Why do your materials break? Tensile testing: inspecting the breaking mechanisms of materials with SEM

By Luigi Raspolini - Sep 27, 2018

Tensile testing is a commonly-used analysis that provides information on the resilience of an object and how much resistance it can offer to traction or compression. Such tests can be performed on a large variety of materials and provide useful information to speculate on the behavior of a material when it undergoes a stress. The main purpose of the tensile test is to evaluate relevant parameters (like the Young's modulus, for example) or to study the how shear stress affects the material. This allows researchers to create models and design better materials. But how can you see what is happening? A scanning electron microscope (SEM) with tensile testing capabilities can provide you with that information.

Tensile testing in a SEM

Tensile testing, in fact, helps to predict when a material will break, but it does not provide information on why or how this happens, unless the test is performed in/under a microscope. Clearly, more information can be retrieved when the resolution of the microscope increases. The combination of a tensile testing unit with an electron microscope can therefore provide unprecedented results and offer a deeper understanding of how a material breaks and what can be done to improve its performance.

Tensile testing in SEM is largely used, for example, to perform cyclical tests on polymers. These tests consist of pulling the material and consequently releasing it several time to evaluate how it wears out over usage.

The tensile testing procedure in SEM works as follows: a small portion of the material is cut into a dog-bone shape, loaded into the tensile unit where it is firmly clamped to two moving jaws, and finally loaded in the SEM. Depending on the material properties, the test can last for a quite long period of time.

The image below shows the results of a cyclic test on a rubber band. The sample was brought from the initial (relaxed) position, to an extension of 1mm. Subsequently it was stretched to 9mm and then relaxed to 1mm again. The last sequence was repeated three times.

Blog tensile testing

Figure 1: The result of a cyclic tension test on a rubber band.

Traction testing of packaging materials

Another typical application of tensile testing is in the development of packaging materials. These materials are usually made with different layers positioned one on top of the other. Every layer has a specific function, and all together guarantee a set of properties that are required for the material function.

In the video below, the packaging of a milk carton was sampled and loaded into a tensile testing machine for a traction test. The test was recorded (a set of images was sequentially collected and put together in a video) and the breaking mechanism is shown here.

The second video shows the other side of the material while the test is being performed.



Specifications of tensile testing equipment

Harder materials, like metals (aluminum, steel), can also be tested. The holders can in fact apply a force as high as 1kN and the tensile test can be performed both inside and outside of the scanning electron microscope using the same holder.The accuracy of the analysis depends on the force resolution of the holder, which is the sensitivity of the holder to changes in the force which is applied to the sample. Typically, the value lays around 1% of the maximum force the holder can apply.

Do you want to learn more about how to perform tensile testing in a Phenom XL scanning electron microscope? Or what kind of results the system can provide? Download the Tensile Sample Holder spec sheet here:

Download the Tensile Holder specification sheet

 


About the author

Luigi Raspolini is an Application Engineer at Thermo Fisher Scientific, the world leader in serving science. Luigi is constantly looking for new approaches to materials characterization, surface roughness measurements and composition analysis. He is passionate about improving user experiences and demonstrating the best way to image every kind of sample.

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