The Blue and Maize Impact Tester
Shaun Eiler
Impact tests can be performed on almost all kinds of materials. Different types of materials will have different impact strengths. For instance, steel will most likely have a greater impact strength then what a piece of wood would have. The main purpose of an impact test is to determine the energy absorbed by a specimen when it is fractured by an impact load (Kazanas, 1979, pg. 298). Toughness is the property that is usually associated when talking about impact tests. Toughness is defined as the capacity of a material to resist fracture under impact loads (Kazanas, 1979, pg. 298). An example of this would be an impact test on plexiglass as compared to regular glass. It takes much more energy to fracture the plexiglass then what it would to fracture the regular glass.
Most of the specimens that are tested are notched. This allows there to be a brittle fracture. A bigger notch will allow the specimen to be broken more easily then if there is little to no notch at all. There are several other things that affect the impact strength of a material. One is temperature and the other is the velocity at which the impact force is come at. The hotter the temperature is, the tougher the material will be. The colder the temperature, the more fragile the material will be. The velocity of the impact force also plays a huge role in how much impact strength a material will have. If the impact force is coming at a high velocity, then the specimen being tested will be broken much easier. If the velocity is low, then the specimen will be harder to break, if it even breaks at all. This could be compared to hitting a baseball. Assuming the ball is coming in at the same speed, if the batter swings the bat hard, the ball will go much further then if the batter just stuck the bat out and made contact with the ball.
There are three different types of impact tests; torsion, tension, and beam. In torsion impact tests, the specimen is subjected to a combination of twisting and impact stresses. Torsion impact tests are rarely used (Kazanas, 1979, pg. 300). In tension impact tests, the specimen is subjected to a combination of tension and impact stresses. Tension impact tests are used to some extent but have not been standardized (Kazanas, 1979, pg. 300).
The most common type of impact test is the beam impact test. There are two different types of beam tests; Izod and Charpy. In the Izod test, the specimen loaded as a cantilever-beam, and is struck on the notch side (Kazanas, 1979, pg. 300). In the Charpy test, the specimen is loaded as a simple-beam and is struck on the side opposite the notch (Kazanas, 1979, pg. 300).
The most common type of impact testing machine is the Universal Impact Testing Machine. The following is a list of the major features of a universal impact testing machine:
· The hammer mounted on the free end of the pendulum arm to provide the energy required to fracture the specimen (Kazanas, 1979, pg. 300).
· The anvil and the vise to support the specimen mounted on the base of the machine (Kazanas, 1979, pg. 300).
· The energy measuring device which indicates the energy absorbed by the specimen after fracture (Kazanas, 1979, pg. 300).
Universal impact testing machines are designed to fracture the specimen with a single blow and to indicate directly the energy absorbed by the specimen (Kazanas, 1979, pg. 300). This energy is the difference between the energy remaining in the hammer after the blow and the total energy of the hammer before the blow (Kazanas, 1979, pg. 300). Foot-pounds or inches-pounds are the two most commonly used units used in measuring the impact force. Universal impact testing machines provide energy limits from 0 to above 300 ft-lb. and striking velocities from 0 to above 20 feet per second (ft. per sec.) (Kazanas, 1979, pg. 301). Different sizes of hammers are used to increase or decrease the amount of force. There is usually a light hammer and a heavy hammer and these are brought to either a low point or a high point. The high position and heavy hammer are used with specimens of very tough materials requiring above 100 ft-lb of energy to fracture (Kazanas, 1979, pg. 301). The low position and light hammer are used with specimens of materials requiring less then 100 ft-lb of energy to fracture (Kazanas, 1979, pg. 301). According to ASTM standards, the report of an impact test should include the following (Kazanas, 1979, pg. 302):
· Type of machine
· Type of specimen
· Striking velocity
· Energy lost to friction and energy of the blow which struck the specimen
· Temperature of the specimen
· Energy absorbed by the specimen (the actual impact value of the specimen)
· Type of fracture
· Number of specimens failing to break at the impact value
Information for this section was found at[1]
Equipment
and Material List
Equipment-
Ruler
Scribe
Dye
Band Saw
Grinder
Wire Brush
Arc Welder
Chop Saw
2 Cans of Spray Paint
2 Bolts
4 Washers
4 Nuts
Bar Clamps
C-clamps
Drill Press
½ bit
Masking Tape
Paper Patterns (flames and “M”)
Material-
6.5’ x 2-15/16” of steel was the primary material used
13” x 10-7/8” steel base
Procedures
of Testing
Three tests were performed. Each
test was done on a 2” x 1/8” sample of cherry.
The needle of the gauge started out at 3-1/16” from the base. This was considered the “0” point. Once the test was performed, the needle was again measured
and then the original starting point of the needle was subtracted from this new
number. This gave the impact number
on my scale. Then the average of
the three samples was taken. For
each test the hammer was brought up to the stop bar and then released.
This ensured that the hammer would have the same velocity for each test
that was performed.
Demonstration
of Testing
The wood sample is first placed into the vise. Two bolts with nuts then tighten the samples down. The zero point is measured to ensure that the measuring arm is at the same place for each test.
The sample must be tightened down so that it does not move around. This also ensures that the hammer will make contact just above the notch. This will help to make the sample easier to break.
Next, the hammer is brought up to the stop bar where it is then released.
The samples are then broken. The following picture shows what each sample looked like once it was broken, and how exactly it was broken.
Data
Recording Sheet
Sample 1-
Size: 2” x 1/8”
Starting Point: 3-1/16”
Finishing Point: 5-7/8”
Impact Strength: 2-13/16
Sample 2-
Size: 2” x 1/8”
Starting Point: 3-1/16”
Finishing Point: 6”
Impact Strength: 2-15/16
Sample 3-
Size: 2” x 1/8”
Starting Point: 3-1/16”
Finishing Point: 5-1/2”
Impact Strength: 2-7/16
Average of the three samples: 2-35/48
Note:
Sample two had the highest impact strength of the three samples.
Important
Terms
Impact Test- The main purpose of an impact test is to determine the energy absorbed by a specimen when it is fractured by an impact load (Kazanas, 1979, pg. 298).
Toughness- is defined as the capacity of a material to resist fracture under impact loads (Kazanas, 1979, pg. 298).
Izod test- the specimen loaded as a cantilever-beam, and is struck on the notch side (Kazanas, 1979, pg. 300)
Charpy test- the specimen is loaded as a simple-beam and is struck on the side opposite the notch (Kazanas, 1979, pg. 300).
Required
Formulas
Finished Height – Starting Height = Impact strength
3 Finishing Heights added together then divided by 3 = Average for the 3 samples
Bibliography
Kazanas, H. C. Technology of Industrial Materials. Chas. A. Bennet Co., INC: Illinois. 1979.
Follow
Up Sheet
1)
What is the difference between an Izod impact test, and a Charpy impact
test?
2)
What are some variables that would have an impact on the data collected
from an impact test?
3)
What do you think the results would be of an impact test if a piece of
pine, cherry, and maple were tested?
4)
What is the main purpose of performing an impact test?
5)
Describe the reason for having a stop bar that controls how high the
hammer can be brought up to.
Follow
Up Sheet With Answers
1)
What is the difference between an Izod impact test, and a Charpy impact
test?
In the Izod test, the specimen loaded as a cantilever-beam, and is struck on the notch side (Kazanas, 1979, pg. 300). In the Charpy test, the specimen is loaded as a simple-beam and is struck on the side opposite the notch (Kazanas, 1979, pg. 300).
2)
What are some variables that would have an impact on the data collected
from an impact test?
The type of wood being tested is one variable. A hard wood is going to have a higher impact strength then a soft wood. The weight of the hammer is another factor. How tight the vise is can also cause problems. If the vise is too loose then the sample could be harder to break and this would throw the numbers off.
3)
What do you think the results would be of an impact test if a piece of
pine, cherry, and maple were tested?
The maple is the hardest wood of the three. It would probably have the highest impact strength. The cherry would be next in hardness. The pine is the softest wood of the group, therefore, it would have the lowest impact strength.
4)
What is the main purpose of performing an impact test?
The main purpose of an impact test is to determine the energy absorbed by a specimen when it is fractured by an impact load (Kazanas, 1979, pg. 298).
5)
Describe the reason for having a stop bar that controls how high the
hammer can be brought up to.
How high the hammer is brought up determines the velocity at which it will strike the samples. If the hammer is brought up to a low point and released it may not even break the sample. If it is brought up to a very high point then it will break the sample easier. Having a stop bar ensures that the hammer will be brought up to the same point and also strike the sample with the same velocity each time.