**3.7 Weld strength**

Weld metal has tremendous strength, generally higher than the raw strength of the base metals that are being joined. This strength can be calculated with the type of load the material will face. Welded components may endure multiple different types of stress in unison. Fortunately, the superposition of loads principle allows the calculations of each type of stress remain separate. Transverse and shear loads represent the two most common load orientations.

The fillet welds represented by the right triangles shown in **Figure 16** are very resistant to the transverse type of loading. However, if the load follows the direction of the arrow and only one fillet weld existed on the right-hand side of the plate, the connection would be weakened and would likely bend over itself and break off at the joint, toward the righthand direction. In general, the best practice would be to weld both sides. However, if the application required only one fillet weld, due to a low load application, then the weld should be placed on the side of the plate that places the weld in tension, not compression. In this case, it would be the arrow side of the plate. If the load was in the opposite direction, then the preferred weld location would move to the other side.

Transverse loading is only when the load applied is perpendicular to the weldment. If the load is present in a direction parallel to the weld, as shown in **Figure 17**, then the

**Figure 16.** *Illustration of transverse loading on weldment.*

**Figure 17.**

*Illustration of shear loading on weldment.*

strength of the weld must be calculated in shear. Shear loading is more detrimental to the structural integrity of a weldment than a transverse load. The decreased structural strength of the weld means that the weld strength cannot be calculated using the full tensile strength of the filler metal. Instead, the AWS recommends that the tensile strength of the filler material be reduced by multiplying by a value of 0.30, reducing the tensile strength by 70% [2]. Otherwise, the strength calculation process is the same.

The calculation of weld strength for transverse load is straightforward, but it is important to emphasize that this is the calculated static strength of the weldment, without measurement against an applied load. Transverse loading requires three basic inputs for determination: filler tensile strength (*TF*), the thickness of base material (*Mt*), and length of weld applied (*LW*). Eqs. (3) and (4), along with Eqs. (5) and (6), are used to successfully calculate the strength of a weld under transverse load [1]:

$$A = W\_t \* L\_W \tag{5}$$

$$F = T\_F \* A \tag{6}$$

where *A* is the sectional area of the weld;

*Wt* is the weld throat size;

*LW* is the length of the applied weld;

*TF* is the tensile strength of the weld filler material; and.

*F* is the resistive force the weld is capable of supporting.

**Table 2** shows the application of this model in calculating the weld strength for material and weldment length using specific values. The tensile strength for the most common MIG filler wire is 483 mPa, and that has been used here. The calculations show that the maximum strength of the weldments under transversal load is 200 and 198 kN in shear load. This number would be an upper bound for loading because the calculation lacks any accounting for imperfections. Any weld imperfections, defects, undercut, material imperfections, material fit-up problems, or dynamic or thermal loading during welding will reduce the strength of a welded joint. This calculation merely provides the theoretic strength of the specific design under specific conditions. In manufacturing, weldment specifications are properly simulated using finite element analysis, tested, and inspected to ensure that the applied loads do not exceed the factor of safety in the design. However, this level of analysis and design is typically not available to repair specialists. They simply tend to increase the factor of safety within their designs, accordingly.

All of the necessary parameters for the basic weldment strength calculations can be easily obtained when conducting field repairs. A tape measure and a phone calculator are the only tools truly needed to execute this calculation in the field. The tensile strength of the filler wire can be found on the wire spool attached to the welding


#### **Table 2.**

*Spreadsheet calculation for weld strength under transverse and shear loading.*

machine. For arc welding applications, the tensile strength is called-out on the electrode. Proper weldment design in repairs is just as critical, perhaps more so than in original manufacture.

Stitch welding is a technique used to connect base material pieces that will see little stress and fatigue in their duty cycles. Additional weld fillet length beyond the structural need is wasteful and costly. Stitch welding is generally a great technique for field repairs that require lengthy connections, eliminating the need to weld a solid bead that would introduce a significant amount of heat into the base material, potentially causing distortion and warpage. Engineering drawings feature unique callouts for stitch welding that include dimensions referencing the length of the stitch weld, the distance between each stitch weld, the weld type, and the size of the weld [15]. The drawings will also feature callouts for the intermittence of the weldment, such as intermittent welds on one or both sides of the joint and the staggered intermittent interval for the weldments [15]. In field repairs, it is usually up to the repairman to evaluate the stitch weld dimensions and the weld run intermittence, depending on the stress of the joint. Significant guidance for the design of the weld can be had through the proper estimation of the needed weld strength.
