61 Chapter 4 Fundamentals of Collision Damage
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stretching flat sheets. A highly plastic metal can be
readily shaped by bending and stretching. The grain
arrangement allows the grains to elongate or compress
without breaking. A highly plastic metal can be formed
into complex shapes, such as fenders and quarter
panels. However, a highly plastic metal is not strong.
Nevertheless, strength can be designed into the part
by adding bends and curves in the part. Like elasticity,
plasticity varies according to metal type and annealing
procedure.
The concept of plastic deformation is illustrated
in Figure 4-4. If the applied force moves the metal
beyond its yield point, it will cause a change in the
grain arrangement in one small area of the metal. This
permanent change in the grain arrangement is called
plastic deformation.
The area where the grain arrangement has
changed is called a buckle or a bend. The differ-
ence between a bend and a buckle is that a bend is d
purposely put into the part when the part is manu-
factured, while a buckle is caused by impact force.
Both bends and buckles are formed by the elongation
of the outer grains and the compression of the inner
grains. The stiffness, or hardness, of the buckle holds
the metal out of place. The metal around a buckle is
not damaged. It is simply forced out of position, or
displaced, by the buckle.
Component Shapes
The shape of body panels and structural compo-
nents will impact the way they react when damaged.
Bends, body lines, and crown are often formed into
panels to increase strength and improve appearance.
To understand a bend, consider a flat piece of metal,
such as the sheet metal blank that a car part is made
from. When viewed from the side, the metal’s grains
are all the same size and are arranged uniformly,
Figure 4-5. During the forming process, some areas
of the sheet metal blank will be bent. A look at a cross
section of the grain arrangement in the bend will reveal
that the outer grains are elongated and the inner grains
are compressed, Figure 4-6. This arrangement is
seen in all bends. In mild bends, the elongation and
compression are less pronounced than in severe
bends. If we were to push on the metal in a bend, we
would find that it is stronger than unbent metal. This is
because the change in grain structure increased the
strength of the metal in the bend area. The increase
in strength is often called work hardening. Work
hardening in severe bends, such as in body lines,
strengthens the panel.
A body line is a styling line in a hood, fender, or
door. The body line makes the panel rigid and less likely
to flex. In addition to being box structures with four bent
corners, unirails—the frame rails on unibody vehicles—
have many bends, or convolutions, in them that provide
structural strength and energy management during a
collision. Many of these unirails are made of mild steel.
The vehicle designers calculate the number and location
of bends to give a part the desired strength.
A bend that is less severe than a body line is called
a crown. Crown is the curvature of a panel. Crown is
found on all outer body sheet metal panels. These are
the four basic types of crown:
■ Low crown—Almost flat, with very little curvature. If
you push in on a low-crown panel, it will flex in and
pop out undamaged.
■ High crown—Highly curved and rigid. You cannot
push the panel in with your finger. It will not flex.
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Figure 4-4. The panel has been moved beyond the
yield point, resulting in plastic deformation.
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Figure 4-5. In unworked steel, the grain pattern is
uniform and the grains are the same size.
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Figure 4-6. The grains are distorted in the bend. The
inner grains are compressed and the outer grains are
elongated.