Phase-Based Motion Magnification in Optical-Flow-Based Fatigue Crack Assessment

Adam Machynia, Z. Dworakowski, K. Dziedziech, M. Dziendzikowski, K. Holak

TL;DR: The results show that motion magnification substantially enhances crack visibility when amplitude maps are derived from the optical-flow magnitude at low excitation amplitudes, extending the regime in which the crack-breathing pattern is discernible.

This paper investigates the role of phase-based motion magnification in a computer-vision-based methodology for marker-free fatigue crack assessment introduced in earlier work. The approach uses dense optical flow and frequency-domain analysis to capture periodic crack-breathing motion. The resulting spatial “amplitude maps” highlight regions dominated by crack-induced cyclic displacement and enable crack visualisation. We present a focused case study examining how motion magnification influences the quality and interpretability of these amplitude maps and under which conditions its use is justified. Experiments are conducted on a cantilever beam with a fatigue crack subjected to harmonic excitation at a known frequency. High-speed camera recordings are acquired across a range of excitation amplitudes, and motion magnification is applied as an optional pre-processing step. For each configuration, dense optical flow is computed and amplitude maps at the excitation frequency are formed using either the optical-flow magnitude or directional components. Maps obtained with and without magnification are compared in terms of crack-related pattern clarity and background artefacts. The results show that motion magnification substantially enhances crack visibility when amplitude maps are derived from the optical-flow magnitude at low excitation amplitudes, extending the regime in which the crack-breathing pattern is discernible. In contrast, for directional optical-flow components, which already provide clear crack visualisation, magnification yields little or no benefit and may degrade map quality at larger displacements. The study demonstrates that motion magnification is a task- and regime-dependent option rather than a default stage in amplitude-map-based crack-assessment pipelines.