What are the differences between displacement, deformation and strain?
Clarify the Concepts Many procurement and laboratory personnel fall into this trap: the universal testing machine itself outputs displacement data. Why install an additional extensometer for tensile testing?
In tensile tests conducted on a Universal Testing Machine (UTM), many users have this confusion. Since the machine's crosshead can output displacement readings, is an extensometer still necessary for tensile testing?
Three concepts frequently appear during equipment selection:
Displacement
Extension
Strain
Extensometer
These parameters are all related to material deformation, yet they represent completely different physical quantities. Understanding their differences helps you properly select test equipment, identify which test scenarios mandate an extensometer, and avoid distorted test data.
01 Differences between Displacement, Extension and Strain
For a standard tensile test, the specimen is clamped between the upper and lower grips of the UTM. Crosshead movement pulls the specimen and induces gradual deformation.
The first data collected by the testing machine is Crosshead Displacement.
Crosshead Displacement:
Crosshead displacement refers to how many millimeters the moving crosshead of the test machine travels. It includes all deformations of the complete test system:
Deformation of the specimen itself
Elastic deformation of grips, machine frame and drive lead screw
Minor slippage between grips and specimen, plus mechanical clearances
For example, if the crosshead travels 20 mm during testing, the software records 20 mm crosshead displacement. Important note: A crosshead travel of 20 mm does NOT equal 20 mm true elongation of the specimen.
In other words, crosshead displacement reflects the total motion of the whole test system, and cannot be directly treated as the true deformation within the specimen gauge length.
Next, distinguish between Extension and Strain. Assume the specimen original gauge length = 50 mm. After tension, the gauge length becomes 55 mm.
① Extension
Definition: Absolute change in length over the specimen gauge section, unit: mm Calculation: Extension = 55 − 50 = 5 mm Mnemonic: How many millimeters the specimen actually stretches.
② Strain
Definition: Ratio of deformation relative to the specimen's original length. Dimensionless, commonly expressed as percentage. Calculation: Strain = Extension ÷ Original Gauge Length In this case: 5 ÷ 50 = 10% Mnemonic: Relative deformation ratio compared to the specimen's original length.
Although displacement, extension and strain all appear on the UTM software interface, they measure different targets. Crosshead displacement is machine-side data; extension and strain are material properties of the specimen.
02 Why Do We Need an Extensometer Even if the Machine Has Displacement Readout?
➡ Precision calculation is required except for simple screening tests Maximum force, break force and rough force-displacement curves can be obtained using crosshead displacement alone. But for accurate characterization of material mechanical properties, relying solely on crosshead displacement introduces significant errors.
In tests for metals, engineering plastics, rubber and composites, we often need to calculate:
Tensile Modulus
Yield Strain
Elongation at Break
Strain at Break
Complete Stress–Strain Curve
These indicators, especially data from the elastic region, heavily rely on precise strain measurement.
➡ Extensometer captures data more directly and accurately An extensometer is clamped directly onto the specimen gauge section. It measures only the true deformation within the specimen gauge length, excluding frame deformation, grip slippage and system clearances.
Concrete example: Original specimen gauge length = 50 mm. Total crosshead travel during test =10 mm. Due to elastic deformation of frame and grips, the true elongation of the specimen gauge length measured by extensometer is only 8 mm.
Strain calculated directly from crosshead displacement: 10 ÷ 50 = 20% Strain calculated from elongation measured by extensometer: 8 ÷ 50 =16%
Noticeable deviation already occurs between the two results. For small-deformation tests such as elastic modulus measurement, errors will be further amplified, leading to completely distorted modulus and yield strength values.
Therefore, mainstream tensile test standards including GB/T 228.1‑2021, ISO 6892‑1 and ASTM E8 mandate the use of extensometers to collect deformation data when determining elastic modulus and proof strength Rp0.2. Crosshead displacement cannot substitute true specimen deformation.
03 When Must an Extensometer Be Used? When Can It Be Omitted?
Cases where an extensometer may NOT be required
Only tensile strength, maximum force and break force are measured; modulus and yield strain are not required
Rough comparative tests for highly extensible soft materials, for internal screening only ⬇ Crosshead displacement can be referenced temporarily
Cases where an extensometer is MANDATORY
Measurement of elastic modulus, Rp0.2 proof strength and yield strain
Generation of standard stress‑strain curves
Standardized testing complying with GB, ISO, ASTM; third-party inspection and R&D research
Performance evaluation for metals and high-strength engineering plastics within small deformation ranges
END
Many customers mistakenly assume that built-in machine displacement equals strain measurement capability. Displacement is machine movement; strain is a material property. To obtain authentic mechanical data of materials, the extensometer is an irreplaceable precision accessory.
