alt="Fig. 2.3"/>
Fig. 2.3. A full-penetration weld should be used to achieve maximum strength. In the bottom left panel, the joint shown is very useful for somewhat thicker material. First, a weld is made in a single V-joint to achieve good penetration. Then the back side is gouged or ground into sound weld metal, making a U-groove. A second weld is then made in the U-groove to fully penetrate into the first deposit.
When you can weld from both sides of the joint, a full-penetration weld is easier to accomplish. For thin material, the edges can be butted together, a weld made on one side, and a weld made on the back side that fully penetrates into the first.
A single V-preparation for butt joints is a proven method accepted for a number of design specifications and can ensure a full-penetration weld is achieved. V-preparation used for a full-penetration joint is particularly useful for thicker materials, such as 3/16- and 1/4-inch plate. By first using a single V-preparation, leaving half the plate thickness as a land accomplishes two things. It ensures good penetration on the first weld and leaves a land under the V that prevents excess penetration where the fit-up is not perfectly tight.
The V can often be made with a grinder, but you must be careful to leave half of the surface as the land. The first weld is placed into the V-groove. It should be made with sufficient current and speed to penetrate about three-quarters of the plate thickness.
Fig. 2.4. These joints are suited for welding sheet metal. The upper left joint is commonly called a joggle joint or flange joint. The official AWS Sheet Metal Code name is an offset lap joint. Whatever it’s called, this is an excellent joint when welding a patch panel. Simple locking-type pliers are available that can progressively form the edges providing a backing for the subsequent weld.
Fig. 2.5. Fabricators, including those making air handling ducts and tractor cabs, weld sheet metal. They have developed a number of joints that make it easier to weld specific sections. Some are useful for specific automotive applications. Flange joints make welding easier and may require less heat input. Backing a weld, such as the corner weld shown, adds strength and allows less precise fit-up.
Then the back side is gouged or ground into sound weld metal by using a grinder held on its side or an air-powered chipper with the proper groove should go sufficiently deep, so the bottom reaches defect-free weld metal in the first-side weld, and it should result in a U-shaped joint.
A second weld is then placed in the U-groove with sufficient current, so it fuses into the groove on the first pass. The resulting weld should overlap about 20 percent of the joint thickness. This overlap eliminates any root defects that may have been created in the first weld.
A J-groove is essentially half a U-groove and can be employed where the edge of a thick plate butts to a vertical member, as might be encountered in a cross-brace attachment to a side frame rail. As with a U-groove, a J-groove minimizes the amount of weld metal and weld heat while still ensuring adequate penetration.
Square butt welds made in sheet metal require very close fitment. Several techniques are employed to make welding these joints easier. One approach is making a joggle or flange joint used to fabricate propane tanks, fire extinguishers, and other thin sheet metal vessel end-cap welds. With this approach, one edge of the joint is formed so the joining plate fits over the bent area. This provides back support for the weld, and it’s more tolerant of slight fit-up variations.
Offset lap joint is the official AWS Sheet Metal Code name for this type of joint. The weld itself is referred to as a flare-bevel weld. Whatever you call it, this is an excellent joint when welding a sheet-metal patch panel. Simple locking pliers are available with dies welded to the grip faces, from companies such as Eastwood, that can progressively form the edges, providing a backing for the subsequent weld. There are air-powered devices that provide the same progressive crimping and make the task for preparing the panel faster.
Fig. 2.6. Tubular joints are used to fabricate chassis and roll cages. The joint names shown are from the AWS Structural Welding Code for Steel. This joint type is a partial-penetration weld because there is an unwelded area at the root of the fillet weld. That creates a stress riser that can cause a crack in the fillet weld when subjected to high-stress, cyclic loading.
A number of industries fabricate sheet-metal parts such as air handling ducts and tractor cabs. A number of joints have been developed to make it easier to weld specific types of sections. Some of these designs, which include flange joints, may be useful for specific street rod applications. These flange joints, as they are referred to, make welding easier and may require less heat input. Melting the edges of a flange butt weld, as shown in Figure 2.5, is easier than making a square butt weld in sheet metal. In addition, the edges can be easily clamped together and the joint tack welded prior to final welding of the seam.
The same fit-up and welding benefits exist for the flange corner weld. Backing a weld with another part, for example in a corner weld, adds strength and is more tolerant of less-than-precise fit-up.
Tubular intersection joints are typically used in race car chassis and roll cage welding, and a number of non-automotive industries use tubular members in construction. They have developed standards that define allowable loads for various intersecting tubular joints. The AWS Structural Welding Code for Steel defines the official names of these intersections. Several of these commonly used for race car fabrication are shown in Figure 2.6.
This type of joint is considered a partial-penetration weld because there is an unwelded notch at the root of the fillet, and this unwelded area creates a stress riser at the weld root. Depending on the loads involved, this stress concentration can cause a crack to propagate into the fillet welds on thinner-wall tubes, such as those used in chassis and roll cage constriction. This is a problem with high-stress and cyclic loading. The allowable stress calculations can reduce the amount some of these joints can be safely loaded by a factor of 70 percent or more. Fatigue is a failure mode in which loads vary in a cyclic manner.
The stress riser, such as the unwelded root of a fillet, can cause a crack to form. Over time, with increasing loading cycles, these small cracks grow bigger and can lead to failure. An advantage of steel is that at a low-enough load level, the crack tip blunts and stops propagating. At that load level, the fatigue life of the structure is said to be infinite. With a fully penetrated weld or base material free from significant defects, that load or stress, to have infinite fatigue life, is about half the material’s ultimate strength. However, with high-stress concentrations, the load to achieve infinite life may be only 20 percent or less of the ultimate strength.
This infinite life characteristic is not applicable to all metals. Aluminum, for example, has no load that eliminates the growth of highly stressed cracks. For highly cyclic loading, such as a rotating member, aluminum is not a good choice.
Race cars often use many complex tube intersections for a lightweight, ridged structure. A NASCAR chassis is shown in the upper left of Figure 2.2 that has six tubes coming into one common point from various angles. To achieve the required welded-joint
