NDT of Fracture Toughness for Pipeline Steels

Fracture toughness is the material parameter that, along with applied loading and flaw geometry, dictates the critical crack size, and therefore controls the flaw size that must be detected during inspections. A major challenge facing pipeline operators is that many pipelines are old, making their structural integrity uncertain, and that material pedigree of the pipeline steel is unknown. Therefore, operations must be based on minimum values of the material parameters. The probability that actual fracture toughness is higher than the minimum is reasonably high, which means that pipelines are operating below capacity. Nondestructive determination of fracture toughness by ball indentation has not sufficiently solved the problem. Nonlinear features of ultrasonic wave propagation are sensitive to material nonlinearities associated with microstructure (based on composition and processing) that dictate fracture toughness. Nonlinear ultrasound-based technology to nondestructively determine fracture toughness of pipeline steels was demonstrated in Phase I. Building upon the Phase I work, the primary goal of the Phase II project is to develop, verify, and deliver a nonlinear guided wave system for nondestructive evaluation of fracture toughness in pipeline steels.