Why Cracked Windshields are Dangerous
Cracked windshields do not offer the same level of security and protection as undamaged or repaired windshields. Based on our tests, cracked windshields, after approximately four months of exposure to the elements, no longer function safely.
When Safety Glass is No Longer Safe.
As tests indicate below, the safety in safety glass goes out the window when cracks appear. An automobile windshield is composed of safety glass, and called such, because it acts as a safety net preventing people from being ejected through the front windshield during impact. The windshield should also shatter into little pieces so that cuts sustained from the broken glass are generally small.
The results of our recent testing would indicate that cracked windshields, after approximately 4 months of exposure, no longer function as they are supposed to. The laminate, used to bond the glass, appears to dry out and lose its flexibility as well as its adhesion to the glass directly at the crack. (See Figure 1.)
This unrepaired crack after only four months of exposure to the elements failed! The five pound ball broke through the glass.
How we conducted our tests.
In recent testing, we sought to find out the effects cracks would have on safety glass. We did this by conducting penetration tests. The ANSI Z26-based penetration tests involved dropping a 5-pound steel ball from 12 feet onto a glass sample held in a wooden frame. This is done to simulate a large ojbect such as a head impacting a car's windshield. The test is considered a failure if the object successfully penetrates the glass and travels beyond.
We purchased some laminated glass samples, cracked one side and placed them outside. Every two weeks we would pick one up and perform a penetration test to see if the laminate was deteriorating from the exposure.
After the first two months, the windshield samples didn't appear to show any signs of laminate deterioration and were still passing the penetration tests. (See Figure 2.)
|This unrepaired glass, at two months exposure, still passes the penetration test. The ball has not passed through the glass.|
At about three months, signs of the effects of exposure on the cracked glass began to indicate conditions for failure, as figures 3 and 4 suggest.
Unrepaired crack after 3 months of exposure. Sample could be folded in half at the crack.
|After 3 months the sample could be ripped in two at the crack.|
At about four months, the samples began to fail the penetration test; they began to crack in half and the ball began falling through the glass. (See Figure 1.) Those samples that didn't crack in half upon impact could easily be pulled apart at the crack with two fingers. These occurrences are failures in a penetration test. We also know from flexural strength tests (ASTMC 158-84) that a cracked, unrepaired laminate has a lower modulus of rupture, which means it breaks easier and in impact in the vicinity of the unrepaired crack will cause rupture at the crack. An Ultra BOND repair cures this problem.
How to Protect Your Customers
Out of curiosity, we repaired some of the glass samples AFTER the 4-month exposure period and then again placed them outside for about another week. A penetration test was performed on these samples. They passed! Remember unrepaired samples of the same batch had just failed a week before. The ball did not fall through, and there was no splitting or spalling of the glass. As a matter of fact, some samples passed the test just one hour after repair.
After four months exposure and then an Ultra BOND repair. The glass passed with flying colors.
It is surprising that some states allow vehicles with cracks in the windshield to be driven. Why pass a law requiring cars to have laminated safety glass if you are going to allow them to crack and lose their function as safety glass? The answer to this, of course, would be to replace the windshield or repair it with the Ultra BOND process.
The Answer is in the Details.
One reason that the Ultra BOND process is a necessity to restore the function of the windshield is because the resin in the middle and edge section of the crack must be at least 150 cps in order to prevent deterioration. (Ultra BOND licensees can use resin of 150 cps or higher in the middle and edge of the crack.) Resins lower than 150 cps do not have enough molecular links to withhold the stress at the edge and will break apart, usually within 6 months, exposing the laminate to the environment again and leading to an unsafe cracked windshield approximately 4 months later.
However, using the proper resin viscosities as we do, the repair will hold and not deteriorate. As a member of the Ultra BOND team and following these procedures correctly, you need not have a safety concern. You can feel comfortable knowing, after you perform a repair, that you have restored the structural integrity and safety functions of that windshield.