A special issue of Journal of Pipeline Engineering
Fracture toughness testing, evaluation, and application for pipeline steels?
THIS SPECIAL ISSUE of the Journal of Pipeline Engineering is dedicated to the topic of fracture-toughness testing, evaluation, and application for pipeline steels. It is well known that fracture toughness is the most important material property required by fracture-mechanics’ methods. For pipeline steels, the commonly-used fracture-toughness parameters are Charpy-V notch (CVN) energy, drop-weight tear test (DWTT) energy, stress-intensity factor K, J-integral, crack-tip-opening displacement (CTOD), and crack-tip-opening angle (CTOA). These parameters have been extensively used in the oil and gas pipeline industry for engineering design and structural-integrity assessment, including material selection; material-performance evaluation; defect assessment; fatigue-life estimation; crack, leak, or rupture determination; engineering-critical analysis; fitness-for-service analysis; fracture-initiation control; and fracture-propagation control. Thus, fracture-toughness testing and evaluation are critical to pipeline steel manufacture and to structural-integrity assessment and management.
More than ten experts in this area were invited to write papers for this special issue, including selected organization representatives, influential scientists, engineers, and academics who have made significant contributions to the development of fracture-toughness test methods and are recognized internationally in the area of fracture-toughness testing and evaluation, and its application to pipeline steels. These authors come from different countries and regions in the world, including North America (USA and Canada), Europe (UK and Belgium), South American (Brazil), and Asia (China). Eleven papers were selected to cover fracture-toughness test methods, test procedures, experimental techniques, experimental evaluations, and standard developments, as well as their applications to transmission pipelines for the six fracture-toughness parameters of CVN, DWTT, K, J, CTOD, and CTOA.
Among the six fracture parameters, the K and J are true fracture parameters and have special significance. The K factor, proposed in 1957 to describe the elastic crack-tip field, symbolized the birth of linear-elastic fracture mechanics, and the J concept, proposed in 1968 to characterize the elastic-plastic crack-tip field, symbolized the birth of elastic-plastic fracture mechanics. Over a subsequent half century, numerous efforts have been made to develop valid fracture-toughness test methods and standards. In the first paper, the undersigned Guest Editor delivers a brief review of historical efforts as well as recent advances in the development of the critical K and J testing, resistance-curve testing, experimental estimation, and standardization by the American Society for Testing and Materials (ASTM). The ASTM standard specimens include the three-point bend and compact-tension specimens.
Fracture toughness is known to depend on the crack-tip constraint due to geometry and loading configurations. To provide a more meaningful measure of toughness, ‘low-constraint’ tests are developing using single-edge notched tensile (SENT) specimens. In the second paper, Dr William Tyson et al. describe the development of SENT tests in terms of J and CTOD for determining crack-growth-resistance curves that can be used to assess the tolerance of weld flaws to tensile loads. The seventh paper by Professor Claudio Ruggieri presents the J-integral resistance-curve testing for pipeline girth welds using the conventional bending and SENT specimens.
In order to better characterize dynamic ductile fracture toughness for pipeline steels, the CTOA was proposed as an alternative fracture parameter, and different CTOA test methods have been developed with use of different laboratory specimens. In the third paper, Dr Su Xu et al. describe the CTOA test procedures and ‘round-robin’ results using the DWTT specimens and a simplified single-specimen method developed at Canmet Materials, with a discussion on the application to determine a critical CTOA with typical results and step-by-step procedures. The sixth paper, by Dr Robert Amaro et al., summarizes the CTOA testing of pipeline steels at quasi-static and dynamic rates using modified double-cantilever-beam (MDCB) specimens that have been done at NIST at its Boulder, Colorado, facility between 2006 and 2012.
The DWTT energy is an apparent toughness parameter that has been used in the pipeline industry since the 1960s. In the fourth paper, Robert Eiber reviews the need, development, and application of DWTT energy for controlling fracture propagation in natural gas transmission pipelines. He summarizes the incidents that started the research leading to the development of the DWTT from 1960 to present. The initial goal of the DWTT was to define the ductile-to-brittle transition temperature of pipeline steels to facilitate the specification of transition temperature below the operating temperature range for linepipe. Today, the need for a measure of the steel toughness has emerged to control ductile-fracture-propagation arrest, leading to examination of the DWTT energy as a substitute for the CVN energy.
In the 1950s, the CVN impact energy was the first apparent fracture-toughness parameter to be used to characterize toughness of linepipe steels. In the fifth paper, Dr Brian Leis reviews the CVN test and standard development, and assesses its use to characterize fracture resistance in applications from vintage to modern toughness pipeline steels. He concludes that where tough materials are involved, alternative testing practices are needed that are better adapted to the specific loading and failure response of the structure of interest.
The eighth paper, by Dr He Li et al., presents fracture-toughness evaluations using the CVN and DWTT energies for the X80 pipeline steels used for the Chinese second west-east gas transmission pipeline. Based on a variety of test data, these authors compare the 2-mm striker and 8-mm striker CVN energies over a range of temperatures, and the DWTT energy with the 8-mm striker CVN energy at the room temperature, and thus obtain useful relationships between these toughness parameters.
The CTOD is an engineering-fracture parameter that was proposed in 1963, and this parameter has been widely used for structural integrity analysis in the oil and gas industry ever since. In the ninth paper, Drs Philippa Moor and Henryk Pisarski present a review of CTOD testing and its application to pipelines in the past, present, and future, and describe the development of standardized fitness-for-service assessment procedures from the use of the CTOD design curve to the failure-analysis diagram approach in the CTOD British Standard.
In addition to the fracture-toughness tests, Professor Rudi Denys developed a curved-wide-plate (CWP) specimen for conducting a quasi-structure test to provide a rational basis for predicting girth-weld integrity for both stress- and strain-based designs, for establishing material requirements, and for validating numerical models or fracture-mechanics’-based defect assessments. These are described in the tenth paper.
The last paper, by the Guest Editor and Dr Brian Leis, discusses the applications of CVN impact energy and DWTT energy on ductile-fracture-propagation control, and reviews the existing ductile-fracture arrest methods in terms of CVN and DWTT toughness parameters for predicting the arrest-fracture toughness of gas transmission pipelines, including modern high-strength pipeline steels.
It is expected that this special issue will serve as a technical document for tracking the historical efforts and developments of fracture-toughness testing, evaluation, and application to pipeline steels, for understanding/using appropriate fracture-toughness parameters as well as the corresponding test methods, and also for further improving the fracture-toughness test standards in the future. As such, it will provide a useful technical source for researchers and engineers in the oil and gas pipeline industry.
Thanks are given to the editor who made this special issue possible. He provided helpful advice in paper solicitation, organization, review, and final submission. His exceptional support made this special issue a record-breaker in the history of this Journal.
Dr Xian-Kui Zhu, Fellow of ASME
Principal Engineer, Structural Integrity and Modelling
Edison Welding Institute
Columbus, OH 43221, USA