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The Dollar Bill ”Ain’t what it used to be!”

In today’s economy this 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-windshield manufacturer networks using an adjusting criteria “the size of a dollar bill” that has been of no value to the consumer for over 20 years.  

Back in 1970’s and 1980s the following statement was published in a few windshield repair training manuals:  “Cracks over six-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). It was accepted and believed, and yet during my research and involvement in the industry for over 24 years, I could never find one document of evidence showing a scientific difference between a crack over 6” whether repaired or not.

Length is not the determining factor for being repairable, contamination is and there is a correlation with length and contamination but six inches is not even in the ballpark. About 80% of cracks under 18-inches are not contaminated but approximately 80% of cracks over 18 inches are contaminated. Length of the crack and the length of time the crack has been there are related. Almost all cracks under 14-inches which is 72% of the repairable crack market are not contaminated, which is why that is a good length for the ROLAGS.

Repairable Crack Market - Survey of 1,142 Crack Repairs

90% were edge cracks
10% were floater cracks
95% of edge cracks were over six inches

Length of cracks repaired:

6 inches and less – 10%
7-14 inches – 62%
15 -18 inches – 17%
over 18 inches – 11%

With the advancements in repair technology and collection of data starting in 1986 we have been able to offer the consumer far more savings and better repairs.

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 ever complain about a repaired crack because it was longer than a dollar bill or six inches and to this day, no one has produced evidence of a difference because the crack is longer than six inches, repaired or un-repaired.

There is documentation; however, that 90% of all cracks are over six inches.  Additionally, 90% of repaired cracks are edge cracks and most windshield replacements are caused from a crack that started at the edge and 95% of edge cracks are over six inches. 10% 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.

So, why are such a large percentage edge cracks?  Because during the manufacturing bending and annealing process a thermal effect causes the edge to be weakened, known as residual stress.  The residual stress is an area about two inches wide (20% 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 an ARG. 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 the1970s when they reduced the thickness of the windshield glass to reduce weight and this is when windshields started to become 30% of comprehensive insurance claims in the United States.

Guardian Industries – Bending Gate Manual - Annealing and Residual Stress
“The peak compressive value will occur at the edge and a band of tension will exist just in from the edge. This tensile stress will then taper to zero usually at maximum 35mm. This phenomenon then causes there to be a perimeter band on the part which is subjected to residual tensile stress and is hence the weakest area of the windshield.”

Now added on to this already existing residual stress, is another stressor, called induced stress.  Induced stress occurs when the windshield is installed onto the vehicle which does two things: first intensifies and expands the residual stress and second adds it’ own stress so that when there is a fracture in this already weakened perimeter area of the windshield it cracks almost immediately to relieve this induced stress (located on the first 4- 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 8-12 inches - the length of approximately 52% of the crack repair market and why 95% of edge cracks are over six inches – the installation/induced stress.

SAE TECHNICAL PAPERS 1999-01-3160
Windshield Investigation – Manufacturing & Installation Stresses.
“Two primary sources of stress on a windshield are manufacturing stress (referred to as residual stresses), and installation stresses (referred to as induced stresses).”

windshield-repair-journal-may-june-2010-clip-image002.jpg

 

windshield-repair-journal-may-june-2010-clip-image004.jpg

O Outer edge of windshield prior to installation
S showing manufacturing stresses. Source - SAE

 

S Same area of windshield after  installation
showing manufacturing  and  installation stresses. Source - SAE

Questions about the Frit – Why is there black paint under outdoor glass where there are already two stresses when glass fracture easier when it is hot? What happens with a single sheet of glass that is varying in temperature?

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, one properly trained in crack repair, if he would rather repair a 12-inch crack or one that is less than 6 inches (dollar bill) and the response will be “12-inches because it is easier”.  Why? Because the stress has been relieved. It is not only easier to repair but once repaired it is more adhesive friendly because adhesives like it when they are not under stress. An edge crack under 6”  is still under negative outward installation stress at the point, which is why 95% are over 6” and 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 over 6” have no stress at the point and less stress in the edge area. It accepts the structural adhesive resins easier, 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. (See the photo of a 12-inch crack repair that held in a rollover crash.) So if one has the skill and technology to properly repair a six inch crack then they can certainly and more easily repair a twelve inch crack.

windshield-repair-journal-may-june-2010-clip-image006.jpg

Repaired 12-inch crack, in between the arrows, did not fail in a Rollover Accident.
windshield-repair-journal-may-june-2010-clip-image008.jpg
 

Jeff and Melody Reddell, owners of Pace Windshield Repair and Replacement who repaired the 12 inch crack in this vehicle that was in a rollover a few months later. Factory seal was saved, no roof-crush, no injuries.

In summary, it is the windshield itself that enables a fracture to crack, and to extend to over 6” 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 over 6” is easier.  So when they say a crack longer than a dollar bill. . . just tell them a dollar bill ain’t what it used to be!

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References

   FAQ
   Before/After Photos
   Incorrect Resin Photos
   Correct Resin Photos
   Fundamentals of Windshield Repair Adhesives
   Repairable Windshield Damage
   Windshields are Made to Crack
   Windshields Repair Standard (ROLAGS)

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