Reliability Engineering Snapshot TM

Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation

Machine Design - Case Study No. 33: Weld Toe Profile Stress Concentration

The importance of smooth transitions between weld bead and base metal is easily discounted and thrown aside, especially whenever time and money are involved. How's that old and classic TV commercial go? "Pay me now or pay me later."

Some people prefer to pay later. Have you ever noticed that we tend to design our problems into the project?

Can you say the word "Grinder"?

What it boils down to is that making a smooth transition from the weld toe to the base metal means grinding, and grinding takes time, and time costs money. If the weld is going to be statically loaded, don't worry about grinding it smooth. On the other hand, if that weld is going to see a fluctuating, reversing, or pulsating load, you better grind that baby smooth. Oh ... and by the way, don't expect the contractor to volunteer this type of work. As I said, grinding costs money. You have to write it into the contract and then you may even need a CWI (certified welding inspector) to be your watchdog, and make sure that it actually gets done.

These pictures show the weld bead profile. The jagged rope-like look didn't help where cyclic loading was concerned. This particular weld was on a piece of equipment that had 40 million cycles on it and now the thing was falling apart, big time. Looking at the weld profile to the left, the welder ground down the wrap around portion of the bead but he didn't follow through into the base metal, or for that matter the rest of the weld. It wasn't specified in the bid contract to grind the welds smooth. The crack had initiated and followed along the bottom toe. The pictures below show how the crack followed either the top toe or bottom toe of the weld. This isn't a strength failure, it is a fatigue failure.

The American Welding Society (AWS) states in section 5.24 "Weld Profiles" subsection 5.24.41 "Flush Surface": "All reinforcement shall blend smoothly into plate surfaces with transition areas free from undercut."

The concern behind this comment is one of sharp corners and induced stress risers versus fatigue strength. Now a rope-like bead profile isn't exactly an undercut, but it is a sharp corner when you come right down and take a close look at it.

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