Fatigue crack growth, free vibrations, and breathing crack detection of aluminium alloy and steel beams

This paper deals with online controlled propagation and vibration-based detection of fatigue cracks in metal beams constituted of two different materials: 6082-T651 aluminium alloy and Fe430 steel. The study addresses the initiation and propagation of cracks in the structures and their influence on the free-vibration dynamic response. One of the original aspects is the introduction of an actual fatigue crack instead of - as is usual - a narrow slot. First, the crack growth is predicted analytically by numerically integrating the Paris-Walker equation. Then, three-point bending tests are performed to obtain edge transverse cracks; two original control procedures enable the tests to be traced, the results of which are compared with the numerical predictions. Second, free vibrations of undamaged and cracked cantilever beams are excited by hammer impact. The experimental results are compared with the numerical solutions of a finite element model including local flexibility increase at crack opening. The differences between the dynamic behaviours of the intact and cracked beams in terms of frequency and damping allow the damage to be detected. Even if this is a 'linear' method, it seems to enable the crack presence to be detected and to account for the so-called 'breathing' crack. These features open the door to future developments towards nonlinear detection methods.