Final answer:
A solidification crack is a typical defect found running along the center line of a weld, occurring during the solidification stage of welding and affecting the mechanical properties of the metal.
Step-by-step explanation:
The kind of defect that can usually be observed running along the center line of a weld is a solidification crack. These cracks occur during the solidification phase of the welding process when the weld metal undergoes volumetric shrinkage. If the liquid film present along the centerline of the weld bead becomes too thin before solidification, the adjoining grains can't fuse properly, leading to a solidification crack. This type of defect is a line defect that weakens the structure along a one-dimensional space and can significantly affect the mechanical properties of the welded metal. Line defects like solidification cracks are important to identify and prevent as they directly impact the durability and strength of the welded joint.