Tensile Test Report

Purpose

To determine the ultimate tensile strength of the material To determine the ductility of the material To determine the elasticity of the material

Equipment

Materials Testing Machine

This machine is used to test the tensile and compressive properties of materials by applying a load to a specimen of the material until it breaks.

Extensometer

This is a device used to measure the change in length of an object. During a tensile test it is used to measure the elongation of the material being tested and feeds this information back to a computer.

Tensile Specimen (Test Piece)

This is a sample of the material being tested. It has two shoulders and a gauge (section) in between. The shoulders are large so they can be readily gripped, whereas the gauge section has a smaller cross-section so that the deformation and failure can occur in this area

Computer

The computer receives data from the testing machine and extensometer and processes this in to a load/extension graph.

Procedure

A sample of material (test piece) with known dimensions such as length and cross sectional area is placed between the grips of the testing machine, which clamp on to the grip section at each end of the material. An extensometer is also attatched to the test piece which is used to test the elongation of the material. The machine is then switched on and will apply a steadily increasing tensile force. During this time the extensometer will measure the change in length of the material and feed this information back to the computer. The results of which are used to form a graph showing the relationship between the load(force) applied and extension of the material.

A tensile force being applied to a test piece

Results/Observations

From the resultant graph I was able to identify the following:

Limit of proportionality

This is the limit at which the load is directly proportional to the elongation of the material and thus obeys Hooke’s Law. Past this limit the graph will no longer show a straight line. For the brass test piece this occurred at around 7200N and is indicated on the graph.

Elastic Limit

The elastic limit is the highest load which a material can withstand without permanent deformation occurring. From this point on the material begins to react more ‘plastically’ to any increase in load. For the brass test piece this occurred at around 7400N and is indicated on the graph.

Waisting

When the test continues beyond the elastic limit of the material, waisting occurs. This is when the extension of the material becomes localized as indicated by the image below. This began to occur with

the brass test piece when its extension reached around 6.5mm and is indicated on the graph.

Tensile Strength

The tensile strength is the maximum load that the material can bear. After this is reached waisting will occur and the load needed for the material to extend is lessened. For the brass test piece the tensile strength was around 8600N.

Fracture

This is the separation of the material. in to two pieces when it has reached breaking stress. This occurred for the brass test piece when it had reached an extension of around 9.5mm and is indicated on the graph.

Ultimate Tensile Strength

This is calculated by dividing the maximum load the material was able to sustain by the cross sectional area of the test piece. The results of this are shown below:

Diameter of test piece = 5.05mm

Radius = Diameter/2 = 2.525mm

Cross Sectional Area = r²π = x2.525²π =20.03mm²

Ultimate Tensile Strength = Max Load/Cross Sectional Area = 8600/20.03 = 429.36 N/mm² (2 d.p)