Volume 12, Issue 3 - May/June 2010

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.


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