Valve Stem Failures due to High Load, Low Cycle Fatigue
Stem separation due to low or high cycle fatigue can result from unexpected loads, improperly designed components, or changes in material properties (embrittlement) or loss of part cross-section (due to erosion).
Figure 1 shows the fractured surface of a gate valve stem. The surface shows signs of an initial ductile fracture followed by a more sudden brittle fracture. The root-cause investigation determined that the horizontal alignment of the valve stem resulted in an unanticipated misalignment between the gate and the backstop bushing. The misalignment of the gate caused a bending moment to act on the stem, which was not accounted for in the original design and structural analysis of the stem. The bending moment was not sufficiently large to cause a one-time overload failure but was sufficiently large to cause fatigue failure.