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Winter 2022 Contents: Issue No. 57
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Shear Failures in Fillet Welds
This nugget reviews shear failures modes in fillet welds with longitudinal and transverse loading.
Download an article on Shear Failures in Fillet Welds
(Below is text of the article without figures; if you would like to download pdf copy with figures and tables, please click on the link above)
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Fusion welding is one of the most common forms of material joining, and about 70% of those fusion welds are fillet welds. Consequently, a good understanding of fillet weld strength and failure modes is essential in design and reliability of such welds. A fillet weld is easier for manufacturing as it is more robust for misalignments and fitup issues compared to butt weld. However, analyzing weld strength of fillet welds is bit more complex compared to butt welds. In this nugget we will review the two primary modes of shear loading on a fillet weld and the corresponding failure modes. Note: This nugget does not deal with torsional loads or fatigue loads on fillet welds.
Fillet welds are made on two types of joints: T-joints and lap joints, as shown in schematics in Figure 1. A fillet welds, as the name implies, forms a fillet at the junction of the two materials to be joined. Fillets can also be formed with brazing and soldering processes with similar joint configurations. In fusion welding, the fillet usually includes filler alloy that is deposited and fuses with both base metals to form the fillet. However, there are exceptions where fillet welds are made without addition of filler material, as is quite common in laser welding and some TIG (GTAW) applications. When any load is applied to a fillet welded joint, the load is transferred from one part to the other through the fillet and hence understanding the failure modes is critical.
Shear loads can be applied to the fillet in two directions, along the length of the weld (longitudinal) and perpendicular to the length of the weld (transverse). Longitudinal load puts the entire weld in shear with the weakest point (or plane) is along the weld throat (Figure 1). Calculation of weld strength is then quite straightforward with strength equal to area of throat multiplied by shear strength with some factor of safety. While the calculation is easy, estimating the shear strength is not easy as the fillet weld now is a mixture of the filler material and some amount of base material. Also, cooling rate and surface profile (convex or concave) can add to residual stresses in the weld. Best option is to make welds and test them to failure by loading them along the length of the weld. Test coupons should be such that loading is in pure shear and there is no torsional component of stress; usually accomplished by two welds on either side of the coupon.
(download pdf for figures)
Figure 1. Schematic shows T-joint and Lap joint with fillet welds; either can be loaded in transverse or longitudinal loading.
Transverse loading is a bit trickier to analyze, and is the cause of quite some misunderstanding in literature, which now includes YouTube videos. As shown in Figure 2, transverse loading can cause failure along three planes: shear along Leg 1, pure tensile along leg 2, and shear along a plane in the fillet zone. If you take a component of the applied force along a plane at an angle to the vertical, and divide by the area of that plane you will get the shear stress at that plane. Plotting shear stress vs angle gives us a shear stress profile which has a maxima at 22.5 degrees from the vertical; stress along the 22.5 degree plane is 20% higher than shear at 0 degrees or 45 degrees (Figure 2). When welds are tested in transverse loading, they do fail close to 22.5 degree plane, as shown in Figure 3. Such failure is contrary to popular belief that all fillet failures in shear will be along the throat at 45 degree
(download pdf for figures)
Figure 2. Potential failure modes in transverse loading of a fillet weld. If there is poor leg fusion the joint can fail along the legs. Typically the joint will fail in shear along a plane in the fillet weld. Graph on the right shows maximum shear stress is at plane at an angle of 22.5 degrees; and not along the throat at 45 degrees as is commonly assumed.
(download pdf for figures)
Figure 3. Weld section of shear failure at a 20 degree plane to the vertical. Such failure is indicative of transverse loading.
angle. It is quite common for design engineers to use the 45 degree plane where the weld throat is measured as the weakest link. However, doing so can jeopardize weld performance as the weld can fail at a 20% lower load at the 22.5 degree or thereabouts in transverse loading. Design engineers would do well to be aware of alternative failure locations in transverse loaded fillet welds for both T-joint as well as lap joints.
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