Fatigue Strength
What is the difference between fatigue strength and ultimate tensile strength?
Ultimate tensile strength is the maximum stress a material can withstand in a one-time pull test, while fatigue strength is governed by cyclic loading and depends on the number of cycles and stress ratio. A part can fail by fatigue at stresses well below ultimate tensile strength.
Why does surface finish affect fatigue so strongly?
Surface roughness acts as a crack nucleation site and reduces the effective fatigue limit. Better finishing can raise fatigue life, while rough machining or abrasive damage lowers it.
Why do compressive residual stresses help?
Compressive residual stress reduces the effective tensile opening stress at the surface, slowing crack initiation and crack propagation. Tensile residual stress does the opposite.
Fatigue strength is the maximum alternating or cyclic stress a material or component can withstand for a specified number of load cycles without fatigue failure; for some materials it is treated as an endurance limit below which infinite life is assumed, while for others it is defined at a stated cycle count such as 10^7 or 10^8 cycles.
On the shop floor, fatigue strength dictates component life under repeated loading. Spindle shafts, toolholders, ball screws, clamps, fixtures, and machine frames see cyclic stress from rotation, vibration, and start-stop duty. The practical goal is keeping local stress at stress concentrators (keyways, fillets, threads, sharp corners, weld toes) below the modified endurance limit. That means applying correction factors for surface finish, size, notch effects, residual stress, and environment. Machining strategy changes this limit measurably; one study on 42CrMo4+QT found fatigue strength from 360 to 510 MPa at one million cycles depending on finishing parameters and coolant. Surface integrity is decisive: smoother finishes and compressive residual stress extend life, while rough feed marks, grinding burn, and tensile residual stress create crack starters. In millwork and edgebanding equipment, repeated clamp actuation and oscillating cutter heads demand attention at holes, fastener runouts, and thin welded transitions.
Sharp geometry at a loaded transition: A shoulder without a fillet, a laser-cut hole with burrs, or an undercut thread root creates a stress concentration. Cyclic stress spikes locally even when average load looks acceptable, causing early crack initiation.
Poor surface finish or grinding damage: A part that passes dimensional inspection can still fail early if machining leaves rough feed marks, chatter, grinding burn, or tensile residual stress. Those surface defects become crack starters under repeated loading.
Ignoring actual duty cycle: A component designed for static strength may be reused in a high-cycle CNC environment where repeated acceleration, deceleration, spindle torque pulsation, and cutting interruption generate far more cycles than assumed. Fatigue failure occurs below yield strength.