Development of a fatigue crack growth monitoring and failure stage determination technique based on nonlinear ultrasonic modulation

Abstract

This thesis presents the development of a fatigue crack growth monitoring and failure stage determination technique in aluminum plates using nonlinear ultrasonic modulation. When two lead zirconate titanate (PZT) transducers apply low frequency (LF) and high frequency (HF) inputs to a structure, the presence of nonlinear sources, such as fatigue cracks, creates nonlinear ultrasonic modulation components at spectral sideband. The amplitude of modulation components increases gradually, as a fatigue crack in a structure under cyclic loadings propagates. When the fatigue crack propagates enough to let the maximum stress intensity factor (SIF, K) meets the fracture toughness of a material, the structure with the fatigue crack starts unstable brittle fracture. As the fatigue crack undergoes brittle fracture, the crack width increases and the crack surface contacts which is a generation mechanism condition of nonlinear ultrasonic modulation components, is not occurred. As a consequence the amplitude of modulation suddenly decreases. Therefore, it can be seen that the point of time at which the modulation components suddenly decreases is immediately after the point at which the SIF at the crack tip reaches the fracture toughness value of a material. Thus, monitoring nonlinear ultrasonic modulation can diagnose the growth of fatigue cracks and the failure stage which the structure begins to brittle fracture. The aluminum 6061-t6 specimens with various thicknesses were tested to verify the developed technique.