Thermoelasticity-based full-field modal analysis and fatigue damage identification

Klemen Zaletelj, Jaša Šonc, L. Capponi, J. Slavič

Visual spectrum cameras have become increasingly popular for non-contact full-field structural dynamics measurements, enabling displacement and deformation identification through techniques such as Digital Image Correlation. However, obtaining strain information from kinematic measurements requires spatial differentiation, which significantly amplifies noise and necessitates known analytical relationships between displacement and stress—particularly challenging for complex geometries. As an alternative, the thermoelastic principle exploits the reversible relationship between cyclic mechanical stress and surface temperature variations under adiabatic conditions. High-speed infrared cameras can directly measure stress-induced temperature oscillations, typically in the millikelvin range, providing full-field stress information without spatial derivatives. For strain-based experimental modal analysis, hybrid identification methods combining thermoelastic measurements with high-dynamic-range reference sensors successfully extracted full-field strain mode shapes from Euler-Bernoulli beams. Critical findings established that minimum surface stress amplitudes of approximately 100 kPa for aluminum and 500 kPa for steel, enable accurate modal identification. These thresholds provide essential design criteria for experiment feasibility assessment. Thermoelasticity was extended to rotating structures by integrating computer vision techniques with infrared imaging. Infrared-compatible ArUco markers enabled motion tracking and compensation, successfully identifying out-of-plane structural dynamics in rotating beams despite in-plane rigid body motion. Sub-pixel transformation accuracy was achieved, with the frames-per-degree ratio emerging as a key performance metric across varying operational conditions. For vibration-fatigue assessment, a novel thermoelasticity-based multiaxial criterion was developed. Since thermal cameras inherently measure the first stress invariant (sum of normal stresses), this criterion provides a natural multiaxial-to-uniaxial conversion for fatigue analysis. Experimental validation on Y-shaped specimens demonstrated accurate identification of critical damage locations when normal stresses dominate, proving highly effective for bending-dominated loads where normal stresses govern crack initiation. The methodology enables close-to-real-time, full-field fatigue damage assessment, opening new possibilities for structural health monitoring applications.