r e c o m m e n d e d practice guide

Installing,
Maintaining, and
Verifying Your
Charpy Impact
Machine

D.P. Vigliotti
T.A. Siewert
C.N. McCowan
N I S T

960-4 Special
Publication
960-4
NIST Recommended Practice Guide

Special Publication 960-4

Installing,
Maintaining, and
Verifying Your
Charpy Impact
Machine
D.P. Vigliotti
T.A. Siewert
C.N. McCowan
Materials Science and
Engineering Laboratory

October 2000

Previously issued as NIST

Technical Note 1500-8

E N T OF C O M
TM M
AR

ER
DE P

CE
ICA
UN
IT

ER

D
E

ST AM
ATES OF

U.S. Department of Commerce

Norman Y. Mineta, Secretary
Technology Administration
Dr. Cheryl L. Shavers, Under Secretary of
Commerce for Technology
National Institute of Standards and Technology
Raymond G. Kammer, Director

i
®

Certain commercial entities, equipment, or materials may be identified in this

document in order to describe an experimental procedure or concept adequately.
Such identification is not intended to imply recommendation or endorsement
by the National Institute of Standards and Technology, nor is it intended to
imply that the entities, materials, or equipment are necessarily the best avail-
able for the purpose.
______________________________________

National Institute of Standards and Technology

Special Publication 960-4
Natl. Inst. Stand. Technol.
Spec. Publ. 960-4
22 pages (October 2000)
CODEN: NSPUE2
U.S. GOVERNMENT PRINTING OFFICE
WASHINGTON: 2000
For sale by the Superintendent of Documents
U.S. Government Printing Office
Washington, DC 20402-9325

ii
 ®

Foreword

This Special Publication is a reprint of NIST Technical Note 1500-8, a TN

series started by the Materials Reliability Division. This TN series describes
their division’s significant research accomplishments in measurement technol-
ogy, reported so that producers and users of materials can improve the quality
and reliability of their products.
This particular Practice Guide provides practical advice on how to correct
problems discovered during the testing of Standard Reference Materials 2092,
2096, and 2098 on Charpy impact machines. Although only a small percent-
age of machines fail to meet the requirements during their annual performance
tests, the failure of a machine can have large economic implications to the
machine's owners, and we try to assist in correcting the problems. From the
study of the fractured specimens and test data that are returned to NIST for
evaluation, we have learned how to identify many of the common problems.
Also, over the years, we have had discussions with thousands of engineers and
technicians at these companies, and have learned the most efficient sequences
for identifying the sources of the problem and for correcting them. Now, we
have collected and organized the various problems and solutions in this
Special Publication. We hope that you will find it useful for installing a new
machine correctly, and then for preparing for the annual verification tests.
More information on the SP 960 series can be found on the Internet at
http://www.nist.gov/practiceguides. This web site includes a complete
list of NIST Practice Guides and ordering information.

iii
®

Abstract:

The quality of the data developed by pendulum impact machines depends on

how well the machines are installed, maintained, and verified. This is the rea-
son that ASTM Standard E 23 Standard Test Methods for Notched Bar Impact
Testing of Metallic Materials specifies annual direct and indirect verification
tests. Each year, NIST provides reference specimens for indirect verification of
over 1000 machines around the world. From evaluation of the absorbed ener-
gies and the fractured specimens, we attempt to deduce the origin of energies
that are outside the ranges permitted by Standard E 23, and report these obser-
vations back to the machine owners. This recommended practice summarizes
the bases for these observations, and hopefully will allow machines to be
maintained at higher levels of accuracy. In addition, we provide details of the
NIST verification program procedures and the production of the specimens.

Key words:
absorbed energy, Charpy V-notch, impact test, machine repair, misalignment,
mounting, pendulum impact test, verification testing, worn anvils

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 Table of Contents ®

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

2. Overview of the NIST Program . . . . . . . . . . . . . . . . . . . . .2

2.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2
2.2 Acceptance Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
2.2.1 Dimensional Tolerances . . . . . . . . . . . . . . . . . . . . .3
2.2.2 Impact Energy Requirements . . . . . . . . . . . . . . . . .3
2.2.3 The Direction in Which the Specimens
Leave the Machine During Impact Testing . . . . . . . .4

3. Machine Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

4. Direct Verification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

5. Indirect Verification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
5.1 Post-Fracture Examination . . . . . . . . . . . . . . . . . . . . . . . .9
5.1.1 Worn Anvils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
5.1.2 Off-Center Specimen . . . . . . . . . . . . . . . . . . . . . . .11
5.1.3 Off-Center Striker . . . . . . . . . . . . . . . . . . . . . . . . .12
5.1.4 Uneven Anvil Marks . . . . . . . . . . . . . . . . . . . . . . .12
5.1.5 Chipped Anvils . . . . . . . . . . . . . . . . . . . . . . . . . . .13
5.1.6 Anvil Relief . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
5.1.7 Damaged Anvils . . . . . . . . . . . . . . . . . . . . . . . . . .14
5.1.8 Bent Pendulum . . . . . . . . . . . . . . . . . . . . . . . . . . .15

6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16

7. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17

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vi
 Introduction ®

1. Introduction
The low cost and simple configuration of the Charpy impact test have made it
a common requirement in codes and standards for critical structures such as
pressure vessels and bridges. For many years, engineers and designers have
recognized that materials behave differently when loaded statically than when
loaded dynamically, and that a number of materials have a brittle-to-ductile
transition temperature. The Charpy impact test is a very cost-effective method
of evaluating the behavior of materials for applications where these attributes
are important.
The history of the pendulum impact test extends back about 100 years. Over
the years, procedure improvements, such as the addition of shrouds to prevent
specimen jamming and the addition of indirect verification to the verification
requirements, have resulted in a simple yet robust test method. The attached
bibliography shows how NIST has contributed to the understanding of the test
method, and for those interested, points to a brief history of the test method.
Accurate impact results can be obtained only from machines that are installed
correctly, then remain in good working condition, such as within the tolerances
specified by Standard E 23. Our indirect verification program is referenced in
Standard E 23, and supplements the direct verification requirements found
there.
Our examination of over 2300 sets of these specimens each year allows us to
identify problems that are often not recognized during routine measurement of
machine dimensions or routine check procedures (such as the free-swing test).
We have learned to recognize which marks on the broken verification speci-
mens indicate factors that could be affecting the results. We can then advise
our customers to recheck or replace the anvils or the striker, tighten bolts,
check bearings, check machine alignment or level, check cooling bath or ther-
mometer, or review testing procedures. This recommended practice describes
the most common problems that we detect, and gives advice on how to avoid
or correct most of them. We have divided the description of impact test prob-
lems into four major sections: Overview of the NIST Program, Machine
Installation, Direct Verification (evaluation of the machine alone), and Indirect
Verification (evaluation of the machine by the testing of specimens).
While the following sections give suggestions to improve the accuracy of your
impact machine, the machine manufacturer and Standard E 23 also are impor-
tant sources of information.

1
® Overview

2. Overview of the NIST Program

The NIST program focuses on evaluating the performance of pendulum impact
test machines through indirect verification (the production and evaluation of
standardized verification specimens that we distribute to customers of our veri-
fication service), as described in ASTM Standard E 23. In chronological order,
the NIST program involves obtaining steel that can be made into verification
specimens, heat treatment and machining of batches of verification specimens,
inspection of representative specimens from each batch to check quality,
assignment of a reference value to each batch, packaging of sets of specimens
for shipment to customers, evaluation of the fractured specimens and customer
test data, and preparation of a certificate of compliance for the customers or
suggestions on how to correct any problems. Further details on these tasks are
found in the following sections.
The basic materials and procedures currently used by NIST have remained
unchanged for the past 10 years, and date back to the procedures maintained
by the U.S. Army at their arsenal in Watertown, Massachusetts (AMMRC).

2.1 Materials
Two materials are currently used to make the specimens for indirect verifica-
tion of Charpy impact machines to E 23 specifications. A 4340 steel is used to
make specimens for the low- and high-energy levels. A type T-200 maraging
steel is used to make specimens for the super-high-energy level.
The steels are purchased as square bar. The bar stock is supplied to subcon-
tractors that machine and heat-treat the impact specimens to meet the NIST
specification.
In these steels, the hardness, impact energy, and strength are interrelated. Since
hardness correlates to impact energy and is a more convenient property to
measure during processing, it is used as the initial process control. The low-
energy specimens are typically heat-treated to attain a room-temperature hard-
ness (HRC) of 45, which corresponds to a Charpy impact energy near 16 J (12
ft-lbf) at -40 ºC (-40 ºF). The high-energy specimens are typically heat-treated
to attain a room temperature HRC of 32, which corresponds to a Charpy
impact energy near 100 J (65 ft-lbf) at -40 ºC (-40 ºF). The super-high-energy
specimens are typically heat-treated to attain a room-temperature HRC of 30,
which corresponds to a Charpy impact energy near 220 J (163 ft-lbf) at room
temperature. Note that the two different steels have different responses to heat
treatment, and are tested at different temperatures.

2
 Materials ®

2.2 Acceptance Criteria

Acceptance of a batch of verification specimens is based on the data obtained
from a pilot lot of 75 specimens, taken from a heat-treatment batch of 1000 to
1200 specimens. Although impact energy is the desired output, three criteria
are combined to determine the acceptability of verification specimens: (1)
dimensional tolerances of specimens, (2) mean and standard deviation of
impact energy, and (3) the direction in which the specimens leave the machine
during impact testing.

2.2.1 Dimensional Tolerances

The dimensional requirements for NIST specimens exceed some of the E 23
specifications for verification specimens. This minimizes variations in impact
energy due to physical variations in the specimens. Also, the notch centering
and the length tolerance for NIST specimens is equivalent to the ISO 164
standard, which permits the specimens to be used in impact machines with
end-centering devices. The NIST requirement for surface finish is equivalent
to the ISO requirement. All of these dimensional requirements can be met with
standard machining practices.

2.2.2 Impact Energy Requirements

NIST lots are currently produced to have average energy values that fall with-
in the ranges of 13 J to 20 J (10 to 15 ft-lbf) for low-energy, 88 J to 136 J
(65 to 100 ft-lbf) for high-energy, and 176 J to 244 J (130 to 180 ft-lbf) for
super-high-energy levels. NIST has never produced a low-energy lot having a
reference energy of less than 15 J (11 ft-lbf). The upper limit of the super-high-
energy range has been limited by NIST because such energies significantly
reduce the final swing velocity for machines with capacities of less than 300 J.
(The standard states that machines should not be evaluated with reference
specimens that exceed 80 % of the machine capacity).
The reference energy for a lot of verification specimens is determined from the
pooled average of 75 specimens (25 specimens tested on each of 3 different
impact machines). In addition to the average impact energy, the standard devi-
ation, and a sample size for the verification set are determined (the number of
specimens required to estimate the performance of a machine with a specified
measurement uncertainty). All 75 specimens are included in these calculations,
except for specimens that are determined to be outliers. An outlier is defined
statistically, but a specimen identified as an outlier is not removed from the

3
® Overview

analysis unless it shows physical evidence of jamming, material flaws, or other

reason for atypical behavior. At present, any lot with more than 5% outliers is
rejected. The E 1271 standard sets a minimum number of specimens for a veri-
fication set at five, and the maximum number of specimens in a set is deter-
mined by a sample size calculation. Occasionally, a batch has such a small
standard deviation that the equation indicates a minimum set size of four.
The average hardness value for a pilot lot is determined for 75 specimens. The
variation in hardness for the lot is also determined, and a lot can be rejected
on hardness data alone if the variation in hardness exceeds 1 HRC of the lot
average. A variation in hardness of 1 HRC results in a very large variation in
absorbed energy. In practice, we find that when the variation in hardness
exceeds 0.5 HRC the quality of the lot is questionable. The correlation
between energy and hardness is much more useful for evaluating variations
of high-energy (lower hardness) specimens, because at high hardness, the
slope of the trend decreases significantly.

2.2.3 The Direction in Which the Specimens Leave the Machine

During Impact Testing
The average hardness of the low-energy specimens also can be used to esti-
mate whether the specimens will have the desired behavior (leaving the
machine in the forward or backward direction). When the average hardness
value falls below 44 HRC, low-energy specimens begin to exit in a forward
direction, which is undesirable.
Low-energy specimens are designed to exit the machine in the direction oppo-
site to that of the pendulum swing to assure that the shrouds around the anvils
of U-type machines are functioning properly. If the shrouds are not adjusted
properly or are of inadequate hardness, the specimens will jam between the
pendulum and anvils, causing artificially high energy values.

4
 Machine Installation ®

3. Machine Installation
This is the detailed procedure developed by NIST to mount the three Master
Charpy Reference Machines. The manufacturer of your machine should be
able to supply procedures that are designed specifically for your machine.
A stable foundation for the impact machine is critical to ensure accurate
results. Energy losses through the foundation must be kept to a minimum. We
recommend using a foundation of high-strength concrete (if your concrete
supplier uses a commercial-grade description to characterize the quality of
the concrete, you should ask for a 7000-pound mix) that measures 1525 mm
(60 in) long by 910 mm (36 in) wide by 450 mm (18 in) thick. Usually you
will need to cut a hole in the floor to accommodate the new foundation. If
other equipment in the area could affect the machine operation, you may want
to isolate it from the floor with expansion-joint material.
Hold-down bolts used to secure the machine to the foundation should be of the
inverted “T” or “J” type. (The next section, on direct verification, describes
problems with the use of lag bolts, which may be tightened up against the base
without gripping the concrete.) The bolts, nuts, and washers should have a
strength of grade 8 or higher. We recommend using bolts or rod with a diame-
ter of 22 mm (7/8 in). At NIST we used 22 mm (7/8 in) grade 8 threaded rod,
cut into pieces that are 610 mm (24 in) long. We then welded 150 mm (6 in
long) pieces of the same threaded rod to the end of the 610 mm (24 in) pieces
to make inverted “T” bolts.
We then positioned the machine over the center of the foundation hole. The
machine was held approximately 100 mm (4 in) above the floor using spacers
suitable to hold the weight of the machine. The “T” bolts were positioned in
the machine-base mounting holes with a nut below and above the base of the
machine. The nuts were tightened to keep the “T” bolts straight while the
concrete was poured. The ends of the T bolts were positioned approximately
25 mm (1 in) from the bottom of the hole. The machine was then leveled on
the spacers. Leveling did not need to be as accurate as the final leveling.
Reinforcement bars were attached to the top of the horizontal rod previously
welded to the bottom of the “T” bolts. The reinforcement bars were attached
in the form of a box connecting the four bolts. A second box formation of
reinforcement rods was attached to the “T” bolts 25 cm (10 in) above the first
box. The concrete was then poured under the machine. The concrete was fin-
ished as level as possible at this time. Before the concrete fully hardened, we
removed enough concrete, to a depth of approximately 25 mm (1 in), from
around each “T” bolt to be able to thread a nut to below the surface of the
concrete, and cleaned the exposed threads. The machine was left in this
position for 72 h.

5
® Machine Installation

After 72 h, the nuts on top of the base plate were removed and the machine
was lifted off the “T” bolts. The bottom nuts were then threaded down into the
cavities that were created before the concrete hardened. The nuts were left
high enough on the “T” bolts to enable the use of an open-end wrench to
adjust them after the machine was positioned on them. At this point, the base
of the machine was coated with a light oil to keep the grout from adhering to
it. The machine was then lifted back onto the “T” bolts and was positioned on
the adjustment nuts. The machine was now ready to be leveled. A machinist’s
level was used to ensure meeting the level tolerance of 3:1000. The critical
leveling procedure was done using the four nuts under the machine. After the
machine was leveled, we wrapped the outside of the nuts with duct-seal putty
to facilitate their removal from the “T” bolts later in the process.
At this point the base of the machine was ready to grout. Heavy cardboard
forms were placed around the base of the machine to keep the grout under the
machine. The grout was pushed under the machine, so that the base of the
machine was in total contact with the grout. The machine was left in this
position for 72 h.
After 72 h, the machine was lifted off the “T” bolts one last time. The grout
was inspected for cavities and for surface contact with the bottom of the
machine. The putty was removed from around the nuts. Some grout leaked
around the putty and had to be chipped away from the nuts to enable them to
turn. The supporting nuts were removed from the “T” bolts. After removing all
debris from the grout, the machine was repositioned on the “T” bolts and rest-
ed on the grout. Washers and nuts were installed on the “T” bolts and were
tightened to pull the machine tightly against the grout. The level was checked
at this point. The “T” bolts were cut off to approximately 12.7 mm (1/2 in)
above the nuts. The nuts were torqued to 380 ft-lb. The final level was
checked at this point.
NOTE: Special non-shrinking grout is recommended. This grout is available at
most industrial hardware stores.
If you have any questions concerning this procedure, please contact Daniel
Vigliotti by phone at (303) 497-3351, by fax at (303) 497-5939, by email at
vigliotti@boulder.nist.gov, or by mail at NIST, Division 853, 325 Broadway,
Boulder, CO 80305-3328.

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 Direct Verification ®

4. Direct Verification
This section of the practice explains the direct verification requirements of
Standard E 23, confirmation that the machine is in good operating condition,
without the use of verification specimens. The direct verification tests are
physics-based tests, which assure that the machine is functioning as closely as
possible to a simple pendulum, with only small losses, due to friction and
windage. Direct verification is most important when the machine is first
installed or when major parts are replaced, but is also important during the
periodic reinspections. While these tests are required for the periodic reinspec-
tions, we recommend that the free-swing test and windage-and-friction test be
performed each day that the machine is used. The records of these tests then
serve as a convenient measure of bearing performance.
Since the Charpy test is a dynamic test with vibration and impact loads, the
hold-down bolts may loosen over time. In extreme cases, this may introduce
error sufficient to cause a machine to exceed the tolerance limits of the indi-
rect verification test. In marginal cases, the movement may still be sufficient
to add a bias to the results that reduces the likelihood of passing. Check the
tightness of all bolts, especially the anvil bolts, the striker bolts, and the base-
plate bolts. The manufacturer can supply the torque values for the anvil and
striker bolts. The base-plate bolts should be torqued to the recommended
torque values for the grade and size of the nuts and bolts. We recommend the
use of “J” or “T” bolts only. We do not recommend lag-type bolts. These are
made to withstand only static loads. We believe that over time, the insert por-
tion of lag bolts can loosen in the concrete. When lag bolts are retightened,
they can pull out of the concrete and be pulled against the base of the machine,
giving the impression of a properly mounted machine. This condition is very
difficult to detect. A machine with this problem will exhibit erroneously high
energy values at the low-energy level. The mounting procedure used to elimi-
nate this problem for our Master Reference Machines was described in the
previous section.
Standard E 23 describes a routine check procedure that should be performed
weekly. It consists of a free-swing check and a friction-and-windage check.
The free swing is a quick and simple test to determine whether the dial or
readout is performing accurately. A proper zero reading after one swing from
the latched position is required on a machine that is equipped with a compen-
sated dial. Some machines are equipped with a non-compensated dial. Such
a dial is one on which the indicator cannot be adjusted to read zero after one
free swing. The user should understand the procedure for dealing with a
non-compensated dial. This information should be available from the
manufacturer.

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® Direct Verification

The friction-and-windage test assesses the condition of the bearings. The pen-
dulum should be released and allowed to swing 10 half cycles (5 full swings).
(We recommend holding the release mechanism down this whole time to avoid
additional friction when the pendulum swings back up to where it may push
on the latch.) As the pendulum starts its 11th half swing, the pointer should be
reset to about 5 % of the scale capacity. Record this value and divide by the 11
half swings. Divide this number by the machine range capacity, then multiply
by 100. Any loss of more than 0.4 % of the machine capacity is excessive, and
the bearings should be inspected.
We suggest that the user develop a daily log or shift log to be kept with the
machine. The log can be used to track the zero and friction values. The log can
also include information such as number of tests, materials tested, mainte-
nance, and any other useful comments.
The anvil and striker radii should be carefully inspected for damage and for
proper dimensions. Damage (chips or burrs) can be detected easily by visual
inspection and by running a finger over the radii to check for smoothness.
Measurement of the dimensions requires more sophisticated equipment. We
find that radius gages are usually inadequate to measure the critical radii. We
recommend making molds of the radii (such as with silicone rubber) or mak-
ing an indentation in a soft, ductile material (such as annealed aluminum), then
measuring the impressions on an optical comparator. Occasionally, even a new
set of anvils and striker may have incorrect radii. We recommend that new
anvils and strikers always be inspected before being installed in the machine.
Since the radii will not have local wear before use (the radii are consistent
along their length), they can be measured directly on an optical comparator or
other optical measurement system.

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 Indirect Verification ®

5. Indirect Verification
Indirect verification uses carefully characterized test specimens to stress the
test machine components to levels similar to those experienced during routine
usage. Since many machine problems, such as loose anvils or striker, cannot
be detected during direct verification, indirect verification serves as an impor-
tant supplemental test of the machine performance.
We recommend using centering tongs, such as those described in ASTM
Standard E 23, to insert the specimens at the center of the anvils. The tongs
should be inspected for wear or damage. A proper set of tongs is critical for
the accurate placement of the specimen. Some machines are equipped with a
centering device. The device should be inspected for wear and proper opera-
tion. We do not recommend the use of centering devices for low-temperature
testing because the centering operation can extend the time between a speci-
men’s removal from the bath and fracture, and so may exceed the five-second
interval allowed for transferring and fracturing the specimen.
Some of the reference specimens are designed to be tested at -40 ºC (-40 ºF).
Since the absorbed energy changes with temperature, accurate temperature
control is necessary to obtain valid test data. The temperature indicator should
be calibrated immediately before testing. Ice water and dry ice are quick and
easy calibration media.

5.1 Post-Fracture Examination

Just matching the reference energies is not sufficient to confirm that the
machine is fully satisfactory. For example, worn anvils can combine with
high-friction bearings to compensate for each other and produce an artificially
correct value during the verification test. These are called compensating errors.
Unfortunately, these errors compensate only over part of the range, so the
machine produces generally inaccurate values. The post-fracture examination
of the NIST standardized verification specimens is a good way to identify such
effects. Therefore, the NIST specimens come with a questionnaire (with criti-
cal questions about the machine and the test procedure) and a mailing label so
the specimens can be returned to NIST. All specimens are examined and com-
pared to the data on the questionnaire before a formal response is sent to the
customers.
Following are the most common of these problems. In many cases, sugges-
tions on how to correct or avoid them in the future are included.

9
® Indirect Verification

5.1.1 Worn Anvils

Most of the wear of an impact test machine occurs on the anvils and striker.
We evaluate this wear by examining the gouge marks that are formed on the
sides of high-energy specimens when they are forced through the anvils.
Anvils that are within the required tolerance of the standard will make a thin,
even gouge mark all the way across both pieces of the broken specimen. As
the anvils wear, they will make a wider, smeared mark across the specimen
halves. Figure 1 shows the change in the gouge marks. When wide, smeared
marks are observed on a customer’s specimens, we recommend that the anvils
be changed, because the reduction in energy needed to push the specimens
through worn anvils eventually drops the machine below the lower tolerance
in the energy range. You can monitor the wear on your machine by retaining
some specimens that are tested with new anvils and comparing them to speci-
mens of similar composition and hardness that are tested as the anvils wear.
For specimens at a similar absorbed energy, the gouge marks will grow wider
and smoother as the anvils wear.

Figure 1. Worn Anvils.

10
 Post-Fracture Examination ®

5.1.2 Off-Center Specimen

An off-center specimen strike occurs when the notch is not centered between
the anvils, so the striker contacts the specimen to the side of the notch. The
low-energy specimen best indicates when an off-center strike occurs. We iden-
tify this condition on the specimens by finding that the gouge marks caused by
the anvils are not equidistant from the machined notch edges, and the striker
gouge mark is offset the same amount from the notch (Figure 2). Also, as seen
in Figure 2, the fracture surface of a correctly tested low-energy specimen is
flat and both halves are even. However, the fracture surfaces of a specimen
that has been tested off-center are on an angle. The more off-center the
strike,the steeper the angle will be. This problem increases the energy needed
to fracture a specimen. The most common causes for this slipping are worn or
damaged centering tongs, a worn or misaligned machine centering device,
careless test procedures, or the use of a cooling fluid that is too viscous at the
test temperature, which causes the specimen to float on the specimen supports.
Most machine manufacturers should be able to provide new centering tongs.
We have found that ethyl alcohol is one of the best cooling media because it
seems to evaporate quickly from the bottom of the specimen to prevent
specimen floating.

Figure 2. Off-Center Specimen.

11
® Indirect Verification

5.1.3 Off-Center Striker

This differs from the off-center specimen in that the notch is centered against
the anvils so the anvil gouge marks are equidistant from the machined notch
edges. However, the striker does not contact the specimen precisely opposite
the notch. Figure 3 shows this appearance. An off-center striker is usually
attributed to the pendulum shaft shifting off center. This shift can be the result
of a loose alignment ring on the shaft or a loose bearing block on the machine.
This problem also increases the energy needed to fracture specimens at all
energy levels.

Figure 3. Off-Center Striker.

5.1.4 Uneven Anvil Marks

Frequent testing of subsize specimens can cause the anvils to wear unevenly.
Figure 4 shows an example of these uneven wear marks at each energy level
of our reference specimens. Since this wear is restricted to a small area that
the full-size reference specimen contacts, there is usually no effect on the
energy required to fracture the specimen. This anvil condition presents two
problems. First, since subsize wear is usually not indicated by a change in the
energy required to break a reference specimen, inspection of the broken speci-
men is required. This wear will cause the anvils to be out of tolerance accord-
ing to the requirements in the standard. This means that the machine does not
meet the direct verification requirements of the standard and is therefore, not
eligible for the indirect verification process. The second, and more important
problem, is that the subsize specimens are being tested in an area of the anvil

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 Post-Fracture Examination ®

that is worn. When the wear is substantial, this condition will produce artifi-
cially low subsized energy values. The anvils should be replaced on a machine
with this condition.

Figure 4. Uneven Anvil Marks.

5.1.5 Chipped Anvils

Sometimes an anvil can be chipped. Figure 5 shows that this condition can be
detected easily on all three energy reference specimens. The low-energy speci-
men is affected the least amount because it is the hardest specimen and there-
fore has a more brittle fracture. The ductile high-energy specimen will produce
higher than normal energy results, and the very ductile super-high-energy
specimens are affected most by a chipped anvil. This condition should be
detected easily by a visual inspection before using the machine. New anvils
are required when an anvil is chipped.

Figure 5. Chipped Anvils.

5.1.6 Anvil Relief

Some Charpy machine manufacturers have designed a machined relief at the
bottom of the anvil (Figure 6). This anvil design does not meet the direct veri-
fication requirements of ASTM Standard E 23. The relief has caused high-
energy results in our ductile high- and super-high-energy specimens. It can
also cause twisting of the specimens, during fracture, that may also contribute
to energy values higher than normal at all energy levels.

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® Indirect Verification

Figure 6. Anvil with Machined Relief.

5.1.7 Damaged Anvils

Under some test conditions, usually for elevated-temperature testing, the anvils
can wear to a rough finish that creates excessive friction (Figure 7). This dam-
aged condition is detected best on the high and super-high specimens. Rough
anvils usually cause the gouge marks to become wider and push the specimen
material to form a ridge that can easily be detected with the fingernail. This
damage usually causes artificially high energy results at the high- and super-
high-energy levels. Damaged anvils must be replaced.

Figure 7. Damaged Anvils.

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 Post-Fracture Examination ®

5.1.8 Bent Pendulum

Figure 8 shows how gouge marks are created by a pendulum bent in the direc-
tion of the swing. This gouge mark is usually deeper on the top edge of the
specimen as it sits in the machine. As shown in Figure 8, the striker contacts
the top edge of the specimen first, causing excessive tumbling and twisting.
This excessive activity can cause the specimen to interact with the striker or
the pendulum after fracture and create additional energy loss. A bent pendulum
can be detected by placing an unbroken reference specimen in the machine
and placing a piece of carbon paper on the surface opposite the notch. At this
point, lightly tap the striker against the specimen. This will make a mark on
the specimen that can be inspected. If the pendulum is not bent, the mark
should appear the same width across the specimen. If the pendulum is bent,
the mark will be wider at one edge and become thinner or even not visible at
the other edge (Figure 9). We recommend that a new pendulum be installed
on a machine with this problem.

Figure 8. Bent Pendulum.

Figure 9. Bent Pendulum.

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® Summary

6. Summary
The condition and accuracy of Charpy machines cannot be checked only by
comparing results of NIST reference specimens to the Master Reference
Machines located at NIST, Boulder, CO. Some machine problems cause artifi-
cially low results while other machine problems cause artificially high results.
In addition, deviations in procedures can cause similar results. These machine
problems and procedural deviations may go undetected for years without some
sort of physical check. For this reason, examination of the broken specimens is
a critical part of the verification process. Many machine problems can be
avoided or corrected with the information presented in this paper. Also, sug-
gested changes in procedure can help to ensure a successful test. To obtain
verification specimens or to clarify procedures for verification testing, you
may use the following information:
Verification specimens can be ordered from the NIST Standard Reference
Materials Program. Phone: (301) 975-6776, fax: (301) 948-3730, or email:
SRMINFO@nist.gov
Questions on verification procedures can be answered by the Charpy Program
Coordinator. Phone: (303) 497-3351, fax: (303) 497-5939, or email:
vigliotti@boulder.nist.gov

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 Bibliography ®

7. Bibliography
1.“Absorbed Energy Differences between 8-mm and 2-mm Charpy V-notch
Striker Radii when using Verification Specimens,” T.A. Siewert and D.P.
Vigliotti, Pendulum Impact Machines: Procedures and Specimens for
Verification, ASTM STP 1248, American Society for Testing and Materials,
1995, 140-152.

2.“Effect of Surface Finish of Charpy Anvils and Striking Bits on Absorbed

Energy,” E.A. Ruth, D.P. Vigliotti, and T.A. Siewert, Pendulum Impact
Machines: Procedures and Specimens for Verification, ASTM STP 1248,
American Society for Testing and Materials, 1995, 91-100.

3.“Notched Bar Impact Testing Standards have Yielded Widespread Benefits

for Industry,” M.P. Manahan, C.N. McCowan, T.A. Siewert, J.M. Holt, F.J.
Marsh, and E.A. Ruth, Standardization News, 27 (2): 30-35, February 1999.

4.“Performance Verification of Impact Machines for Testing Plastics,” T.A.

Siewert, D.P. Vigliotti, L.B. Dirling, and C.N. McCowan, NIST Journal of
Research, 104(6): 1-9, November-December 1999.

5.“The History and Importance of Impact Testing,” T.A. Siewert, M.P.

Manahan, C.N. McCowan, J.H. Holt, F.J. Marsh, and E.A. Ruth, Pendulum
Impact Testing: A Century of Progress, ASTM STP 1380, T.A. Siewert and M.
P. Manahan, Sr., eds., American Society for Testing and Materials, 2000.

6.“Maintaining the Accuracy of Charpy Impact Machines,” D.P. Vigliotti, C.N.

McCowan, and T.A. Siewert, Pendulum Impact Testing: A Century of
Progress, ASTM STP 1380, T.A. Siewert and M. P. Manahan, Sr., eds.,
American Society for Testing and Materials, 2000.

7.“Reference Specimens for Plastics Impact Machines,” C.N. McCowan, D.P.

Vigliotti, and T.A. Siewert, Pendulum Impact Testing: A Century of Progress,
ASTM STP 1380, T.A. Siewert and M.P. Manahan, Sr., eds., American Society
for Testing and Materials, 1999.

8.“Experimental Evidence of the Difference Between Total Absorbed Energy

Measured Using an Instrumented Striker and that Obtained Using an Optical
Encoder,” M.P. Manahan, Sr., T.A. Siewert, C.N. McCowan, and D.P. Vigliotti,
in preparation for Charpy Centenary Conference -CCC2001, Poitiers, France,
2001.

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October 2000