MechE PhD Final Oral Defense: Nicholas Pomianek

TITLE: IMAGING THE DYNAMICS OF COMPLEX CONNECTIONS: ADVANCING MODELING AND EXPERIMENTAL METHODS FOR BOLTED JOINTS WITH DIGITAL IMAGE CORRELATION

ABSTRACT: Bolted joints are a ubiquitous feature of engineered structures, yet an accurate predictive model of their dynamic behavior remains elusive. The central challenge is that friction forces at internal joint interfaces exhibit complex nonlinear behavior but are extremely difficult to measure directly. Exploring this behavior is motivated by the fact that accurate prediction of fatigue life, corrosion resistance, and vibroacoustic behavior in structures that incorporate mechanical joints requires accurate modeling of frictional interactions at joint interfaces. This dissertation develops new experimental and computational methods for characterizing these internal dynamics, advancing both measurement capability and modeling efficiency. First, the effective joint region (EJR) method is introduced, a computationally inexpensive finite element approach that models joints as beam elements with tunable stiffness and damping properties. Particle swarm optimization calibrates these properties against experimental frequency response data, producing models that can be implemented in any commercial finite element package without specialized contact elements. The method is demonstrated on the Brake-Reuß beam with robust agreement across multiple assembly conditions and benchmarked against detailed Abaqus models. Applied to the Brake-Reuß beam benchmark, the method achieves robust agreement with both experimental data and high-fidelity Abaqus simulations across multiple assembly conditions. Second, full-field high-speed 3D digital image correlation (DIC) is applied to a corner-bolted plate system under dynamic load. Full-field measurements of one outer surface of the plate system during an impact hammer tap test are captured and partitioned by spatial region. The modal content of each region is analyzed separately and compared to spatially localize nonlinear behavior for the first time. By systematically varying contact area, a spectrum of friction regimes is studied and a generalized displacement-based zeroed early-time Fourier transform (ZEFFT) approach is developed for DIC measurements. The results establish a clear relationship between contact conditions and dynamic nonlinearity. Third, a four-camera high-speed stereo DIC methodology is developed that images both sides of a bolted joint simultaneously, enabling an inference measurement of the relative motion occurring at the internal interface during vibration. A vectorized relative displacement formulation isolates true joint motion from apparent motion caused by beam bending. Four complementary analysis approaches — RMS mapping, a nonlinearity parameter