Surface Crack Sizing Method Using Rayleigh Waves Generated by Ultrasonic Phased Arrays

Presence of surface breaking cracks on engineering structures such as rails, pressure vessels, turbine blades or pipelines affect the service life. Moreover, if the depth of such cracks is not known, then these parts are typically subjected to direct rejection. Economically this is highly expensive as it can cause the complete disruption of the service. Having an accurate knowledge of crack depth can be used in fracture mechanics analysis to estimate the remaining life of the structure. Among the available nondestructive testing (NDT) techniques, eddy current testing (ECT) is the most widely used method for the detection and sizing of such cracks due to their high sensitivity to surface and near surface defects. However, the size of surface cracks in length and depth requires a complex calibration. Moreover, the depth range is limited by the penetration depth of eddy current. Ultrasonic Rayleigh waves are recently attracting interest for the detection and characterization of surface cracks. The advantages include length and depth sizing as well as the possibility to scan a relatively large area from a limited number of probe positions. The generation of Rayleigh waves can be achieved using piezoelectric transducer, electromagnetic acoustic transducers, air-coupled transducer or laser ultrasound. A considerable research interest on the detection and sizing of surface crack using laser generated Rayleigh waves, has been observed. However, this technique requires extra safety from the users, may require surface preparation and the cost of the equipment is prohibitive. This work presents a method to measure the depth of surface breaking electrical discharged machined (EDM) notches using Rayleigh waves excited and received through conventional ultrasonic phased array probes. Here, the generation of Rayleigh waves is achieved through appropriate delay between the emission of each piezoelectric element of the phased array probe. The time-of-flight (TOF) information of Rayleigh waves and their interaction with the geometry of the notches can be used to size its depth. A two-dimensional finite element (FE) model was used to demonstrate the proposed sizing method. Results obtained from FE simulations show excellent agreement between the measured and simulated true notch depth.