Preload Attenuation Behavior of Bolted Joint Structures Under Transverse Vibration

Bolt self-loosening under transverse vibration is governed by frictional contact and accumulated interfacial slip. In this study, transverse vibration tests were performed on M8 bolted joints under different initial preloads, displacement amplitudes, and excitation frequencies, with three tests per condition. Preload histories were analyzed using an operational four-stage framework comprising an initial abrupt decrease, lower-rate attenuation, a sustained acceleration interval, and a decelerating tail. N2 denotes the operationally identified onset of sustained acceleration, rather than an instantaneous physical discontinuity. Stage-II/III rate contrasts vary between conditions and are modest for several 0.3 mm cases. A model with an independently estimated N2 was favored over the specified alternative with acceleration starting at N1, subject to the comparison assumptions and conditional uncertainty analysis. Quantitative comparisons use repeat-test statistics normalized to each test’s own initial preload. At 5 Hz and 0.3 mm, the mean cycle counts to 50% preload are 206.20 ± 25.35 and 116.25 ± 17.75 cycles for nominal initial preloads of 10 and 12 kN, respectively. At 10 kN and 5 Hz, reducing amplitude from 0.3 to 0.2 mm increases this milestone to 922.25 ± 97.15 cycles. For nominal 12 kN and 0.3 mm, the values at 5, 7 and 10 Hz are 116.25 ± 17.75, 137.34 ± 17.36 and 267.70 ± 32.00 cycles, respectively. Values are means ± sample SDs. In the illustrated records, an independently identifiable later acceleration interval was not observed within 1110 cycles at 15 kN (Group 5) or within 1405 cycles at 0.1 mm (Group 7). The finite element model reproduces the overall staged form but shows substantial transition-time mismatch and is used to interpret representative contact-state evolution. The four-stage framework is a conditional description of the tested histories; it does not establish universally discrete mechanisms or a stage sequence applicable to every condition.