Fatigue Testing of Low-Alloy Steel Fasteners Subjected to Simultaneous Bending and Axial Loads

Abstract

Fatigue testing of high strength, low-alloy steel fasteners has been completed with simultaneous unidirectional bending and axial loads that demonstrates a potentially beneficial effect on fatigue life by accounting for bending. Fatigue tests were performed in load control on fasteners with failures in rolled and cut/machined threads. The objective of the testing was to determine if the fatigue strength of steel fasteners subjected to high bending and axial stresses is higher than in conventional axial, load controlled tests. Test results in the range of 104 to 106 cycles indicate that the fatigue lives of fasteners with a 2:1 ratio of bending to axial stress is from two to ten times as great as that of fasteners subjected to axial loading only. A special fixture design was used for the testing. The fixture permits an adjustable offset between the fastener and clevis centerlines, thus producing an eccentric loading. The eccentric load results in simultaneous application of bending and axial loads. The magnitude of the bending and axial loads was verified by strain gage measurements on the reduced shank diameter of the test specimens. Fatigue life was determined based on failure cycles (specimen separation). Stresses corresponding to failure cycles were based on the sum of the alternating axial and bending stresses at the half-life of the test specimen. A low degree of data scatter and good agreement with published data were observed for the axial load tests performed with the fixture set at zero offset between the clevis and fastener centerlines.