Answer
Edge cracking is a manufacturing defect that occurs during the coiling or stamping of the flat wire, particularly in high-hardness materials like SAE 1090. If the edges of the wire have micro-burrs or 'slitting cracks' from the rolling process, these act as intense stress concentrators. During the forming of the waves, the outer edge of the wave is in tension. If the local stress exceeds the fracture toughness $K_{IC}$, a crack propagates. In service, these cracks grow via fatigue. To analyze this, a cross-sectional metallographic examination is performed. Prevention involves 'edge conditioning' or 'round-edge' processing of the wire before coiling to ensure a smooth, compressive stress state on the edges, which is standard for aerospace-grade springs.
