Failure Analysis
A Ford van had a rear wheel fall off and cause a considerable amount of damage. Upon removing the intact hubcap, we found lug nuts with still intact broken wheel studs. Fatigue cracks were noted on the fracture surfaces. The van had aluminum wheels, which require the studs to be torqued to a specific tightness. Our conclusion was that the mechanic who rotated the tires failed to properly torque the wheel nuts (they were too loose) causing cracks to occur in the stud threads which progressed until failure.
A fractured connecting rod from an eight-cylinder Bugatti race car was submitted for analysis. Examination of the part's fracture face revealed a ¾" long crack. A cross-section of the connecting rod's surface adjacent to the fracture revealed forging laps which were quite soft in comparison to the matrix. Knoop 500 gm. Microhardness Tests were used to detect the soft areas. It was concluded the soft zone adjacent to the ¾" long crack was a stress riser causing a crack to start.
A metals heat treater sent us a very expensive die made from tool steel, which developed a crack after quenching. The groove cut in the die had no radius at the corners. A stress riser was created when the metal cooled and rapidly a crack started - ruining the part.
