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End Cap Failure

Background: The end cap of a large scale pipe tester failed below its load rating. A metallurgical failure analysis was conducted to identify the failure mechanism.

Figure 1: Visual Inspection of As-Received Failed End Cap

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Figure 2: SEM Micrographs and EDS Analysis (wt.%) of Maximum Thickness Loss Regions

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Summary: The end cap failure was associated with sulfide stress corrosion induced initial cracking at the ID near the fillet radius. This initial fracture surface region showed intergranular cracking. A first order approximation Fracture Mechanics calculation, assuming a semielliptical internal surface crack at the failure load conditions, indicates the initial sulfide stress corrosion crack induced a sufficient stress intensification to break the end cap. The small fillet radius (~1/32”) facilitated the formation of the stress corrosion crack by inducing a stress concentration in the order of 2 to 3 times the nominal stress at the cross-section. The stress intensification at the crack of tip controls stress corrosion crack propagation until failure occurs when the fracture toughness of the material is exceeded. Take Away: An initial stress corrosion cracking attack in a localized region can lead to castrophic failure of the component normal operating loads below the rated capacity of a component.
Take Away: Temperature excursions coupled with improper cooling can lead to premature failures of TLEs. The microstructural changes in the material due to thermal exposure can further facilitate the loss of material under synergistic mechanisms of metal dusting and erosion.

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