Fractal-Iwan integrated modeling for nonlinear dynamics of bolted joints with dual rough contact surfaces

Bolted joint structures exhibit complex nonlinear behavior because the real contact interface contains rough asperities, local stick-slip zones, and preload-dependent contact stiffness. This study proposes a dynamic characterization method for bolted joints by coupling measured fractal surface topography with an Iwan-type tangential constitutive model in ABAQUS. Three-dimensional rough surfaces were reconstructed from white-light interferometry data through an improved Weierstrass-Mandelbrot (W-M) function, in which the measured fractal dimension and scale coefficient were used to generate the resolvable surface geometry, while sub-mesh asperity effects were represented through pressure-dependent Iwan parameters. Three models, namely a bonded reference model, a smooth-Iwan contact model, and a rough-Iwan contact model, were compared under quasi-static preload, prestressed modal analysis, and harmonic excitation. The results show that surface roughness reduces the effective contact area and produces a highly non-uniform tangent stiffness field. This non-uniformity changes the modal effective stiffness rather than only the average contact stiffness, which explains the non-monotonic frequency differences among the contact models. In the harmonic response, tangential vibration is mainly governed by frictional energy dissipation, whereas the normal and rotational responses are more sensitive to local contact separation and asperity-scale stiffness concentration. The revised model clarifies the role of rough contact morphology in nonlinear bolted-joint dynamics and provides a basis for subsequent experimental modal validation.