Tensile testing is the most fundamental yet critical test in mechanical property inspection. A metallic specimen is clamped onto a universal testing machine and loaded gradually under tension - it will not fracture instantly.
From minor elongation, irreversible deformation, through to final rupture, the full process falls into four distinct deformation stages.
1. Elastic Deformation Stage
At low applied load, when stress stays below the proportional limit, atoms inside the material only shift slightly. Once the load is removed, the specimen returns fully to its original length with no permanent deformation. Stress and strain follow a linear relationship; the slope of this straight line equals the elastic modulus (E).
Test Note: Deformation in this stage is extremely small. Calculation based only on crosshead displacement brings large errors. An extensometer must be used to capture true elongation over the gauge length to meet requirements of GB/T 228.1 and ISO 6892.
Engineering Value: Steel structures and mechanical components are designed to operate within the elastic range. If stress exceeds this limit, permanent bending remains after unloading, resulting in failed components.
2. Yield Stage
As load rises and stress exceeds yield strength, slip occurs along metal crystal lattices. Load barely increases while the specimen keeps stretching. Mild steel shows a clear yield plateau, with visible 45° slip lines on the surface. Deformation beyond yield is plastic and non-recoverable after unloading. Yield strength is a core acceptance index for high-strength bolts and structural steel.
Engineering Value: Yield strength is the primary design threshold for material selection and a mandatory inspection item for incoming high-strength bolts and construction steel. Design principle: Operating stress shall never exceed yield strength. Once yielding occurs, bolts elongate and loosen, steel beams bend, and failure takes place before fracture, which creates severe safety risks.
Glossary Elastic deformation: Recoverable after unloading (Stage 1 only) Plastic deformation: Permanent after unloading (Stages 2, 3 & 4)
3. Strain Hardening (Work Hardening) Stage
After yielding, lattice dislocations interfere with one another. Higher tensile force is required for further elongation, and the material becomes stronger and harder - known as strain hardening. The specimen elongates uniformly with consistent cross-section reduction, while stress climbs until the peak point: ultimate tensile strength (Rm). Plastic deformation remains if the test is paused and the sample unloaded in this stage. Reloading will show an elevated yield strength, the well-known cold working effect.
Engineering Value: Rm represents the maximum tensile load the material can withstand. Stamping and bending processes leverage this property; stamped steel gains extra local strength after forming. The ratio of tensile strength to yield strength also indicates the material's safety margin.
4. Necking & Fracture Stage
After reaching ultimate tensile strength, deformation becomes non-uniform. The weakest section shrinks rapidly, forming a neck. Cross-sectional area at the neck drops sharply, the sustainable load decreases, and fracture finally occurs at the necked zone. Broken halves can be re-assembled to measure percentage elongation after fracture and percentage reduction of area, two key indicators for ductility.
Engineering Value: These two parameters reflect ductility and resistance to brittle fracture. Two steel grades with identical strength can behave very differently: low-ductility steel fractures suddenly without warning (brittle fracture, extremely hazardous). Ductile steel stretches and necks visibly before rupture, giving clear early warning. Pressure vessels, steel frames and bolts all require adequate ductility to avoid catastrophic brittle failure.
Supplementary Note
Stress-strain curves vary by material, and not all materials exhibit all four stages. High-strength steel, cast iron and other brittle grades often show no obvious yield plateau. Aluminium, copper and many non-ferrous metals have indistinct yield platforms, so Rp0.2 (0.2% proof strength) is adopted for evaluation. Test interpretation and material selection must match the actual curve characteristics.
Accurate acquisition of full stress-strain data demands high force precision and fast sampling rates from the universal testing machine. Extensometers capture tiny elastic elongation, while high-speed data acquisition resolves yield plateaus and necking transients for smooth, repeatable curves.
Nake WDW Series Electronic Universal Testing Machines are engineered for tensile tests of metallic and non-metallic materials. This gantry dual-space servo machine delivers stable performance with fully automated test sequences. It features an 800 mm effective tensile stroke, and interchangeable fixtures support compression, bending, shear and other mechanical tests. Available force ranges cover 10 kN ~ 100 kN, with Class 1 or Class 0.5 accuracy options. The high-resolution acquisition system reliably records stress and strain across all four deformation stages. It complies with GB, ISO and ASTM standards for steel, high-strength bolts, steel strands, pipes and tubular specimens.
For more knowledge of tensile testing and mechanical inspection, or equipment procurement enquiries, please contact Nake.
