Non-Destructive Testing of Fracture Toughness for Pipeline Steels

Main Objective

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. The specific objectives of the proposed project include:

1. Develop and fabricate a nonlinear guided wave transducer, hardware platform, and software package for the nondestructive evaluation of fracture toughness in pipeline steels.
2. Develop a test procedure for the nonlinear guided wave system.
3. Acquire a series of specimens having a range of fracture toughness values for testing and verification of the nonlinear guided wave system.
4. Perform destructive testing, as necessary, to evaluate the fracture toughness of the specimens.
5. Deliver a prototype nonlinear guided wave system for nondestructive evaluation of fracture toughness in pipeline steels, along with appropriate documentation, to the DOT or one of its clients.

Public Abstract

The Office of Pipeline Safety, within the Pipeline and Hazardous Materials Safety Administration (PHMSA), is responsible for regulating the products transported by the 2.8M miles of pipelines throughout the country (see Figure 1). Not only is the U.S. pipeline infrastructure the largest in the world, it is the oldest, making the integrity of the pipelines a critical concern from both safety and economic standpoints. The industry recommended practice (American Petroleum Institute standard RP 579-1) to assess fitness-for-service requires knowledge of the fracture toughness in order to determine the critical flaw size. Unfortunately, too many pipelines have undocumented properties. Moreover, nondestructive determination of fracture toughness for in-service pipeline steel is challenging. Nonetheless, the fitness-for-service analysis of pipelines operating in the U.S. with or without flaws is a critical component of pipeline operations management. If inspection reveals the presence of a flaw, then decisions must be made regarding usage, repair, or replacement. In the case where no flaws are detected, the minimum detectable flaw size (or rather, the largest flaw that might go undetected) needs to be related to the critical flaw size. The caveat is that in fracture mechanics the critical flaw size is related to the fracture toughness of the material, which is a material parameter that is microstructure-dependent. The microstructure in turn, depends on both composition and processing. Laboratory tests of fracture toughness are destructive, but invaluable if available. Unfortunately, material pedigree information is not always available for pipelines. Even if material pedigree is known, then the question changes to whether the material has aged in-service. Aging-related microstructural evolution can occur due to thermal, chemical, or corrosion processes.