Fracture Mechanics .Whilst the Crack Tip Opening Displacement (CTOD) test was developed for the characterisation of metals it has also been used to determine the toughness of non-metallics such as weldable plastics. The CTOD test is one such fracture toughness test that is used when some plastic deformation can occur prior to failure - this allows the tip of a crack to stretch and open, hence 'tip opening displacement

1. Fracture Mechanics
Davalsab.M.L
M.Tech (Machine Design), Reva university
Daval.techno@gmail.com
8904567449
India

2. CTOD testing
Whilst the Crack Tip Opening Displacement (CTOD)
test was developed for the characterisation of metals
it has also been used to determine the toughness of
non-metallics such as weldable plastics.
The CTOD test is one such fracture toughness test
that is used when some plastic deformation can
occur prior to failure - this allows the tip of a crack to
stretch and open, hence 'tip opening displacement
There are two basic forms - a square or a rectangular
cross section specimen. If the specimen thickness is
defined as 'B', the depth (W) will be either B or 2B
with a standard length of 4.6W. A notch is machined
at the centre and then extended by generating a
fatigue crack so that the total 'defect' length is half
the depth of the test piece- see Fig.1. A test on a
100mm thick weld will therefore require a specimen
measuring 100mm thick, 200mm wide and 920mm
long - an expensive operation, the validity of which
can only be determined once the test has been
completed.

3. The test is performed by placing the specimen
into three point bending and measuring the
amount of crack opening. This is done by means
of a strain gauge attached to a clip placed
between two accurately positioned knife edges
at the mouth of the machined notch
As bending proceeds, the crack tip plastically
deforms until a critical point is reached when
the crack has opened sufficiently to initiate a
cleavage crack. This may lead to either partial or
complete failure of the specimen. The test may
be performed at some minimum temperature eg
the minimum design temperature or, more
rarely, at a range of temperatures.
The values that are required for the calculation
of toughness are firstly the load at which
fracture occurs and secondly the amount by
which the crack has opened at the point of crack
propagation

4. Since the length of the crack and the opening
at the mouth of the notch are known it is a
simple matter to calculate the crack tip
opening by simple geometry. Whilst the test is
in progress the results are recorded
automatically on a load/displacement chart.
This illustrates the various shapes of curve that
may be produced –
(a) It is a test where the test piece has
fractured in a brittle manner with little or
no plastic deformation.
(b) exhibits a 'pop-in' where the brittle crack
initiates but only propagates a short
distance before it is arrested in tougher
material - this may occur several times
giving the curve a saw tooth appearance
or after this one pop-in deformation may
continue in a ductile manner as in
(c) which shows completely plastic
behaviour.

6. J-test
ASTM E 1820 has two alternative methods for J tests:
the basic procedure and the resistance curve
procedure. The basic procedure entails monotonically
loading the specimen to failure or to a particular
displacement, depending on the material behavior.
The resistance curve procedure requires that the crack
growth be monitored during the test. The J integral is
calculated incrementally in the resistance curve
procedure. The basic procedure can be used to
measure J at fracture instability or near the onset of
ductile crack extension.
Because crack growth is not monitored as a part of the
basic test procedure, a multiple-specimen technique is
normally required to obtain a J-R curve. In such cases,
a series of nominally identical specimens are loaded to
various levels and then unloaded. Different amounts
of crack growth occur in the various specimens. The
crack growth in each sample is marked by heat tinting
or fatigue cracking after the test. Each specimen is
then broken open and the crack extension is
measured.

9. Fracture testing of Weldments

10. Weldment’s test
consideration
Welded joints, however, have decidedly heterogeneous
microstructures and, in many cases, irregular shapes.
Weldments also contain complex residual stress
distributions. Most existing fracture toughness testing
standards do not address the special problems
associated with weldment testing
Number of factors need special consideration.
• Specimen design and fabrication are more difficult
because of the irregular shapes and curved surfaces
associated with some welded joints.
• The heterogeneous microstructure of typical
weldments requires special attention to the location
of the notch in the test specimen.
• Residual stresses make fatigue precracking of
weldment specimens more difficult.
• After the test, a weldment may be sectioned and
examined metallographically to determine whether
or not the fatigue crack sampled the intended
microstructure.

11. Specimen Design and
Fabrication
The underlying philosophy of the British Standards test procedure on
specimen design and fabrication is that the specimen thickness should be
as close to the section thickness as possible. Larger specimens tend to
produce more crack-tip constraint, and hence lower toughness
Achieving nearly full-thickness weldment often requires sacrifices in other
areas. For example, if a specimen is to be extracted from a curved section
such as a pipe, one can either produce a sub-size rectangular specimen
that meets the tolerances of the existing ASTM standards, or a full-
thickness specimen that is curved.
If curvature or distortion of a weldment is excessive, the specimen can be
straightened by bending on either side of the notch to produce a ‘‘gull
wing’’ configuration, which is illustrated in Figure 7.34. The bending must
be performed so that the three loading points (in an SE(B) specimen) are
aligned

12. Notch location and
orientation
Weldments have a highly heterogeneous microstructure. Fracture
toughness can vary considerably over relatively short distances. Thus, it
is important to take great care in locating the fatigue crack in the correct
region. If the fracture toughness test is designed to simulate an actual
structural flaw, the fatigue crack must sample the same microstructure
as the flaw
Once the microstructure of interest is identified, a notch orientation
must be selected. The two most common alternatives are a through-
thickness notch and a surface notch, which are illustrated in Figure 7.35.
Since full-thickness specimens are desired, the surface-notched
specimen should be a square section (B × B), while the through-
thickness notch will usually be in a rectangular (B × 2B) specimen.
For weld metal testing, the through-thickness orientation is usually
preferable because a variety of regions in the weld are sampled.
However, there may be cases where the surface-notched specimen is
the most suitable for testing the weld metal. For example, a surface
notch can sample a particular region of the weld metal, such as the root
or cap, or the notch can be located in a particular microstructure, such
as unrefined weld metal.
In typical C–Mn structural steels, low toughness is usually associated
with the coarse-grained heat-affected zone (HAZ) and the intercritically
reheated HAZ. A microhardness survey can help identify low toughness
regions because high hardness is often coincident with brittle behavior.

13. Heat affected zone Notch location in the HAZ often depends on the type of weldment. If welds are
produced solely for mechanical testing, for example, as part of a weld
procedure qualification or a research program,
the welded joint can be designed to facilitate HAZ testing. Figure 7.36 illustrates
the K and half- K preparations, which simulate double-V and single-V welds,
respectively. The plates should be tilted when these weldments are made, to
have the same angle of attack for the electrode as in an actual single- or double-
V joint. For fracture toughness testing, a through-thickness notch is placed in
the straight side of the K or half-K HAZ.
In many instances, fracture toughness testing must be performed on an actual
production weldment, where the joint geometry is governed by the structural
design. In such cases, a surface notch is often necessary for the crack to sample
sufficient HAZ material. The measured toughness is sensitive to the volume of
HAZ material sampled by the crack tip because of the weakest-link nature of
cleavage fracture
Another application of the surface-notched orientation is the simulation of
structural flaws. Figure 7.37 illustrates HAZ flaws in a structural weld and a
surface-notched fracture toughness specimen that models one of the flaws.
Figure 7.37 demonstrates the advantages of allowing a range of a/W ratios in
surface- notched specimens. A shallow notch is often required to locate a crack
in the desired region, but most existing ASTM standards do not allow a/W ratios
less than 0.45. Shallow-notched fracture toughness specimens tend to have
lower constraint than deeply cracked specimens, as

14. Posttest Analysis
Correct placement of a fatigue crack in weld metal is usually not difficult
because this region is relatively homogeneous. The microstructure in the
HAZ, however, can change dramatically over very small distances. Correct
placement of a fatigue crack in the HAZ is often accomplished by
trial and error. Because fatigue cracks are usually slightly bowed, the
precise location of the crack tip in the center of a specimen cannot be
inferred from observations on the surface of the specimen. Thus, HAZ
fracture toughness specimens should be examined metallographically
after the test to determine the microstructure that initiated fracture. In
certain cases, a posttest examination may be required in weld metal
specimens.

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