ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation

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Impact Toughness Testing

Introduction

DYNAMIC FRACTURE occurs under a rapidly applied load, such as that produced by impact or by explosive

detonation. In contrast to quasi-static loading, dynamic conditions involve loading rates that are greater than

those encountered in conventional tensile tests or fracture mechanics tests. Dynamic fracture includes the case

of a stationary crack subjected to a rapidly applied load, as well as the case of a rapidly propagating crack under

a quasi-stationary load. In both cases the material at the crack tip is strained rapidly and, if rate sensitive, may

offer less resistance to fracture than at quasi-static strain rates. For example, values for dynamic fracture

toughness are lower than those for static toughness (K

) in the comparison shown in Fig. 1.

Fig. 1 Comparison of static (K

), dynamic (K

), and dynamic-instrumented (K

Idi

) impact fracture

toughness of precracked specimens of ASTM A 533 grade B steel, as a function of test temperature. The

stress-intensity rate was about 1.098 × 10

MPa · s

-1

(10

ksi · s

-1

) for the dynamic tests and

about 1.098 × 10

MPa · s

-1

(10

ksi · s

-1

) for the dynamic-instrumented tests. Source: Ref 1

Because many structural components are subjected to high loading rates in service, or must survive high

loading rates during accident conditions, high strain rate fracture testing is of interest and components must be

designed against crack initiation under high loading rates or designed to arrest a rapidly running crack.

Furthermore, because dynamic fracture toughness is generally lower than static toughness, more conservative

analysis may require consideration of dynamic toughness.

Measurement and analysis of fracture behavior under high loading rates is more complex than under quasi-

static conditions. There are also many different test methods used in the evaluation of dynamic fracture

resistance. Test methods based on fracture mechanics, as discussed extensively in other articles of this Section,

produce quantitative values of fracture toughness parameters that are useful in design. However, many

qualitative methods have also been used in the evaluation of impact energy to break a notched bar, percent of

cleavage area on fracture surfaces, or the temperature for nil ductility or crack arrest. These qualitative tests

include methods such as the Charpy impact test, the Izod impact test, and the drop-weight test. Other less

common tests are the explosive bulge test, the Robertson test, the Esso test, and the Navy tear test (described in

the 8

Edition Metals Handbook, Volume 10, p 38–40).

This article focuses exclusively on notch-toughness tests with emphasis on the Charpy impact test. The Charpy

impact test has been used extensively to test a wide variety of materials. Because of the simplicity of the

Charpy test and the existence of a large database, attempts also have been made to modify the specimen,

loading arrangement, and instrumentation to extract quantitative fracture mechanics information from the

Charpy test. Other miscellaneous notch-toughness test methods are also discussed in this article.

Reference cited in this section

1. Use of Precracked Charpy Specimens, Fracture Control and Prevention, American Society for Metals,

1974, p 255–282

Impact Toughness Testing

History of Impact Testing

Before fracture mechanics became a scientific discipline, notched-bar impact tests were performed on

laboratory specimens to simulate structural failures, eliminating the need to destructively test large engineering

components. The simulation of structural component failure by notched-bar impact tests is based on severe

conditions of high loading rate, stress concentration, and triaxial stress state. These tests have been extensively

used in the evaluation of ductile-to-brittle transition temperature of low- and medium-strength ferritic steels

used in structural applications such as ships, pressure vessels, tanks, pipelines, and bridges.

The initial development of impact testing began around 1904 when Considére discovered and noted in a

published document that increasing strain rate raises the temperature at which brittle fracture occurs. In 1905

another Frenchman, George Charpy, developed a pendulum-type impact testing machine based on an idea by

S.B. Russell. This machine continues to be the most widely used machine for impact testing. In 1908 an

Englishman by the name of Izod developed a similar machine that gained considerable popularity for a period

of time but then waned in popularity because of inherent difficulties in testing at temperatures other than room

temperature.

Impact testing was not widely used, and its significance not fully understood, until World War II when many

all-welded ships were first built (approximately 3000 of them). Of these 3000 ships, approximately 1200

suffered hull fractures, 250 of which were considered hazardous. In fact, 19 or 20 of them broke completely in

two. These failures did not necessarily occur under unusual conditions; several occurred while the ships were at

anchor in calm waters. In addition to ship failures, other large, rigid structures, such as pipelines and storage

tanks, failed in a similar manner. All failures had similar characteristics. They were sudden, had a brittle

appearance, and occurred at stresses well below the yield strength of the material. It was noted that they

originated at notches or other areas of stress concentration, such as sharp corners and weld defects. These

failures were often of considerable magnitude: in one case a pipeline rupture ran for 20 miles.

The Naval Research Laboratory, along with others, launched a study of the cause of these fractures. It was

noted that often, but not always, failures occurred at low temperatures. More detailed historical research

revealed that similar failures had been recorded since the 1800s but had been largely ignored. The results of this

study renewed interest, and further investigation revealed that materials undergo a transition from ductile

behavior to brittle behavior as the temperature is lowered. In the presence of a stress concentrator such as a

notch, it takes little loading to initiate a fracture below this transition temperature, and even less to cause such a

fracture to propagate. These transitions were not predictable by such tests as hardness testing, tensile testing, or,

for the most part, chemical analysis, which were common tests of the times. It was then discovered that a

ductile-to-brittle transition temperature could be determined by impact testing using test specimens of uniform

configuration and standardized notches. Such specimens were tested at a series of decreasing temperatures, and

the energy absorbed in producing the fracture was noted. The Charpy pendulum impact testing machine was

used. At first, test results were difficult to reproduce. The problem was partly resolved by producing more

uniformly accurate test equipment. The notch most often used was of a keyhole type created by drilling a small

hole and then cutting through the test bar to the hole by sawing or abrasive cutting. It was soon found that by

using specimens with sharper notches, better-defined transition temperatures that were more reproducible could

be determined. A well-defined notch with a V configuration became the standard. Steels in particular could then

be tested and the ductile-to-brittle transition temperature obtained.

Two problems remained. First, testing machines had to be standardized very carefully or the results were not

reproducible from one machine to another. The other problem was that the transition temperature found by

testing small bars was not necessarily the same as that for full-size parts.

Fortunately, the problem with standardization was resolved by the Army. They learned that impact testing was

a necessity for producing successful armor plate and gun tubes. Research at the Watertown Arsenal resulted in

the development of standard test specimens of various impact levels. The Army made these available to their

various vendors so that the vendors could standardize their own testing machines. This program was so

successful that such specimens were made available to the public, at a nominal charge, starting in the 1960s.

Next, the manufacturers of testing equipment were pressured into making equipment available that would meet

these exacting standards.

The problem of differing transition temperatures for full-size parts and test specimens was discovered when a

series of full-size parts was tested using a giant pendulum-type impact machine and these results were

compared with those determined using small standard test bars made from the same material. A partial solution

to this problem was the development of the drop-weight test (DWT) and the drop-weight tear test (DWTT).

These tests produced transition temperatures similar to those found when testing full-size parts. Unfortunately,

such tests are adaptable only for plate specimens of limited sizes and have not become widely used.

The Charpy V-notch test continues to be the most used and accepted impact test in use in the industry.

However, the restricted applicability of the Charpy V-notch impact test has been recognized for many years

(Ref 2). Charpy test results are not directly applicable for designs, and the observed ductile-to-brittle transition

depends on specimen size. Nonetheless, the Charpy V-notch test is useful in determining the temperature range

of ductile-to-brittle transition.

Reference cited in this section

2. C.E. Turner, Impact Testing of Metals, STP 466, ASTM, 1970, p 93

Impact Toughness Testing

Types of Notch-Toughness Tests

In general, notch toughness is measured in terms of the absorbed impact energy needed to cause fracturing of

the specimen. The change in potential energy of the impacting head (from before impact to after fracture) is

determined with a calibrated dial that measures the total energy absorbed in breaking the specimen. Other

quantitative parameters, such as fracture appearance (percent fibrous fracture) and degree of

ductility/deformation (lateral expansion or notch root contraction), are also often measured in addition to the

fracture energy. Impact tests may also be instrumented to obtain load data as a function of time during the

fracture event. In its simplest form, instrumented impact testing involves the placement of a strain gage on the

tup (the striker).

Many types of impact tests have been used to evaluate the notch toughness of metals, plastics, and ceramics. In

general, the categories of impact tests can be classified in terms of loading method (pendulum stroke or drop-

weight loading) and the type of notched specimen (e.g., Charpy V-notch, Charpy U-notch, or Izod). The

following descriptions briefly describe the key types of impact tests that are used commonly in the evaluation

of steels or structural alloys.

The Charpy and Izod impact tests are both pendulum-type, single-blow impact tests. The principal difference,

aside from specimen and notch dimensions, is in the configuration of the test setup (Fig. 2). The Charpy test

involves three-point loading, where the test piece is supported at both ends as a simple beam. In contrast, the

Izod specimen is set up as a cantilever beam with the falling pendulum striking the specimen above the notch

(Fig. 2b).

Fig. 2 Specimen types and test configurations for pendulum impact toughness tests. (a) Charpy method.

(b) Izod method

The Charpy V-notch test continues to be the most utilized and accepted impact test in use in the industry. It is

written into many specifications. While this test may not reveal exact ductile-to-brittle transition temperatures

for large full-size parts, it is easily adaptable as an acceptability standard on whether or not parts are apt to

behave in a brittle manner in the temperature range in which they are likely to be used.

The drop-weight test is conducted by subjecting a series (generally four to eight) of specimens to a single

impact load at a sequence of selected temperatures to determine the maximum temperature at which a specimen

breaks. The impact load is provided by a guided, free-falling weight with an energy of 340 to 1630 J (250 to

1200 ft · lbf) depending on the yield strength of the steel to be tested. The specimens are prevented by a stop

from deflecting more than a few tenths of an inch.

This is a “go, no-go” test in that the specimen will either break or fail to break. It is surprisingly reproducible.

For example, Pellini made 82 tests of specimens from one plate of semikilled low-carbon steel. At -1 °C (30 °F)

and 4 °C (40 °F), all specimens remained unbroken. At -7 °C (20 °F), only one of 14 specimens broke;

however, at -12 °C (10 °F), 13 of the 14 specimens broke. At temperatures below -12 °C (10 °F), all specimens

broke.

The drop-weight tear test (DWTT) uses a test specimen that resembles a large Charpy test specimen. The test

specimen is 76 mm (3 in.) wide by 305 mm (12 in.) long, supported on a 254 mm (10 in.) span. The thickness

of the specimen is the full thickness of the material being examined. The specimens are broken by either a

falling weight or a pendulum machine. The notch in the specimen is pressed to a depth of 5 mm (0.20 in.) with

a sharp tool-steel chisel having an angle of 45°. The resulting notch root radius is approximately 0.025 mm

(0.001 in.). One result of the test is the determination of the fracture appearance transition curve. The “average”

percent shear area of the broken specimens is determined for the fracture area neglecting a region “one

thickness” in length from the root of the notch and “one thickness” from the opposite side of the specimen.

These regions are ignored because it is believed that the pressing of the notch introduces a region of plastically

deformed material which is not representative of the base material. Similarly the opposite side of the specimen

is plastically deformed by the hammer tup during impact. The fracture appearance plotted versus temperature

defines an abrupt transition in fracture appearance. This transition has been shown to correlate with the

transition in fracture propagation behavior in cylindrical pressure vessels and piping.

Impact Toughness Testing

Charpy Impact Testing

As previously noted, the specimen in the Charpy test is supported on both ends and is broken by a single blow

from a pendulum that strikes the middle of the specimen on the unnotched side. The specimen breaks at the

notch, the two halves fly away, and the pendulum passes between the two parts of the anvil. The height of fall

minus the height of rise gives the amount of energy absorption involved in deforming and breaking the

specimen. To this is added frictional and other losses amounting to 1.5 or 3J (1 or 2 ft · lbf). The instrument is

calibrated to record directly the energy absorbed by the test specimen.

Methods for Charpy testing of steels are specified in several standards including:

Designation

Title

ASTM E 23

Standard Test Methods for Notched Bar Impact Testing of Metallic Materials

BS 131-2

The Charpy V-Notch Impact Test on Metals

BS 131-3

The Charpy U-Notch Impact Test on Metals

BS 131-6

Method for Precision Determinations of Charpy V-Notch Impact Energies for Metals

ISO 148

Steel—Charpy Impact Test (V-Notch)

ISO 83

Steel—Charpy Impact Test (U-Notch)

DIN-EN 10045

Charpy Impact Test of Metallic Materials

These standards provide requirements of test specimens, anvil supports and striker dimensions and tolerances,

the pendulum action of the test machine, the actual testing procedure and machine verification, and the

determination of fracture appearance and lateral expansion.

The general configuration of the Charpy test, as shown in Fig. 3 for a V-notch specimen, is common to the

requirements of most standards for the Charpy test. Differences between ASTM E 23 and other standards

include differences in machining tolerances, dimensions of the striker tip (Fig. 4), and the ASTM E 23

requirements for testing of reference specimens. The most pronounced difference between standards is the

different geometry for the tip of the striker, or tup. The tup in the ASTM specification (Fig. 4a) is slightly flatter

than in many other specifications (Fig. 4b). From a comparison of results from Charpy tests with the two

different tup geometries, differences appeared more pronounced for several steels at impact energies above 100

J (74 ft · lbf) (Ref 3). From this evaluation, a recommendation was also made to use the sharper and smoother

tup (Fig. 4b) if the national standards are unified further.

Fig. 3 General configuration of anvils and specimen in Charpy test

Fig. 4 Comparison of striker profiles for Charpy testing. (a) ASTM E 23. (b) Other national and

international codes: AS1544, Part 2; BS 131, Part 2; DIN 51222; DS10 230; GOST 9454; ISO R148; JIS

B7722; NF A03-161; NS 1998; UNI 4713-79. Source: Ref 3

There are also three basic types of standard Charpy specimens (Fig. 5): the Charpy V-notch, the Charpy U-

Notch, and the Charpy keyhole specimen. These dimensions are based on specifications in ASTM E 23, ISO

148, and ISO 83. The primary specimen and test procedure involves the Charpy V-notch test. Other Charpy-

type specimens are not used as extensively because their degree of constraint and triaxiality is considerably less

than the V-notch specimen.

Fig. 5 Dimensional details of Charpy test specimens most commonly used for evaluation of notch

toughness. (a) V-notch specimen (ASTM E 23 and ISO 148). (b) Keyhole specimen (ASTM E 23). (c) U-

notch specimen (ASTM E 23 and ISO 83)

The Charpy V-notch impact test has limitations due to its blunt notch, small size, and total energy measurement

(i.e., no separation of initiation and propagation components of energy). However, this test is used widely

because it is inexpensive and simple to perform. Thus, the Charpy V-notch test commonly is used as a

screening test in procurement and quality assurance for assessing different heats of the same type of steel. Also,

correlation with actual fracture toughness data is often devised for a class of steels so that fracture mechanics

analyses can be applied directly. Historically, extensive correlation with service performance has indicated its

usefulness.

The keyhole and U-notches were early recognized (1945) as giving inadequate transition temperatures because

of notch bluntness. Even the V-notch does not necessarily produce a transition temperature that duplicates that

of a full-size part. Under current testing procedures, the Charpy V-notch test is reproducible and produces close

approximations of transition temperatures found in full-size parts. It is widely used in specifications to ensure

that materials are not likely to initiate or propagate fractures at specific temperature levels when subjected to

impact loads.

Equipment

Charpy testing requires good calibration methods. Machine belting should be examined regularly for looseness,

and broken specimens should be examined for unusual side markings. Anvils should also be examined for wear.

Testing Machines. Charpy impact testing machines are available in a variety of types. Some are single-purpose

machines for testing Charpy specimens only. Others are adaptable to testing Izod and tension impact specimens

also. They are offered in a range of loading capacities. The most common of these capacities are 325 and 160 J

(240 and 120 ft · lbf). Some machines have variable load capabilities, but most are of a single-fixed-load type.

When purchasing or using a machine, be sure that the available loading is such that specimens to be tested will

break with a single blow, within 80° of the machine capacity (as shown by the scale on the machine).

While loading capacity depends on the anticipated strength of specimens to be tested, the maximum value of

such specimens is the principal consideration. Very tough specimens may stop the hammer abruptly without

breaking. A number of such load applications have been known to cause breakage of the pendulum arm. On the

other hand, lower-capacity machines may be more accurate and more likely to meet standardization

requirements. For most ordinary steel testing applications, the machine with a capacity of 160 J (120 ft · lbf)

makes a good compromise choice. Testing of a large number of very tough specimens may require a machine

with a capacity of 325 to 400 J (240–300 ft · lbf).

Charpy impact machines are of a pendulum type. They must be very rigid in construction to withstand the

repeated hammering effect of breaking specimens without affecting the operation of the pendulum mechanism.

The machine must be rigidly mounted. Special concrete foundations are sometimes used, but at least the

machine must be bolted down to an existing concrete foundation, which should be a minimum of 150 mm (6

in.) thick. The pendulum should swing freely with a minimum of friction. Any restriction in movement of the

pendulum will increase the energy required to fracture the specimen. This produces a test value that is higher

than normal. There will always be small effects of this type, and they are usually compensated for, along with

windage friction effects, by scale-reading adjustments built into the equipment.

While the pendulum must be loose enough to swing freely with little friction, it must not be loose enough to

produce inaccuracies, such as nonuniform striking of the specimen. The components must be sturdy enough to

resist deformation at impact. This is particularly true of the anvil and pendulum. It is important that the

instrument be level. Some machines have a built-in bubble-type level. Others have machined surfaces where a

level can be used. In operation, the pendulum is raised to the proper height and held by a cocking mechanism

that can be instantly released.

ASTM E 23 specifies that tests should be made at velocities between 3 and 6 m/s (10 and 20 ft/s) and that this

is defined as “the maximum tangential velocity of the striking member at the center of strike.” When hanging

freely, the striking tup of the pendulum should be within 2.5 mm (0.10 in.) of touching the area of the specimen

where first contact will be made. The anvil that retains the test specimen must be made such that the specimen

can be squarely seated. The notch must be centered so that the pendulum tup hits directly behind it.

Most impact testing machines have scales that read directly in foot-pounds (scales also may read in degrees).

As noted, the scale can be adjusted to compensate for windage, pendulum friction, and other variations. The

scale should read zero when the pendulum is released without a specimen being present. Pendulum and anvil

design, configuration, and dimensions are important. It is also important that the broken specimens be able to

fly freely without being trapped in the anvil by the pendulum. Proper anvil design, such as that shown in Fig. 6,

can minimize jamming.

Fig. 6 Typical anvil arrangement with modification that reduces the possibility of jamming

Specimens. As previously noted, there are three commonly used standard Charpy impact test specimens, which

are similar except for the notch (Fig. 5). The V-notch bar is the most frequently used specimen, although some

specific industries still use the other types of test bars. The steel casting industry, for instance, uses the keyhole-

notch specimen more frequently. There are also many varieties of subsize specimens that should be used only

when insufficient material is available for a full-size specimen, or when the shape of the material will not allow

removal of a standard specimen.

It is important that specimens be machined carefully and that all dimensional tolerances be followed. Care must

be exercised to ensure that specimens are square. It is easy to grind opposite sides parallel, but this does not

ensure squareness. The machining of the notch is the most critical factor. The designated shape and size of the

notch must be strictly followed, and the notch must have a smooth (not polished) finish. Special notch-

broaching machines are available for V-notching. A milling machine with a fly cutter can also be used.

In preparing keyhole-notch specimens, the hole should be drilled at a low speed to avoid heat generation and

work hardening. Use of a jig with a drill bushing ensures accuracy. After the hole has been drilled, slotting can

be done by almost any method that meets specifications, but care should be exerted to prevent the slotting tool

from striking the back of the hole. In all cases it is desirable to examine the notch at some magnification. A

stereoscopic microscope or optical comparator is suitable for this examination. In fact, a V-notch template for

use with the optical comparator can be used to ensure proper dimensions.

Specimens must generally be provided with identification markings. This is best done on the ends of the

specimen. In preparing specimens where structural orientation is a factor (e.g., rolling direction of wrought

materials), such orientation should be taken into consideration and noted, because orientation can cause wide

variations in test results. If not otherwise noted, the specimen should be oriented in the rolling direction of the

plate (forming direction of any formed part) and the notch should be perpendicular to that surface (orientation

A in Fig. 7). This produces maximum impact values. All notching must be done after any heat treatment that

might be performed.

Fig. 7 Effect of specimen orientation on impact test results