This paper describes a fatigue lifetime acceptance test criterion by which studs having rolled threads, larger than 1.0 in. (25 mm) in diameter, can be assured to meet minimum quality attributes associated with a controlled rolling process.
This criterion is derived from a stress-dependent, room temperature air fatigue database for test studs having 0.625 in. (16 mm) diameter threads of Alloys X-750 HTH and direct aged 625. Anticipated fatigue lives of larger threads are based on thread root elastic stress concentration factors which increase with increasing thread diameters. Over the thread size range of interest, a 30% increase in notch stress is equivalent to a factor of five (5x) reduction in fatigue life. The resulting diameter-dependent fatigue acceptance criterion is normalized to the aerospace rolled thread acceptance standards for a 1.0 in. (25 mm) diameter, 0.125 in. (about 3 mm) pitch, Unified National thread with a controlled root radius (UNR). Testing was conducted at a stress of 50% of the minimum specified material ultimate strength, 80 ksi, (552 MPa) and at a stress ratio (R) of 0.10. Limited test data for fastener diameters of 1.00 to 2.25 in. (25 to 60 mm) are compared with the acceptance criterion.
Sensitivity of fatigue life of threads to test nut geometry variables was also shown to be dependent on notch stress conditions. Bearing surface concavity of the compression nuts and thread flank contact mismatch conditions can significantly affect fastener fatigue life. Without improved controls these conditions could potentially provide misleading acceptance data. Alternative test nut geometry features are described and implemented in the rolled thread stud specification, MIL-DTL-24789(SH), to mitigate the potential effects on fatigue acceptance data.