A novel method for the severity evaluation of a loose bolt based on the nonlinear output spectrum

Bolted joints are frequently applied to fasten structures because of their obvious advantages in economy and convenience. However, they are also easily subjected to loosening under vibration loads, which may seriously affect structures’ integrity and reliability. Thus, diagnosing loose bolts as early as possible is very important. To consider some issues in existing nonlinear output spectrum-based methods, a new method for the severity evaluation of a loose bolt is presented in this paper. Firstly, the dynamic model of one typical bolted joint is simplified to a single degree-of-freedom system with nonlinear stiffness properties, and the relation between linear/nonlinear stiffness coefficients and output spectra is studied. Thereafter, with a harmonic excitation with a single frequency, linear/nonlinear stiffness coefficients are identified with nonlinear output spectra directly. Based on this, three effective evaluation indexes, which are proportional to bolt pressure, preload and torque, are defined for the severity evaluation of the loose bolt. Results from theoretical analysis, simulation examples and experimental cases show that as the loose severity increases, the first index will decrease, the second index will increase, and the third index will also increase with obvious larger change trend.