Yield Criterion
A yield criterion is a mathematical rule that predicts when a material under combined stress stops behaving elastically and begins permanent plastic deformation. It maps a complex three-dimensional stress state to the onset of yield and compares it with uniaxial yield behavior. In machining, it determines whether cutting forces, clamping loads, or service stresses cause permanent set instead of elastic recovery.
On the shop floor, yield criteria decide whether a machined part stays dimensionally stable after clamps release or forming tools withdraw. During fixture design, jaw pressure, vacuum hold-downs, strap clamps, and locating pins must keep local contact stress below yield; otherwise soft aluminum or thin wall sections take a permanent set. In thin-wall CNC machining, cutting forces and deflection are checked against the elastic range, because exceeding yield causes springback to the wrong size or a bowed workpiece. In metal forming, von Mises and Tresca determine the stress combination at which plastic flow begins, making them standard in FEA and press setup. Even in assembly, overtightening fasteners can crush engineered substrates and destroy tolerance stack-up. The yield criterion is the gate between elastic recovery and permanent distortion, so choosing the right model and yield stress is essential for reliable parts.
- Jaw marks and bowed sealing faces after unclamping: Clamping a soft aluminum or thin-wall steel part too hard concentrates contact stress past yield, leaving permanent plastic distortion. The part then fails flatness and sealing requirements.
- Out-of-flat or shifted hole locations after release: A setup may feel secure during machining, but small bearing areas, vacuum pods, or sharp hard stops can locally exceed yield. Once unclamped, the part distorts permanently, so tolerances shift and locations move.
- Safe on paper, locally yielded in practice: Datasheet tensile yield strength does not describe multiaxial stress states in corners, threads, grooves, or interrupted cuts. Yield criteria bridge that gap; ignoring them invites unexpected local plastic flow.
Why not just compare stress to yield strength?
Because yield strength from a uniaxial test does not capture combined loading; a yield criterion such as von Mises or Tresca converts multiaxial stress into a yield decision.
Which criterion is more common for ductile CNC parts?
von Mises is widely used for ductile metals and is often the default in FEA and forming analysis, while Tresca is more conservative in some shear-dominated cases.
What does 0.2% offset yield mean in practice?
It means the material has undergone a small but permanent plastic strain, which matters when tight tolerances must survive unclamping, assembly preload, or load cycling.