A dynamic structure under vibration loading within its natural frequency range can experience failure due to vibration fatigue. Understanding the causes of such failure requires pinpointing the initiation time and location of fatigue cracks, tracking their propagation, and identifying the frequency range of critical stress responses. This research introduces a novel, thermoelasticity-based method -- the Temperature-Amplitude Spectrum (TAS) method -- for early-stage, full-field, and non-contact crack detection that operates during uninterrupted vibration testing. This method leverages high-speed infrared imaging to analyze the specimen's temperature-amplitude spectrum, capturing comprehensive crack-related information, including initiation and propagation, in real time. Experimentally validated on both 3D-printed polymer and aluminum specimens, the TAS method accurately identified crack locations and paths without complex adjustments to the experimental setup or data processing. This new approach advances vibration-fatigue testing by enabling reliable, high-resolution crack detection and analysis while remaining computationally efficient.