Materials and Science Engineering

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EXPERIMENT 2

NOTCHED BAR IMPACT TESTING OF MATERIALS

Background

Materials sometimes display brittleness which precludes their use in a given design.

Brittleness is characterized by fracturing with low energy under impact. The fracture energy

is proportional to the area under the tensile stress-strain curve and is called the toughness.

Tough steel is generally ductile and requires 100 ft-lbs of energy to cause failure. Brittle steel

does not deform very much during failure and requires less than 15 ft-lbs energy to cause

failure.

Characterizing the toughness of a material is done in several ways. The most common method

is the notched-bar impact test for which two types of specimens prevail, Charpy and Izod.

By subjecting a specimen to an impact load, it will fail if the load exceeds the breaking

strength of the material. By using a swinging pendulum to impart the load, the energy required

to fracture the specimen can be calculated by observing the height the pendulum swings after

fracture, as shown in Figure 2-1.

Figure 2-1: Charpy impact tester, sample positioned in anvil and general testing results.

This test has been used almost exclusively with body-centered-cubic (bcc) crystalline

materials. These materials show a transition from ductile to brittle behavior with temperature

(Figure 2-2). This means that at low temperature the fracture energy is low.

The mechanical behavior of materials often exhibits variations even for seemingly identical

specimens and materials. The steel specimens for this experiment are manufactured from a

single ingot of steel and are machined to a single drawing. Despite these precautions, the

results from identical tests will not always be identical.

Variations in properties are often evaluated by means of statistics to establish the average

value and the possible variance in the results. The amount of shear in the surface failure of

initial heightfinal height

sample

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steels can be determined by looking at the fresh failure surface under low-power magnification

(approx 3X). A smooth surface is characteristic of shear. A fine grained fracture surface is

characteristic of cleavage and brittleness (Figure 2-3). Often failures are mixed (part shear

and part cleavage).

Figure 2-2: Impact Energy transition from ductile to brittle behavior.

If no plastic deformation accompanies fracture, it is generally a brittle fracture, i.e. cleavage.

In the impact test the amount of plastic deformation is characterized by lateral expansion.

Lateral expansion is a thickening of the specimen during fracturing. Looking at half the

failed specimen, the lateral expansion is measured as shown in Figure 2-3.

Figure 2-3: Lateral expansion of Charpy impact specimen.

where lateral expansion = w = wf - wi

wf = final lateral dimension

wi = initial lateral dimension

Technical Approach

The experiment consists of investigating the ductile-to-brittle transition in 1018-steel and the

aluminum alloy (2024 or 6061) as a function of temperature. The Charpy impact specimens of

each material are immersed in a bath for 10 minutes to reach thermal equilibrium at five

separate temperatures ranging from -110 o F (-79

o C) to 212

o F (100

o C). The specimens are

quickly transferred to the Charpy testing machine, fractured, and the impact energy is

measured. After failure the fracture surfaces are examined for evidence of shear or cleavage

failure and lateral expansion at the root of the notch.

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Equipment

Thermocouple

Temperature Baths

Charpy Impact Machine

Tongs

Digital caliper

SAFETY INFORMATION DURING THE COURSE OF THIS EXPERIMENT YOU WILL BE USING A DEWAR OF ACETONE WHICH

HAS BEEN COOLED USING DRY ICE TO THE TEMPERATURE OF DRY ICE (–110 o F or -79

o C). CARE

MUST BE TAKEN TO AVOID EXPOSURE TO SKIN OR EYES. THE EXPOSURE MAY CAUSE BURNS

AND OTHER EFFECTS. STUDENTS ARE ADVISED TO TAKE THE FOLLOWING PRECAUTIONS:

1. Be careful to slowly insert specimens into the baths.

2. Use tongs to insert and remove specimens.

3. Do not drop specimens into bath. (Dewar will break).

4. While transporting the Dewar, use the special cart set up for that purpose.

Procedure

1. Review safety considerations. Label each specimen using a hammer and punch.

2. Identify and measure the initial lateral dimensions of all specimens.

3. Conduct impact tests on 5 as received 1018-steel and five aluminum (2024 or 6061)

specimens heated or cooled to dry ice, antifreeze bath with some dry ice, ice water, boiling

water and room temperature. For consistent results, it is important that all specimens be

positioned identically in the anvil of the Charpy impact tester; use the special tongs provided

to correctly position the specimen. The notch on the specimen must face 180 o opposite to the

hammer.

4. Measure lateral dimensions after impact. Observe the nature of the fracture surface of

steel specimens. The fracture specimens should all be carefully examined and particular

attention should be paid to the type of fracture that is obtained in each particular case. Try to

relate the type of fracture to the energy absorbed by the metal being fractured. A binocular

microscope is available to study these fractures carefully. Look for cleavage and shear and

estimate the fraction area of each.

SAFETY INFORMATION

BEFORE USING THE IMPACT MACHINE, THE INSTRUCTOR WILL SHOW YOU

EXACTLY HOW TO OPERATE IT. ALWAYS TAKE GREAT CARE IN HOW YOU

HANDLE THE MACHINE TO AVOID ACCIDENTS.

If the specimens are transferred rapidly to the machine, it can be assumed that the temperatures

at which they are broken are those of the baths in which they have been held. Desired bath

temperatures are:

dry ice/acetone bath - 110 o F (- 79

o C)

antifreeze/water mix with some dry ice - 40 o F (- 40

o C)

ice/pure water 32 o F (0

o C)

room temperature 72 o F (23

o C)

boiling water 212 o F (100

o C)

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Write up and discussion

Prepare a memo report. Relate the type of fracture observed to the energy absorbed by the metal. Try

to relate the type of fractures observed (shear or cleavage) to the test temperatures. Report the fraction

area of cleavage and shear to nearest 20%. Compare the variation of impact energy with temperature

observed for steel and aluminum.

Plot the results obtained using Excel; graphs should compare (overlay) the 1018-steel and aluminum

(2024 or 6061) samples: 1) impact energies vs temperature

2) changes in width vs temperature

3) shear % vs temperature

Using the attached data from Tables 1 and 2 and the data you collected from testing, calculate the

average value of the fracture energy at each temperature. Graph the average fracture energy versus

temperature for steel and for aluminum. Overlay this with the standard deviation for 13 data

points. The average and standard deviation can be calculated using Excel.

Table 1: temperature and impact energy values for steel.

steel Temp °C

-70 -44 0 25 100

Impact energy (ft-lbs)

5 35 57 78 65

7 32 48 69 83

18 26 55 67 94

14 39 54 68 80

10 27 67 75 95

10 30 63 76 90

5 16 35 76 94

6 16 34 76 103

5 16 61 76 94

6 17 47 66 73

13 19 54 70 85

8 19 41 66 108

Table 2: temperature and impact energy values for aluminum.

Aluminum Temp °C

Impact energy (ft-lbs)

-70 -44 0 25 100

9 10 9 8 8

8 9 8 9 8

9 8 9 8 9

9 8 8 8 9

10 10 10 10 10

9 10 11 10 8

8 7 9 11 11

8 8 7 8 8

11 7 8 7 9

8 7 7 8 7

6 7 7 8 8

7 8 8 8 8

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The Standard Deviation is given by:

where: xi = individual fracture energy value

n = number of values (data points)

Plot the upper and lower values of the standard deviation versus temperature on the chart

with the average value of the fracture energy (one graph for steel, one graph for aluminum). The

additional data from other groups is used to make these calculations.

Glossary of Terms

Understanding the following terms will aid in understanding this experiment:

Body-centered cubic. Common atomic arrangement for metals consisting of eight atoms sitting on the

corners of a cube and a ninth atom at the cubes center.

Brittle. Lacking in deformability.

Cleavage. Brittle fracture along particular crystallographic planes in the grains of the material.

Ductile fracture. Fracture of a material with significant deformation required.

Ductility. The ability of a material to be permanently deformed without breaking when a force is

applied.

Face-centered cubic. Common atomic arrangement for metals consisting of eight atoms sitting on the

corners of a cube and six additional atoms sitting in the center of each face of the cube.

Fracture. Failure or breakage of a material.

Impact energy. The energy required to fracture a standard specimen when the load is suddenly applied.

Impact test. Measures the ability of a material to absorb a sudden application of a load without

breaking. The Charpy test is a commonly used impact test.

Lateral expansion. The lateral change in dimension of a Charpy impact specimen due to fracture. The

dimension measured is the width opposite the v-notch (see Fig. 1-3).

Plastic deformation. Permanent deformation of the material when a load is applied, then removed.

Shear. Deformation due to parallel crystallographic planes.

Shear lip. The surface formed by ductile fracture that is at a 45 o angle to the direction of the applied

stress.

Toughness. A qualitative measure of the impact properties of a material. A material that resists failure

by impact is said to be tough (also given as the total area under the stress-strain curve).

Transition temperature. The temperature below which a material behaves in a brittle manner in an

impact test.

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MSE 227L Name ________________________

Charpy impact testing

Poor Fair Average Good Excellent

Memorandum Format Used 1 2 3 4 5

Spelling, grammar, & punctuation correct 1 2 3 4 5

Report includes: Poor Fair Average Good Excellent

Include table of results (measured values,

averages and standard deviations). 2 4 6 8 10

For aluminum and steel, graph changes in

width vs. test temperature; overlay graphs 2 4 6 8 10

Graph shear lip % vs. test temperature;

overlay graphs for aluminum and steel. 2 4 6 8 10

Graph impact energies vs. test temperature;

overlay graphs for aluminum and steel. 2 4 6 8 10

For steel, graph average fracture energy,

upper and lower standard deviation versus

temperature on the same chart. (3 curves

overlaid on 1 graph)

2 4 6 8 10

For aluminum, graph average fracture

energy, upper and lower standard

deviation versus temperature on the same

chart. (3 curves overlaid on 1 graph)

2 4 6 8 10

Relate the type of fractures observed (e.g.

shear or cleavage) to the test temperatures. 1 2 3 4 5

Compare the variation of impact energy with

temperature observed for 1018-steel and

(2024 or 6061) aluminum. 1 2 3 4 5

Relate type of fracture observed to the

energy absorbed by the metal being

fractured. 1 2 3 4 5

Poor Fair Average Good Excellent

Overall level of effort apparent 1 2 3 4 5

Quality of graphs 1 2 3 4 5

Quality of Abstract 1 2 3 4 5

Quality of work description 1 2 3 4 5

Quality of conclusions 1 2 3 4 5