Fatigue Testing

Fatigue testing is a destructive mechanical test that evaluates how a material or component survives repeated cyclic loading until crack initiation, crack growth, or final fracture. It determines fatigue strength, endurance limit, and fatigue life, commonly using machined specimens with a critical center section and controlled surface finish.

On the shop floor, fatigue testing is a go/no-go validation for machined components exposed to vibration, repeated clamping, cyclic bending, or machine-induced oscillation. Coupons are often CNC-lathed or milled from bar stock or production material, then low-stress ground and polished so the test measures the base material rather than tool marks. Feed, depth of cut, and spindle speed become specimen-making variables because aggressive machining can leave tensile residual stress or grinding burn that shortens fatigue life. A defined critical center section concentrates stress away from the grips, so the failure occurs in the gauge region. Servo-hydraulic or electromechanical test frames apply controlled axial, flexural, or low-cycle loads and generate S-N curves relating stress to cycles to failure. Results qualify material lots, compare machining conditions, and reveal whether a process introduced harmful surface damage or residual stress.

• ⚠️Machining marks at the gauge section: Tool marks, chatter, or poor polishing act as crack initiators and cause premature failure that does not represent the base material.
• ⚠️Wrong specimen geometry or fillet radius: A bad radius transition or inaccurate critical section shifts stress concentration away from the intended test zone, invalidating the result.
• ⚠️Process-induced residual stress or heat damage: Aggressive cutting, improper feeds and speeds, or grinding burn can leave tensile residual stress or microstructural damage that lowers fatigue life and masks true material performance.