Fatigue Test Setup & Results

 Fatigue has never been an issue with the current steel Navy tiedowns because they never lasted long enough in service. The current steel tiedowns have failed in seawater corrosion (often every 4 years) before any could experience enough service spectrum loads to fail in fatigue. Since this new Inconel 718 tiedown is completely impervious to seawater corrosion and can now last for the full 40 year service life of an Arleigh Burke destroyer it would be imprudent not to consider the expected fatigue life of this new tiedown. Since no data is available concerning the probable service load spectrum, engineering estimates had to be drawn upon to arrive at a reasonable load spectrum for this fatigue test. Knowing the purpose of and uses of these tiedowns on a destroyer it was decided that a million cycles at 5400 pounds would adequately prove that this Inconel tiedown would not fail in fatigue during service conditions. From the FEA analysis we previously performed a symmetrical load of 5400 pounds which induces a maximum Von Mises stress of 16.1 ksi which is lower than the endurance limit of this material with a Kt=3 so one would expect this part to be able to endure an infinite number of cycles at this service load. For this fatigue test 1 million cycles of this load were applied which is a reasonable number of load cycles that one could expect this tiedown to experience in service over 40 years. After enduring and passing both cycles of the severe 32,000-pound symmetrical vertical pull static test and cycles of the 25,000 asymmetrical vertical pull static test with no damage or permanent deformation this same tiedown then endured 1 million cycles of an expected service load of 5400 pounds. If this DMLS part was inferior to a machined part we would have expected it to fail, but instead it passed all tests and is even now undamaged, and as good as new, and ready for millions and millions more service load cycles.