Omer W. Blodgett, Sc.D., P.E.

When it comes to fixing welding problems, engineers and manufacturers occasionally find themselves in a position where appearances are deceiving. For instance, engineers may test prototypes, look for cracks, and if they find any, locally strengthen the product. But this "solution" can be a mistake because the point of failure isn't always the source of the problem. Until an engineer correctly identifies the source of a design failure, he can't make progress toward a solution.

Take the example of a rotating truck boom, commonly called a cherry-picker, supporting a 3,000-lb weight at the extreme end of the boom as it rotates through 360°. In this case, the manufacturer made a self-contained unit that slips over the end of a truck frame. In addition to the bucket, boom, and mast, the assembly includes a top horizontal member that sits on the truck frame and a similar bottom horizontal member that rides under the truck frame. In tests of the prototype, the frame of the assembly cracks at the point where the mast and the top horizontal member intersect.

The engineer reinforces the connection with a wrap-around plate and increases the weld size, doubling the strength. He tests the modified design, and, again, the frame fails at the same location.

Since there is evidently more going on than meets the naked eye, the engineer analyzes the varying load conditions that affect the cherry-picker during testing and in service. Two primary forces are involved: the mass weight of the arm, the bucket, and its contents and the radial load due to centrifugal force. Both act to create a bending moment that eventually must be resisted by the mass of the truck. Two outriggers help to resist these overturning moment loads.

The engineer realizes that the bottom of the framework needs reinforcement rather than the welded connection. Therefore, he adds a plate to the lower slender horizontal members to provide that portion of the frame with the compression strength required to resist buckling.

put the top part of the horizontal member of the frame into compression and the bottom is in tension. The slender angles that comprise the lower framework adequately resist the resultant tensile loads.

When the boom travels to the right, the loading is reversed. Now, the upper portion of the frame is in tension, while the bottom part is in compression.

The slender angles that make up the bottom portion buckle slightly under the compression load. When this happens, the load path changes, resulting in a severe bending load between the mast and the top portion of the frame. The bending stresses shift to the joint between the vertical and horizontal frame, which tends to open. The high stresses, combined with the stress concentration created by the weld toe, and the applied cyclic loading, cause the crack at the weld toe.

Omer W. Blodgett, Sc.D., P.E., senior design consultant with The Lincoln Electric Co., struck his first arc on his grandfather's welder at the age of ten. He is the author of Design of Welded Structures and Design of Weldments and an internationally recognized expert in the field of weld design. In 1999, Blodgett was named one of the "Top 125 People of the Past 125 Years" by Engineering News Record. Blodgett may be reached at (216) 383-2225.