A tensile test pulls a bar of known cross-section to fracture while recording force against stretch, and it gives yield strength, tensile strength and elongation directly: stress is the force divided by the original area, so a 10 mm round bar of area 78.5 mm² that carries a 31.4 kN maximum load has a tensile strength of 400 MPa. The result is only valid if the bar breaks in its gauge length, the grips do not slip and the load is applied slowly and in line with the axis. Always shield the rig, because a bar that fails releases its stored energy at once.
Part of Testing and Quality
Tensile testing measures how a material responds to being pulled apart, giving its stiffness, yield strength, tensile strength and ductility from one controlled experiment.
What You Need
The Advanced Materials article describes a workable rig:
- A rigid frame of heavy steel channel or I-beam that will not stretch measurably under test load.
- A loading device, a screw jack or a hydraulic cylinder, built into the frame.
- Two gripping jaws that hold the sample without slipping or crushing it.
- A force measure, a spring scale or load cell.
- An elongation measure, calipers or a dial gauge.
- Standard specimens: flat bars 12.5 mm wide with a 50 mm gauge length, or round bars 6-12 mm in diameter.
A lever and dead weights make a simple force measure without electronics. A 10 : 1 lever carrying 100 kg applies about 1,000 kgf, which is 1,000 × 9.81 = 9,810 N, or 9.81 kN, on the bar. Calibrate against known weights before every series.
Method
- Machine the specimen to size with a smooth finish. File marks and sharp shoulders are notches and cause early failure. Keep the central gauge section uniform.
- Measure the original diameter with a micrometer or calipers at three places and average. Calculate area A₀ = π × (d / 2)².
- Mark the gauge length with two punch marks 50 mm apart.
- Mount in the grips in a straight line with the load axis, so the bar sees only tension and no bending.
- Load slowly and steadily. Record force at regular elongation steps. Note the force where the bar first stretches without more load, the yield point.
- Continue to fracture, recording the maximum force reached.
- Fit the broken pieces together and measure the final gauge length and the smallest diameter at the break.
Worked Example
A 10 mm round bar has A₀ = π × 5² = 78.5 mm². Suppose (illustrative values):
- Yield load 20.0 kN: yield strength = 20,000 / 78.5 = 255 MPa.
- Maximum load 31.4 kN: tensile strength = 31,400 / 78.5 = 400 MPa.
- Final gauge length 61.5 mm: elongation = (61.5 − 50) / 50 = 0.23, or 23%.
- Diameter at the neck 6.0 mm: area = π × 3² = 28.3 mm². Reduction of area = (78.5 − 28.3) / 78.5 = 0.64, or 64%.
Large elongation and reduction of area mean a ductile material that warns before it fails. A brittle one shows almost none, as Tensile Strength describes. Quote the gauge length with every elongation figure, because elongation depends on it.
Reading Where The Bar Broke
- Break in the middle of the gauge: valid.
- Break at the grip or at a shoulder: the test is invalid; the grip crushed or notched the bar.
- Jaws slipped: the force record is wrong. Use serrated grips or longer grip length.
- Flat, bright fracture with no neck: brittle material, or a flaw. Check the surface for a defect.
Turning A Result Into A Working Load
Strength is not a design load. The allowable stress is the failure stress divided by a safety factor, as Safety Factors defines. Using the 255 MPa yield from the example and a factor of 2 gives 127 MPa. For a 12 mm round tie bar, area π × 6² = 113 mm², so the allowable tension is 127 × 113 = 14,400 N, or 14.4 kN, which is about 1,470 kgf. A rig with a crude force measure justifies a bigger factor.
Several coupons per batch are better than one. A hardness reading can later stand in for strength, once you have measured both on your own bars: see Hardness Testing.
A broken bar and stored rig energy
A bar breaking under 30 kN or more releases stored energy as a bang and flying pieces, and a stretched cable, a loaded lever or a pressurised hydraulic line does the same if it lets go. Put a solid shield around the specimen, stand to the side and never stand in line with the bar, keep hands off the specimen while loaded and never exceed the rated capacity of the frame, jack or cylinder.
Common Mistakes
- Bending from misalignment — a lower, scattered result.
- Loading too fast — the reading is higher and the bar behaves more brittly.
- Dividing by the final area — gives true stress. The standard figure uses the original area.
What’s Next
- Learn next: Hardness Testing — a faster, less destructive check
- Combine with: Mechanical Properties — what the numbers mean