Tensile Testing

Standard tensile tests are run on "dog bone" shaped specimens to obtain Young's modulus (stiffness), yield point, and/or strength. Prior to the test, one must prepare the specimen. Thus, we can consider the test as consisting of two separate parts:

1. Preparing (strain gaging) the specimen

2. Testing the specimen

Strain Gages are simple devices used to determine the strain in a specimen. Lets define strain.

The following figure shows the increase in length of the specimen due to a applied tensile load. Let the increase in length be x. Then strain is defined as the change in length divided by the original length.


Dogbone Specimen

Strain = x /L

where L is the original length of the specimen.

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Strain gage

The strain gage converts the increase in length into electrical signals and this is received by the data acquisition system and stored on a personal computer.

Testing

Testing is a common method to understand the way a structure behaves under applied load and to verify the predictions of structural behavior.

This figure illustrates a typical tension test dogbone specimen under a tensile load, applied by a servo hydraulic testing machine. The testing machine is used to apply controlled loads to test specimens and is capable of varying the speed of load application. It can also accurately measure the forces and elongations of the dogbone specimen during the test.
 
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The data acquisition system takes the data from the testing machine and converts the data points into graphs that help the viewer understand the tensile process. For example, this figure shows plots from a tensile test that gives the user an idea between the relationship between the load, deflection, and strain.