A cast aluminum alloy « Y » fitting failed during fire hose pressure testing and injured a firefighter. The failed fire trick fitting was submitted for metallurgical failure analysis.
The, as received, knock-on/quick disconnect coupler and failed « Y » are shown in Photograph A.
Photograph A: Overall view of cast aluminum « Y » fitting that failed during fire hose pressure testing.
The casting in question, after apparently 27 years of useful service, failed in a sudden manner. The casting surface did not contain any manufacturer markings, serial numbers, casting lot numbers, etc. which would identify the manufacturer and/or the year of manufacture.
Testing included stereomicroscopic examination, scanning electron microscopy (SEM), metallography, microstructural examination, chemical analysis and hardness testing. Considerable macro and micro-porosity was observed on the primary fracture face. Photograph B illustrates an area of dendrite structure and porosity surrounded by clear indications of cleavagefractures. However, observing shrinkage and the dendrite structure on a casting fracture surface does not render that casting defective. A true and very valid test of adequacy is the performance history of the casting. As time passes and usage increases, the probability of a significant flaw being present diminishes quickly to zero. Additionally, no evidence of progressive fatigue failure was observed on the casting fracture surface or near any area of casting porosity. Fracture features observed where fracture did occur were cleavage, indicating a sudden failure of a casting material lacking in significant ductility. The lack of significant ductility is not uncommon to cast metals.
Photograph B: Scanning electron micrograph showing dendrite casting porosity (rounded egg shaped structure). The yellow arrows denote areas of cleavage fracture occurring during the sudden overpressure failure.
It was determined that the cast metal « Y » fitting failed due to an over-pressurization from an unknown source.