Tensile test of mechanical materials – stress–strain graph, yield and tensile strength
Design & TechnologyMechanical EngineeringAges 16–17Virtual lab
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Mechanical Engineering
A metal specimen is clamped in a tensile-testing machine: drag the slider (or click Auto pull) to increase the elongation as the specimen stretches, necks and finally fractures. The stress σ = F/S₀ versus strain ε = Δl/l₀ graph is drawn in real time, marking the elastic limit, the yield strength σ_ch and the tensile strength σ_b. Change the material to compare CT38 mild steel, C45 steel, gray cast iron (brittle), aluminum alloy and copper. The Young modulus screen is Searle's experiment: load a long wire, read the extension on a micrometer screw and the diameter with a micrometer, plot load against extension with the elastic region marked, and find E from the gradient.
Lesson: Mechanical materials: mechanical properties of materials — strength, ductility, hardness; the tensile test and the stress–strain graph
What it shows
The tensile test is the standard way to measure how strong and how ductile a metal is: force and extension are converted to stress σ = F/S₀ and strain ε = Δl/l₀, so specimens of different sizes can be compared. At first the metal obeys Hooke's law σ = E·ε; beyond the yield strength it deforms permanently, work-hardens, necks and fractures. The Young modulus screen measures E the school-lab way with Searle's apparatus: a long wire is loaded in steps, the micrometer reads the tiny extension, and E = (F/ΔL)·(L/A) comes from the straight, elastic part of the load–extension graph.
How to use
On the Tensile test tab, choose a Specimen material and click Auto pull, or drag the Elongation slider, and watch the stress–strain graph pass through yielding, work hardening and necking. On the Young modulus (Searle) tab, read the wire's diameter on the micrometer, then raise the Added load step by step and press Record reading each time. Use the gradient of the elastic points, the Wire length L and the area A = πd²/4 to find E, and try copper to see the elastic limit.
Parameters you can change
- Starting screen Tensile test, Young modulus (Searle)
- Specimen material CT38 steel (ordinary carbon steel), C45 steel (quality carbon steel), GX15-32 gray cast iron (brittle material), Aluminum alloy (duralumin), Pure copper (very ductile)
- Specimen diameter d₀ 5–20 mm
- Gauge length l₀ 50–200 mm
- Pull rate (strain per second) 1–20 %/s
- Young modulus screen: wire material Steel, Brass, Copper
- Young modulus screen: test wire Wire A (thin), Wire B, Wire C (thick)
- Young modulus screen: wire length L 1–3 m
Questions to explore
- Why is the graph plotted as stress against strain rather than force against elongation?
- Which of the five materials has the largest elongation at fracture, and what does this tell you about its ductility?
- Judging from the graphs, should a machine part that must withstand impacts be made of gray cast iron or steel?
