This laboratory will study the effect of the environment on the material properties, i.e., the effect of temperature on the material behavior. Depending on the set up, the effect of loading rate could also be demonstrated.

The properties of material may vary with other factors such as temperature, loading rate and environment in general. The temperature and the loading rate have opposite affects on the material properties. The modulus of a material may increase as the temperature lowers yet increases as the loading rate rises. This implies that a design for a particular loading rate or temperature may not work for other conditions.

A way to study the varying properties was by performing impact testing such as Izod, Charpy, or tension impact. If fitted with instrumentation, these tests could yield the material properties just like their slower rate counterpart. However, this does not mean that a more common, non-instrumented version of the impact testing could not yield any useful information. Some of the quantities we are able to determine from the tests are the impact energy, the shear fracture or cleavage fracture of the specimens, and the lateral expansion of the specimens.

If we look at the energy equations, be it the potential or the kinetic energy, we can see that the energy depends on mass. Giving the same material at the different size the amount of mass will be different from one size to the other. The results of the impact test then would be dependent on size of the specimen. They are then, not the material properties in a sense that they are not a constant for the particular material.

• cold bottle for liquid Nitrogen (Dewer)Sp?
• thermocouple
• plastic containers
• hot plate
• thong
• beaker or flasks
• calipers
• shear fracture chart

Balance the potential energy and the kinetic energy to calculate the initial impact velocity. Don't need to know the mass since it will drop out of the equation.

The Charpy Impact and the Izod Impact tests, ASTM E-23, are similar to 3-point bending and cantilever beam tests where the specimen is subjected to a single loading that increases from zero to failure. However, the name "Impact" implies that the load is applied at a higher rate than their common counterparts. Another difference is that the impact test specimen usually has a notch presence to create a multiaxial stress state at the notch root.

There are many extensions to the ASTM standard concerning the instrumentation of the equipment. They are done in order to extract other information such as strain rate history, energy absorption history, and load-time history from the test. The non-instrumented impact test by itself, however, could yield useful information as well. Some of the quantities obtainable from the non-instrumented test are:

1. C_V: The energy absorbed by the specimen during impact.
2. % Shear Fracture: The percentage of the fracture surface that failed in shear.
3. % Lateral Expansion: The percentage change in width of the specimen.

As is probably apparent, the above quantities are not directly applicable for use in design calculations. In addition these values are not independent of specimen size. However, they do enable a comparison among different materials or even effects of the same material subjected to different pre-treatment.

One of the effect often sought after is the effect of temperature on the impact tolerance of a notched material. A transition zone exists in many materials. Above which the impact energy, C_V, is relatively high and then drops across the transition zone. If many impact tests are performed at relatively close temperature increments, it is possible to construct graphs of C_V, % shear fracture, and % lateral expansion as a function of temperature. The transition zone of that material can then be identify. Instead of reporting a range of temperature for the transition zone, a single Transition Temperature for a material in a particular configuration often specified by the one of the following conditions:

1. Temperature at which a certain energy is absorbed during the test. Values of 10 or 15 ft-lbs are commonly used for metals.
2. Temperature at which the fracture surface is 50% "fibrous."
3. Temperature at which a specified lateral expansion (commonly 1%) occurs.

Tasks:

1. Test the Charpy Impact specimens at different temperatures. Note the specimen configuration and pretreatment. Record, measure, and estimate the C_V, lateral expansion, and % shear fracture for each specimen. Also make a careful observation of the fracture surfaces.
2. Calculate % lateral expansion for each specimen.
3. Knowing the drop height of the hammer head, calculate the velocity of the impact head, V_o, at the time of impact. How does the loading rate compare to the 3-point bending or tension test you performed in previous labs?
4. Summarize the quantities you determined (C_V, % shear fracture, % lateral expansion) in your report. Choose the formats that will maximize the clarity of the information you are trying to present.
5. Are there any direct correlations among the different quantities you determined? (Do they yield the same kind of information?)
6. Many empirical relationships had been suggested concerning the loading rate, size, impact energy, etc. Rolfe suggested that the Charpy impact energy (in-lb), the yield strength (psi), and the specimen thickness (inch) are related by the relation:^[1]
   

        (1)

   Instead of σ_Y linearly related to C_V, Wells suggest a different expression:^[2]
   

        (2)

   For both expressions, plot the yield strength, σ_Y, as a function of changing temperature for the corresponding C_V. Comment on the trend of the yield strength as the temperature change. Is it what you expected? Are these two equations valid?
7. Comment on the effect of the size of the specimen. What will happen to the amount of energy the specimen can absorb as the size increases?
8. Discuss how might the specimen preparation (machining, pretreatment, etc.) affect the results. Think of the effect of possible directionality of the grains in the material, the size of the notch, etc.
9. What is your best guess at that the transition temperature is based on the 15 ft-lbs energy absorption criteria? Discuss the impact of this temperature on design.
10. What does the presence of a transition region mean? Identify the range of temperatures where the transition might have occurred. Explain how might the material behave differently across this region.
11. Discuss the advantages and disadvantages of the Charpy Impact test. Also comment on the variability and the uncertainty involve.

References

1. Rolfe, S. T., Gensamer, M., and Barsom, J. M., Fracture-Toughness Requirements for Steel, Presented at the First Annual Offshore Technology Conference, Houston, Texas, 19 to 21 May 1969.
2. Wells, A. A., Fracture Control of Thick Steels for Pressure Vessels, The British Welding Journal, Vol. 15, No. 5, 1968, pp. 221-229

Drop height = 55 inch.