Guided wave evaluation of pipes using the first and second order torsional wave mode

Abstract

Guided wave inspection is a form of ultrasonic testing used for non-destructive testing (NDT). Guided waves are capable of propagating long distances bounded by the geometries of the specimen, such as pipes and plates. The technique is commercially used to detect defects in pipelines and is capable of a full volumetric screening many metres (often up to around 100m) from one location. Fundamental axisymmetric wave modes are used to inspect pipelines and are used to quantify defects and features. However, as the technology has progressed, a demand for improving defect sensitivity, spatial resolution and developing the technology into new fields has been recognised. Operating at medium range frequencies is one possibility that could provide the increase in defect sensitivity and spatial resolution required that may not be achieved at low range frequencies. The use of higher order wave modes could also provide additional information useful for defect sizing. Guided wave inspection is a complex ultrasonic technique due to the many wave modes that exist and testing at medium range frequencies requires some challenges to be overcome. The research presented here investigates the potential of using the second order torsional wave mode at medium range frequencies and provides a new sizing technique that for some applications is likely to offer advancement in guided wave inspection and monitoring. The approach firstly included the design and implementation of a setup for analysing the complex signal responses in order to access the higher order torsional wave mode T(0,2) for defect sizing. An efficient method of using FEA has been presented using segmented models to provide the capability of analysing defects with small increment changes that could not be achieved using a full 3D model of the pipe. Using a pipe segment to virtually represent the full pipe also allowed small changes in defect size to be investigated, which would otherwise be extremely difficult to accurately machine experimentally. The FEA modelling technique is also based on broadband signals in comparison to the conventional approach of using narrowband signals and is capable of obtaining a wide frequency spectrum from one model, which significantly reduces the number of models needed to conduct a frequency analysis. Following on from this work, a high density transducer array was developed and compared against a conventional transducer array used in guided wave inspection for the purpose of medium range frequency inspection, which can also be applied to conventional low range frequency inspection. Finally, a new defect sizing method using T(0,2) is presented, which is capable of predicting the depth using peak amplitude responses from spectral analysis and by comparing this to the cut-off frequency of the remaining wall thickness of the defect. The technique has the potential to improve defect sizing, defect sensitivity, increase spatial resolution, and increase the performance of medium range inspection.