Types of Extensometer

 

An extensometer is an instrument that measures line deformation between two points on components and other objects. The main equipment used in tensile testing includes testing machines and extensometers. The testing machine is a system that applies tensile testing force to the sample, and controls and measures it; the extensometer is a system that measures the extension (or displacement) of the sample. During the testing of material mechanical properties, stress and strain are interdependent. Any material, as long as it is subjected to stress, will definitely produce strain; as long as it produces strain, it must be subject to stress.

Mechanical extensometer

Mechanical extensometers are mainly ball hinge extensometers. Due to its simple structure and easy operation, it is still widely used in tensile property testing.
This instrument is mounted on the specimen through 4 top screws (one top screw at the front and one at the front at positions C and D). When the gauge length l of the specimen is extended by Δl, the upper gauge length fork A can be regarded as not rotating. The lower scale fork B rotates a slight angle with the spherical hinge as the center, and the sample elongation is reflected on the dial indicator. Since the distance from the axis of the dial indicator to the center of the ball hinge is twice the distance from the axis of the specimen to the center of the ball hinge, the reading of the dial indicator when the specimen is extended Δl is 2Δl.
The biggest advantage of this instrument is that it has an automatic averaging structure system, and only one reading device can be used to obtain the pure tensile elongation of the sample that eliminates the influence of eccentric tensile bending. The spherical hinge extensometer uses only a dial indicator and the instrument has a magnification of K=2000 times.
To sum up, since eccentric stretching inevitably occurs during the tensile test, the extensometers used to measure the deformation of the tensile specimen must be installed on each symmetrical side of the specimen. Use the average of these two extensometer readings as the pure tensile elongation of the specimen. (Only one extensometer that automatically takes the average value of both sides is installed)

Electronic extensometer

In recent years, with the rapid development of the computer age, another new type of extensometer has emerged—the electronic extensometer. Electronic extensometers include: resistive extensometers, capacitive extensometers, inductive extensometers, etc.
The electronic extensometer collects the strain signal into the computer so that it can be displayed simultaneously with the material stress. Compared with mechanical extensometers, it reduces human reading errors and improves measurement accuracy. Because electronic extensometers are affordable, easy to install, and easy to use online with electronic universal testing machines, electronic sensors, etc., electronic extensometers are widely used by related industries at home and abroad.
Resistive extensometer
Resistive extensometer is a type of resistive strain sensor and is the most widely used. The measurement principle of the resistance strain extensometer is mainly based on the change in resistance of the strain gauge attached to the elastic element of the extensometer. It is converted into a voltage signal through a conversion circuit, and the strain is measured through computer acquisition and stress synchronous display.
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