M00018310
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ISO FDIS 22889 2013 Edition, June 18, 2013 METALLIC MATERIALS - METHOD OF TEST FOR THE DETERMINATION OF TO STABLE CRACK EXTENSION USING SPECIMENS OF LOW CONSTRAINT
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Availability date: 07/14/2021
Description / Abstract:
This International Standard specifies methods for determining
the resistance to stable crack extension in terms of crack opening
displacement, δ5, and critical crack tip opening angle,
ψc, for homogeneous metallic materials by the
quasistatic loading of cracked specimens that exhibit low
constraint to plastic deformation. Compact and middle-cracked
tension specimens are notched, precracked by fatigue, and tested
under slowly increasing displacement.
This International Standard describes methods covering tests on
specimens not satisfying requirements for size-insensitive fracture
properties; namely, compact specimens and middle-cracked tension
specimens in relatively thin gauges.
Methods are given for determining the crack extension resistance
curve (R-curve). Point values of fracture toughness for compact
specimens are determined according to ISO 12135. Methods for
determining point values of fracture toughness for the
middle-cracked tension specimen are given in Annex D.
Crack extension resistance is determined using either the
multiple-specimen or single-specimen method. The multiple-specimen
method requires that each of several nominally identical specimens
be loaded to a specified level of displacement. The extent of
ductile crack extension is marked and the specimens are then broken
open to allow measurement of crack extension. Single-specimen
methods based on either unloading compliance or potential drop
techniques can be used to measure crack extension, provided they
meet specified accuracy requirements. Recommendations for
single-specimen techniques are described in ISO 12135. Using either
technique, the objective is to determine a sufficient number of
data points to adequately describe the crack extension resistance
behaviour of a material.
The measurement of δ5 is relatively simple and well
established. The δ5 results are expressed in terms of a
resistance curve, which has been shown to be unique within
specified limits of crack extension. Beyond those limits,
δ5 R-curves for compact specimens show a strong specimen
dependency on specimen width, whereas the δ5 R-curves
for middle-cracked tension specimens show a weak dependency.
CTOA is more difficult to determine experimentally. The critical
CTOA is expressed in terms of a constant value achieved after a
certain amount of crack extension. The CTOA concept has been shown
to apply to very large amounts of crack extension and can be
applied beyond the current limits of δ5
applications.
Both measures of crack extension resistance are suitable for
structural assessment. The δ5 concept is well
established and can be applied to structural integrity problems by
means of simple crack driving force formulae from existing
assessment procedures.
The CTOA concept is generally more accurate. Its structural
application requires numerical methods, i.e. finite element
analysis.
Investigations have shown a very close relation between the
concept of constant CTOA and a unique R-curve for both compact and
middle-cracked tension specimens up to maximum load. Further study
is required to establish analytical or numerical relationships
between the δ5 R-curve and the critical CTOA values.