The
property KIc determined by this test method characterizes the resistance of a
material to fracture in a neutral environment in the presence of a sharp crack
under essentially linear-elastic stress and severe tensile constraint, such
that (1) the state of stress near the crack front approaches tritensile plane
strain, and (2) the crack-tip plastic zone is small compared to the crack size,
specimen thickness, and ligament ahead of the crack.
Variation
in the value of KIc can be expected within the allowable range of specimen
proportions, a/W and W/B. KIc may also be expected to rise with increasing
ligament size. Notwithstanding these variations, however, KIc is believed to
represent a lower limiting value of fracture toughness (for 2 % apparent crack
extension) in the environment and at the speed and temperature of the test.
Lower
values of KIc can be obtained for materials that fail by cleavage fracture; for
example, ferritic steels in the ductile-to-brittle transition region or below,
where the crack front length affects the measurement in a stochastic manner
independent of crack front constraint. The present test method does not apply
to such materials and the user is referred to Test Method E1921 and E1820. Likewise
this test method does not apply to high toughness or high tearing-resistance
materials whose failure is accompanied by appreciable amounts of plasticity.
Guidance on testing elastic-plastic materials is given in Test Method E1820.
The
value of KIc obtained by this test method may be used to estimate the relation
between failure stress and crack size for a material in service wherein the
conditions of high constraint described above would be expected. Background
information concerning the basis for development of this test method in terms
of linear elastic fracture mechanics may be found in Refs (1) and (2).
Cyclic
forces can cause crack extension at KI values less than KIc. Crack extension
under cyclic or sustained forces (as by stress corrosion cracking or creep
crack growth) can be influenced by temperature and environment. Therefore, when
KIc is applied to the design of service components, differences between
laboratory test and field conditions shall be considered.
Plane-strain fracture toughness testing is
unusual in that there can be no advance assurance that a valid K Ic will be
determined in a particular test. Therefore, compliance with the specified
validity criteria of this test method is essential.
Residual
stresses can adversely affect the indicated KQ and KIc values. The effect can
be especially significant for specimens removed from as-heat treated or
otherwise non-stress relieved stock, from weldments, from complex wrought
parts, or from parts with intentionally induced residual stresses. Indications
of residual stress include distortion during specimen machining, results that
are specimen configuration dependent, and irregular fatigue precrack growth
(either excessive crack front curvature or out-of-plane growth). Guide B909
provides supplementary guidelines for plane strain fracture toughness testing
of aluminum alloy products for which complete stress relief is not practicable.
Guide B909 includes additional guidelines for recognizing when residual
stresses may be significantly biasing test results, methods for minimizing the
effects of residual stress during testing, and guidelines for correction and
interpretation of data.
This
test method can serve the following purposes:
In research and development, to establish in
quantitative terms significant to service performance, the effects of
metallurgical variables such as composition or heat treatment, or of
fabricating operations such as welding or forming, on the fracture toughness of
new or existing materials.
In
service evaluation, to establish the suitability of a material for a specific
application for which the stress conditions are prescribed and for which
maximum flaw sizes can be established with confidence.
For
specifications of acceptance and manufacturing quality control, but only when
there is a sound basis for specifying minimum KIc values, and then only if the
dimensions of the product are sufficient to provide specimens of the size
required for valid KIc determination. The specification of K Ic values in
relation to a particular application should signify that a fracture control
study has been conducted for the component in relation to the expected loading
and environment, and in relation to the sensitivity and reliability of the
crack detection procedures that are to be applied prior to service and
subsequently during the anticipated life.
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