Material Fatigue & S-N Curve Lab
Cycle a loaded part through repeated stress, watch the waveform run, and read off the S-N curve exactly how many cycles it should survive before it fails.
Change the material's ultimate strength and the applied stress amplitude, and watch a cyclic stress waveform run while the S-N curve marks your predicted fatigue life.
Se = 0.5·Sut, b = -(1/3)log10(0.9Sut/Se), N = 1000·(σa/0.9Sut)^(1/b)About the Material Fatigue & S-N Curve Lab
Free material fatigue & s-n curve lab. Cycle a loaded part through repeated stress, watch the waveform run, and read off the S-N curve exactly how many cycles it should survive before it fails. Drag, change the sliders and see the result live. No sign-up, works on phone and computer. Built for engineering, the material fatigue & s-n curve lab runs instantly in your browser: change a setting or drag an object and the result updates at once, so you learn by trying things out rather than only reading about them.
Cycle a loaded part through repeated stress, watch the waveform run, and read off the S-N curve exactly how many cycles it should survive before it fails. Use it to explore engineering ideas at your own pace, then check what you found against the key ideas further down this page.
How to use the Material Fatigue & S-N Curve Lab
- Use the controls to change Ultimate strength Sut (MPa), Applied stress amplitude σa (MPa). The simulation reacts instantly.
- Press "Reset to defaults", "Lab report" to start, reset or change what is happening.
- Where you see a glowing handle, object, weight or atom, drag it with your mouse or finger. Everything responds in real time.
- Watch the readouts and graphs update as you experiment, and compare what you see with the key ideas below.
Things to try
- Try the 100,000-cycle life challenge at the default 700 MPa steel.
- Drop the stress amplitude below the endurance limit and confirm the infinite-life challenge passes.
- Raise Sut to 1000 MPa and see how much longer the same stress amplitude survives.
- Watch the red dot on the S-N curve slide as you drag the stress amplitude slider.
Key ideas you can learn
- Repeated cyclic stress can crack and break a part at a stress far below what would fail it in a single static pull - this is fatigue failure.
- The endurance limit Se is the stress amplitude below which steel can theoretically survive infinite cycles; for many steels Se is about half the ultimate strength Sut.
- Above the endurance limit, the S-N curve (Basquin's equation) draws a straight line on log(stress) vs log(cycles) axes between (10^3 cycles, 0.9Sut) and (10^6 cycles, Se), letting you read off predicted life N for any stress amplitude.
- Because the S-N relationship is a power law, even a modest reduction in stress amplitude can multiply the predicted fatigue life by a large factor.
Where this is used in the real world
Engineers run this exact S-N/endurance-limit check on rotating shafts, aircraft components, bridge connections and any part under repeated cyclic loading, since fatigue - not one-time overload - is the most common cause of mechanical failure in service.
Who is this simulation for?
Engineering and technology students, makers, robotics clubs and teachers of design and technology. It gives a hands-on feel for how machines behave before you build a real one.
For teachers: project it on the board, let students predict what will happen, then run it together. For students: change one thing at a time and write down what changes.
Frequently asked questions
Why can a part fail from fatigue at a stress far below its ultimate strength?
Fatigue failure is driven by microscopic cracks that form and slowly grow at points of repeated cyclic stress, not by the material yielding all at once; each cycle grows the crack a little further until it is large enough to fracture suddenly, which can happen at stress amplitudes well below the one-time ultimate strength.
Why does lowering the stress amplitude below the endurance limit give theoretically infinite life instead of just a longer one?
For many steels the S-N curve flattens out into a horizontal line at the endurance limit Se; below that stress the microscopic damage per cycle is too small to ever grow into a propagating crack, so the Basquin power-law relationship no longer applies and the part can in theory be cycled forever.
Is the Material Fatigue & S-N Curve Lab free to use?
Yes. It is completely free, with no signup, no download and no ads inside the simulation. It runs in your web browser.
Does the Material Fatigue & S-N Curve Lab work on a phone or tablet?
Yes. It uses touch as well as the mouse, so you can drag objects with your finger. A larger screen makes the controls easier to see.