LLT-tandem inspection of thick-section welds using phased-arrays probes

Abstract

The examination of welds for planar discontinuities whose plane lies perpendicular to the inspection surface is an inspection problem for which tandem ultrasonic testing has long been used as a solution. There exist standards that set out how to design a conventional tandem examination with 45° transverse-wave probes as well as how to design an LLT-tandem inspection in which angled longitudinal waves reflect from the far surface of a component so that they can undergo mode conversion on the face of a defect perpendicular to the surface and return to a receiving probe as transverse waves. For the End-of-Manufacture inspections of the high-integrity components of Hinkley Point C power station, the LLT-tandem technique has been applied to thick-section welds with internal cladding, by using phased-array probes to create a series of “crossing depth” configurations that cover the thickness of the weld. The considerations used for the design of the probes are explained. The resulting design covers the thickness with a 5 mm pitch with a single mechanical arrangement of two separate probes. The design enables coverage of welds with thickness exceeding 200 mm. Other multi-leg reflection modes are possible and the data analysts for these inspections were required to understand and recognise the other modes that could occur. The LLT-tandem inspections form part of multi-technique inspections for a number of high-integrity component welds that have been successfully subjected to Inspection Qualification prior to being used for the final examination of these welds.