On-line detection of cracks using surface wave propagation techniques

Surface acoustic waves (SAW) can be used to detect a crack in a component by monitoring the transmitted energy between a sending and a receiving transducer. In this paper, it will be shown that the success of the technique largely depends on the stress in the vicinity of the crack (when the crack is closed the SAW energy is almost completely transmitted and consequently the crack cannot be detected). This means that the use of the SAW method is not possible during a fatigue test (i.e. on-line), because in that case the stress, and consequently the transmitted SAW energy, typically varies in cycles. In this paper, a novel approach will be proposed to circumvent this problem. During one cycle of a fatigue test several pulses will be transmitted and received. Then a parameter that describes the statistical distribution of this set of transmission ratios is calculated. It will be shown that this parameter is a good indicator of the crack length that is not sensitive to the stress state of the structure. Therefore, the method can be used on-line during the fatigue test. The proposed method will be validated on a steel beam with a propagating crack.