The tensile test is the most widely used mechanical test in the world for characterising materials. A single test provides the data that every engineer uses on a daily basis: elastic modulus, yield strength, tensile strength and ductility.
In this article, we examine what the tensile test is, how it is performed according to ISO and ASTM standards, which laboratory best practices should be followed and why this data is crucial for design and quality control.
The tensile test is a destructive mechanical test. A specimen with a standardised geometry is “pulled” along its axis under an increasing load until it breaks.
During the test, the machine continuously records the applied force and the elongation of the specimen. From this data, the stress-strain curve is obtained: the material’s true mechanical identity card.
In summary, the tensile test answers three fundamental questions:
The performance of the test is strictly codified to ensure repeatability and comparability of results between different laboratories. The main standards are:
UNI EN ISO 6892-1: the reference standard in Europe for metallic materials
ASTM E8/E8M: the American equivalent for metallic materials
ISO 6892-2: tensile test at elevated temperature
UNI EN ISO 527: tensile test for plastics and composites
ISO 6892-3 and ISO 6892-4: tests at low temperature and in liquid helium
Values obtained according to different standards are not always directly comparable.
Let us look step by step at how the test works in the laboratory.
The specimen is taken from the material to be tested and machined to a standardised geometry: cylindrical for bars and forgings, flat for sheets and strips. The gauge length L₀ is proportional to the initial cross-sectional area S₀; for standard ISO specimens, L₀ = 5.65·√S₀.
Machining is critical: roughness, surface defects and misalignment can distort the results.
The specimen is gripped by the jaws of a universal testing machine, equipped with a load cell calibrated according to ISO 7500-1. An extensometer, either contact or optical, is applied to the gauge section to measure the actual elongation of the material.
A practical tip: to measure the elastic modulus and Rp0.2, crosshead displacement is not sufficient; an extensometer is always required.
The machine applies the load at a controlled rate. ISO 6892-1, Method A, specifies control by strain rate. The resulting curve shows four stages:
Elastic region: stress and strain are proportional, according to Hooke’s law; the slope is the material’s Young’s modulus (E).
Yielding: permanent plastic deformation begins.
Strain hardening: the material continues to withstand load up to the maximum stress, Rm.
Necking and fracture: deformation concentrates in a section that narrows until fracture occurs.
The test determines the parameters found in every inspection certificate (EN 10204 3.1) and every material datasheet:
Elastic modulus E [GPa]
Yield strength Re, ReH/ReL or Rp0.2 [MPa]
Tensile strength Rm [MPa]
Percentage elongation at break A [%]
Reduction of area Z [%]
To ensure repeatable results and proper comparability, several key criteria must be followed:
Always specify the reference standard in technical specifications: ISO and ASTM results are not directly comparable.
Control the test speed: almost all materials are sensitive to strain rate; higher speeds increase the measured yield values.
Ensure correct alignment: a misaligned specimen is subjected to parasitic bending, which causes apparent yielding to occur earlier.
Respect the sampling direction: in rolled products, properties vary between the rolling direction and the transverse direction.
Check the fracture location: a fracture outside the gauge section invalidates the test measurement.
The tensile test is central to design because it provides direct knowledge of a material’s mechanical behaviour when subjected to stress.
The test provides fundamental properties such as elastic modulus, yield strength, tensile strength and elongation at break, which indicate how stiff, strong and deformable a material is. This data is essential for selecting the most suitable material, correctly sizing a component and predicting its behaviour in service.
In this way, the designer can avoid sudden failures, ensure structural safety and optimise the weight, cost and performance of the final product.
The tensile test is the foundation of mechanical characterisation: a standardised, fast and cost-effective test that provides the data on which design, simulation and quality control are based. Understanding how the test is performed, which standards regulate it and how to interpret the results makes it possible to design using reliable data, choose the right suppliers and prevent in-service failures.
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ReH is the upper yield strength, measurable only in materials that show a clear yield point, typically annealed carbon steels. Rp0.2 is the proof strength that produces a residual plastic strain of 0.2%, and is used for all materials with a gradual elastic-plastic transition. They are not numerically equivalent, but they play the same role in verification.
Yes. The specimen is taken to fracture. When the component cannot be sacrificed, indirect methods are used, such as hardness tests, which can be correlated with Rm using ISO 18265 tables, or miniaturised tests such as the small punch test.
With the speeds prescribed by ISO 6892-1, a test on steel typically lasts from one to a few minutes. Tests at very low strain rates, creep-like tests or tests on highly ductile materials may take longer.
ISO 7500-1 requires verification of the force-measuring system at least once a year, as well as after significant work on the load cell or frame. Extensometers are calibrated according to ISO 9513.
The tensile test is used to measure the fundamental mechanical properties of a material: stiffness, through Young’s modulus E; strength, through yield strength Rp0.2 and tensile strength Rm; and ductility, through elongation A% and reduction of area Z%.
The dimensions depend on the standard and the type of material. According to ISO, for cylindrical specimens L₀ = 5d is often used: for example, a 10 mm specimen has L₀ = 50 mm. For flat specimens, typical widths are 12.5, 20 or 25 mm, with gauge lengths often equal to 50 or 80 mm.
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