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This material first appeared in the I-CAR Advantage Online, which is
published and distributed free of charge. I-CAR, the Inter-Industry Conference
on Auto Collision Repair, is a not-for-profit international training
organization that researches and develops quality technical education programs
related to collision repair. To learn more about I-CAR, and to subscribe to the
free publication, visit www.i-car.com.
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Some believe that straightening damage to exterior panels is becoming
somewhat of a lost art form. In the pre-unibody era, a large percentage of
damaged panels were repaired instead of replaced. For a variety of reasons, many
more damaged panels are now being replaced rather than repaired. This doesn’t
mean that repairers don’t still need to have metal straightening as part of
their skill set and knowledge base. Straightening an exterior panel when it’s
practical can have advantages, some of which may be overlooked at first
thought.
This article, part one of a two-part article on straightening exterior
panels, discusses the science and theory behind metal shaping and how it relates
to straightening damaged panels. Part two of this series will show practical
application of this science when straightening a panel and specific procedures
used during the straightening process.
Intrusiveness of a Repair
Something that should be considered when making a repair or replace decision
is the intrusiveness of the repair. Replacing welded-on exterior panels requires
disturbing many factory spot welds, and the factory-applied corrosion
protection. While the straightening process will also disturb some corrosion
protection, it typically does not in the flange areas where it is the most
difficult to properly restore. Because of this, there may be an advantage to
straightening a welded-on panel when possible. Depending on the location of the
damage on the panel, straightening may also avoid the need to blend the finish
into adjacent panels.
So how do you make a decision whether to repair or replace a panel, and how
do you proceed once the decision has been made to straighten a panel? First you
need to understand a little about the properties of metal, how it is formed, and
what happens to it when it is damaged.
Metal Properties
Metals, both steel and aluminium, are made up of a basic crystalline
structure. The atoms of the metal are bonded together in lattice-like
arrangements that form crystals. These individual crystals are then arranged in
a granular structure that makes up the metal. The crystalline and granular
structures of metal are in a relaxed state when the metal has not been formed or
damaged.
To understand what happens to metal when it is formed or damaged, you need to
understand the difference between elastic and plastic deformation. When an
external force is applied to the metal, it will bend. Elastic deformation
is when the metal returns to its original shape after the force is removed.
There is no permanent change to the crystalline structure of the metal. Once the
metal is bent beyond its elastic limit, it is said to be in the plastic
deformation range. In the plastic deformation range, the metal will not
return to its original shape when the force that is bending it is removed. There
has been a permanent change in the crystalline lattice structure of the metal.
The individual atoms that make up the crystals have shifted in position in
relation to each other and will not shift back when the force is removed. This
also causes a change to the grain structure of the metal compressing some grains
and stretching others. This change to the grain structure is what causes stress
build-up and work hardening of the metal.
If the grains are stretched into the plastic range in one area only, this
will place a stress or force on the metal around the stretched area. If the
force is great enough, it will also cause the metal around the stretched area to
move. This movement will be in the elastic range so it wants to return back to
its original shape, but cannot because of the force placed on it by the
stretched metal. This places the panel under stress and makes it unstable and
unpredictable. This is exactly what causes what we know as the "oil canning"
effect.
Two- and Three-Dimensional Shape
Exterior panels are a combination of two-dimensional shape, created by simple
bending and rolling of the metal, and three-dimensional shape, created by
shrinking and stretching the metal. Two-dimensional shape typically does not
require plastic deformation to create, but three-dimensional shape typically
does.
When panels are hand fabricated, the three-dimensional shape is created first
and the two-dimensional shape is refined or added. Panels that are made by
pressing into a die create both the two- and three-dimensional shapes at the
same time.
A simplified way to think of this is that two-dimensional shape is the length
and height of the panel and the basic curvature in one direction only.
Three-dimensional shape is the refined shape made up of compound curves, such as
the end of a quarter panel around the tail lamp pocket, or a curvature that
forms a crown in both the length and height directions at the same time.
Plannishing for Stress Relief
To avoid causing areas of high stress and oil canning, panel fabrication
requires both stretching and shrinking of the metal. Still, when a panel is
formed, there may be areas of spot stress and work hardening. To form a panel
that is stress-free, all of the metal movement done to put three-dimensional
shape into the panel needs to be in the plastic range. If it is not, the panel
has to undergo some form of stress relieving after forming. Heat treatment is an
example of how some formed panels may be stress relieved. Another method of
stress relieving an unstable panel is to lightly plannish every part of it.
"Plannishing" is basically slightly stretching the entire panel evenly by
hammering lightly using the on-dolly technique. This smoothes any small
irregularities in the panel and evens the stresses in the metal. This is the
method often used to stress relieve hand-fabricated panels. If a panel is
stress-free, you could cut it in half and both halves would maintain their
shape. If there are stresses in areas of the panel and it is cut through, the
stressed areas will move and change shape after it is cut.
Applying the Theory
How does all of this apply to repairing a damaged panel?
When straightening collision-damaged exterior panels, two-dimensional shape
is restored first and three-dimensional shape is refined and added, the exact
opposite of the fabrication process. Restoring two-dimensional shape makes up
the majority of the straightening work during collision repairs to exterior
panels.
As an example, this truck that has been hit in the rear with the bumper
pushed into the lower part of the bedside. The buckle over the wheel opening is
a change in two-dimensional shape. The damage behind the bumper is a change in
both two- and three-dimensional shape. The majority of the damage to this
bedside is a change to two-dimensional shape or length. The change in length has
placed forces on the entire panel that have caused elastic deformation, creating
the buckle. The first step in repairing this type of damage is to restore the
length to the bedside by pulling. This will remove most, if not all, of the
buckle over the wheel opening. Next, the dent behind the bumper could be pushed
back into its basic shape from behind. Only after these two operations should
any stretched areas, or changes to three-dimensional shape be repaired.
Let's look at another example; this time a vehicle that has been hit from the
side and has the quarter panel pushed in above the wheel opening. As with the
truck example, the majority of this damage is a change in two-dimensional shape.
The first step in the repair is to push the damage out from the backside,
restoring the two-dimensional shape to the panel. Then any stretched areas canbe
repaired by shrinking the metal, restoring any change to the three-dimensional
shape.
It is important that whenever restoring three-dimensional shape that all
metal movement be in the plastic deformation range. This usually requires
shrinking the metal more so than stretching. Collision forces will typically
stretch the metal much more than shrink it, so repairing the damage requires
removing the stretched areas and the stress that has been introduced. A large
amount of change to the three-dimensional shape of a panel, basically meaning a
lot of stretched metal, like on the side of this truck, will typically make a
panel difficult to repair. Repairable panels typically do not have a large
amount of three-dimensional shape change or stretched metal.
The Paper Prop
Using a plain sheet of paper, hold the paper up and push the ends of it close
together. The majority of the "damage" to the paper is a change in
two-dimensional shape. The original shape of the paper is easily restored simply
by pulling the ends back to the original position.
Now take the same sheet of paper and fold it up into a ball, causing large
amounts of both two- and three-dimensional shape change. The paper is now very
difficult to restore back to original shape and condition. The creases in the
paper are similar to stretched areas of metal and will hold incorrect shape
unless completely removed.
Conclusion
Exterior panels are a combination of two-dimensional shape, created by simple
bending and rolling of the metal, and three-dimensional shape, created by
shrinking and stretching the metal. During fabrication, three-dimensional shape
is created first, followed by two-dimensional shape. For repair, the process is
reversed.
Damaged panels with mostly two-dimensional shape loss are the best candidates
for straightening. Panels that have large amounts of three-dimensional shape
loss are difficult to straighten because they typically require a lot of metal
shrinking to restore the proper shape. Think about the forces acting on the
panel and be sure to remove any outside forces created from misalignment of the
vehicle structure before attempting to straighten the panel itself.
Next week: specific procedures used during the straightening process.
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