Science of Engineering Materials

Equipment required:

Charpy impact testing machine

Several constant temperature baths

Thermometers(thermocouples)

Materials required:

Low carbon steel Charpy impact test specimens

Ice, liquid nitrogen

Suggested reading:
Textbook - Pg. 178-184,199-203

Introduction

The strength of a material is the critical stress required to initiate failure. Strength is not only mechanical parameter that determines the usefulness of a material for a specific application. Some glasses are as strong as steel, but nobody would recommend the use of glass for car bumpers! The reason is that steel can absorb a large amount of energy before it fails whereas glass can not. The parameter describing the ability of a material to absorb energy is toughness.

The ultimate mechanical failure is fracture. We commonly categorize fracture as being either ductile or brittle. Little energy is required to fracture brittle materials, such as glass, polystyrene, and some of the cast iron. Conversely, tough materials, such as rubber and many steels, absorb considerable amounts of energy in the fracture process.

Brittle fracture requires energy to separate atoms and expose new surfaces along the fracture path. Ductile failure requires not only the energy just mentioned but much more additional energy to deform plastically the material ahead of the fracture.

One measure of the toughness is the area of the stress-strain curve. The unit for toughness are therefore the product of stress and strain:

i.e. energy per unit volume. In reality, the energy consumption is very nonuniform within the fractured test piece; negligible is absorbed in non-deformed regions, while a major quantity is absorbed in the vicinity of the fracture. Furthermore, this distribution is strongly affected by the size and shape of the test specimen. Notches are especially critical in determining the energy requirements for fracture. A notch is machined in the side of the test specimen and serves as a stress concentrator, ensuring that fracture will occur at that location.

Many materials show an abrupt drop in ductility and toughness as the temperature is lowered. In glass and other amorphous (i.e.. non-crystalline) materials, this change corresponds to the glass-transition temperature. Metals are crystalline but they may instead exhibit a dutile-brittle transition temperature, Tdt, that divides a lower temperature regime where the fracture is said to be nonductile from a higher temperature range where considerable plastic deformation accompanies failure.

BCC metals such as low carbon steel, become brittle at low temperature or at very high strain rates. FCC metals, however, generally remain ductile at low temperatures. Almost any metal may be made brittle by the presence of inclusions or dissolved gasses, by the neutron irradiation, or by the presence of brittle second phase particles along the grain boundaries. Many polymers become brittle at low temperature and exhibit a glass transition temperature much like that observed in glasses.

In metals, plastic deformation at room temperature occurs by dislocation motion. The stress required to move a dislocation depends on the atomic bonding, crystal structure, and obstacles such as solute atoms, grain boundaries., precipitate particles and other dislocations. If the stress to move dislocation too high, the metal will fail instead by the propagation of cracks that is the failure will be brittle. THUS, EITHER PLASTIC FLOW OR CRACK PROPAGATION WILL OCCUR, DEPENDING ON WHICH PROCESS REQUIRES THE SMALLER APPLIED STRESS. In BCC metals, the flow stress reduced, whereas the crack propagation stress is relatively independent of temperature. Thus the mode of failure changes from plastic flow at high temperature to brittle fracture at low temperature. In FCC metals, dislocation movement remains high at low temperature. and the material remains relatively ductile.

The mechanism of the ductile-brittle transition in polymers is quite different. Above the glass transition temperature, Tg , polymers can exhibit either viscous flow or highly elastic behavior because thermal activation ensures that molecular mobility is high (primary bond rotation and breaking/reforming of secondary bonds). Below Tg the molecular configuration is "frozen in" and brittle fracture occurs by crack propagation.

The test principle is straightforward. A charpy V-notch test specimen is placed on an anvil in the path of a pendulum. When the pendulum hits the specimen, the material is subjected to elastic deformation, plastic deformation, and finally, fracture is rapid succession. The pendulum transform its potential energy to kinetic energy and back to potential energy again while swinging back and forth. Its energy is only potential when it is at its highest point, h1, before it is released. At its highest point the potential energy is

Ep = mgh1

Where m is the mass of the hammer at the end of the pendulum, g is the acceleration due to graviation and h is the vertical distance between the highest and the lowest position of the pendulum. When the pendulum swings through and breaks the specimen it will lose part of its energy, and consequently swing up to a lesser height, h2. The energy E, used to fracture the specimen is therefore

E = mg (h1-h2).

The scale on the impact machine is calibrated in such a way that the energy expended in the fracture process can be read directly form the scale.

Reference.

Background information on this experiment can be found in the following books:

J. F. Shackelford, Introduction to Materials Science for Engineering, 2nd. Ed, Chapter 7, Section 3.

L. H. Van Vlack, Elements of Materials Science and Engineering, 6th. Ed.,Chapter 8, Section 4.

W. F. Smith, Principles of Materials Science and Engineering, Chapter 6, Section 9.

THE EXPERIMENT

The objective of this experiment is to determine the ductile-brittle transition temperature, Tdt, for the low carbon steel and to give you an appreciation for the large decrease in toughness that occur in certain materials when the temperature is reduced. You will; therefore determine the impact energy for the type of specimen provided, at different temperature. The following temperature baths should be prepared:

Hot water both (about 100 °C)

melting ice (0 °C)

liquid nitrogen (-195.8 °C)

Place two specimens in each bath and allow it to come to the temperature of the bath before testing.

BE VERY CAREFUL WITH THE LIQUID NITROGEN. CONTACT WITH LIQUID NITROGEN OR WITH OBJECTS AT LIQUID NITROGEN TEMPERATURE WILL FREEZE YOUR SKIN INSTANTANEOUSLY AND CAUSE BLISTERS WHICH HEAL VERY SLOWLY.

Move the pendulum of the impact tester to its highest position and ascertain that it is surely locked in this position. Set the plastic pointer to 15 joules (straight down). Now release the pendulum by turning the black knob clockwise.

BE SURE THAT NOTHING, INCLUDING YOUR ARMS AND HANDS, ARE IN THE PATH OF THE PENDULUM. IT CAN EASILY BREAK YOUR ARM!

Stop the pendulum with beak by pushing the red knob up. The pointer on the dial should read zero, because the pendulum will not have lost any of its energy. If it does not read zero contact the TA.

We are now ready to do the actual experiment. The specimens should be transfered from the bath to the anvil as fast as possible, otherwise the temperature of the specimen will change too much (for the low temperatures the specimens will warm up somewhat anyway). Use tongs to transfer the specimen and let the tongs' temperature equilibrate in the bath before removing the specimen. The specimen should be placed on the anvil with its notch on the side opposite where the hammer will hit, and the notch should be in the middle between the supports. Release the pendulum as soon as possible. Read the amount of energy lost in the fracturing process and inspect and describe the fracture surface. The surface should change from dull for ductile fracture, to shiny for brittle fracture. Explain this behavior in your report.

For your report you should plot the impact energy as a function of temperature and draw a smooth curve through your data points. Look at note at bottom of lab for the data. (warning:there may be substantial scatter in your data and you should see a TA if you are not sure how to draw the best curve through your data points). From this curve you should determine Tdt. Several different definitions are used for the transition temperature. We define it as the temperature for which the impact energy is halfway between the impact energy for the highest temperature and the impact energy for the lowest temperature. This definition is usually expressed as the temperature corresponding to the average energy of the upper and lower shelf values.

Do the following demonstration experiment and describe in your report what you observe. Take the rubber tubing and hit the end with the heavy object (hammer). Cool the end of the tubing in liquid nitrogen. Be sure to let the temperature equilibrate by keeping the end long enough in the nitrogen to stop the violent boiling. Now hit the cold rubber again. Be sure to wear safety glasses for this phase of the experiment and warn the persons standing around you. Hit the rubber again after it has warmed up.

CLEAN UP AFTER YOUR EXPERIMENT. ASK THE TA HOW TO DISPOSE OF THE LIQUIDS IN THE TEMPERATURE BATHS.