Designed to measure specimen strain in a single element of the surface parallel to the axis of the specimen, rebar extensometers use hardened, pointed screw clamps to positively connect the instrument to hardened reinforcing bar specimens. These extensometers should be removed before specimen fracture, as the violent fracture of rebar will cause damage to the instrument.
Rebar extensometers are engineered to conform to ASTM E 83, and they use a linear variable displacement transformer (LVDT) to measure strain. Rebar extensometers use a pivot and lever arm system to transfer axial deformation of the specimen to linear motion of the LVDT. The length of the lever arm mechanically magnifies any elongation of the specimen, thereby providing extremely accurate measurements of specimen strain.
Principle of Operation
When in place, the extensometer is in contact with the specimen at four points: two gage screws connected to the lower measuring yoke, and two gage screws connected to the upper pivoting yoke. The lower yoke pivots axially in the direction of specimen deformation to mechanically transmit specimen strain to the LVDT. The upper yoke pivots laterally to provide a mechanically averaged measurement of specimen deformation.
When tension is applied to the specimen, strain is registered by the lower yoke. This yoke pivots about a fixed point to push on the core of the LVDT. The LVDT converts the mechanical motion of its core to a proportional electrical signal. This signal is then sent to the controller via the instruments connector.
Extensometers are equipped with connectors that interface with current Instron controller platforms. All connectors are equipped with auto-calibration, and have a user code to uniquely identify the instrument on the system being used. Extensometers can also be equipped with connections to interface with Satec legacy controllers, such as MATS II, Apex, Vertex, Mark III, and Mark IV.