Development of a Portable Phased Array Ultrasonic Test Methodology for Non-uniform, Thick Ablative Nozzle Liners Used in Launch Vehicles

Ablative throat insert liners made of silica phenolic and carbon phenolic composite materials are used in liquid and solid stage nozzles respectively in ISRO’s launch vehicles. These thick liners (thickness at the throat region can be from 50 to 150 mm) operate in extreme thermal environments. In the case of solid motors, these liners can also be subjected to erosion due to solid propellant particle impingement. Non-destructive testing (NDT) plays a significant role in the quality assurance of these mission-critical liners. Ultrasonic normal beam pulse-echo testing used to be performed at the cylindrical configuration of these throat insert liners. After the ultrasonic testing, the liners used to be machined to their final configurations. Subsequently, a new manufacturing process that enabled the realization of near-net-shaped liners with multiple taper angles on the inside surface was implemented. Though the new process offered significant savings in material, machining and realization times, NDT assessment using the standard ultrasonic pulse-echo testing could not be carried out because of the non-uniform thickness of these liners. A search for alternate options led to the identification of phased array ultrasonic testing (PAUT) as a potential method. However, a proper evaluation procedure was required to assess the suitability of the application. This paper explores the ultrasonic couplant restrictions, criteria used for the reference specimen, initial studies and the considerations that led to the identification of additional requirements, which helped in evolving a test methodology that could provide satisfactory full volume coverage.