Method for evaluation of surface crack size of wind turbine main shaft by using ultrasonic diffracted waves

Abstract

The structural damage in the main shaft threatens the service safety of the wind turbine directly. The mechanical properties of the in-service main shaft tend to degrade severely due to the transverse cracks extending from the outer circumference. In the health monitoring of the shaft, it is of great importance not only to identify the cracks, but also to accurately evaluate the crack size. In this research, a novel quantitative method is proposed for the transverse crack characterization. The diffracted waves from the crack tip received in the central hole of the shaft are used to construct elliptic trajectories of beam path. The crack position and its depth can be determined by the intersection of the trajectories. Furthermore, a crack size quantification model is established, and the numerical demonstration is given for evaluation of crack position and depth. Meanwhile, an acoustic finite element model is developed, in which the path of the acoustic waves radiated from the end face of the shaft is analyzed in detail. The quantitative method is well confirmed by the simulation of cracks at different locations with different depths. Considering the accuracy of crack evaluation from a large size zone, the time of diffracted echo is calibrated by measuring the time shift of the transducers. A shaft sample with a transverse crack was used to implement the experimental verification. Totally, the sizing error is less than 5 mm, which indicates that this proposed method is effective for the evaluation of surface cracks in the main shaft.

A feasibility study on monitoring of weld fatigue crack growth based on coda wave interferometry (CWI)

“…Recently, various nondestructive testing (NDT) methods [8,9] and structural health monitoring (SHM) approaches [10,11] are applied to detect metal cracks [12] in the early stage of welded structures, including penetration testing (PT) [13], magnetic testing (MT) [14], x-ray radiography [15], eddy current testing (ECT) method [16,17], acoustic emission (AE) [18], ultrasonic techniques [19] and other methods [20]. However, the PT and MT are mainly for the defects on the surface or near surface of materials and not applicable to buried or undersurface defects.…”