ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation

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While correlation exists between full-size specimens and subsize specimens, such correlation is not direct.

Many specifications (ASTM and ASME, for example) specify differing acceptable values for various specimen

sizes (Table 1).

Table 1 Conversion table for subsize Charpy impact-test specimens

Minimum average impact

strength for three specimens

Minimum impact strength for one

specimens or for set of three specimens

Size of

specimens

(a)

, mm

J ft · lbf J

ft · lbf

10 × 10 (full size) 20.3 15.0 13.6

10.0

10 × 7.5 16.9 12.5 11.5

8.5

10 × 5 13.6 10.0 9.5

7.0

10 × 2.5 6.8 5.0 4.7 3.5

(a) Insofar as possible, full-size Charpy keyhole specimens should be used. However, where absolutely

necessary, it is permissible to use specimens with width (in direction of the length of the notch; see ASME

Section VIII, U-84, Unfired Pressure Vessels) reduced in accordance with the above tabulation.

Calibration. ASTM E 23 goes into considerable detail to ensure proper calibration of testing machines. Other

relevant standards for qualification or calibration of the test machines are:

ASTM E

1236

Standard Practice for Qualifying Charpy Impact Machines as Reference Machines

BS 131-7

Verification of the Test Machine Used for Precision Determination of Charpy V-Notch

Impact

BS-EN

10045-2

Charpy Impact Test on Metallic Materials Part 2: Method for the Verification of Impact

Testing Machines

ISO 148-2 Metallic Materials—Charpy Pendulum Impact Test Part 2: Verification of Test

Machines

These publications should be consulted for a basic understanding of machine calibration. Calibration and test

variables are also reviewed in Ref 4 and 5. These publications help identify causes of improper results.

Standard test bars for calibration can be purchased from: Director, Army Materials and Mechanics Research

Center, Attention AMXMR-MQ, Watertown, MA 02172 (formerly known as the Watertown Arsenal). Standard

specimens are tested as per instructions, and the results, along with a filled-out questionnaire and the broken

specimens, are returned. A report is then sent stating if the machine meets calibration standards and, if not, what

should be done to ensure qualification.

Test Method

Once the equipment has been properly set up and calibrated and the specimens have been correctly prepared,

testing can be done. Prior to each testing session, the pendulum should be allowed at least one free fall with no

test specimen present, to confirm that zero energy is indicated. Specimen identification and measurements are

then recorded along with test temperature. The pendulum is cocked, and the specimen is carefully positioned in

the anvil using special tongs (Fig. 8) that ensure centering of the notch. The quick-release mechanism is

actuated, and the pendulum falls and strikes the specimen, generally causing it to break. The amount of energy

absorbed is recorded (normally in foot-pounds), and this data is noted adjacent to the specimen identification on

the data sheet. The broken specimens are retained for additional evaluation of the fracture appearance and for

measurement of lateral expansion where required. The broken halves are often placed side by side, taped

together, and labeled for identification.

Fig. 8 Use of tongs to place a specimen in a Charpy impact testing machine for testing

The release mechanism must be consistent and smooth. Test specimens must leave the impact machine freely,

without jamming or rebounding into the pendulum; requirements on clearances and containment shrouds are

specific to individual machine types. The test specimen must be accurately positioned on the anvil support

within 5 s of removal from the heating (or cooling) medium; requirements for heating time depend on the

heating medium. Identification marks on test specimens must not interfere with the test; also, any heat treatment

of specimens should be performed prior to final machining.

A daily check procedure of the apparatus must be conducted to ensure proper performance. Verification of the

testing system is required using Army Materials and Mechanics Research Center (AMMRC) standardized

specimens; verification should be completed at least once a year or after any parts are replaced or any repairs or

adjustments are made to the machine. An operational testing sequence is recommended, as well as specifics on

dial energy reading, lateral expansion measurement (technique and measuring fixture), and fracture appearance

estimation.

Test Temperature. Specimen temperature can drastically affect the results of impact testing. If not otherwise

stated, testing should be done at temperatures from 21 to 32 °C (70–90 °F). Much Charpy impact testing is done

at temperatures lower than those commonly designated as room temperature. Of these low-temperature tests,

the majority are made between room temperature and -46 °C (-50 °F), because it is within this range that most

ductile-to-brittle transition temperatures occur. A certain amount of testing is also done down to -196 °C (-320

°F) for those materials that may be used in cryogenic service. Some additional testing (mainly research) is done

at the liquid helium and liquid hydrogen temperatures (-269 and -251 °C, or -452 and -420 °F). Such testing

requires special techniques and will not be discussed here. For testing at temperatures down to or slightly below

-59 °C (-75 °F), ethyl alcohol and dry ice are most commonly used. This combination solidifies at around -68

°C (-90 °F). A suitable insulated container should be used to cool the test specimens (a container insulated with

a layer of styrofoam works fine). A screen-type grid raised at least 25 mm (1 in.) above the bottom of the

container allows cooling liquid to circulate beneath the specimens. A calibrated temperature-measuring device,

such as a low-temperature glass or metal thermometer or a thermocouple device, should be placed so as to read

the temperature near the center of a group of specimens being cooled. The solution should be agitated

sufficiently to ensure uniformity of bath temperature. The cooling liquid should cover the specimen by at least

25 mm (1 in.). The specimen-handling tongs should be placed in the same cooling bath as the specimens. When

the specimens have been placed in the alcohol bath along with the tongs, chips of solid CO

(dry ice) can be

added and the solution agitated. Experience will dictate the amount of dry ice required to reach a certain

temperature. Once the temperature is reached, it seems to hold steady with only an occasional addition of a

small chip of dry ice. The specimens in a liquid bath should be held within +0 and -1.5 °C (+0 and -3 °F) of test

temperature for at least 5 min prior to testing. The specimens should then be removed one at a time with the

cooled tongs and tested within 5 s of removal from the bath. Watch the temperature between tests because the

tongs can raise the bath temperature if left out of the bath too long. The commercial cooling baths that are

available range from insulated stainless steel containers to containers with self-contained refrigeration units.

Also available are thermocouple devices that can be placed in the cooling bath and will give a digital

temperature readout. Dry ice cannot be stored for any length of time, but there is a device that produces

“instant” dry ice from a CO

compressed gas bottle. Testing between -59 and -196 °C (-75 and -320 °F)

requires a liquid medium that will not solidify at these temperatures. Various liquids are available. One that has

been successfully used is isohexane (adequate ventilation should be provided and care exercised to avoid

inhalation of the volatile organic fumes). Liquid nitrogen replaces the dry ice as a coolant material, and the

procedure is then similar to that for dry ice and alcohol. It is wise to keep handy a large, easy-to-handle piece of

metal to serve as a temperature moderator in case the temperature becomes lower than desired. It can be

plunged into the bath and, acting as a heat sink, can cause the temperature to rise quickly.

High-Temperature Testing. Occasionally, high-temperature impact testing is performed. This can be done using

an agitated, high-flashpoint oil (heat treating quenching oils may work) or other liquid medium that is stable at

the desired test temperature. The bath and specimens are then held at temperature in a furnace or oven for at

least 10 min prior to testing.

Test Results

Results of impact testing are determined in three ways. In the first method, already discussed, they can be read

directly from the testing machine (in joules or foot-pounds). This is the most commonly specified test result. It

is desirable to test three specimens at each test temperature; the average value of the three is the test result used.

If a minimum test value is specified for material acceptance, not more than one test result of the three should

fall below that value. If the value of one of the three specimens is about 6 J (5 ft · lbf) lower than the average, or

lower than the average value by greater than of the specified acceptance value, the material should be either

rejected or retested. In retesting, three additional specimens must be tested, and all must equal or exceed the

specified acceptance value. Since it is often required or important to determine the ductile-to-brittle transition

temperature, impact test results are plotted against test temperature. Somewhere in that transition zone between

the high-energy and low-energy values is an energy value that can be defined as the transition temperature.

When the transition is very pronounced, this value is easily determined. However, because the more common

case is a less sharply defined transition, an energy value may be specified below which the material is

considered to be brittle (below the ductile-to-brittle transition temperature). Such a value may vary with

material type and requirements, but the value of 20 J (15 ft · lbf) is often used as a specified value.

Fracture Appearance Method. Other methods of specifying ductile-to-brittle transition temperature are

sometimes presented along with the energy values obtained. The first of these auxiliary tests is the fracture-

appearance method. The fractured impact bars are examined and the fractures compared with a series of

standard fractures or overlays of such fractures. By this method the percentage of shear fracture is determined.

The amount of shear fracture can also be determined in another way. This is done by carefully measuring the

dimensions of the brittle cleavage exhibited on the specimen fracture surface (Fig. 9), and then referring to

Table 2. These methods are described in detail in ASTM A 370. The percentage of shear can be plotted against

test temperature and the transition temperature can be ascertained using the shear percentage value specified.

Table 2 Tables of percent shear for measurements made in both inches and millimeters for impact-test

specimens

Because these tables are set up for finite measurements or dimensions A and B (see Fig. 9), 100% shear is to be

reported when either A or B is zero.

Dimension A, in.

Dimension

B, in.

0.05 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 0.28 0.30 0.32 0.34 0.36 0.38

0.40

0.05 98 96 95 94 94 93 92 91 90 90 89 88 87 86 85 85

0.10 96 92 90 89 87 85 84 82 81 79 77 76 74 73 71 69

0.12 95 90 88 86 85 83 81 79 77 75 73 71 69 67 65 63

0.14 94 89 86 84 82 80 77 75 73 71 68 66 64 62 59 57

0.16 94 87 85 82 79 77 74 72 69 67 64 61 59 56 53 51

0.18 93 85 83 80 77 74 72 68 65 62 59 56 54 51 48 45

0.20 92 84 81 77 74 72 68 65 61 58 55 52 48 45 42 39

0.22 91 82 79 75 72 68 65 61 57 54 50 47 43 40 36 33

0.24 90 81 77 73 69 65 61 57 54 50 46 42 38 34 30 27

0.26 90 79 75 71 67 62 58 54 50 46 41 37 33 29 25 20

0.28 89 77 73 68 64 59 55 50 46 41 37 32 28 23 18 14

0.30 88 76 71 66 61 56 52 47 42 37 32 27 23 18 13 9

0.31 88 75 70 65 60 55 50 45 40 35 30 25 20 18 10 5 0

Dimension A, mm

Dimension

B, mm

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5

1.0 99 98 98 97 96 96 95 94 94 93 92 92 91 91 90 89 89 88

1.5 98 97 96 95 94 93 92 92 91 90 89 88 87 86 85 84 83 82

2.0 98 96 95 94 92 91 90 89 88 86 85 84 82 81 80 79 77 76

2.5 97 95 94 92 91 89 88 86 84 83 81 80 78 77 75 73 72 70

3.0 96 94 92 91 89 87 85 83 81 79 77 76 74 72 70 68 66 64

3.5 96 93 91 89 87 85 82 80 78 76 74 72 69 67 65 63 61 58

4.0 95 92 90 88 85 82 80 77 75 72 70 67 65 62 60 57 55 52

4.5 94 92 89 86 83 80 77 75 72 69 66 63 61 58 55 52 49 46

5.0 94 91 88 85 81 78 75 72 69 66 62 59 56 53 50 47 44 41

5.5 93 90 86 83 79 76 72 69 66 62 59 55 52 48 45 42 38 35

6.0 92 89 85 81 77 74 70 66 62 59 55 51 47 44 40 36 33 29

6.5 92 88 84 80 76 72 67 63 59 55 51 47 43 39 35 31 27 23

7.0 91 87 82 78 74 69 65 61 56 52 47 43 39 34 30 26 21 17

7.5 91 86 81 77 72 67 62 58 53 48 44 39 34 30 25 20 16 11

8.0 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

Fig. 9 Sketch of a fractured impact test bar. The method used in calculating percent shear involves

measuring average dimensions A and B to the nearest 0.5 mm (0.02 in.) and then consulting a chart

(Table 2) to determine the percent shear fracture. (Courtesy of ASTM)

Unlike Charpy energy, fracture appearance is indicative of how a specimen failed. It is therefore useful when

attempting to correlate results of Charpy testing with other toughness test methods that use different specimen

geometries and loading rates. However, the fracture-appearance method can also be subjective. In one round-

robin test survey of 20 specimens (Ref 6), results showed that agreement was best when operators are

experienced, samples are close to the fracture-appearance transition, and when simple, two-dimensional figures

are used for assessment.

Lateral-Expansion Method. The other auxiliary method of determining transition temperature is the lateral-

expansion method. This procedure is based on the fact that protruding shear lips are produced (perpendicular to

the notch) on both sides of each broken specimen. The greater the ductility, the larger the protrusions. This

lateral expansion can be expressed as a measure of acceptable ductility at a given test temperature. The broken

halves from each end of each specimen are measured. The higher values from each side are added together, and

this total is the lateral-expansion value. A minimum value of lateral expansion must be specified as a transition

value. These test results are then plotted against test temperature and a curve interpolated. The impact energy

(in joules or foot-pounds) is also reported. These methods are described in detail in ASTM A 370 and E 23.

Applications

Test criteria for Charpy V-notch impact testing usually involve:

• A minimum impact energy value

• Shear appearance of fractured test bars expressed in percent

• Lateral expansion

For steels, the minimum acceptable values most commonly specified for these three evaluation methods are,

respectively: 20 J (15 ft · lbf), 50% shear, and 1.3 mm (50 mil). As a general rule of thumb, Charpy V-notch

impact strengths of 14 J (10 ft · lbf) and lower are likely to initiate fractures. An impact strength of 27 J (20 ft ·

lbf) is likely to propagate brittle fracture once initiated, and values well above 27 J (20 ft · lbf) are necessary to

arrest fracturing once it has been initiated.

Charpy impact testing does not produce numbers that can be used for design purposes, but is widely used in

specifications such as ASTM A 593, “Specification for Charpy V-Notch Testing Requirements for Steel Plates

for Pressure Vessel.” Other applications are briefly described below.

Nuclear Pressure Vessel Design Code. For nuclear pressure vessels, the American Society of Mechanical

Engineers (ASME) Boiler and Pressure Vessel Code (Ref 7) and the Code of Federal Regulations (Ref 8)

currently use fracture mechanics principles that dictate toughness requirements for pressure vessel steels and

weldments. The specified toughness requirements are obtained using Charpy V-notch test specimens coupled

with the nil-ductility transition temperature (NDTT) per ASTM E 208. The actual approach involves a

reference temperature, designated RT

NDT

, and the reference fracture toughness curve, K

. The reference

fracture toughness curve defined in Appendix G, Section III, of the ASME Code uses an experimentally

determined relationship between toughness and temperature that is adjusted along the temperature axis

according to an index reference temperature.

The reference toughness curve, K

, is assumed to describe the minimum (lower bound) fracture toughness for

all ferritic materials approved for nuclear pressure boundary applications having a minimum specified yield

strength of 345 MPa (50 ksi) or less. The value of RT

NDT

is obtained by measuring the drop-weight nil-ductility

transition temperature and performing standard Charpy V-notch tests. The nil-ductility transition temperature is

determined initially, and then a set of three Charpy V-notch specimens is tested at a temperature that is 33 °C

(60 °F) higher than the nil-ductility transition temperature to measure the temperature, T

, which ensures an

increase in toughness with temperature. Charpy energies of 68 J (50 ft · lbf) and lateral expansion of 0.89 mm

(35 mil) are used to ensure this condition.

The nil-ductility transition temperature becomes the RT

NDT

temperature if the Charpy results equal or exceed

the above limits. If the Charpy values at T

or the nil-ductility transition temperature plus 33 °C (60 °F) are

lower than required, additional Charpy tests should be performed at higher test temperatures, usually in

increments of 5.6 °C (10 °F), until the requirements are satisfied and T

is measured. The RT

NDT

temperature

then becomes the temperature (T

) at which the criteria are met minus 33 °C (60 °F). Thus, the reference

temperature is always either greater than or equal to the nil-ductility transition temperature.

Steel Bridge Toughness Criteria. The American Association of State Highway and Transportation Officials

(AASHTO) has adopted Charpy impact toughness requirements for primary tension members in bridge steels

based on section thickness, yield strength, and expected service temperature. They are based on the fracture

toughness corresponding to the maximum loading rate expected in service (Ref 9).

Correlations with Fracture Toughness. Empirical attempts have been made to correlate the Charpy impact

energy with K

to allow a quantitative assessment of critical flaw size and permissible stress levels. Most of

these correlations are dimensionally incompatible, ignore differences between the two measures of toughness

(in particular, loading rate and notch acuity), and are valid only for limited types of materials and ranges of

data. Additionally, these correlations can be widely scattered. However, some correlations can provide a useful

guide to estimating fracture toughness; in fact, the preceding design criteria for nuclear pressure vessel and

bridge steels are partially based on such correlative procedures.

Some of the more common correlations are listed in Table 3 (Ref 9, 10, 11, 12, 13, 14, 15, and 16) with

appropriate units. Note that some of the correlations attempt to eliminate the effects of loading rate; the

dynamic fracture toughness, K

, is correlated with Charpy energy. Other attempts have been made to improve

and explain some of the correlations (see, for example, Ref 17). A study has also been conducted using a

portion of the Charpy energy to separate initiation and propagation components in the Charpy test (Ref 18). The

results from this study for an upper-shelf J

correlation for pressure vessel steels were not significantly better

than the Rolfe-Novak correlation listed in Table 3. A statistically based correlation for lower-bound toughness

has also been developed for pressure vessel steels (Ref 19, 20). Thus, simple and empirical correlations can be

used as general guidelines for estimating K

or K

within the limits of the specific correlation.

Table 3 Typical Charpy/K

correlation for steels

Correlation

Transition temperature regime

Barsom (Ref 9)

/E = 5 (CVN)

Barsom-Rolfe (Ref 10)

/E = 2(CVN)

3/2

= psi

E = psi

CVN = ft · lbf

Sailors-Corten (Ref 11)

/E = 8 (CVN)

= 15.873(CVN) 3/8

= ksi

CVN = ft · lbf

Begley-Logsdon—three points (Ref 12)

)

= 0.45 σ

at 0% shear fracture temperature

)

From Rolfe-Novak Correlation at 100% shear fracture

temperature

)

= [(K

)

+ (K

)

] at 50% shear fracture temperature

= ksi

Marandet-Sanz—three steps (Ref 13)

100

= 9 + 1.37 T

= 19 (CVN) 1/2

Shift K

curve through T

100

point

100

= °C, for which K

= 100 MPa

= °C, for which CVN = 28J

= MPa

CVN = J

Wullaert-Server (Ref 14)

Ic,d

= 2.1 (σ

CVN) 1/2

Ic,d

= ksi

CVN = ft · lbf

= ksi corresponding to approximate

loading rate

Upper-shelf region

Rolfe-Novak—σ

> 100 ksi (Ref 15)

/σ

)

= 5 (CVN/σ

- 0.05)

= ksi

CVN = ft · lbf

= ksi

Ault-Wald-Bertolo—ultrahigh-strength steels (Ref 16)

/σ

)

= 1.37 (CVN/σ

) - 0.045

= ksi

CVN = ft · lbf

= ksi

1.0 ksi = 6.8948 MPa; 1.0 ksi = 1.099 MPa ; 1.0 ft · lbf = 1.356 J; CVN is the designation for

Charpy impact energy; σ

is the yield stress; and E is the Young's modulus.

As previously described, a lower-bound K

toughness curve is shifted relative to a reference temperature,

NDT

, and used to define the ductile-to-brittle transition. The RT

NDT

is a critical value and is defined very

conservatively in terms of Charpy and dynamic tear specimen results. Continued application of these

requirements is now a principal limitation to continued operation of several commercial nuclear power plants

(Ref 21). Recent work by ASTM Committee E-8 has proposed a method to obtain a new reference temperature

and a method to define, using a probabilistic approach, a median ductile-to-brittle transition curve from a set of

six properly tested small samples that would, in many cases, be precracked Charpy specimens. Statistical

confidence bounds would then be available for this median transition “master curve,” which would be specific

to the particular nuclear plant of interest and could be used to assure that the pressure vessel had adequate

toughness for continued operation.

A generalized prediction method to predict K

transition curves has also been developed with data from various

steels including 2.25Cr-1Mo, 1.25Cr-0.50Mo, 1Cr and 0.50Mo chemical pressure vessel steels, and ASTM A

508 C1.1, A 508 C1.2, A 508 C1.3 and A 533 Gr.B C1.1 nuclear pressure vessel steels (Ref 22). This method

consists of a master curve of K

and a temperature shift, ΔT, between fracture toughness and Charpy V-notch

impact transition curves versus yield strength relationship for T

, where T

is the temperature showing 50% of

the upper-shelf K

value. The K

transition curves predicted using both methods showed a good agreement

with the lower bound of measured K

values obtained from elastic-plastic, J

, tests.

References cited in this section

3. O.L. Towers, Effects of Striker Geometry on Charpy Results, Met. Constr., Nov 1983, p 682–685

4. J.M. Holt, Ed., Charpy Impact Test: Factors and Variables, STP 1072, ASTM, 1990

5. T.A. Siewert and A.K. Schmieder, Ed., Pendulum Impact Machines: Procedures and Specimens for

Verification, STP 1248, ASTM, 1995

6. B.F. Dixon, Reliability of Fracture Appearance Measurement in the Charpy Test, Weld. J., Vol 73 (No.

8), Aug 1994, p 39–46

7. “Rules for Construction of Nuclear Power Plant Components,” ASME Boiler and Pressure Vessel Code,

Section III, Division 1 , Appendices, Nonmandatory Appendix G, American Society for Mechanical

Engineers, 1983

8. Energy (Title 10), Domestic Licensing of Production and Utilization Facilities (Part 50), Code of

Federal Regulations, U.S. Government Printing Office, 1981

9. J.M. Barsom, The Development of AASHTO Fracture Toughness Requirements for Bridge Steels, Eng.

Fract. Mech., Vol 7 (No. 3), Sept 1975, p 605–618

10. J.M. Barsom and S.T. Rolfe, Correlations Between K

and Charpy V-Notch Test Results in the

Transition Temperature Range, Impact Testing of Materials, STP 466, ASTM, 1979, p 281–302

11. R.H. Sailors and H.T. Corten, Relationship Between Material Fracture Toughness Using Fracture

Mechanics and Transition Temperature Tests, Fracture Toughness, Proceedings of the 1971 National

Symposium on Fracture Mechanics—Part II, STP 514, ASTM, 1972, p 164–191

12. J.A. Begley and W.A. Logsdon, “Correlation of Fracture Toughness and Charpy Properties for Rotor

Steels,” WRL Scientific Paper 71-1E7-MSLRF-P1, Westinghouse Research Laboratory, Pittsburgh, PA,

July 1971

13. B. Marandet and G. Sanz, Evaluation of the Toughness of Thick Medium-Strength Steels by Using

Linear Elastic Fracture Mechanics and Correlations Between K

and Charpy V-Notch, Flaw Growth

and Fracture, STP 631, ASTM, 1977, p 72–95

14. R.A. Wullaert, Fracture Toughness Predictions from Charpy V-Notch Data, What Does the Charpy Test

Really Tell Us?: Proceedings of the American Institute of Mining, Metallurgical and Petroleum

Engineers, American Society for Metals, 1978

15. S.T. Rolfe and S.R. Novak, Slow-Bend K

Testing of Medium-Strength High-Toughness Steels, Review

of Developments in Plane-Strain Fracture Toughness Testing, STP 463, ASTM, 1970, p 124–159

16. “Rapid Inexpensive Tests for Determining Fracture Toughness,” National Materials Advisory Board,

National Academy of Sciences, Washington, D.C., 1976

17. What Does the Charpy Test Really Tell Us?: Proceedings of the American Institute of Mining,

Metallurgical and Petroleum Engineers, American Society for Metals, 1978

18. D.M. Norris, J.E. Reaugh, and W.L. Server, A Fracture-Toughness Correlation Based on Charpy

Initiation Energy, Fracture Mechanics: Thirteenth Conference, STP 743, ASTM, 1981, p 207–217

19. W.L. Server et al., “Analysis of Radiation Embrittlement Reference Toughness Curves,” EPRI NP-

1661, Electric Power Research Institute, Palo Alto, CA, Jan 1981

20. Metal Properties Council MPC-24, Reference Fracture Toughness Procedures Applied to Pressure

Vessel Materials, Proceedings of the Winter Annual Meeting of the American Society for Mechanical

Engineers, American Society of Mechanical Engineers, New York, 1984

21. J.A. Joyce, Predicting the Ductile-to-Brittle Transition in Nuclear Pressure Vessel Steels from Charpy

Surveillance Specimens, Recent Advances in Fracture, Minerals, Metals and Materials Society/AIME,

1997, p 65–75

22. T. Iwadate, Y. Tanaka, and H. Takemata, Prediction of Fracture Toughness K

Transition Curves of

Pressure Vessel Steels from Charpy V-Notch Impact Test Results, J. Pressure Vessel Technol. (Trans.

ASME), Vol 116 (No. 4), p 353–358

Impact Toughness Testing

Instrumented Charpy Impact Test

The use of additional instrumentation (typically an instrumented tup) allows a standard Charpy impact machine

to monitor the analog load-time response of Charpy V-notch specimen deformation and fracturing. The primary

advantage of instrumenting the Charpy test is the additional information obtained while maintaining low cost,

small specimens, and simple operation. The most commonly used approach is application of strain gages to the

striker to sense the load-time behavior of the test specimen. In some cases, gages are placed on the specimen as

well, such as for the example shown in Fig. 10 (Ref 23).

Fig. 10 Charpy specimen with additional instrumentation at the supports

General Description

Instrumentation of the tup provides valuable data in terms of the load-time, P-t, history during impact.

Extensive efforts have been made to help determine the dynamic fracture toughness, K

, over a range of

behavior in linear-elastic, elastic-plastic, and fully plastic regimes. An overview of these efforts is given in Ref

24.

Figure 11 schematically illustrates the change in Charpy behavior as a function of temperature for a medium-

strength steel. As shown, instrumentation clearly allows the various stages in the fracture process to be

identified. The energy value, W

, is associated with the area under the load-time (P-t) curve up to maximum

load, P

. This impulse value is converted to energy by using Newton's second law, which accounts for the

pendulum velocity decrease during the deformation-fracture process. This velocity decrease is proportional to

the instantaneous load on the specimen at any particular time, t

; the actual energy absorbed, ΔE

, simplifies to

(Ref 25):

(Eq 1)