University of New Mexico

Civil Engineering Department

Civil Engineering Materials Laboratory, CE 305L

IMPACT TESTS

General

Impact tests provide information on the resistance of a material to sudden fracture where a sharp stress riser or flaw is present. In addition to providing information not available from any other simple mechanical test, these tests are quick and inexpensive. The data obtained from such impact tests is frequently employed for engineering purposes.

Various standard impact tests are widely employed in which notched specimens are broken by a swinging pendulum. The most common tests of this type are the Charpy V-notch test and the Izod test which are described in ASTM E23, Standard Test Methods for Notched Bar Impact Testing of Metallic Materials. Another test method, although not a standard test method, is the tension impact test .

Many materials, including metals, exhibit marked changes in impact energy with temperature. It is known that there tends to be a region of temperatures over which the impact energy increases rapidly from a lower level that may be relatively constant to an upper level that may also be relatively constant. Such temperature-transition behavior is common for metal materials. This temperature dependence for various steel alloys with the same hardness but different carbon contents is graphically shown in Figure 1. This figure shows the impact energy obtained from Charpy V-notch impact specimens as a function of temperature. The temperature-transition behavior is of engineering significance since it aids in comparing materials for use at various temperatures. In general, a material should not be severely loaded at temperatures where it has low impact energy.

These types of impact tests have given way to testing methods that make use of fracture mechanics. Fracture mechanics methods allow more sophisticated analysis of materials containing cracks and sharp notches. However, the advantages of fracture mechanics are achieved at the sacrifice of simplicity and economy. Impact tests such as the Charpy, Izod, and tension impact have thus remained popular despite their shortcomings, as they serve a useful purpose in quickly comparing materials and obtaining general information on their behavior.

Objectives

The objective of this experiment is to evaluate the energy absorbing characteristics of metal materials at room temperature using the Charpy, Izod, and tension impact methods.

Equipment

1. Tinius Olsen “Change-O-Matic” impact testing machine, with Change-O-Matic head for testing:

•  Charpy, simple beam specimens

•  Izod, cantilever specimens

•  Tension Impact specimens

Specimens

1. Charpy Impact Specimens, AISI O1 Tool Steel

   a) Un-Heat Treated (~ 6 HRC)

   b) Heat Treated to (~ 60 HRC)

2. Charpy Impact Specimens, ASTM-A36 Structural Steel

The specimen geometry for the Izod, Charpy, and Tension Impact are shown in Figure 2.

Procedure

Download the procedures for impact tests here

Download Excel spreasheet for Impact data

•  Safety

1. Safety glasses shall be worn by all participants.

•  Prior to performing any operations, such as changing striking bits or placement of specimens, pull the pendulum arm back and engage the safety latch, prohibiting movement of the pendulum arm.

3. Avoid placing fingers in pinch areas.

4. All participants shall remain behind the caution tape during the actual test.

5. Make sure the safety latch is in the clear when raising the pendulum arm into the test position.

6. The test operator shall apply brake upon breakage of the test specimen. All other participants shall remain clear until the brake has brought the pendulum to a complete stop.

The impact tester is dangerous with the potential for bodily injury. Pay attention at all times and observe these safety considerations.

B. Charpy Impact Test

•  Set the latching mechanism in the upper position (264 foot-pound range).

2. Rotate the pendulum and secure with the safety latching mechanism.

•  Select the Charpy striking bit in the Change-O-Matic head; make sure this position is firmly tightened in place.

•  Place the Charpy specimen horizontally across supports with the notch away from the hammer. Make sure the specimen is centered within the anvil jaws (see Figure 3).

•  Slide the indicator pointer to the left until it indicates the maximum energy range on the upper Charpy-Tension scale.

•  Raise the pendulum arm to the right until it is firmly supported by the latching mechanism. Caution: Make sure the safety latch is in the clear when raising the pendulum arm into this test position.

•  Ensure all participants are clear and behind the caution stripe. The test conductor shall then release the pendulum by pushing up on the release knob. The hammer will drop and attain a striking velocity of 16.8 ft/s, striking the specimen, with a swing through dependent on the amount of energy absorbed by the test specimen. The indicator will move and stop when peak swing through is registered, providing a direct reading of the energy absorbed by the specimen. Read the indicated value from the Charpy scale and record.

•  Apply the brake until the pendulum has returned to its stable hanging vertical position.

•  Remove the specimen from the testing area and observe the failure surface.

Leave pendulum in the down hanging vertical position until another test is to be performed.

View a Quicktime Movie of an impact test

Required

1. Report impact energy for all specimens tested and ambient temperature conditions.

2. Calculate an average impact energy value for each specimen type evaluated. Report this value to the nearest 1ft•lb.

•  Calculate the standard deviation for each specimen type evaluated. Report this value to the nearest 1ft•lb.

•  Calculate the coefficient of variation for each specimen type evaluated. Report to the nearest 1%.

•  Compare your results to known values and discuss.

•  Discuss the ductile nature of A36 steel in the context of its relative value compared to other impact specimens evaluated. Also, discuss its ductile behavior relative to ductility measurements obtained from previously performed tension tests.

References

1. _______, Standard Test Methods for Notched Bar Impact Testing of Metallic Materials , ASTM Designation: E 23, American Society for Testing and Materials, West Conshohocken, PA, Vol. 03.01.

2. N.E. Dowling, Mechanical Behavior of Materials , 2^nd Ed., Prentice Hall, 1999.

3. _______, Change-O-Matic Impact Testing Machine , Instruction Booklet No. 65-2-2, Tinius Olsen Testing Machine Company, Willow Grove, PA.

Figure 1. Temperature Dependence of Charpy V-Notch Impact Resistance for Different Alloys Hardened to HRC 34 (N.E. Dowling).

Figure 2. Izod, Charpy and Impact Specimens (Tinius Olsen).

Figure 3. Specimens and Loading Configurations for (a) Charpy V-Notch and (b) Izod Tests (per ASTM E 23).

DISCUSSION OF RESULTS

•  Discuss the relative toughness and hardness values obtained for all materials tested.

•  Describe the fracture surface of the different materials tested.