Lessons · Engineering · Things break at loads they carried yesterday
Things break at loads they carried yesterday
A part loaded repeatedly can fail at a stress far below the one that would break it once. Fatigue is driven by the range of stress and the number of cycles, and cracks start where stress concentrates: a sharp corner, a keyway, a hole, a weld toe.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
In beta. This lesson was written for Hone and has not yet been checked by a licensed engineer. Practice material, not professional advice. What that means.
What it is for
It accounts for most in-service mechanical failures, and it is the one that surprises people, because the part carried that load thousands of times before. Nothing changed except the count.
How to think about it
Look for the range of stress rather than the peak, and hunt for concentrations. A generous radius where a shaft steps down is often worth more than a stronger material, because the crack was always going to start at the corner.
Worked example
Failure below the single-load strength, after many cycles.The count is the variable.
Driven by stress RANGE, not the maximum alone.A load that cycles is worse than one that sits.
Cracks start at concentrations: corners, keyways, holes.Geometry decides where.
A radius at a step is cheap and effective.It spreads what a sharp corner concentrates.
Your turn
Where does a fatigue crack usually start?
A fatigue crack starts where stress
Solve one, graded on the server
The trap
Specifying a stronger material and keeping the sharp corner. Fatigue life is governed more by the concentration than by the grade, and the crack starts in the same place.