A tensile test, also known as a tension test, is the most fundamental and important testing method for evaluating the mechanical properties of materials. In simple terms, a standard metal bar (specimen) is clamped in a testing machine and pulled axially at both ends until it fractures. During this process, the applied force and corresponding elongation are recorded to determine whether the material is strong and ductile. For pressure vessel steels (such as Q345R, 304 stainless steel), tensile testing is a mandatory inspection item to verify that the material meets the design requirements for strength and toughness.
1. Test Principle and Procedure
- Sampling: A standard-sized specimen (usually dumbbell-shaped, narrower in the middle and thicker at both ends) is cut from a steel plate or forging.
- Mounting: The specimen is secured in the upper and lower grips of a universal testing machine.
- Loading: The machine applies an axial tensile force to the specimen at a constant speed.
- Recording: Sensors record the applied load and the elongation of the specimen in real time.
- Fracture: When the load reaches a certain level, the specimen necks (thins locally) and eventually fractures.
- Calculation: Based on the recorded data, a force-elongation curve is plotted, and various mechanical property indicators are calculated.
2. Key Indicators Measured by Tensile Test
The following four core indicators are the main focus and will be shown on the material test certificate:
A. Strength Indicators – How “strong” the material is
Yield Strength (ReL or σₛ)
- Definition: The minimum stress at which the material begins to undergo plastic deformation (permanent set that does not recover after unloading).
- Significance: The primary basis for pressure vessel design. Vessel wall thickness is calculated using yield strength to ensure no deformation under working pressure.
Tensile Strength (Rm or σᵦ)
- Definition: The maximum stress a material can withstand before fracture.
- Significance: Represents the ultimate load-carrying capacity of the material.
B. Plasticity Indicators – How “ductile” the material is
Elongation after Fracture (A)
- Definition: The percentage ratio of the increased gauge length after fracture to the original gauge length.
- Significance: Measures ductility. Higher elongation means lower risk of brittle fracture and improved safety of pressure vessels.
Reduction of Area (Z)
- Definition: The percentage ratio of the reduced cross-sectional area at the fracture to the original cross-sectional area.
- Significance: Also a measure of plasticity, reflecting the material’s ability to absorb energy before fracture.
3. Stress-Strain Curve
The tensile process typically consists of four stages:
- Elastic Stage: The specimen elongates under load and recovers completely when unloaded. Stress is proportional to strain (follows Hooke’s law).
- Yield Stage: The curve shows a serrated plateau, indicating the onset of irreversible plastic deformation.
- Strain-Hardening Stage: After yielding, the material becomes stronger due to microstructural changes, requiring higher force for further elongation.
- Necking and Fracture Stage: After reaching peak load, localized necking occurs at the weakest section, force drops rapidly, and the specimen fractures.
4. Why Pressure Vessels Emphasize Tensile Tests
Pressure vessels contain high internal pressure, often combined with high temperature or corrosive environments, imposing strict requirements on tensile properties:
- Prevent rupture: Insufficient tensile strength may cause catastrophic bursting under overpressure.
- Prevent excessive deformation: Low yield strength can lead to bulging or permanent deformation under normal operating pressure.
- Prevent brittle fracture: Low elongation (brittleness) may result in sudden, glass-like fracture under shock loads such as earthquakes or rapid temperature drops, without prior deformation warning.
5. Relevant Standards
In China, tensile testing of metallic materials is mainly performed in accordance with the national standard GB/T 228.1-2010 Metallic materials — Tensile testing — Part 1: Method of test at room temperature.
