|
Ask the Doctor
pros who know
The Dollar Bill Isn’t What It Used to
Be!
by Richard Campfield
In today’s economy, the above phrase is even truer than before, and we
believe it and accept it. How many times have you heard “it’s not worth
the paper it was printed on?” And yet, here we are today with the insurance
industry and others using “the size of a dollar bill” as an adjusting
criteria that has been of no value to the consumer for more than 20 years.
Back in the 1970s and 1980s the following statement was published in a
few windshield repair training manuals: “Cracks over 6 inches are not
repairable because the stress is greater than the resin can withstand.”
This statement was not correct; in fact it is just the opposite. In all
my research and involvement in the industry during the last 24 years,
I have never found one document of evidence showing a scientific difference
between a crack longer than 6 inches—whether repaired or not.
I believe length is not the determining factor for repair-ability; contamination
is. There is a correlation between length and contamination, but 6 inches
is not even in the ballpark. About 80 percent of cracks shorter than 18
inches are not contaminated, but approximately 80 percent of cracks longer
than 18 inches are contaminated. Likewise, the length of the crack and
the length of time the crack has existed are related. Almost all cracks
shorter than 14 inches (which is 72 percent of the repairable crack market)
are not contaminated, which is why I think that is a good length for the
Repair of Laminated Glass Standard (ROLAGS) to address. (Recently, the
ROLAGS Committee voted to adjust its scope, to address cracks of 6 inches
or less, and to begin development of a second standard for cracks 6 inches
or longer. Prior to that, it had covered repair of cracks up to 14 inches.)
Consumer Feedback
Getting back to the length of a dollar bill again, I can honestly say
that I have never heard of one consumer or crack repair technician complaining
about a repaired crack because it was longer than a dollar bill (or 6
inches). To this day, I know of no evidence of a difference because the
crack is longer than 6 inches, repaired or un-repaired.
There is documentation, however, that 90 percent of all cracks are longer
than 6 inches. Additionally, 90 percent of repaired cracks are edge cracks
and most windshield replacements are caused from a crack that started
at the edge. Also, 95 percent of edge cracks are longer than 6 inches.
Ten percent of the repairable crack market consists of floater cracks,
but we will cover the floater crack in another issue. This is about the
edge crack.
“I have never found
one document of evidence showing a scientific difference between a crack
longer than 6 inches—whether repaired or not.”
Edge Cracks
So, why are such a large percentage of windshield cracks edge cracks?
This happens because during the manufacturing bending and annealing process,
a thermal effect causes the edge of the glass to be weakened, known as
residual stress. The residual stress is an area about two inches wide
(20 percent of the exposed surface area of the average windshield) around
the perimeter and is 1,000 psi or less on an OE and usually higher on
replacement glass. This is why this area fractures twice as easily as
the rest of the windshield. This defect became exposed after the first
gas crunch in the 1970s when manufacturers reduced the thickness of the
windshield glass to reduce weight; this is when windshield claims began
to comprise 30 percent of all comprehensive insurance claims in the United
States.
Induced Stress
In addition, there is another stressor called induced stress. Induced
stress occurs when the windshield is installed in the vehicle, intensifying
and expanding the residual stress and adding its own stress. Then, when
a fracture occurs in this already weakened perimeter area of the windshield,
it cracks almost immediately to relieve this induced stress (located on
the first 4 to 6 inches), and stops when the lamination stress pulls the
crack back together. The inward positive lamination stress becomes greater
than the negative installation stress and stops the crack. This occurs
when the crack is at approximately 8 to 12 inches—the length of approximately
52 percent of the crack repair market. Installation/induced stress is
also why 95 percent of edge cracks are longer than six inches.
The SAE Technical Papers 1999-01-3160, titled “Windshield Investigation
– Manufacturing Installation Stresses,” describes the aforementioned stresses
as follows:
“Two primary sources of stress on a windshield are manufacturing stress
(referred to as residual stresses), and installation stresses (referred
to as induced stresses).”
We have discussed the reasons why a crack is likely to occur and why
it will quickly and easily go beyond the “dollar bill” size. What about
repair-ability as far as technician skill is concerned? Well, if you ask
a technician properly trained in crack repair if he would rather repair
a 12-inch crack or one that is shorter than 6 inches (or dollar bill size),
the response likely will be “12 inches, because it is easier.”
Stress Relief
Why? Because the stress in that crack has been relieved. It is not only
easier to repair but, once repaired, it is more adhesive-friendly because
adhesives react positively when they are not under stress. An edge crack
smaller than 6 inches is still under negative outward installation stress
at the point, which is why 95 percent of cracks are longer than 6 inches;
the lamination stress and induced stress are still fighting for control
at the edge, making it more temperamental to repair.
On the other hand, cracks longer than 6 inches have no stress at the point
and less stress in the edge area. It accepts the structural adhesive resins
more easily, and, when the correct resin properties are used, the cured
resin is under less stress and is being helped by the positive inward
lamination stress. A properly repaired long-edge crack is structurally
superior to a properly repaired chip repair, because chip resins are adhesives
and crack resins are structural adhesives.
In summary, it is the windshield itself that enables a fracture to crack
and to extend to more than 6 inches in length. Fixing a cracker longer
than the size of a dollar bill is easier than fixing one that is smaller,
and you are basically using the same methods, tools and resins to perform
both types of repairs, but, still, cracks longer than 6 inches are easier
to repair. So when you hear insurers, customers, etc., refer to “a crack
longer than a dollar bill,” just tell them, “a dollar bill ain’t what
it used to be!” n
Richard Campfield is the founder and president of Ultra Bond Inc.
in Grand Junction, Colo. Mr. Campfield’s opinions are solely his own and
not necessarily those of this magazine.
AGRR
© Copyright 2010 Key Communications Inc. All rights reserved.
No reproduction of any type without expressed written permission.
|
