Page 319 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)

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7 Fracture Toughness

Testing of Metals

A fracture toughness test measures the resistance of a material to crack extension. Such a test may
yield either a single value of fracture toughness or a resistance curve, where a toughness parameter
such as K, J, or CTOD is plotted against the crack extension. A single toughness value is usually
sufficient to describe a test that fails by cleavage, because this fracture mechanism is typically
unstable. The situation is similar to the schematic in Figure 2.10(a), which illustrates a material
with a flat R curve. Cleavage fracture actually has a falling resistance curve, as Figure 4.8 illustrates.
Crack growth by microvoid coalescence, however, usually yields a rising R curve, such as that
shown in Figure 2.10(b); ductile crack growth can be stable, at least initially. When ductile crack
growth initiates in a test specimen, that specimen seldom fails immediately. Therefore, one can
quantify ductile fracture resistance either by the initiation value or by the entire R curve.
A variety of organizations throughout the world publish standardized procedures for fracture
toughness measurements, including the American Society for Testing and Materials (ASTM), the
British Standards Institution (BSI), the International Institute of Standards (ISO), and the Japan
Society of Mechanical Engineers (JSME). The first standards for K and J testing were developed
by ASTM in 1970 and 1981, respectively, while BSI published the first CTOD test method in 1979.
Existing fracture toughness standards include procedures for K , K-R curve, J , J-R curve,
Ic
Ic
CTOD, and K testing. This chapter focuses primarily on ASTM standards, since they are the most
Ia
widely used throughout the world. Standards produced by other organizations, however, are broadly
consistent with the ASTM procedures, and usually differ only in minute details. The existing
standards are continually evolving, as the technology improves and more experience is gained.
The reader should not rely on this chapter alone for guidance on conducting fracture toughness
tests, but should consult the relevant standards. Also, the reader is strongly encouraged to review
Chapter 2 and Chapter 3 in order to gain an understanding of the fundamental basis of K, J, and
CTOD, as well as the limitations of these parameters.

7.1 GENERAL CONSIDERATIONS

Virtually all fracture toughness tests have several common features. The design of test specimens is
similar in each of the standards, and the orientation of the specimen relative to symmetry directions
in the material is always an important consideration. The cracks in test specimens are introduced by
fatigue in each case, although the requirements for fatigue loads varies from one standard to the next.
The basic instrumentation required to measure load and displacement is common to virtually all
fracture mechanics tests, but some tests require additional instrumentation to monitor crack growth.

7.1.1 SPECIMEN CONFIGURATIONS

There are five types of specimens that are permitted in ASTM standards that characterize fracture
initiation and crack growth, although no single standard allows all five configurations, and the
design of a particular specimen type may vary between standards. The configurations that are
currently standardized include the compact specimen, the single-edge-notched bend (SE(B)) geom-
etry, the arc-shaped specimen, the disk specimen, and the middle tension (MT) panel. Figure 7.1
shows a drawing of each specimen type.

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