Where do fatigue cracks typically initiate in aircraft structures?

Fatigue cracks typically initiate at stress concentrations and high-stress locations: fastener holes (most common, especially in lap joints), fillets and corners, material defects or inclusions, surface scratches or corrosion pits, thickness changes, repair areas, and manufacturing defects. In pressurized fuselages, critical locations include window and door corners, longitudinal lap joints, and circumferential splices. Understanding crack initiation sites guides inspection programs (where to look) and damage tolerance design (ensuring cracks remain detectable before becoming critical). Proper manufacturing and surface treatment delay initiation.