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The Case of the Spontaneous Windshield Breakage

Cracked windshield
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Question regarding spontaneous windshield breakage:

Dear Glass Detective,

I have a crack that formed on the lower passenger side corner of my windshield. There is a small star pattern with a dominant horizontal crack. The star is centered over a heating element in the windshield designed to prevent ice from building up on the windshield wipers. I am looking for an opinion on whether the star pattern necessitates an external trauma? Toyota thinks so, so they won’t replace the windshield under warranty. They say the star must mean trauma. No chip present, if so minuscule (less than 1mm diameter and that’s debatable). The star sits under the top half of the wiper blade, which would shield it from road debris (unless the wipers were on). I don’t recall hearing anything impact the windshield while driving. The plane of the wiper bladed coming down on the windshield is about 1/2cm below the center of the star, so the wiper blade coming down could not have caused the trauma. My hypothesis is that the heating element caused the crack, but I don’t know id a focal area of heat can cause a star pattern. What do you think caused the crack–heat or mystery trauma?

Thank you,

Micha J.

Answer to question regarding spontaneous windshield breakage:

Thank you for contacting the Glass Detective regarding your request for some help in determining what may have broken the windshield on your Toyota. I have carefully read and reread your very informative and concise e-mail about the matter and I think I might possibly understand the answer you would most likely want to get … although what I am about to state may not get you there. With this in mind, here are my responses/comments to your questions based on 47 years in the glass business and hundreds of personal experiences with a great deal of reading on this subject as well. All comments will be mine and mine alone but you may feel free to share them as you deem appropriate. Here we go:

  1. I believe, based on what you have written, that the outer piece of glass (in the windshield) was the only piece to break or was the first to break. Windshields are made with two pieces of glass laminated together with an inner layer of vinyl (a sandwich if you will with the glass being the bread and the vinyl being the peanut butter in between the two pieces of bread). Feel free to substitute a different food type for the inner material (vinyl) if you’d like. I am just partial to peanut butter.
  2. If, as we are supposing above, the outer piece broke first, the most likely cause for the breakage was something hitting the outside of the windshield with enough force to break it. Often, the inner lite of glass will not break simply because the outer one does.
  3. Very often the break will start at an origin spot caused by road debris of some sort. A small pebble, a bird, a piece from a tree, or whatever. An innocent looking piece of debris can easily break the outer lite of glass when the car is moving because if you are going 60 miles an hour, the object hitting the glass is getting its force from the car’s speed (at point of impact). Even at low speeds, certain objects can easily break the outer glass in a windshield.
  4. The break process you describe (star pattern that then runs horizontally as a crack) is extremely common. The star is where the glass initially got broken and then opened up and “ran” to (usually) the nearest glass edge.
  5. The heating element may or may not have contributed to the “running out” of the break. If it was extremely cold and you put the defroster on high, it is possible that the thermal shock or temperature differential accelerated the “run” from the break origin. But in all likelihood, the heat element had nothing to do with the initial break. If so, the inner piece of glass would most likely have broken first and a “star” would most likely not be present. Thermal cracks usually run out pretty quickly and the origin is typically at an edge … no star present.
  6. You may not have heard anything hit the windshield if you had the radio on, wipers going, were talking with another passenger or on your cell phone … and shame on you if you were on your cell phone.
  7. Lastly, I can accept and understand the manufacturer’s claim that the break was an “external trauma.” This occurs thousands upon thousands of times all over the world every year. I’m sorry but you are now one of those “occurrences” in my opinion.

Thank you for contacting the Glass Detective and I hope the above comments are of some help to you.

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Lyle Hill

Lyle Hill has been in the glass and metal industry for more than 40 years. In this time he has managed glass retail, contract glazing, mirror, architectural window, window film, and automotive glass businesses throughout America. He obtained an MBA from IIT with a focus on Technology and Engineering Management. Hill is also a columnist for glass industry trade magazines and often called the “face” of the glass industry. He has also authored books including “The Broken Tomato and Other Business Parables,” which is available through Amazon. Find out more about Lyle on Linkedin.

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4 Responses

  1. The next question is why window glass is so fragile in certain makes of cars when in earlier years windshields could withstand relatively large rock impacts without fractures or small fractures that didn’t spread.

    1. Thanks for the question Glen. I cannot confirm whether or not this is indeed a verified trend. However, we do know that in an effort to increase gas mileage, vehicle manufacturers look to save weight through various strategies. One possible target could be installing thinner, lighter windshields.

  2. If I use an electric heater (i.e. a printer head heating element) to de-ice or defog my windshield, will it crack the glass if it around 20 degrees F? I live near Chicago and have to park outside, but I’m thinking of making a defogger piece that will quickly heat the frozen windshield? This piece will be an aluminum strip about a 1″ x 36″ that will be touching the windshield on the inside of the glass. The aluminum strip will have a pair of those heating elements attached to it to heat pretty quick, less than 2 minutes to reach 100 degrees.Do you know the maximum rate of temperature increase I can go without causing the windshield to crack?

    1. Larry,

      That’s very inventive! We have an office in Chicago, so we can appreciate your need for faster defrosting during those harsh winter months. The manner in which you describe heating the windshield sounds a bit aggressive though and we wouldn’t recommend it. You may wish to try some safer tips for de-icing your car’s windshield which we wrote about in this blog:

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