instrumented impact testing: influence of machine variables and

OPEN REPORT

SCK•CEN-BLG-1058

Instrumented Impact Testing:

Influence of Machine Variables and

Specimen Position

Work performed during a 3-month secondment at NIST,

Boulder CO (USA), June-August 2008

Enrico Lucon

Institute for Nuclear Material Science, SCK•CEN, Mol

(Belgium)

Chris N. McCowan and Ray A. Santoyo

National Institute for Standards and Technology,

Material Reliability Division, Boulder CO (USA)

September, 2008

SCK•CEN Boeretang 200

BE-2400 Mol

Belgium

OPEN REPORT OF THE BELGIAN NUCLEAR RESEARCH CENTRE

SCK•CEN-BLG-1058

Instrumented Impact Testing:

Influence of Machine Variables and

Specimen Position

Work performed during a 3-month secondment at NIST,

Boulder CO (USA), June-August 2008

Enrico Lucon

Institute for Nuclear Material Science, SCK•CEN, Mol

(Belgium)

Chris N. McCowan and Ray A. Santoyo

National Institute for Standards and Technology, Material

Reliability Division, Boulder CO (USA)

September, 2008

Status: Unclassified

ISSN 1379-2407

SCK•CEN Boeretang 200

BE-2400 Mol

Belgium

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SCK•CEN Open Report BLG-1058 – Page 1 of 45

Preface

This work was performed during my secondment at NIST (National Institute of

Standards and Technology) in Boulder, Colorado (USA), between June and August 2008.

An abridged version of this report was submitted to Journal of Testing and Evaluation for

publication on 25 August 2008. The report is also published as a NIST Internal Report (NISTIR).

Abstract

An investigation has been conducted on the influence of impact machine variables and specimen

positioning on characteristic forces and absorbed energies from instrumented Charpy tests.

Brittle and ductile fracture behavior has been investigated by testing NIST reference samples of

low, high and super-high energy levels. Test machine variables included tightness of foundation,

anvil and striker bolts, and the position of the center of percussion with respect to the center of

strike. For specimen positioning, we tested samples which had been moved away or sideways

with respect to the anvils. In order to assess the influence of the various factors, we compared

mean values in the reference ("unaltered") and "altered" conditions; for machine variables, t-test

analyses were also performed in order to evaluate the statistical significance of the observed

differences. Our results indicate that the only circumstance which resulted in variations larger

than 5% for both brittle and ductile specimens is when the sample is not in contact with the

anvils. These findings should be taken into account in future revisions of instrumented Charpy

test standards.

Keywords

Instrumented Charpy tests, impact machine variables, specimen positioning, t-test analysis.


Table of Contents

Preface ................................................................................................................... 1

Abstract.................................................................................................................. 1

Keywords............................................................................................................... 1

1 Introduction..................................................................................................... 3

2 Material and experimental ............................................................................... 4

2.1 Loosening machine bolts .......................................................................... 5

2.1.1 Foundation bolts................................................................................. 5

2.1.2 Anvil bolts ....................................................................................... 13

2.1.3 Striker bolts...................................................................................... 18

2.2 Changing the position of the Center of Percussion (COP)....................... 23

2.3 Specimen positioning.............................................................................. 28

2.3.1 Specimen not in contact with the anvils ........................................... 28

2.4 Lateral specimen offset ........................................................................... 34

2.5 Discussion .............................................................................................. 39

3 Conclusions................................................................................................... 43

References............................................................................................................ 44


1 Introduction

Impact testing has a history extending back to the 1850's [1], when engineers realized that

loading rate can significantly affect material properties. For structural steels, the main result of

increasing loading rate is to raise the yield and tensile strength, which in turn causes a decrease

in impact energy required for cleavage fracture, an increase in energy required for ductile

fracture and a shift in the ductile-to-brittle transition temperature. Loading rate must therefore be

considered when designing structures subject to dynamic loading, such as buildings in

earthquakes, ships and overland vehicles in collisions, aircraft landing gear and, at the highest

rates, structures subjected to ballistic projectiles and explosive shocks.

The conventional, non-instrumented Charpy test, which has recently celebrated its

official centennial [2,3], can provide useful comparative information, but generally cannot offer

any detailed insight into the failure mechanisms, or yield quantifiable material properties. This is

also true for some other impact test methods, such as the Pellini drop-weight and the Izod

pendulum test.

Instrumenting the machine striker in order to measure and record the force applied to the

notched sample during impact provides a much clearer picture of the events occurring, providing

additional insight on the behavior of material under impact loading. Instrumented Charpy tests

are currently regulated by test standard ISO 14556 [4], while standardization is also in progress

within ASTM [5].

For fracture mechanics-based structural integrity evaluations, using fatigue-precracked

Charpy (PCC) specimens is particularly useful. At the time of writing, ISO and ASTM

standardization of fracture toughness tests at impact loading rates is ongoing [6,7].

In the nuclear field, the effects of neutron exposure on the fracture properties of the

reactor pressure vessel can be assessed more reliably by using the results of instrumented Charpy

tests than by applying the current regulatory practice, which relies on an empirical correlation

between fracture toughness and the temperature corresponding to a fixed Charpy energy level

(41 J). This is done in the framework of the so-called Enhanced Surveillance Approach [8-11],

which includes the determination of characteristic force values from instrumented Charpy tests

and the derivation of alternative index temperatures [12,13].

Numerous studies have been conducted in the past [14-17] in order to investigate the

experimental factors, related to both the machine and the specimen, which can affect the results

of a Charpy test in terms of absorbed energy, as provided by the machine dial or the optical

encoder (dial energy, KV). In this study, we have examined the influence of some of these

machine/specimen variables on the results of instrumented Charpy tests, namely in terms of

characteristic forces at general yield (Fgy), maximum forces (Fm) and absorbed energies

calculated from the instrumented test record (Wt). KV values have been also included in the

analyses, as well as the ratio between Wt and KV.1

From the test machine point of view, we focused our attention on the bolts that connect

the pendulum to its foundation and those that hold the anvils and the instrumented striker in

place. Tests were run with the bolts partially loose (anvil, striker) or completely loose

(foundation), and the results were compared to data obtained with all the bolts tightened to the

manufacturer’s specifications. Additionally, the center of percussion (COP) was moved away

from the center of strike (COS) by adding weights to the pendulum hammer.

As far as specimen positioning is concerned, we investigated the effects of a lateral offset

with respect to the centered position of the notch between the anvils and of a gap between the

1

Note that Fgy values were not considered in the case of low-energy specimens, since general yield does not occur or

cannot be unambiguously defined in case of fully brittle behavior.


sample and the anvils. Again, results were compared to those obtained under ideal conditions

(i.e. specimen centered and flush against the anvils).

The ultimate goal of this investigation was to find out whether, with respect to the

"conventional" dial energy KV, instrumented Charpy forces and energies are less, equally or

more sensitive to experimental variations that could eventually occur during Charpy tests.

2 Material and experimental

NIST-certified impact-verification specimens of different energy levels were used in this

study. For all investigated variables, low (~ 15 J) and high (~ 100 J) energy specimens were

used; super-high energy (~ 240 J) samples were tested only when looking at the effect of

foundation bolts.

Tests were performed on an instrumented Tinius-Olsen Model 84 pendulum with 407.6 J

capacity, located at NIST in Boulder, Colorado. The instrumented striker conforms to the

requirements of ASTM E 23 (i.e. 8 mm radius of the striking edge).

Tests were conducted at room temperature (RT, ~ 21°C), in order to avoid issues related

to temperature control. However, when investigating the effect of loosening the foundation bolts,

an additional set of low-energy specimens was tested at -40°C. This is the prescribed temperature

for both low- and high-energy specimens, when tested for the verification of impact test

machines.

The overall test matrix is given in Table 1. In the reference condition (i.e. with the

machine fully compliant to ASTM E 23), sets of 5 specimens were tested for each energy level,

except for the low-energy samples, where two sets were tested (one at RT and one at -40°C).

Five to seven specimens per energy level and per condition were tested when investigating bolt

loosening and center of percussion. Eight or nine tests per energy level were conducted to

evaluate specimen positioning effects.

Table 1 - Overall test matrix.

Low energy Variable considered

RT -40°C

High

energy

Super-high

energy

Reference tests 5 5 5 5

Loose foundation bolts 5 5 5 5

Loose anvil bolts 5 - 6 -

Loose striker bolts 5 - 5 -

COP position changed 7 - 5 -

Lateral specimen offset 9 - 9 -

Specimen/anvils distance 8 - 8 -

TOTAL 44 10 43 10

To provide additional benchmarking, results are compared to the outcome of a recent

instrumented Charpy round-robin exercise conducted by NIST among several European and US

laboratories (6 participants) using the same low- and high-energy specimens tested here (10 tests

per material and laboratory) [18]. For the comparisons at room temperature, we used grand mean

values for force from [18] and absorbed energy values. For tests at -40°C, and with super-high

energy specimens, no force data were available, so absorbed energy values were considered.


2.1 Loosening machine bolts

ASTM E 23 requires the impact machine to be securely bolted to a concrete floor at least

150 mm thick or to a foundation having a mass at least 40 times that of the pendulum. The direct

verification for parts requiring annual inspection includes ensuring that the foundation bolts and

the bolts that attach the anvils and striker to the machine are tightened to the manufacturer's

specifications.

2.1.1 Foundation bolts

The machine used in this study is bolted to a concrete foundation having a mass of

approximately 1500 kg, over 50 times that of the pendulum. The T-bolts securing the machine

to the foundation are tightened to a torque value of 515 J (380 ft-lb), which is higher than the

manufacture specification.

Charpy tests on low, high, and super-high-energy specimens were performed with the

foundation bolts completely loosened. This is obviously an extreme situation which is unlikely to

occur in real life.

The results (instrumented forces, instrumented absorbed energy, dial energy and Wt/KV

ratio) and the comparison with reference data are shown in Table 2 and Figure 1 through Figure

7. In the Figures, solid lines connect average values and error bars (often hardly visible)

correspond to ± 2σ.

Note that, in the case of super-high-energy specimens, the acquisition time used was too

short and the tails of the instrumented curves were truncated. As a consequence, Wt and Wt/KV

values cannot be considered reliable and are, therefore, reported in italics; however, the

comparison with the reference condition remains meaningful.

Table 2 – Effect of loosening the foundation bolts. NOTE: only NIST test results are given.

Baseline Foundation unbolted

Fgy Fm Wt KV Fgy Fm Wt KV Material

(kN) (kN) (J) (J) Wt/KV


- 32.15 18.36 19.65 0.934 - 32.38 18.78 19.48 0.964

- 32.38 18.64 19.68 0.947 - 31.90 18.54 19.48 0.952

- 32.74 19.21 20.25 0.949 - 32.64 18.80 19.78 0.950

- 31.00 18.62 19.08 0.976 - 31.90 19.35 20.21 0.957

Low Energy - RT

- 32.16 18.25 19.05 0.958 - 32.33 18.82 19.51 0.965

Average - 32.09 18.62 19.54 0.953 - 32.23 18.86 19.69 0.958

Standard dev. - 0.653 0.372 0.497 0.015 - 0.323 0.298 0.316 0.007

- 31.49 16.64 16.70 0.996 - 31.87 15.72 15.78 0.996

- 31.55 16.84 16.89 0.997 - 30.70 16.33 16.37 0.998

- 31.35 16.75 16.70 1.003 - 31.05 16.45 16.50 0.997

- 31.60 16.98 17.03 0.997 - 31.98 17.01 17.29 0.984

Low Energy -40°C

- 31.14 16.66 16.76 0.994 - 31.37 17.02 16.99 1.002

Average - 31.43 16.77 16.82 0.998 - 31.39 16.51 16.59 0.995

Standard dev. - 0.185 0.140 0.143 0.003 - 0.541 0.541 0.584 0.007


21.19 24.56 104.81 107.54 0.975 20.94 24.79 105.76 110.37 0.958

20.81 24.59 107.86 113.01 0.954 20.96 24.78 105.69 110.49 0.957

20.91 24.55 106.04 109.08 0.972 20.98 24.78 105.61 110.61 0.955

20.97 24.55 105.78 111.53 0.948 20.99 24.77 105.54 110.73 0.953

High Energy - RT

20.95 24.56 105.47 108.88 0.969 21.01 24.77 105.46 110.85 0.951

Average 20.97 24.56 105.99 110.01 0.964 20.98 24.78 105.61 110.61 0.955

Standard dev. 0.140 0.016 1.141 2.211 0.012 0.027 0.009 0.120 0.190 0.003

20.20 25.63 221.51 237.97 0.931 20.52 25.65 225.13 243.21 0.926

20.75 25.55 231.83 252.49 0.918 20.52 25.64 228.33 246.61 0.926

20.43 25.55 226.32 244.32 0.926 20.64 25.60 221.77 239.44 0.926

20.70 25.61 222.74 239.96 0.928 20.33 25.57 221.21 235.61 0.939

Super High

Energy - RT

20.76 25.66 227.11 244.83 0.928 20.76 26.12 241.24 260.65 0.926

Average 20.57 25.60 225.90 243.91 0.926 20.55 25.72 227.54 245.10 0.928

Standard dev. 0.246 0.049 4.064 5.602 0.005 0.160 0.228 8.177 9.614 0.006

10

13

16

19

22

25

Fo

rce a

t g

en

era

l y

ield

(k

N)

High energy RT

Super high energy RT

Reference Foundation

unbolted

Fgy grand mean

from NIST

round-robin

± 95% repr. limit

(high energy)

Figure 1 - Effect of loosening the foundation bolts on forces at general yield. NIST round-robin

information only refers to high-energy specimens.


20

23

26

29

32

35

Ma

xim

um

fo

rce (

kN

)

Low energy RT

Low energy -40°C

High energy RT



unbolted

Fm reference

value from NIST

round-robin

± exp. uncert.

Figure 2 - Effect of loosening the foundation bolts on maximum forces. NIST round-robin

information refers to low and high-energy specimens tested at RT.

10

13

16

19

22

25

Wt (J

)

Low energy RT

Low energy -40°C


unbolted

Wt data from

NIST round

robin ± 1.4 J

Certified

value ± 1.4 J

(ASTM E23)

Figure 3 - Effect of loosening the foundation bolts on instrumented absorbed energies for low-

energy specimens. NOTES: tests at -40°C were not performed in the NIST round robin; certified

values only refer to tests at -40°C.


100

125

150

175

200

225

250

Wt (J

)

High energy RT



unbolted

Wt data from

NIST round

robin ± 5%

Figure 4 - Effect of loosening the foundation bolts on instrumented absorbed energies for high

and super-high-energy specimens.

10

13

16

19

22

25

KV

(J

)

Low energy RT

Low energy -40°C


unbolted

KV data from

NIST round robin

± 1.4 J

Certified value

± 1.4 J (ASTM E23)

Figure 5 - Effect of loosening the foundation bolts on dial energies for low-energy specimens.


100

120

140

160

180

200

220

240

260

KV

(J

)


High energy RT


unbolted

KV data from

NIST round robin

± 5%

Certified value

± 5% (ASTM E23)

Figure 6 - Effect of loosening the foundation bolts on dial energies for high and super-high-

energy specimens.

0.75

1

1.25

Wt/K

V

Low energy -40°C

Low energy RT

High energy RT


unbolted

Figure 7 - Effect of loosening the foundation bolts on the ratio between instrumented and dial

energy. NOTE: super-high energy specimens aren't shown since test records are incomplete.


None of observed variations appears particularly substantial. However, we used a simple

statistical test (unpaired t-test) to determine whether the differences between mean values are

statistically significant or not at the 95% confidence level. The degree of statistical significance

depends on the value of the two-tailed probability P2 using a threshold value 0.05, as follows:

• P > 0.05 ⇒ not significant

• < P < 0.05 ⇒ significant

• < P < 0.01 ⇒ very significant

• P < 0.001 ⇒ extremely significant

The results of the t-test for the loose foundation bolts are shown in Table 3. No influence

of the loosening of the foundation bolts is detected, except for the maximum forces of the high-

energy specimens.

Table 3 - Results of the statistical t-test for the loose foundation bolts.

Specimens / Energy level Parameter P Result (difference is…)

Low energy (RT)

Fm

Wt

KV

0.6701

0.2887

0.5845

Not significant

Not significant

Not significant

Low energy (-40°C)

Fm

Wt

KV

0.9035

0.3147

0.4169

Not significant

Not significant

Not significant

High energy

Fgy

Fm

Wt

KV

0.8789

< 0.0001

0.4800

0.5609

Not significant

Extremely significant

Not significant

Not significant

Super-high energy

Fgy

Fm

Wt

KV

0.9176

0.2985

0.6995

0.8170

Not significant

Not significant

Not significant

Not significant

It is also interesting to observe that all test results (reference and unbolted) are in

excellent agreement with the NIST round-robin results. Note that, for the super-high-energy

specimens, the test machine would pass the indirect verification even with the foundation

completely unbolted (Figure 6). Coincidentally, the low energy level at -40°C (Figure 5) would

fail the indirect verification in the reference condition but remarkably would pass with the

foundation bolts completely loose! In this particular case, the absorbed energy is slightly

decreased for the unbolted condition. This is contrary to what might be expected, but the scatter

for the unbolted result is much higher than for the bolted condition.

The ratio between dial and instrumented energy (Figure 7) is fairly consistent between

low-and high energy samples, and it is very close to unity for the tests conducted at -40°C.

A direct comparison of the instrumented test records (Figure 8 through Figure 11, using

only one curve per condition (for the sake of clarity) does not offer any additional insight.

2 In statistical hypothesis testing, P is the probability of obtaining a value of the test statistic at least as extreme as the one that

was actually observed, given that the null hypothesis (i.e. no difference between the means) is true.


0

5

10

15

20

25

30

35

0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0.0045 0.005

Displacement (m)

Fo

rce

(kN

)

Baseline

Unbolted

Figure 8 - Effect of loosening the foundation bolts on the instrumented test records of the low-

energy specimens (RT).

0

5

10

15

20

25

30

35

0 0.001 0.002 0.003 0.004 0.005

Displacement (m)

Fo

rce

(k

N)

Baseline

Unbolted

Figure 9 - Effect of loosening the foundation bolts on the instrumented test records of the low-

energy specimens (-40°C).


0

5

10

15

20

25

30

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016

Displacement (m)

Fo

rce (

kN

)

Baseline

Unbolted

Figure 10 - Effect of loosening the foundation bolts on the instrumented test records of the high-

energy specimens.

0

5

10

15

20

25

30

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016

Displacement (m)

Fo

rce

(k

N)

Baseline

Unbolted

Figure 11 - Effect of loosening the foundation bolts on the instrumented test records of the super-

high energy specimens. Note the curves truncated at 0.014 m.


2.1.2 Anvil bolts

The torque applied to the anvil bolts for the baseline condition is 36.6 J (27 ft-lb), as

specified by the manufacturer. The applied torque on the bolts was reduced to 2.8 J (25 in-lb), a

condition which might be described as "finger-tight".

Five tests on low-energy specimens and five tests on high-energy specimens were

performed at RT with loosened anvils bolts and the results compared to the baseline data (Table

4 and Figure 12 through Figure 16).

Table 4 – Effect of loosening the anvil bolts.

Baseline Anvil bolts loosened




- 32.15 18.36 19.65 0.934 - 33.55 18.80 19.94 0.943

- 32.38 18.64 19.68 0.947 - 33.78 18.64 19.31 0.965

- 32.74 19.21 20.25 0.949 - 32.58 18.57 19.61 0.947

- 31.00 18.62 19.08 0.976 - 33.63 19.48 19.78 0.985 Low Energy

- 32.16 18.25 19.05 0.958 - 32.79 18.67 19.61 0.952

Average - 32.09 18.62 19.54 0.953 - 33.27 18.83 19.65 0.958

Standard dev. - 0.653 0.372 0.497 0.015 - 0.542 0.372 0.234 0.017

21.19 24.56 104.81 107.54 0.975 20.66 24.35 106.90 110.58 0.967

20.81 24.59 107.86 113.01 0.954 20.66 24.36 100.57 104.19 0.965

20.91 24.55 106.04 109.08 0.972 21.02 24.47 107.57 111.36 0.966

20.97 24.55 105.78 111.53 0.948 20.92 24.36 104.69 108.34 0.966

20.95 24.56 105.47 108.88 0.969 20.75 24.36 107.43 112.58 0.954

High Energy

20.81 24.47 104.69 107.85 0.971

Average 20.97 24.56 105.99 110.01 0.964 20.80 24.40 105.31 109.15 0.965

Standard dev. 0.140 0.016 1.141 2.211 0.012 0.145 0.058 2.660 3.021 0.006


0

5

10

15

20

25

Fg

y (

kN

)

Reference Anvil bolts

loosened

Fgy grand mean

from NIST

round-robin

± 95% repr. limit

Figure 12 - Effect of loosening the anvil bolts on forces at general yield for the high-energy

specimens.

20

23

26

29

32

35

Fm

(k

N)

Low energy

High energy

Reference

Fm reference

value from NIST

round-robin

± exp. uncert.

Anvil bolts

loosened

Figure 13 - Effect of loosening the anvil bolts on maximum forces.


0

20

40

60

80

100

120

Wt (J

)

High energy

Low energy

Reference

Wt data from

NIST round

robin

± 1.4 J or 5%

Anvil bolts

loosened

Figure 14 - Effect of loosening the anvil bolts on instrumented absorbed energies.

0

20

40

60

80

100

120

KV

(J

) High energy

Low energy

Reference

KV data from

NIST round robin

± 1.4 J or 5%

Anvil bolts

loosened

Figure 15 - Effect of loosening the anvil bolts on dial energies.


0.75

1

1.25

Wt/K

VLow energy

High energy

Reference Anvil bolts

loosened

Figure 16 - Effect of loosening the anvil bolts on the ratio between instrumented and dial energy.

The results of the t-test to assess the statistical significance of the observed differences

between mean values are provided in Table 5.

Table 5 - Results of the statistical t-test for the loosened anvil bolts.


Low energy

Fm

Wt

KV

0.0144

0.3851

0.6719

Significant

Not significant

Not significant

High energy

Fgy

Fm

Wt

KV

0.0918

0.0002

0.6078

0.6113

Not significant


Not significant

Not significant

The effect of the loosened anvil bolts appears significant only on maximum force data.

However, it is noted that Fm increases for the low-energy specimens and decreases for the high-

energy specimens (Figure 13).

Wt/KV ratios remain substantially constant and the difference between dial and

instrumented energies does not exceed 5% (Figure 16).

The direct comparison of instrumented curves (Figure 17 and Figure 18) does not offer

additional insight.


0

5

10

15

20

25

30

35

0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0.0045 0.005

Displacement (m)

Fo

rce

(k

N)

Baseline

Anvil bolts loosened

Figure 17 - Effect of loosening the anvil bolts on the instrumented test records of the low-energy

specimens.

0

5

10

15

20

25

30

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014

Displacement (m)

Fo

rce (

kN

)

Baseline


Figure 18 - Effect of loosening the anvil bolts on the instrumented test records of the high-energy

specimens.


2.1.3 Striker bolts

The bolts holding the instrumented striker in place were loosened from manufacture

specification of 74.6 J (55 ft-lb) to 4.5 J (40 in-lb), which again might be described as "finger-

tight".

Five tests on low-energy specimens and five tests on high-energy specimens were

performed at RT with loosened striker bolts and the results compared to the baseline data (Table

6 and Figure 19 through Figure 23).

Table 6 – Effect of loosening the striker bolts.

Baseline Striker bolts loosened




- 32.15 18.36 19.65 0.934 - 32.95 19.12 19.51 0.980

- 32.38 18.64 19.68 0.947 - 33.82 19.32 19.31 1.001

- 32.74 19.21 20.25 0.949 - 33.51 19.69 19.71 0.999

- 31.00 18.62 19.08 0.976 - 33.54 18.93 20.14 0.940

Low Energy

- 32.16 18.25 19.05 0.958 - 33.82 19.59 19.78 0.990

Average - 32.09 18.62 19.54 0.953 - 33.53 19.33 19.69 0.982

Standard dev. - 0.653 0.372 0.497 0.015 - 0.355 0.317 0.311 0.025

21.19 24.56 104.81 107.54 0.975 21.19 24.64 104.30 106.83 0.976

20.81 24.59 107.86 113.01 0.954 21.42 24.58 104.29 108.50 0.961

20.91 24.55 106.04 109.08 0.972 21.36 24.57 106.86 111.76 0.956

20.97 24.55 105.78 111.53 0.948 21.33 24.80 107.29 111.32 0.964

High Energy

20.95 24.56 105.47 108.88 0.969 21.20 24.59 104.58 107.07 0.977

Average 20.97 24.56 105.99 110.01 0.964 21.30 24.64 105.46 109.10 0.967

Standard dev. 0.140 0.016 1.141 2.211 0.012 0.101 0.096 1.483 2.326 0.009


0

5

10

15

20

25

Fg

y (

kN

)

Reference Striker bolts

loosened

Fgy grand mean

from NIST

round-robin

± 95% repr. limit

Figure 19 - Effect of loosening the striker bolts on forces at general yield for the high-energy

specimens.

20

23

26

29

32

35

Fm

(k

N)

Low energy

High energy

Reference

Fm reference

value from NIST

round-robin

± exp. uncert.

Striker bolts

loosened

Figure 20 - Effect of loosening the striker bolts on maximum forces.


0

20

40

60

80

100

120

Wt (J

)

High energy

Low energy

Reference

Wt data from

NIST round

robin

± 1.4 J or 5%

Striker bolts

loosened

Figure 21 - Effect of loosening the striker bolts on instrumented absorbed energies.

0

20

40

60

80

100

120

KV

(J

) High energy

Low energy

Reference

KV data from

NIST round robin

± 1.4 J or 5%

Striker bolts

loosened

Figure 22 - Effect of loosening the striker bolts on dial energies.


0.75

1

1.25

Wt/K

VLow energy

High energy

Reference Striker bolts

loosened

Figure 23 - Effect of loosening the striker bolts on the ratio between instrumented and dial

energy.

The statistical significance of the observed differences between mean values was

assessed by means of the t-test at a confidence level of 95% (Table 7).

Table 7 - Results of the statistical t-test for the loosened striker bolts.


Low energy

Fm

Wt

KV

0.0025

0.0114

0.5880

Very significant

Significant

Not significant

High energy

Fgy

Fm

Wt

KV

0.0025

0.1263

0.5456

0.5429

Very significant

Not significant

Not significant

Not significant

In comparison with foundation and anvil bolts, loosening the striker bolts seems to have

somewhat more detectable effects, particularly for the low-energy specimens.

The difference between dial and instrumented energies remains within 5% regardless of

the torque applied to the striker bolts (Figure 23).

Once again, no additional insight is provided by comparing instrumented test records for

the two conditions (Figure 24 and Figure 25).


0

5

10

15

20

25

30

35

0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0.0045 0.005

Displacement (m)

Fo

rce (

kN

)

Baseline

Striker bolts loosened

Figure 24 - Effect of loosening the striker bolts on the instrumented test records of the low-

energy specimens.

0

5

10

15

20

25

30

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014

Displacement (m)

Fo

rce (

kN

)

Baseline


Figure 25 - Effect of loosening the striker bolts on the instrumented test records of the high-

energy specimens.


2.2 Changing the position of the Center of Percussion (COP)

The center of percussion (COP) of an object is defined as the point where a perpendicular

impact will produce translational and rotational forces which perfectly cancel each other out at

some given pivot point so that the pivot will not be moving momentarily after the impulse.

Centers of percussion are often discussed in the context of a bat, racquet, sword or other long

thin objects. For such objects, the COP may or may not be the "sweet spot" depending on the

pivot point chosen.

ASTM E 23 requires the COP of the pendulum hammer to be within 1% of the distance

between the axis of rotation and the center of strike (COS) on the specimen (corresponding to the

length of the pendulum). This requirement ensures that minimum force is transmitted to the point

of rotation. Moving COP and COS away from each other is expected to cause additional

vibrations and therefore increase vibrational energy losses, particularly when testing brittle

materials.

The location of the COP is determined from the equation:

2

2

4π

gpL = (1)

where g is the local gravitational acceleration in m/s², p is the period of a complete oscillation

measured with a suitable time-measuring device in s, and L is the distance between the axis of

rotation and the COP in m.

The length of the pendulum used in this study is 900.65 mm, and the original position of

the COP is 898.79 mm from the axis of rotation. The COP-COS distance is therefore 1.86 mm,

or 0.2% of the pendulum length.

By adding 2 kg to the pendulum mass, we reduced the oscillation period and therefore

shortened the distance between axis of rotation and COP to 893.73 mm and increased the

COP-COS distance to 6.95 mm (0.77% of the pendulum length3). Subsequently, tests were run

on low and high-energy specimens and results were compared to data from reference tests and

the NIST round robin (Table 8 and Figure 26 through Figure 30).

Table 8 – Effect of moving the COP away from the COS4.

Baseline COP moved




- 32.15 18.36 19.65 0.934 - - - 18.94 -

- 32.38 18.64 19.68 0.947 - 31.98 17.88 19.40 0.922

- 32.74 19.21 20.25 0.949 - 31.43 17.89 19.47 0.919

- 31.00 18.62 19.08 0.976 - 31.62 17.77 19.30 0.921

- 32.16 18.25 19.05 0.958 - 31.56 17.67 18.98 0.931

- 31.94 17.64 - -

Low Energy

- 31.08 17.90 - -

Average - 32.09 18.62 19.54 0.953 - 31.65 17.80 19.22 0.923

Standard dev. - 0.653 0.372 0.497 0.015 - 0.235 0.104 0.244 0.005

3 It was not possible to add more weight to the pendulum mass, and therefore increase further the COP-COS distance.

4 For one of the low energy tests, instrumented data were lost. For two additional low energy tests, an incorrect value of pendulum length was

entered and therefore KV is not reliable.


Baseline COP moved




21.19 24.56 104.81 107.54 0.975 20.84 24.55 109.39 115.61 0.946

20.81 24.59 107.86 113.01 0.954 20.98 24.45 108.40 113.87 0.952

20.91 24.55 106.04 109.08 0.972 20.77 24.20 105.58 111.37 0.948

20.97 24.55 105.78 111.53 0.948 21.00 24.39 107.95 113.83 0.948 High Energy

20.95 24.56 105.47 108.88 0.969 20.96 24.35 102.91 107.06 0.961

Average 20.97 24.56 105.99 110.01 0.964 20.91 24.39 106.85 112.35 0.951

Standard dev. 0.140 0.016 1.141 2.211 0.012 0.100 0.129 2.608 3.319 0.006

0

5

10

15

20

25

Fg

y (

kN

)

Reference COP moved

away

Fgy grand mean

from NIST

round-robin

± 95% repr. limit

Figure 26 - Effect of changing the COP position on forces at general yield for the high-energy

specimens.


20

23

26

29

32

35

Fm

(k

N)

Low energy

High energy

Reference

Fm reference

value from NIST

round-robin

± exp. uncert.

COP moved

away

Figure 27 - Effect of changing the COP position on maximum forces.

0

20

40

60

80

100

120

Wt (J

)

High energy

Low energy

Reference

Wt data from

NIST round

robin

± 1.4 J or 5%

COP moved

away

Figure 28 - Effect of changing the COP position on instrumented absorbed energies.


0

20

40

60

80

100

120

KV

(J

) High energy

Low energy

Reference

KV data from

NIST round robin

± 1.4 J or 5%

COP moved

away

Figure 29 - Effect of changing the COP position on dial energies.

0.75

1

1.25

Wt/K

V

Low energy

High energy

Reference COP moved

away

Figure 30 - Effect of changing the COP position on the ratio between instrumented and dial

energy.


Table 9 shows the results of the statistical analyses conducted using the t-test.

Table 9 - Results of the statistical t-test for the modified COP location tests.


Low energy

Fm

Wt

KV

0.1453

0.0006

0.2268

Not significant


Not significant

High energy

Fgy

Fm

Wt

KV

0.4866

0.0175

0.5212

0.2259

Not significant

Significant

Not significant

Not significant

The most significant effect is observed on instrumented energies for the low-energy

specimens which, contrary to expectations, decreased when the COP was moved away from the

COS. As a consequence, the ratio between instrumented and dial energies falls below 0.95

(Figure 30).

Figure 31 and Figure 32 compare instrumented test records for low- and high energy

samples.

0

5

10

15

20

25

30

35

0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0.0045 0.005

Displacement (m)

Fo

rce

(k

N)

Baseline

COP moved away

Figure 31 - Effect of changing the COP position on the instrumented test records of the low-

energy specimens.


0

5

10

15

20

25

30

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014

Displacement (m)

Fo

rce

(k

N)

COP moved away

Baseline

Figure 32 - Effect of changing the COP position on the instrumented test records of the high-

energy specimens.

2.3 Specimen positioning

The influence of specimen impact position was investigated by moving the samples away

from the correct location (i.e. flush against the anvils and with the notch centered between the

anvils). Conditions with specimens moved away from flush, and specimen notch positions

moved off center were evaluated.

For every selected distance, multiple impact tests on low- and high-energy specimens

were performed. Due to the limited number of data available for each condition (from 2 to 4), we

decided not to perform any statistical analysis (t-test).

2.3.1 Specimen not in contact with the anvils

In everyday laboratory practice, it is not uncommon that a Charpy specimen is not fully

in contact with the anvils, particularly in the case of manual positioning and when the tests are

performed at low or high temperatures, since the operator must fulfill the < 5 s transfer time

requirement (ASTM E 23).

A sample which was not against the anvils can be easily detected from its instrumented

test record, which shows significantly amplified dynamic oscillations up to general yield and a

characteristic delay in the rise of the force signal with respect to the onset of data acquisition.

For our study, we selected three values of the distance between specimen and anvils: 0.5

mm, 1 mm and 2 mm. Up to 5 tests per condition were performed, although instrumented data

were lost for several tests as will be discussed later. The results are summarized and compared

with the reference data and NIST round-robin results in Table 10 and Figure 33 through Figure

37.


Table 10 – Effect of the distance between specimen and anvils.

Low Energy High Energy

Fm Wt KV Fgy Fm Wt KV Distance

from anvils (kN) (J) (J)

Wt/KV (kN) (kN) (J) (J)

Wt/KV

32.15 18.36 19.65 0.934 21.19 24.56 104.81 107.54 0.975

32.38 18.64 19.68 0.947 20.81 24.59 107.86 113.01 0.954

32.74 19.21 20.25 0.949 20.91 24.55 106.04 109.08 0.972

31.00 18.62 19.08 0.976 20.97 24.55 105.78 111.53 0.948

0 mm

(reference)

32.16 18.25 19.05 0.958 20.95 24.56 105.47 108.88 0.969

Average 32.09 18.62 19.54 0.953 20.97 24.56 105.99 110.01 0.964

Standard dev. 0.653 0.372 0.497 0.015 0.140 0.016 1.141 2.211 0.012

Fm Wt KV Fgy Fm Wt KV Distance from

anvils (kN) (J) (J) Wt/KV


33.98 19.73 20.61 0.957 19.54 24.92 108.95 114.19 0.954

34.78 21.22 22.70 0.935 19.22 24.52 107.48 112.30 0.957

34.83 20.22 21.38 0.946 19.49 24.79 107.84 110.50 0.976 0.5 mm

34.86 20.45 21.52 0.950

Average 34.61 20.41 21.55 0.947 19.42 24.74 108.09 112.33 0.962

Standard dev. 0.423 0.621 0.863 0.009 0.172 0.204 0.766 1.845 0.012




33.70 22.10 22.76 0.971 19.94 24.82 114.48 118.62 0.965 1 mm

35.62 21.04 22.53 0.934 19.66 24.84 110.03 113.53 0.969

Average 34.66 21.57 22.65 0.952 19.80 24.83 112.26 116.08 0.967

Standard dev. 1.358 0.750 0.163 0.026 0.198 0.014 3.147 3.599 0.003




35.62 24.54 25.48 0.963 20.45 24.84 114.76 117.02 0.981

35.02 25.10 26.23 0.957 22.00 24.87 112.22 113.29 0.991 2 mm

20.66 25.14 111.75 112.88 0.990

Average 35.32 24.82 25.86 0.960 21.04 24.95 112.91 114.40 0.987

Standard dev. 0.424 0.396 0.530 0.004 0.841 0.165 1.619 2.281 0.006


0

5

10

15

20

25

0 0.5 1 1.5 2

Specimen distance from anvils (mm)

Fg

y (

kN

)

Fgy grand mean

from NIST

round-robin

± 95% repr. limit

Figure 33 - Effect of the distance between specimen and anvils on forces at general yield for the

high-energy specimens.

20

25

30

35

40

0 0.5 1 1.5 2


Fm

(k

N)

Low

energy

High

energy

Fm reference

value from NIST

round-robin

± exp. uncert.

Figure 34 - Effect of the distance between specimen and anvils on maximum forces.


0

20

40

60

80

100

120

140

0 0.5 1 1.5 2


Wt (J

)

Low

energy

High

energy

Wt data from

NIST round robin

± 1.4 J or 5%

Figure 35 - Effect of the distance between specimen and anvils on instrumented absorbed

energies.

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2


KV

(J)

Low

energy

High

energy

KV data from

NIST round robin

± 1.4 J or 5%

Figure 36 - Effect of the distance between specimen and anvils on dial energies.


0.75

1

1.25

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Specimen distance from the anvils (mm)

Wt/K

V

Low

energy

High

energy

Figure 37 - Effect of the distance between specimen and anvils on the ratio between

instrumented and dial energies.

Increasing the distance between specimen and anvils results in a generalized increase of

the characteristic values of force and energy, as well as the Wt/KV ratio. The increment is

particularly significant for maximum force values measured from low-energy specimens (Figure

34), where the enhanced dynamic oscillations lead to a significant overestimation of Fm. For the

same reason, the uncertainty associated with Fgy values from high-energy specimens increases,

while ringing effects are sufficiently dampened once maximum force is reached.

In general, brittle specimens are more affected than ductile specimens.

As previously mentioned, the occurrence of instrumented data loss was problematic for

the low energy samples at 1 mm and 2 mm distance: data were not acquired in 5 out of 9 tests,

whereas in only one instance (out of 6 tests performed) high energy data were lost. No

acquisition failure was recorded for specimens placed 0.5 mm away from the anvils. It is

therefore clear that distances of 1 mm or more increase the likelihood of acquisition failure,

probably due to false triggering.

Two interesting features emerge from the comparison of instrumented test records

(Figure 38 and Figure 39):

• the traces of the offset low-energy specimens are clipped at a force level of 35.6 kN, which

corresponds to the threshold of the acquisition system (Figure 38);

• curves for the reference condition do not reach this force level;

• on the displacement axis, the rise of the force signal roughly corresponds to the initial

distance between specimen and anvils.

In all cases, large inertia peaks are observed at the beginning of the instrumented traces,

caused by the specimen rebounding between anvils and striker before full contact is achieved.


0

5

10

15

20

25

30

35

0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0.0045 0.005

Displacement (m)

Fo

rce (

kN

)

Reference (0 mm)

0.5 mm

1 mm

2 mm

Figure 38 - Effect of the distance between specimen and anvils on the instrumented test records

of low-energy specimens. Note the clipped force signals above 35.6 kN.

0

5

10

15

20

25

30

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014

Displacement (m)

Fo

rce (

kN

)

Reference (0 mm)

0.5 mm

1 mm

2 mm

Figure 39 - Effect of the distance between specimen and anvils on the instrumented test records

of high-energy specimens.


2.3.2 Lateral specimen offset

Annex A1 of ASTM E 23 prescribes that the specimen be positioned against the anvils

such that the center of the notch is located within 0.25 mm of the midpoint between the anvils.

The use of self-centering tongs is recommended to achieve this.

In our study, we selected three values for the specimen lateral offset: 0.5 mm, 1 mm and

2 mm. All distances were outside the ASTM E 23 tolerance.

For each condition, three tests on low-energy and three test on high-energy specimens

were conducted. No failed data acquisition was experienced. Results and comparison with

reference values and NIST round-robin data are provided in Table 11 and Figure 40 through

Figure 44.

Table 11 – Effect of lateral specimen offset.

Low Energy High Energy

Fm Wt KV Fgy Fm Wt KV Lateral offset

(kN) (J) (J) Wt/KV


32.15 18.36 19.65 0.934 21.19 24.56 104.81 107.54 0.975

32.38 18.64 19.68 0.947 20.81 24.59 107.86 113.01 0.954

32.74 19.21 20.25 0.949 20.91 24.55 106.04 109.08 0.972

31.00 18.62 19.08 0.976 20.97 24.55 105.78 111.53 0.948

0 mm

(reference)

32.16 18.25 19.05 0.958 20.95 24.56 105.47 108.88 0.969

Average 32.09 18.62 19.54 0.953 20.97 24.56 105.99 110.01 0.964

Standard dev. 0.653 0.372 0.497 0.015 0.140 0.016 1.141 2.211 0.012


(kN) (J) (J) Wt/KV


32.32 18.46 18.94 0.975 21.06 24.56 108.62 113.61 0.956

33.14 18.93 19.21 0.985 20.98 24.57 108.30 113.08 0.958 0.5 mm

32.60 18.95 19.91 0.952 21.05 24.37 106.40 111.28 0.956

Average 32.69 18.78 19.35 0.971 21.03 24.50 107.77 112.66 0.957

Standard dev. 0.417 0.277 0.501 0.017 0.044 0.113 1.200 1.221 0.001


(kN) (J) (J) Wt/KV


33.50 19.36 20.11 0.963 21.17 24.60 105.67 111.73 0.946

32.51 18.53 18.98 0.976 21.10 24.59 108.45 112.87 0.961 1 mm

33.30 18.80 19.04 0.987 20.97 24.62 110.12 114.68 0.960

Average 33.10 18.90 19.38 0.975 21.08 24.60 108.08 113.09 0.956

Standard dev. 0.523 0.423 0.636 0.012 0.101 0.015 2.248 1.488 0.009


(kN) (J) (J) Wt/KV


33.64 19.50 20.68 0.943 21.24 24.72 103.45 113.77 0.909

34.49 20.67 21.01 0.984 21.24 24.93 103.39 109.52 0.944 2 mm

32.79 18.88 19.44 0.971 21.54 24.92 107.92 114.10 0.946

Average 33.64 19.68 20.38 0.966 21.34 24.86 104.92 112.46 0.933

Standard dev. 0.850 0.909 0.828 0.021 0.173 0.118 2.598 2.554 0.021


0

5

10

15

20

25

0 0.5 1 1.5 2

Lateral specimen offset (mm)

Fg

y (

kN

)

Fgy grand mean

from NIST

round-robin

± 95% repr. limit

Figure 40 - Effect of lateral specimen offset on forces at general yield for the high-energy

specimens.

20

25

30

35

40

0 0.5 1 1.5 2


Fm

(kN

)

Low

energy

High

energy

Fm reference

value from NIST

round-robin

± exp. uncert.

Figure 41 - Effect of lateral specimen offset on maximum forces.


0

20

40

60

80

100

120

140

0 0.5 1 1.5 2


Wt (J

)

Low

energy

High

energy

Wt data from

NIST round robin

± 1.4 J or 5%

Figure 42 - Effect of lateral specimen offset on instrumented absorbed energies.

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2


KV

(J

)

Low

energy

High

energy

KV data from

NIST round robin

± 1.4 J or 5%

Figure 43 - Effect of lateral specimen offset on dial energies.


0.75

1

1.25

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2


Wt/K

V Low

energy

High

energy

Figure 44 - Effect of lateral specimen offset on the ratio between instrumented and dial energies.

Moving the specimen off-center appears to have a systematic, though moderate effect on

instrumented forces. The force values steadily increase with the offset (Figure 40 and Figure 41).

The effect is less pronounced and not systematic for energy values and Wt/KV ratio (Figure 42 to

Figure 44), with both Wt and KV being slightly lower for 2 mm than for 1 mm for the high-

energy specimens.

No substantial features emerge from the comparison of instrumented test records (Figure

45 and Figure 46). For low energy tests, the amplitude of dynamic oscillations seems to increase

with respect to the baseline condition, but it is not directly correlated to the magnitude of the

specimen offset.


0

5

10

15

20

25

30

35

0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0.0045 0.005

Displacement (m)

Fo

rce

(k

N)

Reference (0 mm)

0.5 mm

1 mm

2 mm

Figure 45 - Effect of lateral specimen offset on the instrumented test records of low-energy

specimens.

0

5

10

15

20

25

30

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014

Displacement (m)

Fo

rce

(k

N)

Reference (0 mm)

0.5 mm

1 mm

2 mm

Figure 46 - Effect of lateral specimen offset on the instrumented test records of high-energy

specimens.


3 Discussion

Table 12 summarizes the results of the statistical t-test applied to the results of the tests

conducted on low, high and super-high-energy specimens in order to investigate the effect of test

machine variables. In the Table, the following convention is used to indicate the statistical

significance (at the 95% confidence level) of the observed differences between mean values:

Symbol Meaning

NS Not significant

* Significant

** Very significant

*** Extremely significant

Table 12 - Statistical significance of the various machine parameters based on the t-test results.

Test machine

variable

Test

parameter

Low energy

specimens (RT)

Low energy

specimens (-40°C)

High energy

specimens

Super-high energy

specimens

Foundation bolts

Fgy

Fm

Wt

KV

-

NS

NS

NS

-

NS

NS

NS

NS

***

NS

NS

NS

NS

NS

NS

Anvil bolts

Fgy

Fm

Wt

KV

-

*

NS

NS

-

-

-

-

NS

***

NS

NS

-

-

-

-

Striker bolts

Fgy

Fm

Wt

KV

-

**

*

NS

-

-

-

-

**

NS

NS

NS

-

-

-

-

COP position

Fgy

Fm

Wt

KV

-

NS

***

NS

-

-

-

-

NS

*

NS

NS

-

-

-

-

From the examination of Table 12, it is generally clear that the investigated effects are

neither particularly substantial nor systematic, nor is the consistency between the different

parameters and energy levels. Therefore, it is unlikely that a user could detect a given effect from

a simple examination of an individual test record, without resorting to a more detailed statistical

analysis. Force values (namely maximum forces) seem to be slightly more sensitive than energy

values.

As far as specimen position is concerned, our results show that moving the samples away

from the anvils is more influential than displacing it sideways. The effects are clearly visible on

the instrumented test record and are particularly significant when the distance between specimen

and anvils exceeds 0.5 mm. Brittle specimens are more affected than ductile specimens, and for

our test system clipping of the force signal due to saturation of the strain gage signal was

observed. Moreover, the likelihood of losing the instrumented data due to mistriggering is

greatly increased. A lateral offset of the sample affects forces more than energies, but the

consequences are not very pronounced.

Table 13 and Figure 47 through Figure 49 summarize the outcome of our investigation

for brittle and ductile Charpy tests. Data are presented in terms of average forces and energies

measured after "altering" one of the experimental parameters, as a function of the average values

for the reference ("unaltered") condition.


Table 13 - Average values of characteristic forces and absorbed energies obtained in this study.

Fracture

behavior Material/Tests Variable considered

Fgy

(kN)

Fm

(kN)

Wt

(J)

KV

(J)

Reference condition 32.09 18.62 19.54

Foundation bolts 32.23 18.86 19.69

Anvil bolts 33.27 18.83 19.65

Striker bolts 33.53 19.33 19.69

Moving COP 31.60 17.79 19.22

Not against anvils (0.5 mm) 34.61 20.41 21.55

Not against anvils (1 mm) 34.66 21.57 22.65

Not against anvils (2 mm) 35.32 24.82 25.86

Lateral offset (0.5 mm) 32.69 18.78 19.35

Lateral offset (1 mm) 33.10 18.90 19.38

Low energy (RT)

Lateral offset (2 mm) 33.64 19.68 20.38

Reference condition 31.43 16.77 16.82

Brittle

Low energy (-40°C) Foundation bolts

31.39 16.51 16.59

Reference condition 20.97 24.56 105.99 110.01

Foundation bolts 20.98 24.78 105.61 110.61

Anvil bolts 20.80 24.40 105.31 109.15

Striker bolts 21.30 24.64 105.46 109.10

Moving COP 20.91 24.39 106.85 112.35

Not against anvils (0.5 mm) 19.42 24.74 108.09 112.33

Not against anvils (1 mm) 19.80 24.83 112.26 116.08

Not against anvils (2 mm) 21.04 24.95 112.91 114.40

Lateral offset (0.5 mm) 21.03 24.50 107.77 112.66

Lateral offset (1 mm) 21.08 24.60 108.08 113.09

High energy

Lateral offset (2 mm) 21.34 24.86 104.92 112.46

Reference condition 20.57 25.60 225.90 243.91

Ductile

Super-high energy Foundation bolts 20.55 25.72 227.54 245.10


16

20

24

28

32

36

16 20 24 28 32 36

Reference values (Fm - kN; Wt,KV - J)

Alt

ered

va

lues

(F

m -

kN

; W

t,K

V -

J)

Foundation unbolted



Moving COP away from COS

Sample 0.5 mm from anvils

Sample 1 mm from anvils


Lateral offset 0.5 mm

Lateral offset 1 mm

Lateral offset 2 mm

Wt, KV

Fm

−10%

+10%

−5%

+5% 1:1

Figure 47 - Effect of test-machine variables and specimen positioning for brittle specimens (low-

energy level at RT and -40°C).

16

20

24

28

16 20 24 28

Reference force values (kN)

Alt

ered

fo

rce

valu

es

(kN

)

Foundation unbolted








Lateral offset 1 mm

Lateral offset 2 mm

−10%

+10%

−5%

+5% 1:1

Fgy

Fm

Figure 48 - Effect of test-machine variables and specimen positioning on characteristic forces for

ductile specimens (high and super-high energy).


100

125

150

175

200

225

250

100 125 150 175 200 225 250

Reference energy values (J)

Alt

ere

d e

ne

rgy

valu

es (

J) Foundation unbolted








Lateral offset 1 mm

Lateral offset 2 mm

−10%

+10%

−5%

+5% 1:1

Low

energy

Super-high

energy

Figure 49 - Effect of test-machine variables and specimen positioning on absorbed energies for

ductile specimens (high and super-high energy).

In the case of brittle behavior (Figure 47), most of the investigated variables tend to

increase maximum forces and absorbed energies as a result of additional vibrational energy

losses, except when the COP is moved away from the COS. In general, altering the specimen

position before impact is more influential than loosening the test machine bolts.

The variations observed with respect to the reference condition are relatively moderate,

i.e. less than 5%, except when the specimen is moved away from the anvil by more than 0.5 mm.

For specimen/anvil distances of 1 and 2 mm, absorbed energies increase by much more than

10%.

Fully ductile specimens (Figure 48 and Figure 49) are generally less sensitive than brittle

specimens to the investigated variables. In particular (Figure 48), maximum forces are

essentially unaffected (variations well below 5%), whereas for general yield forces only the

distance between specimen and anvil causes variations between 5% and 10%. In terms of

absorbed energies (Figure 49), from both the instrumented test record and from the machine

encoder, an increase is generally observed, but no larger than 5-6% of the reference value.


4 Conclusions

Our investigation on the effect of test-machine variables (bolting of foundation, anvils

and striker; position of the center of percussion) on instrumented Charpy test results leads to the

general conclusion that most of the observed variations are lower than 10% and generally neither

consistent nor systematic, particularly in case of fully ductile behavior. As a consequence, they

are unlikely to be detected from the instrumented test record or from the value of absorbed

energy yielded by the machine encoder.

The most notable effect is when the specimen is not in contact with the anvils;

particularly for brittle tests (such as the low-energy NIST-reference samples) and when the

sample is more than 0.5 mm from the anvils, absorbed energies can be overestimated by

considerably more than 10%. It would be advisable that instrumented Charpy test standards

require data from Charpy tests in which the gap between specimen and anvils was more than 0.5

mm to be discarded. This requirement would be relatively easy to implement, since (a) the test

record of such a test can be easily recognized (delay between test initiation and force signal rise;

very pronounced dynamic oscillations prior to general yield), and (b) the displacement

corresponding to the rise of the force signal provides a rough estimate of the initial distance

between specimen and anvils.

One the most significant outcomes of our study is that a "good" machine has a high

probability of still delivering high quality results, including passing the indirect verification of

ASTM E 23, even when it is unbolted from its foundation, the striker and anvils are just "finger

tightened" and the specimens are offset from the midpoint between the anvils. However, we

expect that these results are strongly influenced by machine design. So, the magnitude of the

effects reported here may differ substantially from results obtained on a machine having a

different design.

Finally, we can say that instrumented forces are more sensitive than absorbed energies to

the variables considered, whereas calculated (Wt) and dial (KV) energies are equally

(in)sensitive. This implies that, if Charpy tests are performed using an instrumented striker,

effects can be detected which would not be noticed if one looked at only energy values returned

by the machine encoder.


References

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instrumented impact testing: influence of machine variables and

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