Detection of Fatigue Crack in Metallic Structures with In-Situ Phased Array Ultrasonics

Much of the world’s infrastructure is made from metal. These structures are subject to cyclic loads which can eventually cause the formation of micro-cracks. This would be considered as stage 1, the initiation stage, of the fatigue crack propagation process. Further loading increases the fatigue crack size as it enters stage 2, the propagation stage. If no action is taken the crack becomes so large that failure occurs at stage 3, the fracture stage. Therefore, it is important to detect fatigue cracks before they become too large and cause destruction by employing structural health monitoring methods like ultrasonic testing. Damage indices extracted from ultrasonic signals have been used in the literature to characterize and locate damage. They were based on amplitude and energy of the ultrasonic signals in the time domain and frequency domain, as well as fusions of multiple damage indices. In this research, a guided wave ultrasonic in phase array was used to detect a fatigue crack in-situ in a modified ASTM E647 compact tension specimen, where the crack was designed to grow at the notch tip. A numerical model was created in COMSOL Multiphysics to confirm the formation of the crack at the notch tip first. Dynamic models were also created at frequencies of 400 and 300 kHz to extract the signals needed for the damage index calculations based on amplitude, energy, frequency and phase shift. The experimental study transmitted ultrasonic signals at 400 and 300 kHz to the structural steel specimen after the crack grew at each 10,000 cycle interval. A new damage index using the surface wave arrival as the reference signal was introduced. The damage indices were calculated and compared to the numerical results to determine the most effective damage index for tracking the progress of the fatigue crack. DIs for receivers closest to the transmitter based on amplitude, energy, frequency and phase shift were effective at 400 kHz within the time range of the expected crack reflection.