Overview: The document describes the Model 3550 extensometer, designed for measuring axial and torsional deflections in specimens tested in axial/torsional machines. It is primarily used on round specimens in bi-axial test machines capable of simultaneous axial and torsional loading.

Specifications: The extensometer supports bi-directional displacement, suitable for cyclic testing under fully reversed loading conditions. It is self-supporting on specimens ranging from 0.375 to 1.000 inches (9.5 to 25.4 mm) and features conical point contacts made from tungsten carbide. The design minimizes crosstalk between axes and provides high accuracy measurements, featuring a dual flexure design.

Temperature and Compatibility: The standard temperature range is -40 °C to 100 °C, with options extending from -265 °C to 175 °C. Both inch and metric versions are available, with various gage lengths and measuring ranges.

Features: The extensometer includes a full bridge, 350 ohm strain gaged design compatible with most test systems. It meets ASTM class B-1 and ISO 9513, class 0.5 accuracy requirements, and comes with a high-quality foam-lined case.

Technical Specifications: Recommended excitation is 5 to 10 VDC, with a maximum of 12 VDC or VAC. The output is 2 to 4 mV/V, with linearity ≤0.15% of the full-scale measuring range. Operating force is typically less than 30 g, and crosstalk is less than 0.5%.

Options: Connectors are available to interface with nearly any brand of test equipment. Additional options include a shunt calibration module and the Model 3590AT axial/torsional calibrator.

Model Variants: Various models are available with different axial gage lengths, axial strain, and strain angles. The document lists specific models with their respective specifications.

Design and Measurement: The extensometer is designed to directly measure shear strain without needing to know the specimen's radius. The relationship between shear strain and the angle of twist is provided, illustrating the variation of the angle of twist with changes in specimen diameter.