Extensometers

An extensometer should be used whenever accurate strain measurement is required that cannot be reliably inferred from crosshead displacement alone. Crosshead displacement includes contributions from machine compliance, grip seating, and load train deflection — all of which can significantly overestimate true specimen strain, particularly in stiff materials or at low strain values. Extensometers are essential for determining modulus of elasticity, yield strength, and other strain-dependent properties, and are required by most material test standards including ISO 6892-1, ASTM E8/E8M, and ISO 527. As a general rule, if your test result depends on an accurate stress-strain curve, an extensometer should be used.

Gauge length is the defined reference distance between the two contact points of an extensometer on an unstrained specimen. Strain is calculated as the change in that length divided by the original gauge length. Many test standards specify a required gauge length relative to specimen dimensions — for example, ISO 6892-1 typically uses a proportional gauge length of 5.65√S₀. Selecting the correct gauge length for your specimen and standard is essential for producing compliant, comparable results.

Whether to remove an extensometer before fracture depends on the material and how it fails. For ductile materials with gradual failure, the extensometer can generally remain on through fracture. For brittle or high-strength materials that fail explosively, the energy released at break can damage the device — in these cases removal before failure is recommended. With standard clip-on extensometers this typically requires pausing the test to remove the device manually before proceeding. The AutoX750 and AutoXBiax can be configured to retract automatically at a defined threshold, removing the need to interrupt the test, and video extensometers require no removal as they maintain no physical contact with the specimen.

Rubber and elastomers undergo very large deformations — often 500–1000% strain — which rules out most standard clip-on extensometers due to their limited travel range. For these materials, a video extensometer is a popular choice, tracking two dots applied to the specimen surface optically with no physical contact, no influence on material behavior, and no risk of sensor damage through to fracture. Instron's XL Long Travel extensometer is also a viable option, offering up to 750mm of mechanical travel, though it is a more straightforward solution compared to the flexibility and measurement range a video extensometer provides. Both approaches are consistent with ISO 37 and ASTM D412 requirements for tensile testing of vulcanized rubber and elastomers.

Extensometers are calibrated and classified to ISO 9513 (internationally) or ASTM E83 (US), which define accuracy classes — such as Class 0.5, Class 1, and Class 2 — specifying permissible errors across the measurement range. The class required for your test is typically specified by the relevant material test standard. For example, ISO 6892-1 requires Class 1 or better for most applications. Instron extensometers are supplied with calibration certificates traceable to national standards, with recalibration services available to maintain ongoing compliance.

An axial extensometer measures strain along the loading axis — the primary direction of deformation in a tensile or compression test. A transverse extensometer measures strain perpendicular to the loading axis, capturing the reduction in width or diameter as the specimen is stretched. Used together, axial and transverse measurements allow calculation of Poisson's ratio — the ratio of lateral to axial strain. Instron offers several ways to capture both simultaneously in a single test: dedicated transverse clip-on extensometers paired with an axial device, biaxial and averaging clip-on configurations, the AutoXBiax automatic contacting extensometer, and AVE3 video extensometer, which can measure axial and transverse strain optically without any physical contact with the specimen.

An automatic contacting extensometer attaches to and detaches from the specimen automatically under software control, eliminating manual handling between tests. It is the ideal choice for high-throughput production testing environments and automated test cells where minimizing cycle time, reducing operator variability, and maintaining consistent gauge length positioning are priorities. Instron's AutoX750 and AutoXBiax are compatible with a wide range of universal and static hydraulic testing systems and fully integrate with Bluehill software for seamless test control.

It depends on the extensometer type. Clip-on extensometers are typically fixed to a specific gauge length — labs testing to multiple standards or specimen geometries will generally need separate devices for each gauge length. Video extensometers are the most flexible option, capable of measuring across a wide range of gauge lengths, elongations, and material types without any hardware changes — the device optically tracks two dots applied directly to the specimen surface, so gauge length is simply determined by dot placement. The AutoX750 supports automatic gauge length positioning within its travel range, making it well suited to labs testing a variety of specimen dimensions without manual adjustment between tests.

Both devices measure strain, but they work in fundamentally different ways. A strain gauge is a small resistive element bonded directly to the specimen surface — as the material deforms, the gauge deforms with it, changing its electrical resistance in proportion to strain. Proper installation requires a skilled technician: surface preparation, precise adhesive application, and correct wiring are all critical to getting reliable data. An extensometer is a separate instrument that measures the change in distance between two defined points on the specimen, either through mechanical contact or optical tracking.

Strain gauges are well suited to measuring strain at a specific point on a structure or complex geometry. Extensometers, however, are the standard choice for materials testing where a defined gauge length, easy setup between tests, and compliance with test standards such as ISO 9513 and ASTM E83 are required — without the preparation time or specialist expertise that strain gauges demand.

The most common sources of extensometer error include incorrect gauge length setting, inconsistent knife-edge placement on the specimen, and slippage of the contact points during the test — all of which introduce variability into strain measurements. For clip-on extensometers, insufficient or excessive clamping force can cause the device to slip or influence the specimen's behavior. Misalignment between the extensometer and the loading axis can introduce bending errors, particularly in compression testing — averaging extensometers are specifically designed to compensate for this. Environmental conditions, particularly temperature fluctuation and airflow in the vicinity of the specimen, can affect image stability and introduce error in video extensometer measurements. Instron's AVE3 video extensometer incorporates integrated lighting and airflow management technology designed to reduce the impact of these variables on measurement accuracy. Regular calibration to ISO 9513 or ASTM E83 is the most effective way to identify and correct systematic errors.

Extensometers measure engineering strain — the change in gauge length divided by the original gauge length. Engineering strain is the standard output required by most material test standards, including ISO 6892-1 and ASTM E8/E8M, and is sufficient for most materials testing applications where deformations are relatively small. True strain, which accounts for the continuously changing gauge length as the specimen deforms, can be calculated from engineering strain values in post-processing if required. For large deformation testing — such as elastomers or highly ductile metals — the difference between engineering and true strain becomes more significant and should be considered when interpreting results.