Designed to measure specimen strain in a single element of the surface parallel to the axis of the specimen, snap-on extensometers utilize a clamp spring and squeeze plates to provide the user with easy one-handed operation. These extensometers should be removed before specimen fracture.
Snap-on extensometers are engineered to conform to ASTM E83, and they use a linear variable displacement transformer (LVDT) to measure strain. Snap-on extensometers utilize a bell crank to translate deformation of the specimen to the LVDT. An electrical output is provided by the LVDT that is proportional to the mechanical travel.
Principle of Operation
When in place, the extensometer is in contact with the specimen at four points: the fixed knife edge, the moveable knife edge, and the two clamping edges of the back support. When testing round specimens, the back support has clamping edges that are vee-shaped, and when testing flat specimens, the back support has clamping edges that are rounded. For durability, knife edges are hardened to at least HRC 58.
When tension is applied to the specimen, strain is registered by the moveable knife edge. This knife edge is fastened to the bell crank. As the moveable knife edge travels with specimen elongation, it moves the bell crank and the LVDT core. The LVDT converts the mechanical motion of the core to a proportional electrical signal. This signal is then sent to the controller via the connector. For the T1M, T3M, and T3M Mill extensometer models, the moveable knife edge can be interchanged between a straight knife edge, which is typically used for round specimens, or a conical point, which is typically used for flat specimens.
Extensometers are equipped with connectors that interface with current Instron controllers. All connectors are equipped with auto-calibration, and most have an auto-identification feature that allows the controller to recognize the extensometer that is being used. Instruments not equipped with the auto-identification feature can be set up with user codes for convenient use of the instrument. Extensometers can also be equipped with connections to interface with Satec legacy controllers, such as MATS II, Apex, Vertex, Mark III, and Mark IV.