Material Fatigue & S-N Curve Lab Community
Questions, answers and tips from people who use the 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..
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From the tool guide: common questions about Material Fatigue & S-N Curve Lab
The items below are not posts by community members. They come from the written guide for this tool and are here to get a conversation started. Use “Ask a follow-up” to open a thread about any of them.
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.
From the tool guide Ask a follow-upWhy 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.
From the tool guide Ask a follow-upKey ideas
- 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.
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.
Where this is used
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.
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