Feel the Strain

9 December 2008 (Last Updated December 9th, 2008 18:30)

Measuring materials and components requires careful selection of an extensometer, but how should this be determined? Zwick's David Phillips examines the benefits of contact and non-contact measuring devices.

Feel the Strain

Mechanical testing on biomaterials, as well as materials and components used in the manufacture of medical devices, raises many challenges for those seeking to measure strain. The diverse applications require multifaceted technical requirements for strain measurement devices and as a result there is no single device which satisfies all needs.

The requirements for an extensometer are determined primarily by the characteristics of the material to be tested. These define the gauge length, accuracy, test sequence and environmental conditions. However, the right choice of extensometer cannot be limited to the basic material characteristic alone. It is must also be decided whether an extensometer can be connected directly to the specimen without influencing the load measurement or causing damage.

"The right choice of extensometer cannot be limited to the basic material characteristic alone."

Very thin specimens, such as latex for gloves or human, animal and artificial biomaterials, can be sensitive to clamping forces. Small wire specimens do not provide enough visible area for reliable non-contact measurements.

A high stiffness in the initial extension range, followed by high plasticity requires more than one extensometer. The first measures small strains (typically up to 5mm) very accurately in the elastic range, and the second measures very high extensions (≥ ³500mm). Transparent specimens or those with smooth surfaces are not suitable for non-contact measurements without using measuring marks.

An important consideration is the behaviour when the specimen fails. Metals and hard plastics will slip through the knife edges of a contact extensometer without damaging them, and rotatable knife edges should be used to reduce the risk of damage. High extension or flexible specimens can damage or destroy the knife edges and even the extensometer due to whiplash, splintering or delamination of specimens. These require non-contact measurement.

Travel, gauge length and accuracy

With contact extensometers, the measurement travel is normally an engineered and fixed value dependent on the range of the measurement transducer and, with fulcrum hinged sensor arms, the leverage ratio. The initial gauge length is set manually with fixed steps or automatically over a defined range.

Non-contact extensometers that use a video camera must have the field of view larger than the required range plus the initial gauge length. Since the specimen portions which are outside the gauge length, and the machine components themselves deform in the direction of loading, the position of the measuring marks on the specimen changes during the test. For extension and/or gauge lengths which are expected to be outside of the field of view, the objective lens must be changed or the distance between the specimen and the video camera must be increased. All these actions decrease the measuring accuracy.

'Accuracy' is a commonly used qualitative term. To qualify the integrity of a measured signal standards use quantitative terms such as 'resolution', 'deviation' or 'uncertainty' and definitive values are given for these. Requirements for the accuracy of extension measurements are normally given in application-specific test requirements and international standards.

"An important consideration is the behaviour when the specimen fails."

Easily assembled devices and automated sequences reduce personnel time and effort, and improve the quality of test results because subjective influences are minimised. High initial acquisition costs can be quickly amortised, especially when the extensometer can be used for a wide range of applications. When using a video extensometer the time and costs for marking the specimen must be considered, as well as any potential human error.

Contact extensometers

Clip-on extensometers are mounted directly onto the specimen. The mechanical parts which transfer extension from the specimen to the internal transducer are short and stiff. There is practically no movement between the specimen and the extensometer, so measurement accuracy is exceptionally high. The range of a clip-on extensometer is limited to a few millimetres and applies a load directly to the specimen. The application and removal is normally manual to minimise errors, but certain products are equipped with motorised application and removal systems (see Figure 1).

Feeler arm extensometers offer automatic operation and a large range with high accuracy. They are suitable for many different applications, such as testing of textile materials like bandages. Precision designs with a smooth and balanced mechanical operation apply minimum loading to the specimen. Since the feeler arms are in contact with both sides of the specimen, bending strains are largely compensated for.

"Feeler arm extensometers offer automatic operation and a large range with high accuracy."

Clip-on and feeler arm extensometers are in direct mechanical contact with the specimen via knife edges perpendicular to the gauge length. The contact force from the knife edges can cause a microscopic indentation into the specimen surface which gives a precisely positioned contact point. Because of direct contact, feeler arm extensometers can be damaged or even destroyed by whiplash.

Non-contact extensometers

The main advantage of non-contact video and laser scanning extensometers is that they can be used without damage even when testing specimens that exhibit whiplash. They require measurement marks to be attached to the specimen which are optically distinct from the surrounding area.

The measurement marks are clipped, tacked or glued onto the specimen, or the specimen is marked with a coloured pen. This introduces additional sources of error as the marks can become indistinct, move or fall off the specimen surface as it deforms during loading. The application of the measurement marks can also introduce higher costs, as well as result inaccuracies. However, for carrying out tests on biomaterials in a water bath, such non-contact extensometry offers benefits not previously possible.

The position of measurement marks on the specimen is evaluated by software algorithms which determine a certain area around an optical centre point. This becomes the gauge length and as the specimen is loaded, the movement of the marks is converted to extension values. Special lighting for surface or background illumination of the specimen optimises the contrast to the measurement mark.

"Non-contact extensometers are required when the specimen is sensitive to notching knife edges or when the extensometer might be damaged."

During the deformation of the specimen, lighting changes and surrounding influences (such as reflections) can influence the optical centre point. This is often the cause of scatter in test results.

Recent developments in laser interferometry have discovered a new breed of extensometer. Able to accurately measure from less than one micrometre to almost one metre, these devices measure strain without contact and without marks. They use the specimen's surface structure as a 'fingerprint' to generate a virtual mark.

Laser light directed on the specimen is reflected in various directions corresponding to the surface structure and creates a specific pattern of speckles. Selected measurement points are constantly followed and converted to direct extension values. The change in the surface structure, which is the basis of the speckle pattern, is continuously evaluated during specimen deformation (see Figure 2).

Wide-ranging application

Contact extensometers measure extension accurately and are cost effective, but clip-on extensometers require much more manual intervention. Feeler arms extensometers offer high accuracy, excellent repeatability and ease of use. Non-contact extensometers are required when the specimen is sensitive to notching knife edges or when the extensometer might be damaged. They are also still relatively expensive and time consuming to set up and calibrate, especially when testing different specimen types.

There is no such device as a universal extensometer. The large range of applications demands various devices with different functions and characteristics, and the extensometer must be selected for each application.