Stress Amplitude
What is the formal relation for stress amplitude in a symmetric cycle?
For a cycle with maximum stress σmax and minimum stress σmin, the stress amplitude is σa = (σmax - σmin)/2, while mean stress is σm = (σmax + σmin)/2. Fatigue data are plotted as stress amplitude against cycles to failure on an S-N curve.
Why do machining choices affect a part's fatigue life under stress amplitude?
Machining determines surface integrity and residual stress. Tensile residual stress from aggressive roughing or poor thermal control adds to the externally applied cyclic stress amplitude, reducing the effective fatigue margin and causing earlier crack initiation than pure design calculations indicate.
How is stress amplitude used in qualification testing of machined parts?
Components are subjected to repeated loading at defined amplitudes, and cycles to failure are plotted on S-N curves to verify endurance life. This testing validates design allowables and shop-floor decisions such as surface finish, fillet radii, and residual-stress control.
Stress amplitude is half the difference between maximum and minimum stress in a cyclic load, defining the alternating component of fatigue loading. On S-N curves, stress amplitude is the vertical axis and predicts cycles to failure; lower amplitudes correspond to longer fatigue life. It is separate from residual stress locked into machined components.
On the shop floor, stress amplitude becomes a practical concern whenever a machined part sees repeated service loading: spindle torque reversals, press-fit cycling, vibration, bending, or pressure pulsation. Validation engineers use it to set design allowables and acceptance criteria for thin walls, fillets, and cross-drilled holes, where cyclic loads concentrate and fatigue cracks often start. For nickel alloys like Hastelloy, S-N behavior is directly tied to cyclic stress amplitude, and small amplitudes can still cause failure in aggressive environments. This means finish-machined geometry, tool marks, surface integrity, and residual stress state all matter because they determine how a given applied amplitude translates into real fatigue damage. That is why roughing and finishing strategy, stress relief, clamping sequence, and coolant choice are tightly controlled in precision machining; they influence residual stress and warpage, which in turn shrink or preserve the fatigue margin under cyclic loading.
Treating stress amplitude as the whole fatigue picture: Mean stress shifts the S-N curve, so identical amplitudes can yield very different lives; ignoring this creates non-conservative predictions and unexpected service failures.
Honing or finish cutting with tensile residual stress: Aggressive roughing or poor coolant control leaves tensile stress near the surface, which stacks with applied cyclic amplitude and accelerates crack initiation far earlier than the S-N estimate predicts.
Neglecting notch and radius effects: Sharp filets, slots, threads, and section changes concentrate local stress above the nominal amplitude, so a paper-safe component can still crack at the notch root under cyclic service loading.