63 Chapter 4 Fundamentals of Collision Damage
Copyright Goodheart-Willcox Co., Inc.
again be damaged. Which ball would cause the most
damage? Most everyone will agree that the bowling
ball would do the most damage. All three balls would
be moving at approximately the same speed, but the
heaviest ball would have the most force and, there-
fore, cause the most damage. The damage would be
caused by the energy from the balls rearranging the
grain structure of the hood. The golf ball would not
cause damage because the force of the golf ball would
not exceed the yield point of the metal. The yield point
is the minimum amount of force required to cause a
permanent change in grain arrangement. The force
of an impact must be greater than the yield point for
permanent deformation to occur. The force of the base-
ball was great enough to rearrange the grain structure
in the hood, but because there was less force caused
by the baseball than the bowling ball, the grain rear-
rangement would not be as extensive as that caused
by the bowling ball.
The other component of force is speed. If we
compare two objects of the same weight with one
moving twice as fast as the other, the faster-moving
object has twice the force of the slower moving object.
Therefore, the force that a vehicle has at the time of
collision depends on the weight and the speed of the
vehicle.
When force first acts on an object, the object may
simply move. This is sometimes seen in a collision.
When an object impacts a fender, the fender may
move, or shift. Movement may not be possible because
the fender is held in place by bolts, braces, and adja-
cent panels. If the fender cannot move and the force is
greater than the yield point, deformation will result.
There are two ways force can impact on a panel,
longitudinally and laterally. In Figure 4-9, a longitudinal
force (force applied from one end) is applied to a piece
of sheet metal. If the sheet metal panel is not attached
to anything, the force will simply move the entire panel
to the left and there will be no distortion of the panel.
In Figure 4-10, the panel is attached, preventing any
movement. A force greater than the yield point is applied
at A. The attachment areas at B and C act as pivot
points. The metal will fold between points B and C. As
a fold is created, the grains in the buckle become work
hardened and more resistant to further bending. A
new buckle will form in the weaker metal ahead of the
first buckle (between D and E). The second buckle will
become work hardened. Assuming the force is removed
at this point, no further damage will occur.
An examination of the damaged panel shows that
the overall length is reduced, but the surface area has
not been reduced. If we were to measure the total
length of the panel by measuring each fold, we would
find that the total length has not changed.
To demonstrate this concept, hold a sheet of
computer paper lengthwise with one hand at either end.
Move one hand toward the other. Note how the paper
folds. On sheet metal, these folds are called simple
rolled buckles. Next, simulate a body line by making
a longitudinal fold in the paper. Again, hold the paper
lengthwise with one hand at either end and move one
hand toward the other. As you will notice, the folded
paper is much stiffer than the flat sheet. Note how the
distortion takes place at the fold. See Figure 4-11. This
type of damage is called a collapsed-hinge buckle and
can often be found when a fender is hit from the front.
The longitudinal force causes collapsed-hinge buckling
at the body line. The metal at the body line is very stiff.
If a buckle forms at a body line the damage will be even
stiffer.
If the panel is not attached to anything, lateral
force (force applied from the side) will simply move the
panel, causing no damage. In Figure 4-12, a low-crown
Goodheart-Willcox Publisher
Figure 4-9. Longitudinal force applied to the
unattached fender simply moves the fender.
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Figure 4-10. The panel buckles first at B-C, where the
attachment points act as pivot points. When the buckle
at B-C resists additional movement, another buckle
forms between D-E.
Overall length
A
C
E
Buckle
B
D
Total length
A
D
B
Longitudinal
force
Entire panel moves