Persistent slip band
A persistent slip band is a fine line on the metal surface caused by cyclic plastic deformation. Dislocations emerge at the surface and leave behind a step that concentrates stress. That's where most fatigue cracks start.
Physical-Chemical Mechanism of Action
Under cyclic loading, dislocations in a metal grain move back and forth on specific slip planes. When they emerge at the surface, they create tiny steps – called extrusions and intrusions. With each cycle, these steps grow. They act like notches and significantly increase local stress.
Metallurgical Relevance & Risks in Welding
Welds often have residual stresses and sharp transitions at the weld toe. Under service load, persistent slip bands tend to form there. A slip band quickly becomes a microcrack – and then a fatigue crack. This is especially critical in hard, martensitic microstructures: they can't relieve slip bands through plastic deformation. Excessive surface roughness on the weld also promotes slip band formation.
Workshop Practice & Action Instructions
Reduce all notches on the weld. Grind weld transitions smooth (roughness Rz < 20 µm). Shot peening or needle peening introduces compressive residual stresses in the surface layer, which significantly delays slip band formation. Control the cooling time t8/5: for unalloyed steels, hold t8/5 between 10 and 25 seconds to avoid martensite. For existing slip bands, only complete grinding out and re-evaluation of the weld helps.