Selecting A Non-Contacting Extensometer for 1D, 2D or 3D Strain Measurement

Non-contacting extensometers provide precise strain measurement without having to place the extensometer on the specimen. They are ideal for fragile specimens, those susceptible to localized damage and specimens that release large amounts of stored energy during breaks. These extensometers work by using lasers or cameras to measure visual changes in the specimen.

Given options that can accommodate everything from 1D axial strain to those that can handle 3D digital image correlation, how do you decide which non-contacting strain measurement device is best for your budget and application?

Considerations When Choosing A Non-Contacting Extensometer

Specimen Material

The material type influences the degree of precision required, and therefore the type of lens needed. Materials that experience less than 10% strain, such as some metals or composites, need to have a more precise lens to capture the movement than materials that experience greater than 10% strain, such as elastomers or plastics.

Specimen Quantity & Variety

When testing many specimens, a non-contacting extensometer will increase testing efficiency because you will not have to detach and re-attach the extensometer to each specimen, allowing you to complete more tests in less time. Even if you are just measuring axial strain, a non-contacting option like the AOX Optical Extensometer will save time. It is always on and always measuring, so it doesn’t have to be started and stopped for each specimen.

Most clip-on extensometers can accommodate a single gage length, but a non-contacting solution will allow you to choose various gage lengths, giving you the ability to use one measurement device for many specimen types.

Specimen Geometry & Variability

Non-standard specimen shapes can deform and fail in unique ways, so digital image correlation (DIC) will give you the most accurate data when measuring one-of-a-kind or non-standard specimens.

Composites are good candidates for non-contacting strain measurement because they do not deform consistently across the specimen or from specimen to specimen. Non-contacting extensometers will provide more accurate data and some models will allow for crack growth measurement as well. Switching to a non-contacting extensometer could save time in test setup too as marking a specimen takes far less time than gluing on strain gages. 

Field of View Requirements

How much does your extensometer need to capture? The extensometer’s field of view should be large enough to cover the specimen gage length and the expected expansion of the specimen.

Non-ambient Temperatures

There are special considerations for testing in non-ambient conditions. For example, when using a non-contacting extensometer with an environmental chamber, it is critical that the working distance is large enough so that the camera can be positioned in front of the chamber window when the chamber door is closed. There are also best practices for lighting and specimen marking among other things. Please contact us for more details on using non-contacting extensometers when testing at non-ambient temperatures.

Future Growth Expectations

Perhaps the types of specimen you are testing today require a less robust strain measurement solution, but what are your plans for future testing? Will you be testing more unusual specimen or a higher volume of specimen? How soon will you need additional capabilities?

Non-Contacting Strain Measurement Solutions