile stress develops on its outer surface due to the tensile world getting cracked.

Concave Fillet welds are made in more than one pass, the first pass is slightly convex in nature and subsequent passes are built up from the concave Fillet weld. 

Apart from that concave Fillet welds are more suitable when repetitive stress takes place.

General specification of fillet weld 

Size of weld 

The size of weld is specified as the leg length of the weld so that the size of the weld can be defined as the minimum Leg length of the fillet weld. 

Leg length –

Leg length is the distance from the root to the toe of the weld and it is measured by the largest inscribed right angle triangle within the weld. But this inscribed triangle within the weld often gives different or unequal sides of the triangle. 

A diagram of leg length is shown in the figure.

Maximum size:

The maximum size of the weld depends on the thickness of the thinner member to be joined. 

The maximum size of the weld is obtained by subtracting 1.5 mm from the thickness of the thinner member to be joined.

Mathematically, 

 maximum size of the weld = thickness of thinner member-1.5 mm.

When weld is applied to section a with a rounded toe there the maximum size of maximum size of the weld should not exceed 75% of the thickness of the section at the toe.

Minimum size

The minimum size of the weld is specified so as to have adequate heating to expand the thicker member. If a very small weld is provided on a thicker member then the heat generated in depositing the small size of weld may not be sufficient enough to expand the thicker base plate.

 So that IS-800:2007 specified the minimum size of weld must not exceed the thickness of the thinner member jointed and also must not be less than 3 mm.

The minimum size of the weld as per IS 800 2007 is given in the table below.

Effective throat thickness:

Effective throat thickness is the shortest distance from the root of the weld to the hypotenuse of the inscribed right-angle triangle within the weld.

In any case, effective throat thickness should not be less than 3 mm and should not exceed 0.7t or 1.0t under certain situations where t is the thickness of thinner member jointed. 

Mathematically,

Effective through thickness = K x Size of the weld.

S = size of the fillet weld 

K = constant that depends on the angle between the joining (fusion) surface and is given below table-

Note:  If the angle between the fusion face is less than 60° or more than  120°, fillet welding is not recommended.

Effective length:

Effective length is the length equal to the overall length provided – twice the size of the weld. The deduction of 2S is made to allow for creators to be formed at the end of the welded length. 

In order to relieve INS of weld length from high stress concentration of an in returns are provided of size equal to twice the size of the weld. 

Note: The actual length of the weld must not be less than four times of size of the weld.

The effective area of  fillet weld:

The effective area of the weld is equal to the effective length of the fillet weld multiplied by the effective throat thickness.

Mathematically,

       A_eff = l_eff Χ t_t

A_eff =Effective area, l_eff = effective length, and t_t = Effective Throat thickness

Overlap 

The overlap of the plate to be joined by weld should not be less than 4 times the thickness of the thinner member to be jointed or 4 mm whichever is greater. As shown in the figure below –