Fatigue
Why does fatigue failure happen below yield strength?
Fatigue is driven by repeated cyclic damage accumulation, not a single overload event. Microscopic cracks can initiate at surface defects and grow over many cycles until final fracture, even when the maximum stress never reaches the material's yield point.
What machining variables most affect fatigue life?
The key variables are surface roughness, residual stress state, cutting speed, feed rate, tool geometry, and coolant strategy. These factors control subsurface damage and crack-initiation risk, thereby directly influencing fatigue life.
Why are compressive residual stresses beneficial?
Compressive residual stresses tend to close crack tips and slow crack growth, which increases fatigue life. In contrast, tensile residual stresses open cracks and accelerate propagation, reducing the component's service life.
Material fatigue is the progressive damage mechanism in which parts accumulate internal cracks under repeated cyclic loading, even when the applied stress remains below the static yield strength. In CNC machining, crack initiation typically begins at surface defects, tool marks, or residual-stress-affected zones. Operator fatigue, a separate meaning, involves mental and muscle exhaustion from sustained attention and repetitive tasks, increasing error rates and reducing productivity.
In the CNC shop, fatigue is a critical design and process-control concern because machined surface integrity directly governs crack initiation and fatigue life. Rougher surfaces and tensile residual stresses shorten component life, while compressive residual stresses improve it. Practical control means managing tool condition, feed and speed, coolant application, surface finish, and residual stress to minimize microcrack formation, work hardening, and subsurface damage. In millwork and assembly, fatigue is discussed for fastener joints, hinges, brackets, and dowel connections under cyclic service loading—repetitive opening, vibration, or flexing can loosen joints or initiate cracks at weak interfaces. For operators, controlling mental and muscle fatigue requires task rotation, anti-fatigue mats, scheduled breaks, and ergonomic layout changes to reduce input errors, missed inspection cues, and handling mistakes during long shifts.
Tool marks and poor surface finish: Rougher machined surfaces act as stress concentrators, becoming initiation sites for cracks that drastically reduce fatigue life, especially in notch-sensitive materials.
Tensile residual stress from machining: Conditions that leave tensile residual stress on the surface accelerate crack propagation under cyclic loading, shortening component life and increasing failure risk.
Ignoring operator fatigue in precision work: Long monitoring periods, awkward postures, and repetitive tasks increase mental and muscle fatigue, raising the risk of programming mistakes, missed alarms, and handling errors.