ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation
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110)
210–410 (30–60) 350
250
410–620 (60–90) 400
300
620–830 (90–
120)
450
350
P-2 19 × 51 × 127 (¾ × 2 ×
5)
102
(4.0)
1.5 (0.06)
830–1030 (120–
150)
550
400
210–410 (30–60) 350
250
410–620 (60–90) 400
300
620–830 (90–
120)
450
350
P-3
15.9 × 51 × 127 ( × 2
× 5)
102
(4.0)
1.9 (0.075)
830–1030 (120–
150)
550 400
(a) Initial test of a steel with a given strength level should be conducted with the drop-weight energy stated in
this column. In the event that insufficient deflection is developed (no-test performance), an increased drop-
weight energy should be employed for other specimens of the given steel.
The same piece of test equipment is used for both the drop-weight test and the drop-weight tear test. The
difference lies in the anvil that holds the specimen. The principal requirement is that sufficient impact can be
generated to produce cracking. This is commonly provided by a vertical structure on which weights can be
attached using guides. The weights are raised to a measured height, quickly released, and guided to strike the
specimen properly.
The Drop-Weight Test
The DWT specimen and procedure are shown in Fig. 13 and are described in ASTM E 208. The crack inducer
is a bead of hard-facing metal approximately 76 mm (3 in.) long. The specimen, 89 × 356 × 19 mm (3½ × 14 ×
¾in.), is placed, weld down, on rounded end supports and is struck by a 27 kg (60 lb) falling weight with
sufficient energy to bend the specimen about 5°. A cleavage crack forms in the bead as soon as incipient yield
occurs (at about 3° deflection), thus forming the sharpest possible notch, a cleavage crack in the test specimen.
A series of specimens is tested over a range of temperatures to find the nil-ductility transition temperature.
Fig. 13 Drop-weight test method
The weld bead is deposited on one side of the specimen at the center using a copper template. The weld bead is
purposely a hard, brittle deposit (the Murex-Hardex N electrode is recommended). A notch is made in the weld
bead, but not in the specimen itself. The specimen, after being cooled to the desired temperature, is placed in
the anvil with the notched weld deposit facing downward. The weight is dropped, striking the back side of the
specimen (the amounts of weight and height depend on the strength of the material being tested; see Table 4).
The specimen is allowed to deflect slightly under the impact load, controlled by deflection stops. This initiates a
crack at the notch in the weld bead. When the crack reaches the specimen material it will be either propagated
or arrested. The specimen is then examined to see whether or not it has fractured. A specimen is considered to
be broken if the crack extends to one or both sides of the specimen surface with the weld bead. If the crack does
not propagate to the edge it is considered a “no break.” If the weld notch is not visibly cracked, or if complete
deflection does not occur (determined by mark transfer on the deflection stops), it is considered a “no test.” The
nil-ductility transition temperature (NDTT) is the maximum temperature at which the specimen breaks. When
minimum temperatures are set in material specifications, at least two specimens must be tested at the specified
temperature. All specimens tested shall show a “no-break” performance.
Drop-Weight Tear Test Procedure
The drop-weight tear test (ASTM E 436) is similar in some ways to the drop-weight test. The transition fracture
appearance occurs at the same temperature as for full-size parts. It has the same sudden change from shear to
cleavage as that observed in full-scale pieces of equipment. The test is relatively simple in terms of both
specimen preparation and lack of sensitivity to specimen-preparation techniques. The results vary with
specimen thickness in the same manner as actual parts do. The principal short coming, as in the drop-weight
test, is that testing is confined to plate material between 3 and 19 mm (0.125 and 0.750 in.) thick. The test
specimen is even larger than the DWT specimen: it is 76 mm (3 in.) wide by 305 mm (12 in.) long. Tests are
made with the same apparatus used for the drop-weight test, but the test fixture for holding the specimen is
altogether different. A large pendulum-type machine can also be used, but the vertical weight-dropping
apparatus is more commonly used. Up to 2700 J (2000 ft · lbf) of energy may be required.
Test-Specimen Anvil. The holder for the test specimen must support the specimen on edge (305 mm, or 12 in.,
long edge) in such a manner that rotation will not occur when the specimen is struck. This usually requires
adjustable supports for differing specimen thicknesses. Hardened supports at each end suspend the specimen,
and a centering guide at one end centers the specimen. A slot in the center of the anvil allows downward
clearance for the breaking specimen (Fig. 14).
Fig. 14 Drop-weight tear test specimen and support dimensions
Specimen Preparation. Test specimens can be removed by sawing, shearing, or flame cutting. Specimen
dimensions and tolerances are shown in Fig. 14. A notch is impressed at the center of one of the 305 mm (12
in.) edges. This is accomplished by using a sharp tool-steel chisel that is hardened. The sharp edge should have
an angle of 45 ± 1 or 2°. The depth of the notch is 0.5 ± 0.05 mm (0.020 ± 0.002 in.). The combination of the
sharpness of the notch (radius of about 0.013 mm, or 0.0005 in.) and the cold working that occurs as the result
of impressing it produces cleavage fractures under the notch.
Test Procedure. Specimens are tested at various temperatures. They are brought to the desired temperature by
immersing them in a cooled solution and holding for at least 15 min at temperature. The bath should be
agitated, and if several specimens are cooled simultaneously they should be separated by several specimen
thicknesses. Specimens should be broken within 10 s after they are removed from the bath. The cooled
specimen is inserted in the anvil so that the notch is directly beneath the point of load application, and the test
load, which must be only of sufficient magnitude to produce a fracture, is suddenly applied. If the specimen
buckles under the test load, the test is considered to be invalid. Otherwise, the specimen fractures and separates
as it moves into the slotted anvil without the two pieces being jammed against one another.
Test Results. The test is evaluated by examining the broken pieces. The idea is to determine the percentages of
the fracture surface that exhibit ductile shear and brittle cleavage. The two regions are very different in
appearance, and the transition from one to the other is abrupt. There are two methods of making this evaluation.
One is for percentages of shear from 45 to 100% (Fig. 15), and the other for percentages from 0 to 45% (Fig.
16). The acceptance criterion is percentage of shear at a specific temperature. The temperature at which 50%
shear occurs is sometimes considered the ductile-to-brittle transition temperature. Such tests have often been
used for evaluation of line-pipe material for natural-gas transmission lines. The specimens from the curved pipe
may be flattened prior to testing.
Fig. 15 Method of calculating percent shear for drop-weight tear testing by examination and
measurements made of the fractures. This method applies when the percent shear is between 45% and
100%. %SA is percent shear area; A is the width of the cleavage fracture at the one “t” line beneath the
notch, in.; and B is the length of the cleavage fracture in between the two “t” lines, in.
Fig. 16 Method of shear calculation for drop-weight tear testing when the percent shear is less than 45%
Impact Toughness Testing
Other Impact Tests
Izod impact testing uses a specimen with a V-notch (Fig. 17) that is similar to the Charpy V-notch specimen.
The principal difference is that the specimen is gripped at one end only, allowing the cantilevered end to be
struck by the pendulum (Fig. 18). An advantage of this method is that several notches can be made in a single
specimen and the ends broken off one at a time. The disadvantage that has caused it to lose popularity is that the
required time for and method of clamping the specimen in an anvil preclude low-temperature testing. Izod
specimens can also be round. Many testing machines can be used for both Charpy and Izod testing.
Fig. 17 Izod specimen
Fig. 18 Cross section depicting clamped specimen and contact point for testing. All dimensional
tolerances are ±0.05 mm (0.002 in.) unless otherwise specified. The clamping surfaces of A and B are flat
and parallel within 0.025 mm (0.001 in.). Finish on unmarked parts is 2 μm (63 μin.). Striker width must
be greater than that of the specimen being tested.
Impact toughness values from the British Standard Izod test are compared with various other methods of testing
in Fig. 19. These graphs, derived from a large number of test results on carbon and low-alloy steels, are only
intended to show comparative trends. The curves in Fig. 19 should not be used for comparing or compiling
specifications (Ref 59).
Fig. 19 Impact values obtained with the British Standard Izod test and other test methods. The inner
dotted band represents the area within which 50% of the results may be expected to fall, while the wider
full band covers approximately 80% of results. Source: Ref 59
The one-point bend test uses a single-edge cracked specimen and the same testing arrangement as a
conventional three-point bend test, except that the end supports are removed (Fig. 20). The specimen holder
used in a Charpy or Izod test is replaced by a simple frame that supports the specimen, while allowing it to
move freely in the horizontal plane. Depending on the design of the original pendulum and hammer, the impact
tester may require retrofitting with a new hammer and striker that will not interfere with the specimen edges or
the support frame. When the hammer strikes the specimen, the center portion of the specimen is accelerated
away from the hammer; the end portions of the specimen lag behind because of inertia. This causes the
specimen to bend and to load the crack tip.
Fig. 20 Experimental setup used to perform the one-point bend test
The primary advantage of the one-point bend test is that the measured stress-intensity history incorporates
dynamic effects completely. Therefore, no limits need to be imposed on the impact velocity and the test
duration to fracture. Use of the one-point bend test currently is restricted to small-scale yielding conditions (Ref
60).
Dynamic Notched Round Bar Testing. The dynamic notched round bar specimen is a long cylindrical bar with a
fatigue precrack (Fig. 21). During the test, the specimen is loaded in tension at one end by an impact of
sufficiently large magnitude that the resulting stress pulse produces a fracture at the notch. In principle,
therefore, the dynamic notched round bar test is more amenable to analysis than the Charpy test because the
fracture process is completed before the stress pulse has sufficient time to be reflected from the farthest end of
the bar.
Fig. 21 Typical apparatus for dynamic fracture initiation experiment. Source: Ref 61
The Charpy test is a simple, low-cost test that rapidly detects changes in ductility. However, the Charpy test
does have certain disadvantages for quantitative assessments. Fracture in the Charpy specimen does not occur
under plane-strain conditions. Furthermore, the state of stress at the fracture site is unknown and quite complex
due to multiple pulse reflections from its various surfaces. For these reasons, it is difficult to interpret Charpy
results in terms of elastic or elastic-plastic fracture toughness parameters, although, as previously described,
instrumented impact testing of precracked Charpy V-notch specimens provides useful results for evaluations of
dynamic fracture toughness, as described in more detail in Ref 62.
Dynamic notched round bar testing yields data from which a reliable value of the dynamic critical stress-
intensity factor K
Id
can be calculated easily. Hence, results are immediately related on a quantitative basis to
fracture mechanics parameters. However, the test setup is rather elaborate, and more material is required for
each specimen compared to Charpy testing. As a result, the technique is not suitable for routine testing. It may
be used, however, when a precise evaluation of the fracture initiation properties of a particular material is
required, perhaps as a function of temperature as well as of loading rate.
In this test, measurements of the average stress across the fracture plane and of crack-opening displacement,
both as functions of time, are easily obtained. Various methods can be used to measure crack-opening
displacement, but the stress across the fracture plane is most easily determined by using electric resistance
strain gages applied to the surface of the bar downstream from the fracture site. In this respect, and several
others, the dynamic notched round bar test resembles the Kolsky (or split-Hopkinson) bar used in dynamic
plasticity. Another example of using a notched round bar in evaluation of dynamic fracture toughness is given
in Ref 63, where the K
Id
toughness of A533-B reactor-grade steel was determined over the temperature range
from 3 to 50 °C (37–122 °F), by dynamic loading of notched round bar specimens with axial precompression of
the notch.
The Schnadt specimen, details of which are shown in Fig. 22, has been used primarily in Europe for testing ship
plate. In the Schnadt test, five test pieces are used with different notch radii, ranging from no notch to a severe
notch made by pressing a sharp knife into the bottom of a milled groove. A hardened steel pin is inserted in a
hole parallel to and behind the notch, replacing the material normally under compression in the Charpy or Izod
tests. The specimen is broken by impact as a three-point-loaded beam.
Fig. 22 Details of the Schnadt notched-bar impact-test specimen
References cited in this section
59. J. Woolman and R.A. Mottram, The Mechanical and Physical Properties of the British Standard DN
Steels, Vol 1, Appendix UI, The British Iron and Steel Research Association, 1964, p 442
60. J.H.J. Giovanola, One-Point Bend Test, Mechanical Testing, Vol 8, ASM Handbook, 1985, ASM
International, p 271–275
61. L.S. Costin, J. Duffy, and L.B. Freund, Fracture Initiation in Metals Under Stress Wave Loading
Conditions, Fast Fracture and Crack Arrest, STP 627, G.T. Hahn and M.F. Kanninen, Ed., ASTM,
1977, p 301–318
62. R.H. Hawley, J. Duffy, C.F. Shih, Dynamic Notched Round Bar Testing, Mechanical Testing, Vol 8,
ASM Handbook, 1985, ASM International, p 275–284
63. G.R. Irwin, J.W. Dally, X.-J. Zhang, and R.J. Bonenberger, Lower-Bound Initiation Toughness of
A533-B Reactor-Grade Steel, Rapid Load Fracture Testing, STP 1130, ASTM, 1991, p 9–23
Impact Toughness Testing
Acknowledgments
Contents of this article were adapted from the following:
• Impact Testing, by R.C. Anderson, in Inspection of Metals: Destructive Testing, ASM International,
1988, p 121–170
• Charpy Impact Testing, by William Seaver, in Mechanical Testing, Vol 8, ASM Handbook, 1985, p
261–269
Impact Toughness Testing
References
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Ic
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Ic
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