Tensile Testing of Materials Assignment

Assignment Task

Aim

The aim of this part is to characterize various specimens by means of the tensile test.

Introduction

There are various tests to characterise the properties of a material and they are classified in two main categories: destructive and non-destructive tests. The latter are mainly utilized to check for defects whereas the former is used to obtain the material strength and stiffness. Amongst the destructive material tests, the uniaxial tensile test is the most important of them all (see Figure 1).

A tensile test is conducted by means of a specialised machine, called the tensile testing machine . In such machines, a controlled loading of the sample is possible. The deformation of the sample is its response to the applied load. During the test, the values of the force and deformation are continuously recorded. Engineering stress ( σ ) can be calculated using the recorded forces:

where F is the instantaneous load applied perpendicular to the specimen cross section in units of newtons, A 0 is the original cross-sectional area before any load is applied (m ² ). The units of engineering stress is pascal (Pa). Engineering strain (ε) is the change in dimension per unit length. Engineering strain can be calculated by knowing the deformation at every instance.

Where is the original length the gauge length before any loading is the instantaneous length and is the deformation elongation or change in length at some instant as referenced to the original length. The percentage of strain is the engineering strain times 100.

During the test, the applied force and the displacement, which is the response of the sample to the applied load, is constantly measured. The tensile testing machine to be used is depicted in Figure 2.

How a tensile test works

The force is measured by the load cell, located at the lower part of the machine whereas the deformation of the gauge length is measured by an extensometer (see Figure 2a). Namely, the deformation is measured along a specific length in the middle of the tensile sample. The measured load is displayed by the force gauge. The load cell is integrated into the machine but the extensometer must be attached to the specimen exactly on the marked gauge length. Once the extensometer is installed, the setup is ready to be used (see Figure 4).

The data acquisition unit collects the force and deformation readings and exports them to a PC. This is done by means of two USB connectors and an intermediary measurement amplifier (see Figure 5). The components of the data acquisition unit is provided by National Instruments which also includes the LabVIEW software (see Figure 6). The button starts the data capture; however, a holding time has to be granted until the initial transient oscillation effects are set. Thus, a green indicator will indicate the start of the tensile test.

The tensile test is performed by gradually loading the sample by constantly turning the handwheel clockwise, until the tensile sample fails. Try to apply a constant strain rate by smoothly and continuously turning the handwheel.

The data recording process must be stop only after the failure of the sample. In the next step, remove the extensometer and finally, remove the fractured sample. Restore the initial distance between the clamping jaws to make it ready for further tests.

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