Crack Flank Displacement

Crack flank displacement basically means the two sides of a crack shift laterally relative to each other under load. That's a sign that the crack is under stress and can propagate. When welding, you mainly need to watch out for this if the weld or heat-affected zone is brittle or cools too quickly. Then the part can simply tear further under load. So control the cooling to keep the structure tough, and ensure a clean, crack-free weld cross-section.