A first width W1 in the first direction DR1 of the second protrusion portion PRT2 may be greater than a second width W2 in the first direction DR1 of the third protrusion portion PRT3. Accordingly, a first gap GP1 between an end of the third protrusion portion PRT3 of the (h+1)-th joint unit JU and a lower end of the body portion BDP of the h-th joint unit JU may be greater than a second gap GP2 between an end of the second protrusion portion PRT2 of the (h+1)-th joint unit JU and an upper end of the body portion BDP of the h-th joint unit JU.

The first gap GP1 may be defined by a gap between the lower ends of the joint units JU adjacent to each other. The second gap GP2 may be defined by a gap between the upper ends of the joint units JU adjacent to each other. In one exemplary embodiment, for example, the first gap GP1 may be in a range from about 0.3 mm to about 0.8 mm. The second gap GP2 may be greater than about zero (0) mm and equal to or smaller than about 0.1 mm.

The first protrusion portion PRT1 may include a concave portion CNC extending in the first direction DR1 from the one end of the body portion BDP and a convex portion CNV extending in the first direction DR1 from the concave portion CNC. When viewed from a plan view in the second direction DR2, upper and lower surfaces of the concave portion CNC may have a concave shape. When viewed from the plan view in the second direction DR2, the convex portion CNV may have an oval-like shape.