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 No. 29: Benefits of Using Liquid Dye Penetrant to Find Cracks


Extraneous Weld Bead Starts Stress CrackUsing liquid dye penetrant to detect cracks is easy, cheap, quick, and can be a life saver. It should be in every inspector's tool box.

The reinforcing ring shown to the left had failed once in its lifetime. It was nearing that point in time when fatigue was going to play a big factor in squeezing whatever useful life was left out of it. The failure analysis had discovered that the cause of failure started from the bottom of a weld bead where there was a lack of fusion. The disturbing fact was that there shouldn't have been any weld beads on the outer diameter of this reinforcing ring. A typical weld bead is shown in the picture to the left (white line highlight weld centerline). Closer inspection revealed about a dozen of these haphazard weld beads. It almost gave the appearance of cosmetic blemish cleaning of rolling tears by the fabricator, done after rolling and prior to finishing. This was never substantiated. Regardless, they needed to be removed.

They were ground out and then checked with dye-penetrant. The penetrant (bottom middle picture) is absorbed into any cracks that might exist. The excess is then wiped off and a "developer" is sprayed onto the clean metal surface (bottom right picture). If there are any cracks the dye will be pulled out by the developer and a red line will appear. The deeper the crack, the more dye penetrant is pulled out by the developer, and the bigger the red line. A deep crack will appear as a large blotch.

We were lucky and found no cracks. The ground surface was not built back up with weld metal.

Weld Bead Removed Dye Pentrant Test - Dye Application Dye Penetrant Test - Developer Application


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