What is the working principle of a tensile testing machine

Source：www.cshuaqiang.com Release date： 2026-05-07

The core principle of a tensile testing machine is to apply controllable tensile force to the specimen using a servo motor or hydraulic system, while synchronously collecting force and deformation through force sensors and extensometers, and converting mechanical indicators such as strength and modulus according to Hooke's law.
Below are four parts to explain: mechanics foundation, overall workflow, core system, and key indicators:
1、 Fundamentals of Mechanics: Hooke's Law and Strain
Within the elastic range, the material stress (σ) is proportional to the strain (ε): σ=E ·ε (E is the elastic modulus). The tensile machine measures the E and yield strength during the elastic stage, as well as the tensile strength and elongation after fracture.
2、 Whole machine workflow
Sample installation: The upper and lower ends of the sample (such as plastic, metal, fabric) are fixed coaxially with fixtures to avoid eccentric force.
load:
Electronic (commonly used): servo motor → ball screw → horizontal beam moves down uniformly, with adjustable speed of 0.001~1000mm/min.
Hydraulic type (large tonnage): hydraulic pump → oil cylinder → beam tension, suitable for high-strength materials such as steel bars and steel structures.
Measurement:
Force value: The load sensor (strain gauge+Wheatstone bridge) converts the tension into an electrical signal with an accuracy of ± 0.5%.
Deformation: The extensometer directly tests the elongation within the standard length of the sample; Or use an encoder to measure the displacement of the crossbeam (roughly).
Data processing: The system real-time draws force displacement/stress-strain curves, automatically calculates and outputs the results after the sample fractures.
3、 Four core systems
Drive system: servo motor+screw (precision) or hydraulic pump station (high load), providing stable and controllable tension.
Measurement system: force sensor (measuring force), extensometer/encoder (measuring deformation), are key to accuracy.
Control system: PLC/microcomputer closed-loop control, loading according to set speed/force value, real-time feedback adjustment.
Fixture system: wedge-shaped, flat mouth, winding, etc., suitable for different specimens to prevent slipping or breakage.
4、 Key output indicators
Tensile strength: The greater stress that occurs before the specimen fractures.
Yield strength: The stress at which a material begins to undergo plastic deformation.
Elastic modulus E: The stress-strain slope during the elastic stage, which characterizes the stiffness.
Elongation at break: elongation at break/original gauge length x 100%, characterizing toughness.

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