An inlet valve for the dishwasher in a personal residence leaked water causing damage to the surrounding structure. The homeowner contacted the retailer from whom the appliance was purchased, and a claims procedure was initiated to determine the cause of failure. The analysis was restricted to nondestructive evaluation.
A crack in the brass inlet fitting on the valve was observed in the threaded section where a brass elbow had been threaded into the fitting. The crack in the inlet fitting was a straight longitudinal-radial crack that extended along the entire length of the fitting and through its full thickness.
Only small areas of the crack surface were exposed to allow microscopic examination. The nature of the crack in the subject fitting indicated that brittle intergranular fracture was the predominant mechanism for this failure.
Intergranular fractures in brass alloys most commonly occur from the stress corrosion cracking failure mechanism. A simple overload failure, such as from excessive tightening of the elbow into the fitting, would be expected to produce a dimpled fracture morphology and some plastic deformation of the fitting. Neither of which was observed in this case.
Stress corrosion cracking is caused by a combination of a mildly corrosive environment, a constant tensile stress, and a susceptible metal alloy. In this case, the corrosive environment could be the water inside the fitting or an external contaminant, such as a household cleaner. The constant tensile stress is expected in the threaded connection and would be oriented in the direction consistent with the observed cracking. The fitting material (a leaded yellow brass alloy) is known to be very susceptible to stress corrosion cracking, even in relatively benign environments, including some potable water supplies.
The work scope for this analysis consisted of light microscopy and scanning electron microscopy (SEM). For specific details on these methods, please see the Handbook of Analytical Methods of Materials on our website or contact us for more information.
Just because an analysis is limited to nondestructive evaluation doesn’t mean a failure mechanism cannot be determined. The most important parts of any failure investigation are a visual examination and engineering experience. MEE brings the expertise and breadth of scientific experience paired with state of the art specialized equipment to every project. In this case, MEE even made a house call to inspect the failed dishwasher.