Stress intensity factor
The stress intensity factor K describes the stress concentration at a crack tip. It depends on the tensile stress, crack length, and component geometry. If K exceeds the material toughness K_IC, the crack propagates rapidly.
Physikalisch-chemischer Wirkmechanismus
At a crack tip, stresses concentrate. There, the stress becomes theoretically infinite. The stress intensity factor K indicates the strength of this stress concentration. K is calculated from the nominal stress, crack length, and a geometry factor. The unit is MPa·√m. Every material withstands up to a critical value K_IC. If K is less than K_IC, the crack remains stable or grows only slowly. If K is greater than K_IC, the crack propagates suddenly.
Metallurgical Relevance & Welding Risks
Welds often contain small cracks or flaws. Additionally, welding residual stresses create high tensile stresses. Both increase the stress intensity factor K. In the heat-affected zone, martensite can form. Martensite has low fracture toughness K_IC. Then even a small crack is enough to drive K above K_IC. The crack grows brittle and without warning. Hydrogen in the weld metal intensifies this hazard.
Werkstattpraxis & Handlungsanweisungen
Avoid cracks by clean joint preparation and suitable filler materials. Maintain the specified preheat temperature. This slows cooling and prevents hard martensite. Reduce welding residual stresses by a proper welding sequence or stress relief annealing. If you find a crack after welding, measure its length and evaluate it using a fracture mechanics method. If K exceeds the allowable value, the crack must be completely removed and rewelded.