The Utilization of Ultrasonic Characteristics for Identifying Precursors of Low-Cycle Fatigue Crack Formation in AA5083-H112 Aluminum

Abstract

This study investigates the evolution of fatigue-induced damage in AA5083-H112 aluminum alloy under low-cycle fatigue using linear and nonlinear ultrasonic techniques in conjunction with X-ray diffraction (XRD) and optical microscopy. Three independent tensile specimens and three independent fatigue specimens were tested to verify the repeatability of the experimental results. Residual stress and full width at half maximum (FWHM) gradually increased during the early fatigue stage because of dislocation accumulation, followed by a decrease as fatigue damage progressed. Acoustic velocity exhibited significant changes during the early fatigue stage, whereas the root mean square (RMS) amplitude and the time of flight (TOF) varied predominantly during the later stages of fatigue. Optical microscopy revealed potential microcrack initiation sites at approximately 2200 fatigue cycles, followed by clearly identifiable microcracks at 2700 cycles. During this transition, the nonlinear ultrasonic SPC-I exhibited a pronounced increase during the transition from potential microcrack initiation to visible microcrack formation, indicating high sensitivity to fatigue damage evolution prior to the development of visible surface cracks. The combined results demonstrate that the integration of SPC-I with acoustic velocity, TOF, residual stress, FWHM, and optical microscopy provides a reliable and complementary approach for monitoring fatigue damage evolution, particularly during the transition from microstructural damage accumulation to microcrack formation.