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 quality it is essential to have very good fit-up with minimum gaps. Time spent in joint preparation saves time in welding and produces the best quality structure. In Chapter 4, examples are shown of both proper and improper joint fit-up. In some instances small grinding wheels or abrasive cartridge rolls may be employed to achieve the desired maximum gaps of about .010 inch for thin tube walls such as .040 inch. For .062 and thicker wall tubes, .020-inch maximum gaps should produce satisfactory welds.
Fig. 2.7. The AWS Specification for Automotive Weld Quality—Arc Welding of Steel, defines the names for this series of sheet-metal weld joints. Arc and plug welds are commonly used for street rod fabrication. A plug weld is made through a premade drilled or punched hole. For thicker materials, a fillet weld can be made in an elongated slot.
Gussets can be used on tube joint intersections to stiffen the assembly. They are particularly useful for some high-strength materials such as 4130 chrome-moly, where a somewhat lower-strength, more ductile welding rod and smaller weld size can be offset with the added strength supplied by a gusset. An example of the use of a gusset is shown on a NASCAR roll cage in the lower right of Figure 2.2.
A fillet weld is a triangular-shaped deposit commonly used for many joints where two materials to be joined intersect at angles. In instances where two flat plates are joined there is little joint fit-up required. However, for fillet welding intersecting tubes, the complex joint geometry must be properly cut and matched to achieve the needed maximum gap of .010 to .020 inch. It is also important to ensure the bottom of the fillet weld, at the intersection of the shapes being joined, is melted and fused. It is possible to make a fillet weld having a good surface appearance that is not properly fused in this bottom area, called the root of the fillet.
Fillet welds are often partial-penetration welds and require a reduction in allowable loads because of the gap left at the weld root. The reduction factors depend on the exact joint, the amount of penetration, the type of loads involved, and design specification requirements.
Fig. 2.8. Fillet welds are considered partial-penetration welds and require a reduction in allowable loads because of the unwelded area at the root. The amount of reduction depends on specification requirements. A single fillet has the highest stress concentration because of the loading. The double fillet is better to use, and the full-penetration double-fillet weld in which the root gap is eliminated is best.
A single fillet has the highest stress concentration because of the loading. If loaded so that the joint is bent toward the weld, the stress at the root of the weld is significantly increased.
A double fillet weld is better because, although an unwelded area exists, when a side load is applied the stresses are shared by the two fillet welds and the root stress concentration is not as high as with a single fillet.
A full-penetration double fillet is the best joint because there is no unwelded area.
The commercial automotive industry has developed its own standards for the types of welded joints. These sheet-metal weld joints are defined in the AWS Specification for Automotive Weld Quality—Arc Welding of Steel. They include plug and spot welds, which are commonly used for street rod welding.