Deformation measurement is a very important point in the testing of mechanical material properties, it is related to many very important indicators, such as: B. Modulus of elasticity, elastic range, strain, stress at constant strain, strain at constant stress, bending deflection, compression, etc. At present, electronic tensile testing machines have three methods of measuring deformation: displacement, extensometer, and large deformation. They each have their own characteristics, and their scope and purpose are different. Put simply:
(a) Displacement: Mainly used for compression testing, bending testing and deformation of film.
(b) Extensometer: In the early days there were mainly mirror extensometers, lever extensometers, butterfly extensometers (see figure below); Due to the limited measurement resolution of the mechanical structure strain gage, it is difficult to achieve ideal measurement performance, and most current testing machines are generally equipped with differential transformer type extensometers and resistance extensometers. The extensometer is mainly used to test the material's tensile modulus of elasticity, fracture toughness KIC test and the elongation of the hard plastic at a small elongation. Two-dimensional extensometers are added that can be used for n-value and R-value testing of sheet metal: The HiAdding double ended extensometer is mainly used to accurately measure elastic modulus and can be used for strain or tension testing. Some extensometers are shown in the figure below.
c) Large Deformation: The large deformation extensometer is mainly used to test the strain, constant strain stress, constant stress strain and other parameters of soft plastic or rubber specimen.
d) According to the electronic tensile testing machine, the deformation measurement of the electronic torsion testing machine The device mainly uses a rotary encoder to measure the number of revolutions of the rotating shaft of the testing machine and the change in torsion angle generated by the specimen. for torsion meter The variation in torsion angle in the range of (0-12)0 produced by the measurement sample.
e) Non-Contact Extensometer
In recent years, non-contact extensometer products suitable for high temperature conditions and deformation detection of rubber materials have been released. Non-contact extensometers are mainly used in tests with high test requirements where contact extensometers cannot be installed, such as: B. High temperature testing of metals and tensile testing of foils. The main feature of the non-contact extensometer is that it can eliminate the stress concentration and stress error of the extensometer, it can perform continuous strain measurement for the entire test (including the rupture range), the initial gauge length can be set arbitrarily, and the corresponding size of the gauge lengthn error does not occur, no influence, accurate and reliable measurement results.