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Impact Testing

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Author: Michael Smetana

Objective

                The objective of this experiment is determine the functionality of the IZOD impact testing machine, and determine the Stress that a notched piece of low density carbon steel could handle under the load before it bends and becomes plastic under sheer load.

Materials Used

                The materials used were four (4) 3.2mm x 3.2mm pieces of Carbon Steel Key stock (original length were about one meter),Ruler, Needle Nose Pliers, a table vice, hacksaw, Triangular File, and an IZOD impact testing machine built by Baldwin.

 Types Of Impact Testers

Background

                There are many different types of testing machines for this simple test that was preformed.  The type of machine used and that machine focused on was the IZOD Impact Testing Machine.  Among the other machines that could have chosen to complete this same project is the Charpy or the Gardner machines.  Each of those machines do quite possibility the same type of examination of the carbon steel and determine the load that these materials can take a high rate of speed.

                The fracture of a material is different when tested at a low strain rate then it is compared to a high strain rate.  If a high strain rate or Intense Blow is impacting a material it is more likely to behave brittle or become more of a plastically deformed material.  Loading impacted suddenly will not allow the material to have time for its molecules to align and slide past each other, thus causing a sudden brittle fracture.

                The testing of how sudden impact affects different materials can change the way something is built.  In harsh or very cold environments different materials need to be used to make the same structure that someone could build and use in the warm sunny areas.  Temperature is a big factor because materials generally have a breaking point and that breaking point is significantly reduces when the molecules inside that structure are slowed down by the Freezing Arctic Winters or even the Snow Storms as they separate the roads up in the North.  These are the reasons to effectively use and pick the correct materials based on different locations and the strict loading that will be put on them.  How will the metal on the front bumper of your car perform when you are in a car accident?  Impact testing gives a good insight to these types of questions.

Procedure

                The first step of this project was to create the sample and strike in the impact tester.  For this some Low density Carbon Steel Key stock was cut it into lengths approximately 80-87mm in length.  The Key stock was approximately 3.2mm x 3.2mm in Width and Depth.  To cut the steel there was a Vice and a Hacksaw.  As most teams continued to try to use the hacksaw to cut this steel our team found a pair of Needle Nose Pliers which had a Cutting indentation section.  It was found that if squeezing fairly hard and slightly bending either end of the Key stock it will break at the cutting point.  After cutting 4 pieces of stock our then was required to notch each piece at approximately one third to one fourth of the way through the sample.  Our Team first started by measuring approximately 20-24mm from one end of the sample and making a mark with a pencil.  Then started a small grove with the hacksaw and competed the filing with the Triangular File until the Notch was about half way through the Steel and it had created a "V" Shape Cut.  After pushing the file back and forth across the key stock it was found to be more consistent and less time consuming to slide the key stock across the file.  It seems it was easier to get a better hold onto the smaller lighter pieces, and by doing it this way made the process go much faster. 

                After each cut was made in all four pieces our team marked each sample with a pencil, Sample one with one line, Sample two with two lines, up to sample four.  Then our team measured each sample and wrote down the results for the position of the cut and the overall length of the sample.  (Had this lab actually been a real test for strength we would need all of these information including the height the hammer hit the sample and the position the sample was held in the machine.)

                The next step in the process was to Error set the IZOD impact testing machine.  To do this our team basically Dry ran the machine.  Meaning the machine was run without a sample to determine the loss created by air, bearings, or other random parts of the machine.

                Then after error testing the IZOD Impact Testing machine all for samples one at a time was put in the machine to determine the amount of force, in foot pounds, that the machine exerted to bend or Break the sample.  For all four samples data was recorded.

Results

Bar Number

Length of Bar

Position of Notch

Impact Force Reading

Error of Machine

1

87mm

23mm

1.987 ft-lbs

0.59 ft-lbs

2

84mm

22mm

1.68 ft-lbs

0.59 ft-lbs

3

83mm

23mm

1.82 ft-lbs

0.59 ft-lbs

4

83mm

23mm

1.70 ft-lbs

0.59 ft-lbs

 

                In order to properly get the units for this amount of impact it is required to take the impact force reading then subtract the error, multiply by eight and then convert to Joules.  The results are below.

Bar Number

Reading - Error X 8 = Energy Required  Ft-lbs

Energy Required Joules

1

11.397

15.15

2

8.72

11.82

3

9.84

13.34

4

8.88

12.04

 

Energy Absorbed Durring Impact Testing

Discussion

                Since our team kept track of the length of each piece of key stock inserted in the IZOD Impact Tester, it was found that there has been no direct correlation between the over all length of the material and how fast it fractured.  To relate the material length to fracture more information, such as the striking distance and the height each piece of key stock was sitting in the Impact Tester, would be required. 

I believe that the amount of force required to fracture each piece of Carbon Steel was dependant on the depth of the Notch filed into the material.  Since the distance of the notch was always about the same from the striking point and we had no way of actually recording this we would have no way of telling if the energy required was greater if closer or farther from the notch.  Since the distance was so close to the impact point it and each test was approximately the same we can assume that the notch played the largest role in the energy required.  Since the first piece of Carbon Steel appeared to be the strongest the first assumption is that the notch was smaller due to human interaction and attempting to rush the first piece.

The purpose for using the IZOD Impact testing machine verses the Charpy or other types of Impact Testing machines is because of the availability of the machine for use in the UCF lab, and it properly demonstrates the Error, and Swing through Techniques of these types of machines.  It seems that the Charpy Impact test is quite like the IZOD Impact test because it seems to operate the same way with a big swinging hammer that come down and hits the sample at a high rate of speed, but instead of keeping the sample Vertical in the vice and swinging through it.  The A machine puts support on both sides of the sample horizontally and strikes in the middle but on the opposite side of the notch.

Since our group was the last group to perform this experiment we were able to look at other groups results such as another groups Error at the beginning of there experiment versus the error at the beginning of our experiment.  It seems the error Varied from 0.44 to 0.59 in only a few Swings.  Meaning the percentage of Error increased by approximately 25% in only 8 to 10 swings.  This means that the error from the beginning to the end of our Testing samples could have increased even more, and this is a possible reason why the Number 1 sample versus the Number 4 samples are so very different.

Conclusion

                In theory the notch in the Carbon Steel would be the greatest factor in determining it's over all strength, where as in our experiment the attempt was to make the notch approximately 1.5mm deep, but it was very difficult to be precise.  The Machines Accuracy was at question when dealing with sample of this size.  Possibly, if larger samples were under load the machines error wouldn't be quite so large.

                In conclusion I enjoyed this lab because it was very hands on, and I learned something that I have never seen before.  I learned how to use and calculate amounts of energy required for failure of a specific metal, and the overall temperature of the environment can greatly affect materials rigidity.


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