Standardization News

by Cicely Enright

The master curve concept - and the standard that details the procedure - characterizes the ductile to brittle fracture toughness of ferritic steels.

The standard is E1921, Test Method for Determination of Reference Temperature, To, for Ferritic Steels in the Transition Range; An Introduction to the Development and Use of the Master Curve Method, is the manual about its background and its application.

Before E1921, before the master curve, cost and materials requirements complicated, if not prohibited, the use of fracture mechanics to determine the ductile-brittle toughness of ferritic steels. "Without the master curve, the surveillance specimens used in nuclear reactors could not be used to determine the materials fracture toughness," says Kim R. W. Wallin, Ph.D., academy professor at the Academy of Finland, Helsinki, Finland, who is considered to be the father of the master curve.

In 1997, the publication of E1921 completely changed the situation to a manageable one.

"Its [E1921's] efficiency is in being able to determine the fracture toughness curve from brittle to ductile behavior with a small number of relatively small specimens," says Randy Nanstad, Ph.D., leader, Nuclear Materials Science and Technology Group, Materials Science and Technology Division, Oak Ridge National Laboratory, and a member of Committees E08 on Fatigue and Fracture and E10 on Nuclear Technology and Applications.

The standard, which is under the jurisdiction of Committee E08, presents a procedure to enable fracture toughness estimation - possibly even from operating structures - through extracting small, 0.2 in. to 0.4 in. (5 mm to 10 mm), metal specimens. Wallin explains that the master curve method applies the primary results from theoretical cleavage fracture models and expresses those results in an easy-to-use engineering framework. According to the procedure, notched and fatigue precracked bend or compact specimens are tested at temperatures where cracking may develop, and the fracture toughness is calculated from recorded force versus displacement data.

"This is the only method with the tools and procedures to characterize ductile to brittle transition behavior of ferritic steels," says Mikhail Sokolov, Ph.D., task leader for fracture mechanics in the NMST Group, ORNL, co-chairman of the E08 task group responsible for E1921 and an E10 member. "This particular standard provides a powerful statistical apparatus to describe this ductile-to-brittle transition in consistent ways so that different steels can be compared based on one parameter."

The predominant use for the master curve method comes with nuclear reactor applications; the test provides an indicator of potential reactor structure problems. However, use of the procedure has been expanding into the petrochemical industry and can potentially be applied to ferritic steels for other purposes as well.

Useful background about and the application of E1921 can be found in the compact, information-filled manual by Donald E. McCabe, John G. Merkle and Wallin, a book that gives guidance about test result accuracy as well as possible situations where the method may not work.

The manual provides comprehensive coverage of the master curve: nomenclature, KJc data validity requirements, test specimens and equipment, specimen precracking and side-grooving, test practices, fracture toughness calculations, determination of scale parameter and reference temperature, development of tolerance bounds, concepts under study and applications.

A new version of the manual today might be a bit different because "the master curve method is still a living procedure in constant development," says Wallin. But the work continues to be a significant guide for engineers with limited experience with elastic-plastic fracture mechanics or advanced statistical methods.