108 Steels: Metallurgy and Applicationsanvil and an expanding die. When the punch has penetrated the double thickness
of steel to be joined, the displaced element of steel is compressed between the
punch and the anvil. This causes the element of the upper sheet to spread beneath
the lower sheet thus forming the joint. It is found that the strength of press-
formed joints depends on the thickness and strength of the steel being joined.
The maximum load that can be generated in a shear test is higher in the direction
perpendicular to the two cuts, but the elongation to failure is several times greater
in the direction parallel to the cuts than in the perpendicular direction.
The method has been adapted by the use of multi-point tooling to make a large
number of separate joins by means of one movement of the tool. The orientations
of each joint may be selected to give the optimum directionality of the strength
of the final composite joint.
Joining using fasteners
Fasteners include screws and rivets. Formerly, these required holes to be drilled
prior to the insertion of the screw. Self-drilling and self-tapping screws, however,
are now available which greatly simplify assembly since access to the reverse
side of the steel is not needed. A similar benefit may be obtained, however, using
compressive fasteners which have a nut which forces the legs apart to clamp two
steel sheets together.
A recent advance is the development of self-piercing rivets which are now
coming into common use. The basic form of this device is semi-tubular, as
illustrated in Figure 1.109(a). The joining process itself occurs in two stages but
these are combined into a single continuous operation. In the first stage, the rivet
is pressed into the two sheets to be joined and its shank cuts a hole through
the top sheet and partially through the bottom sheet. In the later stages of travel
through the bottom sheet, the sheet material is displaced by the die causing the
shank of the rivet to flare out and lock itself into the steel. A typical cross-section
of a riveted joint is illustrated in Figure 1.109(b).
Figure 1.109 Diagrams illustrating the cross-section of a self-piercing rivet (a) before
and (b) after riveting 158