Trunnion Rod Failure Analysis

In the 1960's and 70's, a number of locks and dams were built on rivers across the United States. The foundations for the trunnions supporting the tainter gates on these structures used high strength rods to pre-stress the concrete foundations. At the time of construction, the yield strength level of the trunnion rods was below the known limit for stress corrosion cracking (SCC) in steel. These rods have recently begun to fail, which is a serious issue as rod failure is violent, with the strength to punch through steel covers.

Initial failure analyses only reported a brittle type fracture. Two of the rod failures were then made available to Metallurgical Consulting for a more detailed evaluation. Cleaning revealed a distinct thumbnail type pre-existing crack origin, which was followed by a cleavage brittle fracture. Numerous other stable cracks were also observed at small corrosion pits.

The cracking mode was determined to be stress corrosion cracking, propagating out of corrosion pits. The long period before failures was consistent with the absorption of hydrogen from the corrosion pits; this was followed by hydrogen driving the mode for SCC. Thus there is a long incubation period for SCC to develop. Compounding this scenario was the fact that the rods were supposed to have been protected from corrosion; however, no protective coating was found. Mechanical properties including fracture toughness were determined by the Automated Ball Indentation procedure by Advanced Technology in Oak Ridge, Tennessee. These tests provided data showing that the rods met strength specifications and provided fracture toughness, KIc, data to estimate applied stress levels. One rod with the distinct pre-existing flaw was determined to have failed at or above recorded pretension loads. The second rod was determined to have failed during original installation.

For more information on this failure analysis project, contact Metallurgical Consulting directly.

Images From The Metallurgical Services Provided For This Trunnion Rod Failure Analysis

(click on thumbnail to enlarge)

Figure 1: A distinct crack origin is shown.

Figure 2: A SEM micrograph of the crack origin shows several cracking phases.

Figure 3: Corrosion pit with a SCC crack propagating out of the pit.

Figure 4: Trunnion rod microstructure was fully pearlitic.