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TWI Knowledge Summary
Fracture toughness testing
by Henryk Pisarski
Background
Standards
Test specimens
Instrumentation and loading
Fracture toughness parameters
References
Background
The resistance to fracture of a material is known as its fracture toughness. Fracture
toughness generally depends on temperature, environment, loading rate, the
composition of the material and its microstructure, together with geometric effects
(constraint).
[1]
These factors are of particular importance for welded joints, where the
metallurgical and geometric effects are complex
[2,3]
Fracture toughness is a critical input parameter for fracture-mechanics based fitness-for-
service assessments. Although fracture toughness can sometimes be obtained from the
literature, or materials properties databases, it is preferable to determine this by
experiment for the particular material and joint being assessed.
Various measures of 'toughness' exist, including the widely used but qualitative Charpy
impact test. Although it is possible to correlate Charpy energy with fracture toughness, a
large degree of uncertainty is associated with correlations because they are empirical. It
is preferable to determine fracture toughness in a rigorous fashion, in terms of K (stress
intensity factor), CTOD (crack tip opening displacement), or J (the J integral); see also
What is a fracture toughness test? Standards exist for performing fracture mechanics
tests, with the most common specimen configuration shown in )LJ (the single-edge
notch bend SENB specimen, sometimes referred as a SE(B) specimen). A sharp fatigue
crack is inserted in the specimen, which is loaded to failure. The crack driving force is
calculated for the failure condition, giving the fracture toughness.
Fig.1. Fracture mechanics testing
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Standards
Various national Standards have been developed for fracture toughness testing:
z The British Standard BS 7448
[4]
includes four parts, for testing of metallic materials,
including parent materials (Part 1), weldments (Part 2), high strain rates (dynamic
fracture toughness testing, Part 3), and resistance curves (R-curves for ductile
tearing, Part 4). BS 7448: Part 2 is the first Standard worldwide to apply specifically
to weldments.
z A series of American standards (ASTM) cover K, CTOD, J testing, ASTM E1290 (CTOD
testing), ASTM E1820 (K, J & CTOD, including R-curves) and ASTM E1921 (J testing to
determine T0 for ferritic steels). None specifically address testing of welds. ASTM
E1823 provides a useful summary of terminology.
[5-9]
z A series of international (ISO) standards are being developed. ISO 12135 covers all
aspects of fracture testing (K, J-integral & CTOD) of plain material. Standards are
being prepared on testing of welds (ISO/FDIS 15653) and stable crack growth in low
constraint specimens (ISO/22889). The latter is mainly concerned with testing thin,
sheet material.
z The European Structural Integrity Society (ESIS) has published procedures for R-
curve and standard fracture toughness testing of metallic materials.
[10-11]
A draft
unified testing procedure (ESIS P3-04), which includes weld testing, is being
developed. (These are not standards in the usual sense, but rather testing protocols
that have been agreed by experts. Unfortunately, currently there is no formal
mechanism for keeping these protocols up to date).
z Increasing use is being made of single edge notch tension (SENT or SE(T)) specimens
to determine fracture toughness of girth welds in submarine pipelines. Currently,
there is no testing standard but a DNV recommended practice does provide a testing
protocol
[12]
. This is designed for ductile materials and the protocol describes a
method for the determination of a J R-curve.
Although different standards have historically been published for determining K, CTOD
and J-integral, the tests are very similar, and generally all three values can be
established from one test. See Are there any differences between fracture toughness
tests carried out to BS7448 and E1820?
Test specimens
The most widely used fracture toughness test configurations are the single edge notch
bend (SENB or three-point bend), and the compact (CT) specimens, as shown in )LJ.
The compact specimen has the advantage that it requires less material, but is more
expensive to machine and more complex to test compared with the SENB specimen.
Also, special requirements are needed for temperature control (e.g. use of an
environmental chamber). SENB specimens are typically immersed in a bath for low
temperature tests. Although the compact specimen is loaded in tension, the crack tip
conditions are predominantly bending (high constraint). If limited material is available, it
is possible to fabricate SENB specimens by welding extension pieces (for the loading
arms) to the material sample. (Electron beam welding is typically used, because the weld
is narrow and causes little distortion).
Fig.2. Examples of
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Other specimen configurations include centre-cracked tension (CCT) panels, single edge
notch tension (SENT) specimens, and shallow-crack tests. These specialised tests are
associated with lower levels of constraint, and can be more structurally representative
than standard SENB or CT specimens. SENT specimens are being used to determine
fracture toughness of pipeline girth in submarine pipelines, especially where the
installation method involves plastic straining. Further information can be found in a DNV
recommended practice
[12]
and Appendix A of a DNV standard
[13]
. The primary purpose
of the tests is to define flaw acceptance criteria when the results are used with an
appropriate assessment procedure.
The position and orientation of the specimen is important. In particular, the location and
orientation of the notch is critical, especially for welded joints. Typically, the notch
(fatigue pre-crack) is positioned such that a chosen microstructure is sampled. The
orientation of the notch is defined with respect to either the weld axis for welded joints,
or the rolling direction or forging axis for other components.
In standard SENB & C T specimens (see )LJ), the notch depth is within the range 45-
70% of the specimen width, W, giving a lower-bound estimate of fracture toughness,
because of the high level of crack tip constraint generated by the specimen design.
A notch is machined into the fracture toughness specimen blank, following which a
fatigue crack is grown by applying cyclic loading to the specimen. In order to minimise
the time spent in fatigue pre-cracking, specialised high frequency resonance or servo-
hydraulic machines are often used for this process.
Since specimens taken from as-welded joints will contain residual stresses arising from
the welding process, there is a risk of non-uniform fatigue pre-cracks which would
invalidate the test result. To counter this, various options are recommended in BS7448
Part2. The most commonly used method which has also been found to provide the most
consistent results is local compression. This involves controlled plastic compression of
the sides of the specimen after notching but before fatigue pre-cracking.
The fracture mechanics test standards include many checks to ensure that results are
credible. These include restrictions on the fatigue crack size, plane and shape, together
with limitations on the maximum allowable fatigue force (this is to ensure that the crack-
tip plastic zone produced during fatigue pre-cracking is small in comparison with the
plastic zone produced during testing). Many of these checks can only be performed after
testing.
Instrumentation and loading
During fracture toughness testing, the force applied to the specimen and specimen
displacements and loading rate (using load cells and displacement transducers), together
with the temperature are recorded.
One of the displacements is the crack-mouth opening. This is measured using a clip
gauge either attached to knife edges mounted at the crack mouth (see )LJ) or integral
knife edges machined into the notch. These gauges comprise two cantilevered beams on
which are positioned four strain gauges. By measuring the elastic strains and calibration
common fracture
toughness test
specimen types
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crack-mouth opening displacement is obtained. Analysis of the relationship between
applied force and crack mouth opening displacement form the test enables fracture
toughness to be determined in terms of K, CTOD and J-integral.
Fracture toughness tests are performed in universal hydraulic test machines, generally
using displacement control.
Fracture toughness parameters
The following are the fracture toughness parameters commonly obtained from testing
z K (stress intensity factor) can be considered as a stress-based estimate of fracture
toughness. It is derived from a function which depends on the applied force at
fracture. K depends on geometry (the flaw depth, together with a geometric function,
which is given in test standards for each test specimen geometry).
z CTOD or (crack-tip opening displacement) can be considered as a strain-based
estimate of fracture toughness. However, it can be separated into elastic and plastic
components. The elastic part of CTOD is derived from the stress intensity factor, K. In
some standards, the plastic component of CTOD is obtained by assuming that the
specimen rotates about a plastic hinge. The plastic component is derived from the
crack mouth opening displacement (measured using a clip gauge). The position of the
plastic hinge (defined by r
p
) is given in test standards for each specimen type.
Alternative methods exist for estimating CTOD, which make no assumption regarding
the position of the plastic hinge. These require the determination of J from which
CTOD is derived.
[6,7]
CTOD values determined from formulations assuming a plastic
hinge
[4]
may differ from those determined from J.
[6,7]
z J (the J-integral) is an energy-based estimate of fracture toughness. It can be
separated into elastic and plastic components. As with CTOD, the elastic component is
based on K, while the plastic component is derived from the plastic area under the
force-displacement curve.
It should be noted that all three fracture parameters can be related to one another.
However, the relationship is not unique and depends on material tensile properties and
specimen geometry.
For more information please contact us.
References
Items 1-3 are not in the public domain but are available to TWI Industrial Member
companies.
1. M G Dawes: 'An Introduction to K, CTOD and J Fracture Mechanics Analyses and
Toughness, and the Application of these to Metal Structures'.
2. H G Pisarski: 'Update on Fracture Toughness Test Methods for Welded Joints'.
3. H G Pisarski: 'A Review of HAZ Toughness Evaluation'.
4. BS7448: 'Fracture Mechanics Toughness Tests'
z Part 1:1991: 'Method for determination of Kic, Critical CTOD and Critical J
values of metallic material'.
z Part 2: 1997: 'Method for Determination of Kic, Critical CTOD and Critical J
Values of Welds in Metallic Materials'.
z Part 3: 2005 'Method for determining dynamic fracture toughness' (to be
published in 2005)
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z Part 4 1997: 'Method for Determination of Fracture Resistance Curve and
Initiation values for Stable Crack Extension in Metallic Materials'. British
Standards Institute, London.
5. ASTM E399-09: 'Standard Test Method for Plane Strain Fracture Toughness of Metallic
Materials'. American Society of Testing and Materials, Philadelphia, 2009.
6. ASTM E1290-09: 'Standard Test Method for Crack-Tip Opening Displacement (CTOD)
Fracture Toughness Measurement'. American Society of Testing and Materials,
Philadelphia, 2009
7. ASTM E1820-09: 'Standard Test Method for Measurement for Fracture Toughness'.
American Society of Testing and Materials, Philadelphia, 2009.
8. ASTM E1823-09: 'Technology Relating to Fatigue and Fracture Testing'. American
Society for Testing and Materials, Philadelphia, 2009.
9. ASTM E1921-09 'Standard Test Method for Determination of Reference Temperature,
T0, for Ferritic Steels in the Transition Range'. American Society of Testing and
Materials, Philadelphia, 2009
10. ESIS P1-92: 'ESIS Recommendation for Determining the Fracture Resistance of
Ductile Materials' European Structural Integrity Society, 1992.
11. ESIS P2-92: 'ESIS Procedure for Determining the Fracture Behaviour of Materials'.
European Structural Integrity Society, 1992.
12. DNV-RP-F108: 'Fracture control for pipeline installation methods introducing cyclic
plastic strain'. Det Norske Veritas, January 2006.
13. DNV-OS-F101: 'Submarine pipeline systems'. Det Norske Veritas, October 2007.
Last Reviewed 2010 / Copyright 2010 TWI Ltd
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