Analysis of Multiple Hydraulic Piston Cap Screw Failures

This failure analysis involves seven one-inch alloy steel socket head cap screws from the end cap of a hydraulic cylinder used in a steel mill payoff reel. The fracture of these bolts caused a significant loss in revenue due to the unplanned downtime on the continuous annealing line. Visual inspection of the fracture surfaces revealed classic indicators of fatigue such as ratchet marks, thumbnail cracks, and beach marks on all seven bolts. Low magnification observations found surface discontinuities on one of the bolts that were non-conforming to ASTM A574 visual inspection requirements. Rockwell hardness testing found that three of the bolts had average HRC values below that of the material grade limits reported by the supplier, suggesting a possible material or heat treatment error. Scanning electron and optical microscopy of the fracture surfaces revealed possible inclusions at the base of several ratchet marks, which may have decreased fatigue life due to the presence of microstructural flaws. The estimated fatigue life was calculated for nominal operating conditions using the Paris Law approximation with modifications for crack closure and threshold behavior. In combination with fatigue life, an analysis of the bolt pre-load was also conducted. These analyses determined that the fatigue failures were not the result of inadequate pre-load, which would have resulted in flange separation and loss of hydraulic pressure prior to bolt failure. Recommendations to prevent future failures are to investigate material quality control measures such as grade marking, requiring the use of a shop traveler to track visual inspections and assembly procedures, comparing the quality of in-use bolts to bolts from other suppliers, and considering a safe-life maintenance schedule to mitigate future failures.