Preventing Weld Failures by Early Detection and Repair of Cracks

Metal structures can fail prematurely because of problems resulting from the welding process. It’s important to remember that failure of a structure or part can include serious deterioration or cracking that renders the affected area unreliable or unsafe for use.

There are three goals in failure analysis:

1. Find the primary cause of the failure.
2. Consider other contributing factors and assess their relevance
3. Prevent similar failures through corrective action

Failure analysis begins with a thorough visual inspection, an overlooked source of clues. The next step uses non-destructive testing methods such as ultrasonic inspection or radiography. These two strategies allow evaluation and detection of cracks before failure occurs.

Once a fracture has occurred, mechanical testing tells the engineer whether the fracture is ductile, brittle or caused by fatigue. Macro-etch techniques are used during this process to clean and examine fracture surfaces more closely.

The case under analysis was a weld fault, specifically an arc strike on a carbon steel plate.

Common weld defects (such as craters, undercut and arc strikes) can significantly reduce service life and affect the crack propagation rate. These faults can also cause premature failure especially when structures are loaded cyclically and exposed to low service temperatures.

The resulting analysis of weld failures shows that most cracks are caused by fatigue fracture and can develop at loads much smaller than service loads.

But the good news is that cracks occur quite slowly over time. With regular inspections, cracks can be detected and repaired before a structure experiences failure.

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