Fatigue Fatigue Striations
Fatigue Striations are fine, parallel ripples on the fracture surface of a material. They form with each fatigue cycle due to plastic deformation at the crack tip. They prove: the crack grew by fatigue, not by a single overload.
Physical-Chemical Mechanism of Action
A crack grows step by step under cyclic loading. At the crack tip, plastic deformation alternates: when loaded, the crack opens and the material deforms plastically. When unloaded, the crack closes. A fine Fatigue Striation remains. Each Fatigue Striation corresponds to one load cycle. The Fatigue Striations lie perpendicular to the crack propagation direction. They are only visible at high magnification.
Metallurgical Relevance & Risks in Welding
Weld seams have notches (weld toe, lack of fusion). Stress concentrates at these notches. Under dynamic loading (machines, vehicles, bridges), fatigue cracks start there. Fatigue Striations on a fracture surface show: the part failed from many small load cycles, not a single overload. The risk: the crack grows unnoticed until the remaining cross-section suddenly breaks. Fatigue Striations are a warning sign of design or fabrication errors.
Workshop Practice & Action Guidelines
Fatigue Striations are only visible under a scanning electron microscope. Macroscopically, there are beach marks that look similar but are coarser. If fatigue is suspected, the practitioner must carefully examine the fracture surface. Prevention measures: smooth weld transitions (grinding, TIG remelting). Avoid lack of fusion. Reduce stress concentrations. Introduce compressive residual stresses (shot peening). Control notch radius and surface roughness.