Lab 1: Tension Test

Andrew Thiher

2/17/2012

Introduction:

In this lab, we tested the material properties of 836 cold rolled steel, T351-2024 aluminum, 110 copper, grey cast iron, and high-density polyethylene (HDPE) by subjecting each material to a tensile force until it fractured. Using data gathered, we graphed the stress-strain graph and determined specific properties for each material. These properties included whether the specimen was ductile or brittle, yield stress, and tensile strength. For equations used, see appendix 2.

Experimental Apparatus:

Each material was prepared for the experiment by marking the initial length as two inches by means of a gage punch and hammer. Additionally, calipers were used to measure the initial diameter. The sample was then screwed into the tensile testing machine and an extensometer was attached to measure elongation and collect data. The extensometer remained attached until the material reached its proportional limit, at this point it was removed, and the remaining data was collected using machinist scales prepared in .05 inch increments. Each time the gauge length reached the preset machinist scale increment, the live load from the computer was recorded. This process was repeated until the specimen fractured. Once the specimen fractured, the final gauge length and smallest diameter in the necking region were recorded.

Calculations:

[pic]

|Property |Cold Steel |110 Copper |T351-2024 Al |Cast Iron |HDPE (Plastic) |
|Modulus of Elasticity (E) |1.17*10^7 |7.75*10^6 |6.59*10^6 |3.48*10^7 |-1.04*10^9 |
|Yield Stress (psi) |87502 |41185 |34372 |41185 |N/A* |
|Ultimate Stress (lbs) |19061 |10234 |13166 |10234 |N/A* |
|% Elongation |23.125 |18.3 |26.335 |2.0 |N/A* |
|% Area Reduction |60.89 |66.46 |24.16 |66.46 |N/A* |

*Many properties for plastic were unavailable because the plastic specimen did not fracture during the experiment.

Equations used in appendix 2.

Ductile or Brittle Analysis:

Cold Steel proved to be quite ductile. It did not fail under low amount of stress, elongated under tension, it had significant necking before fracture, and it had the highest ultimate stress of all the materials tested. All these things are characteristics of a ductile material.

Copper and Aluminum also demonstrated ductile behavior because they had high ultimate stresses, necking before fracture, and they elongated under tension.

Plastic was also a ductile material. As tension was applied the plastic specimen elongated and the diameter decreased. The material didn’t fracture during the test.

Cast Iron was the only material that demonstrated characteristics of a brittle material. Elongation during the test was very small, there was no measureable necking, and the specimen failed abruptly and much more quickly than the other materials tested.

Results:

Of the materials tested, steel had the greatest yield strength. The steel, aluminum, and copper all behaved similarly under tension. Each one had high yield strength, and they each underwent elongation and necking before they fractured. Cast iron was the lone material that was not ductile. When subjected to tension it broke abruptly, with very little to no elongation or necking. The plastic specimen was unique in that it did not fracture during the experiment. When subjected to tension the material elongated and decreased in diameter greatly until it simply pulled apart. The yield strength of a material is important in engineering design because anytime something is being created or built, it is important to choose the appropriate material for the job. Knowing how a material acts under tension or compression is important to choosing a material to build with. If a material with a yield strength that is too low is selected, it could lead to failure.

The tensile strength was highest in steel, followed by aluminum and copper. Cast iron had the lowest tensile strength of the metals, and plastic was the lowest overall. A low tensile strength for plastic was expected. The significance of tensile strength is similar to that of yield strength. One would not want to build something that will be under high tension or compression with a material, such as plastic, with a very low tensile strength. To do so would very likely cause failure of the structure.

The Modulus of Elasticity for the four metals tested was relatively close for each of the four metals tested. Of the metals, cast iron had the highest Modulus of Elasticity, followed by steel, then copper, with aluminum narrowly having the lowest score. Plastic had the highest Modulus of Elasticity. This can be attributed to the rapid elongation that the specimen underwent while under tension. The higher the Modulus of Elasticity, the more stress it takes to attain the same level of strain in a material. This means that a given material can sustain greater forces without permanently deforming than a material with a lower Modulus of Elasticity.

Plastic was easily the most ductile material that was tested. It never fractured and elongated greatly. All the other metals also proved to be ductile in nature except for cast iron. The cast iron was a brittle material because it fractured quickly, unexpectedly, and with very little elongation or necking. Ductility vs. brittleness is worth noting because it is always important to know how a material reacts under tension. Choosing between a ductile or brittle material gives engineers freedom to customize their product to fit their needs.

Plastic had the most unique surface in that because there was no fracture, the two ends had virtually no diameter. From this we could infer, if we had not seen the failure occur, that the material had simply been pulled apart, leaving the ends string like. Steel, aluminum, and copper all had sharp, jagged edges and necking had occurred in each. This type of surface is indicative of a ductile material. For images of the fracture surface, see appendix 1.

Conclusion:

Tension tests were performed on five different materials: steel, cast iron, aluminum, copper, and plastic. Each specimen was subjected to a tensile force until failure. After testing, a number of observations can be made. First, cast iron was the only material that proved to be brittle, the rest were ductile. Plastic was unique in that it did not fracture like the other materials did. Aluminum could sustain the highest load and also had the highest tensile strength and yield strength.

Knowing these kinds of properties for materials is very important to engineering design. Through this knowledge, engineering designers can choose the ideal material for their work. It gives them the flexibility and wherewithal to produce the best product possible. For objects that will be undergoing high stress, choosing a material such as aluminum or steel would be ideal due to their high tensile strength. But if the project called for a lighter material that did not need to sustain high amounts of pressure or tensile force, plastic would likely be more ideal.

References:

Hibbeler, R. C., Mechanics of Materials 8th ed. Prentice Hall, Person, Boston, 2011

Callister, W.D., Jr., Materials Science and Engineering: An Introduction. 4th ed. New
York:Wiley, 1997. See Sections 6.2, 6.3, 6.5, 6.6.

Phillips, James W., ed. Behavior of Engineering Materials: Laboratory Notes 1996-1997
(TAM 224/CE210). Urbana-Champaign: University of Illinois/College of Engineering,
1996. See Lab 5: The Tension Test.

Appendix 1:

*All images have been deleted to reduce size

Figure 1.1: Steel

Figure 1.2: Aluminum

Figure 1.3: Copper

Figure 1.4: Plastic

Appendix 2:

2.1: Modulus of Elasticity: E = Normal Stress/Normal Strain

2.2: Elongation: Elongation = (Length Final – Length Initial)/Length Initial

2.3: Reduction of Area: RA = (Area Initial – Area Final)/Area Initial

2.4: Ultimate Stress: Ultimate Stress = Normal Stress*Area Initial

2.5: Tensile Strength: Tensile Strength = Force/Area

2.6: Yield Stress: Yield Stress = E*(Normal Strain + .002)

Data Sheet for Specimens:

Material Length and Diameter

|Material: |Steel |Cast Iron |Plastic |Copper |Aluminum |
|Length Initial |2 |2 |2.007 |2 |2 |
|Length Final |2.423 |2.004 |9.8 |2.364 |2.460 |
|Diameter Initial |.502 |.5025 |.503 |.5025 |.503 |
|Diameter Final |.3140 |.291 |0 |.291 |.438 |

*all measurements in inches

Material Load

|Material: |Steel |Cast Iron |Plastic |Copper |Aluminum |
|Load Initial |0 |0 |0 |0 |0 |
|Load Final |19484 |10413 |1806 |10413 |15450 |

*all measurements in lbf