Investigation of a Failed Titanium Valve Stem

A titanium valve stem fractured during service; this presentation discusses the results of an investigation by MEE and supplemental testing conducted by this author. The techniques utilized in the investigation included digital photography, light microscopy, scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS), metallography, tensile testing, microhardness testing, and chemical analysis.

Visual inspection of the fracture surface showed radial lines emanating from multiple origins, ratchet marks, and beach marks. Observation in the SEM revealed microscopic striations. These features are collectively indicative of a fatigue fracture, which occurs due to cyclic loading. The failure features suggested that initiation and propagation of the fatigue crack could not have occurred solely from load cycles imparted by the opening and closing of the valve, so recommendations included identifying sources of vibration.

An additional contributing factor to the failure was surface damage to the shaft in the fracture area due to contact with the valve packing. Recommendations also included further investigation of conditions contributing to the wear damage.