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Impact Laboratory

Laboratory Performed on September 30, 2008 by the Late Tuesday Lab Group:

Ryan Carmichael, Zach Eichenwald, Anne Krikorian, Jeff Santner, Anson Stewart, Scott Taylor,

and Meghan Whalen

Report by Ryan Carmichael and Anne Krikorian

E59 Laboratory Report Submitted October 21, 2008

Department of Engineering, Swarthmore College

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Abstract___________________________________________________________

In this lab, we performed a Charpy impact test on three different types of steels (hot-

rolled C1018, cold-finished C4140 alloy steel, and cold-finished C1095 carbon steel) testing

each variety at three different temperatures (22.6C, 0.1C, and -80.0C). From our results

(shown below), we determined that the C1095 steel exhibited only brittle fracture, while both the

C1018 and the C4140 steels exhibited a transition from ductile failures to brittle failures.

Theory____________________________________________________________

Impact tests determine impact toughness, a material property, most commonly by

measuring the work required to fracture a test

specimen under impact. Impact tests are

useful in the analysis and prediction of the

behaviors of different materials under impact

stresses or dynamic loading. However, such

tests cannot directly predict the reaction of a

material to real life loading. Instead, the

results are used for comparison purposes.

Impact

Toughness (22.6C)

Percent Ductility (22.6C)

Impact Toughness

(0.1C)

Percent Ductility (0.1C)

Impact Toughness (-80.0C)

Percent Ductility (-80.0C)

C1018 Blue HR 132 ft-lb* 70% 134 ft-lb* 50% 10 ft-lb 0% C4140 Green CF 73 ft-lb 60% 44 ft-lb 50% 9 ft-lb 0%

C1095 CFW1 4 ft-lb 0% 2 ft-lb 0% 1 ft-lb 0%

Table 1: Impact Toughness and Percent Ductility Results *Specimens were not completely fractured by the testing apparatus

Figure 1: Charpy Test Specimen Top View

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In the United States, the most common impact test is the Charpy impact test. The Charpy

impact test is a high strain rate test that measures the work required to rupture a specimen in

flexure. Charpy specimens are uniform, rectangular prismatic specimens with one notch per

specimen to encourage rupture (see Figure 1 above). The Charpy testing machine (Figure 2

below) is comprised essentially of a hammer with a striking head (a wedge shaped head was used

in this laboratory) attached to a nearly frictionless pendulum with a known potential energy.

When released from a known height, the hammer strikes the Charpy specimen placed in the

anvil, usually fracturing it. The testing machine then records the amount of energy used to

fracture the specimen by

determining the difference in

potential energy of the hammer

before release and at the peak of

its upswing after rupture. This is

given by the equation fracture

energy = mg(h - h), where h is

the original height of the

hammer, h is the peak of the

first upswing, m is the mass of

the hammer, and g is the

acceleration due to gravity.

In general, impact causes a region of plastic deformation to occur around the notch in the

test specimen, followed by strain hardening. The stress and strain then increase until the

specimen ruptures. The energy required to fracture the specimen (the impact toughness) provides

valuable information about how the material will behave under sudden impacts, although there

Figure 2: Diagram of Charpy Impact Machine (formerly Troxell Figure 13.1)

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are limitations on the applicability of the findings. Like hardness tests, impact tests do not result

in a number that definitively describes the materials toughness. Instead, impact tests yield

comparative data, which is interpreted in combination with an analysis of the broken surfaces of

the test specimens themselves. The test is popular in manufacturing because it is a fast,

inexpensive, and easy way to compare the properties of manufactured materials to each other;

the test is most useful for comparing batches of steels and for preliminary selection of materials

in design, but does not predict the behavior under impact of a large structural element made from

the material.

Impact tests are also useful for determining transition temperatures. These temperatures-

or ranges of temperatures are dependant on the material they describe, and record the region

where the material transitions from breaking mostly due to ductile shear to breaking mostly from

brittle fracture as shown in

Figure 3. This is significant

because when the material

reaches a temperature below its

transitional temperature, it takes

significantly less work to

fracture it than it would at a

temperature just above the

transitional temperature. As a

result, it is common for failures

to occur at or below the transition temperature. The work required to rupture a material at very

high temperatures is also small, but as these temperatures are rarely reached in normal scenarios,

Figure 3: Illustration of Transition Temperature (formerly

Troxell 13.11)

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and so do not play as large a role in impact failures of materials as the lower temperatures that

are more common.

When a specimen is tested, the energy that is transferred in the impact test may be

absorbed in a few different ways: through elastic deformations, plastic deformations, hysteresis,

friction, or inertia. In Charpy tests specifically, the most significant of these are elastic and

plastic deformations, with plastic deformation usually accounting for the majority of the

absorbed energy. The amount of energy required to achieve fracture is reliant on the ductility of

the material (which changes greatly with temperature), and the unknown proportion of work

done in elastic deformation to work done in plastic deformation which necessitates the physical

examination of each broken specimen. Post-fracture visual analysis can provide information on

what percent of the area was ductile during impact and what percent of area was brittle; this is

shown by the break patterns displayed in the broken surfaces as seen in Figure 4 below.

Figure 4: Guide for Determining Fracture Appearance

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The performance of a specimen in a Charpy impact test is, however, influenced by many

factors beyond material composition and temperature: yield strength and ductility, placement,

size, and shape of notches, strain rate, and fracture mechanism all affect the performance of a

sample. In an attempt to eliminate some of these variables, the ASTM E23 standardizes the size

and shape of Charpy specimens, fracture mechanism (which varies according to materials

tested), placement (which is still variable due to human error), and strain rate (which, as an effect

of gravity is constant at any particular testing location). When as many of the factors are held

constant as possible, the results of an impact test reflect the toughness of the material, although

even then the values found are useful only to compare to other results, and not as a simply

defined property that can be stated universally as a single value.

Procedure_________________________________________________________

Initially, we prepared the specimens. After noting their composition, we placed the

specimens in the two different low-temperature baths for more than five minutes, so that

one sample of each material was at room temperature (22.6C), in ice (0.1C), and in dry

ice (-80.0C).

We next recorded the frictional losses of the machine by releasing the pendulum without

a specimen on the anvil, and also examined the striking hammer (we used a wedge-

shaped hammer).

Following this, we placed a sample on the anvils (using tongs to avoid altering the

temperatures), making sure it was centered with the notch facing away from the surface

the pendulum would strike.

Having set this up, and making sure that everyone was a safe distance from the path of

the pendulum, we released the striker to rupture the specimen. For the six tests at cold

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temperatures, care was taken to allow no more than 5 seconds to pass between the

specimen being removed from the ice and fracture occurring.

After the impact, we retrieved the specimen and examined the fractured surface, raised

and locked the pendulum for the next test, and reset the energy indicator after recording

the energy value displayed.

We repeated this sequence with each of the nine test specimens.

Results____________________________________________________________

From our raw lab data seen in Appendix A, we subtracted the frictional loss of the

Charpy testing machine from the dial readings to determine the impact toughness seen below.

From these values we see that the C1095 steel exhibited only brittle fracture, while both the

C1018 and the C4140 steels showed a transition from ductile failures to brittle failures. To

determine the percent ductility of the fracture, we examined the specimens after testing under a

microscope, and compared their broken surfaces to those in Figure 4 to get a numerical value.

Impact

Toughness (22.6C)

Percent Ductility (22.6C)

Impact Toughness

(0.1C)

Percent Ductility (0.1C)

Impact Toughness (-80.0C)

Percent Ductility (-80.0C)

C1018 Blue HR 132 ft-lb* 70%* 134 ft-lb* 50%* 10 ft-lb 0% C4140 Green CF 73 ft-lb 60% 44 ft-lb 50% 9 ft-lb 0%

C1095 CFW1 4 ft-lb 0% 2 ft-lb 0% 1 ft-lb 0%

Table 1: Impact Toughness and Percent Ductility Results *Specimens were not completely fractured by the testing apparatus

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Disscussion________________________________________________________

As seen in our results, the C1095 CFW1 steel was 0% ductile at all three test

temperatures, while C1018 HR and C4140 CF steels both showed a transition from greater than

50% ductility to 0% ductility. At 0 C, both the C1018 and the C4140 specimens exhibited 50%

ductility, indicating that 0 C is or is close to the transition temperature of each of the metals.

This means that, in practical applications, if the metal reaches a temperature of 0 C or lower, a

failure will be much more likely to occur as the decrease in ductility lessens the amount of

energy that can be absorbed before fracture. While static tests would show some changes in

strength and ductility, only impact tests will show the abrupt change in susceptibility to sudden

loading and the development of more velocity-sensitive behaviors. The more comprehensive

view of the changes in behavior that occur at different temperatures is the advantage of impact

tests over static testing.

Unfortunately, we were unable to find standard Charpy values for our metal specimens.

We did find standard Izod values for our metals; however, these values were not for a range of

temperatures, and, thus, could not be accurately converted to either verify or contradict our test

results. While we could not compare our results, we do know that error may have occurred in

many different places. In the procedure used, we deviated from the ASTM E23 guidelines by

neglecting to place the tongs used to transport the chilled samples in the cold bath for five

minutes before their use. Mechanically, the specimens may not have been precisely centered due

to human error. Thermally, we may have exceeded our goal time of five seconds from removal

of the specimen to testing, which could have caused significant changes in the temperatures of

the samples. There are also general sources, such as friction within the pivot bearing, air

resistance, frictional resistance of the pointer, and the kinetic energy transmitted to the specimen;

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while we did try to account for the first three of these by adjusting our values by the result of a

test run without a specimen, there may have been variation. Also, any variations or impurities in

the samples themselves could affect the results and prevent them from matching published

values.

Conclusion_________________________________________________________

In conclusion, our results were consistent with reasonably expected behaviors for steels.

Two of the steels tested (C1018 and C4140) exhibited the typical transition from ductility to

brittle fracture, while the third (C1095) was brittle at all temperatures we tested. However, as a

large range of results would lie within the expected ranges of different types of steels,

considering that composition, heat-treating, and many other factors result in a wide variety of

possible results, this may not be a significant verification of our work. Whether or not we

accurately determined the Charpy values for our specimens, it stands that we could not determine

an absolute impact resistance of a specimen via our tests, which provide only comparative results

and not clear, universal values.

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References_________________________________________________________

ASTM Standard E 23-07, Standard Test Methods for Notched Bar Impact Testing of Metallic

Materials, West Conshohocken, PA: ASTM International, 2007.

Davis, Harmer Elmer, G. Hauck, and G. Troxell. The Testing of Engineering Materials. Boston:

Mcgraw-Hill College, 1982.

Siddiqui, Faruq. Mechanics of Solids: Impact Test. Swarthmore College, 2008.

Elgun, Serdar. "Impact Test." 20

October 2008.

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Appendix A: Raw Lab Data and Specimen Pictures____________________________

C1095 cold finished carbon steel:

22.6C 0.1C -80.0C C4140 cold finished alloy steel:

22.6C 0.1C -80.0C

C1018 hot-rolled alloy steel:

22.6C 0.1C -80.0C

Dial Reading (22.6C) Dial Reading

(0.1C) Dial Reading

(-80.0C)

C1018 Blue HR 136 ft-lb (unbroken) 138 ft-lb (unbroken) 14 ft-lb (broken)

C4140 Green CF 77 ft-lb (broken) 48 ft-lb (broken) 13 ft-lb (broken)

C1095 CFW1 8 ft-lb (broken) 6 ft-lb (broken) 5 ft-lb (broken)

Table 2: Raw Lab Data

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Appendix B: Lab Handout______________________________________________

ENGR 059 MECHANICS OF SOLIDS FMAS

IMPACT TEST

Experiment 5

Purpose To introduce the student to impact-testing equipment and procedures. In this experiment we will determine relative impact resistance of steel, in the form of notched-bar Charpy specimens. Preparatory Reading

Class handout ASTM Standard E23 (Please read this prior to lab. Copy the shear fracture chart) Testing of Engineering Materials, Troxell et al. (Chapter on Impact tests). Optional Reading: Metals Handbook 9th Ed (red vols.) Vol. 1 pp.689-701, Vol. 8 pp. 261-268.

Equipment

Universal pendulum impact tester; standard Charpy and Izod impact specimens; micrometers, ruler, weighing scale; low-temperature bath.

Instructions

1. Meaasure the lateral dimensions of the specimens at a full section and at the notch. Weigh the specimens and place some of them into the bath. 2. Note the operation of the test on a sample specimen. CAUTION: USE EXTREME CARE WITH THIS MACHINE. DO NOT STAY IN THE PATH OF THE PENDULUM AT ANY TIME UNLESS IT IS LOCKED OR SECURED. Note the type, model and capacity of machine used. Note the frictional losses (if any) by using the machine without a specimen and letting the pendulum swing freely. Note the different striking hammers and use the appropriate hammer for the appropriate test. 3. Place a specimen accurately in position on the anvils. Note the temperature of the specimen. Raise the pendulum to its upper position and let it fall to rupture the specimen. 4. Study and note the shape of the fractured surface, its inclination with respect to the axis of the piece, its texture, and its relation with respect to the notch. Use the chart in E23 to determine percentage of shear fracture area. 5. Repeat with other specimens.

Report:

1. Draw a neat sketch of the experimental setup. 2. Explain briefly the principles of the test and the term "transition temperature" in the theory section 4. Compare published values of tests with your lab data. Find the percentage of the broken surface idue to fracture and shear. 5. Discuss the significance and advantages of impact tests compared with static tests. 6. Can the absolute impact resistance of a specimen be determined by the test procedure used? Explain.