Material fatigue

Material fatigue (called metal fatigue when it concerns metals) is the progressive damage of a material under repeated, that is cyclic, loading. A part that would easily withstand a single load of the same magnitude cracks after enough cycles, even at stresses below the yield strength. Fatigue is one of the most common causes of machine part failures in service.

A fatigue fracture develops in three stages: crack initiation (usually at the surface, at a stress concentration such as a notch, hole, sharp transition, thread or weld), gradual crack growth with every cycle, and finally a sudden overload fracture of the remaining cross section. The fracture surface typically shows a smoother crack growth zone with beach marks and a rough final fracture zone.

Fatigue properties are described by the S-N curve (Wöhler curve), which plots stress amplitude against the number of cycles to failure. For most steels, the curve flattens to an almost horizontal line somewhere between 1 and 10 million cycles, and the corresponding amplitude is called the endurance limit (fatigue limit). For smooth steel specimens in rotating bending, it is roughly 0.4 to 0.5 times the ultimate tensile strength; for a real part it is much lower because of notches, surface finish and size. Aluminum alloys have no distinct endurance limit, so their fatigue strength is stated for a chosen number of cycles. Engineers also distinguish low-cycle fatigue (typically up to tens of thousands of cycles, with plastic deformation) from high-cycle fatigue.

Fatigue life is further reduced by tensile mean stress, a rough surface, corrosion and elevated temperature, and increased by compressive residual stress in the surface (for example after shot peening or deep rolling). Under variable service loads, cycles are counted with the rainflow method and damage is summed using the Palmgren-Miner rule.

Fatigue has to be considered wherever loads repeat: rotating shafts, springs, connecting rods, welded vehicle and machine frames, parts exposed to vibration, pressure vessels with cyclic filling, snap fits and flexible plastic features. Cycle counts add up fast: a part vibrating at 50 Hz reaches one million cycles in about 5.5 hours of operation.

A fatigue assessment therefore belongs in the structural analysis of most moving and vibrating structures, and for safety-relevant parts it is complemented by fatigue testing.

A typical mistake is designing a part for static strength only. Fatigue cracks start in the details: sharp internal corners, abrupt changes in cross section, cross holes, weld starts and stops, threads and rough or damaged surfaces. Use generous fillet radii, keep welds away from the most highly stressed areas, and specify surface roughness on critical surfaces and, where needed, a surface treatment that introduces compressive stress.

Do not count on a true endurance limit for aluminum, plastics or 3D-printed parts. Plastics also heat up at higher loading frequencies, and in printed parts cracks readily propagate along the layers. Use fatigue strength values for the specific material, condition and manufacturing process, and verify critical parts by testing several specimens, because fatigue test results show wide scatter.