lab report

ABSTRACT

The report describe the results obtained from a tensile test. This was in determination of the ultimate tensile strength of both metals and polymeric materials. It is common knowledge that materials have certain unique properties but assigning the exact values to them requires a well outlined laboratory procedure. The materials under testing were; steel, aluminum, high density polyethylene, and low density polyethylene. The results obtained were analyzed and presented in graphical form.

INTRODUCTION

The Ultimate Tensile strength of a material is the maximum amount of stress that a given material can tolerate when pulling forces are applied at both ends, without failing. Ultimate Tensile strength is differentiated from compressive strength in that the former is acted upon by forces that pull a material outwards on both sides while the latter is acted upon by forces that push a material inwards. (David, 2008) Tensile strength is determined by using a cylindrical sample of known length and cross sectional area and subjecting it to tensile forces in a tensile machine. Most commonly used tensile machines include: lab master z-direction tensile tester and universal tester. (David, 2008) When the tests are carried out, the stress-strain relationships is easily determined hence making it easy to deduce the ultimate tensile strength of the material specimen and its young’s modulus.

When designing for rigid structures, the properties of various construction materials need to be determined. (David, 2008)The reason being materials are subject to external forces when used in construction of structures. Different materials have different properties due to differences in their molecular structure. This fact is evident in the case of metals and polymeric samples which exhibit different physical and chemical properties due to variances in molecular structure and in extension atomic composition. This makes them react differently to when subjected to external forces. Metal are generally stronger than polymers hence used in area which experience heavy loading in a structure. In order for one to find the most suitable material for a particular project, material property needs to be known, with the most natural property being tensile strength.

Steel and aluminum are the most commonly used metals in construction projects. They have both high electrical and thermal conductivity, relatively high densities, high melting points, and both high ultimate and yield strength. (Gordon, 1976)Steel is mostly used together with concrete to form rigid structure such as buildings and dams. Aluminum is mostly used in construction of planes, electric cables and engine blocks. Polymeric materials have both low electrical and thermal conductivity, relatively low densities, low melting points and both low ultimate and yield strength. Low density polyethylene is used in making of playground slides, six pack rings, and plastic wraps. High density polyethylene is used in making of fuel tanks for vehicles, 3-D printer filaments, and swimming pool installations. (David, 2008)

THEORETICAL

After carrying out tensile tests, the data obtained needs to be analyzed. This is achieved by use of engineering formulae related to stress and strain. They include:

Formulae

Where;

Where;

Where;

EXPERIMENTAL

Equipment

1. Vernier calipers.

2. Tensile testing machine.

3. Computer (installed with DOS).

PROCEDURE

1. Visual inspection of the sample was done to identify possible flaws.

2. The width, thickness, and length of the metal and polymeric samples were measured.

3. The machine’s computer was calibrated with the results obtained in procedure two and prepared to receive data and present it graphical form.

4. Steel specimen was placed in the tensile machine and clamped in place.

5. The machine was activated and force and elongation readings displayed were noted and recorded.

6. Procedure two, three, and four was repeated for aluminum, low density polyethylene, and high density polyethylene specimens.

7. The data was analyzed by and presented in graphical form.

RESULTS

DISCUSSION

Materials behave differently when acted upon by external forces. Some deform more than others while some fracture almost immediately. This behavior is governed by the properties each material possesses. In order to determine these properties, a series of tests is carried out each material. An example of such a test is the tensile test. Specimens of known dimensions are tested and in this case their thickness were constant at 0.12 inches. The materials whose properties were to be determined were steel, aluminum, high and low density polyethylene. The first two are known as metals while the last two are known as polymeric.

When steel was subjected to the test, the ultimate tensile stress was found to be 23 ksi while that of Aluminum was found to be 4.6 ksi. Though both of them are metals, they possess different tensile strength due to variation in the constitution and atomic structures. However both experience are reasonable length of linear relationship in stress-strain graphs. When high density polyethylene was subjected to test, its ultimate tensile strength was found to 0.896 ksi while that of low density polyethylene was found to be 0.494 ksi. The difference is brought about by the difference in intermolecular forces. High density polyethylene has a higher tensile strength as compare to low density polyethylene, because the former has strong intermolecular forces due to little branching while the latter has weak intermolecular forces.

During testing, it is found that both the metals and polymeric materials undergo necking beyond a certain elongation. The necking is caused by localized and increased flow in the length of the sample when the maximum true stress is reached. It is at a point when the localized flow cannot be reimbursed by further strain hardening. The polymeric materials further experience drawing. It is phenomenon where the neck spans the full length of the specimen. The cross section areas of the specimens affect the ultimate tensile strength of materials but their lengths do not. It is due to increase in interplanar bond between adjacent cross sections. These bonds are stronger in specimens with large cross section areas. This is the reason for keeping the thickness constant.

However, the values obtained in this experiment were significantly smaller as compare to the standard values. This could be as a result of human error during the experiment.

CONCLUSION

In conclusion, it is found that metals have higher ultimate strengths as compared to polymeric materials. The order of strength in descending order is steel, aluminum, high density polyethylene, and low density polyethylene respectively. The shear graph for the materials turned out as expected. They obeyed hook’s law at first and when the yield strength was reached, they formed a curve to signify permanent deformation. The necking of the metals occurred as anticipated with the outer region showing signs of shear failure and the middle region showing signs of tensile failure.

REFERENCES Anderson, T. (1991). Fracture Mechanics: Fundamentals and Applications. Boca Raton: CRC Press. David, R. (2008). Mechanical Properties of Materials. New York: CRC press. Gordon, J. (1976). The New Science of Strong Materials. Princeton University Press. Hertzberg, R. (1976). Deformation and Fracture Mechanics of Engineering Materials. New York: Wiley Publishers. Knott, J. (1973). Fundamentals of Fracture Mechanics. New York: Halsted Press.