Chipped automobile windshields are vulnerable to cracking when exposed to extreme heat or cold. Summer sun can raise temperatures inside an automobile to over 120 degrees while summers nights can be cool.
Automobile windshields are made of two pieces of glass laminated together to make up our modern windshield. Being two pieces of glass, the inside glass and outside glass, they are made of the same glass, no differences inside to outside separated by the laminate, except one faces out and is exposed to the elements the other faces inward and is in a protected environment inside the auto. This can cause temperature differences from the inside glass to outside glass that puts stress on the windshield as the windshield glass expands or heats and cools at different rates, for instance a parked car exposed to the sun with a nearby shade tree. The inside temperature of the car is 125 degrees the outside temperature of the glass is 95 degrees and a cool afternoon breeze is starting to blow cooling the outside glass, but the inside of the car is still 122 degrees then the sun moves behind the tree and the outer glass cools even more putting more stress on the windshield as the inside glass is hot and the outside glass exposed to the cool breeze is much cooler.
So now that we understand when inside or outside windshield glass temperatures are higher or lower than the other this creates stress, now add installation stresses as the windshield is bonded to the automobile frame and cured. Through in a rock chip and the potential outcome will be a cracked out windshield that has to be replaced.
Most rock chips have cracks some visible some not, but left alone most all rock chips and cracks will spread from the stresses discussed earlier. Heating and cooling, expanding and contracting of the windshield day after day is all that is required to make rock chips expand into long cracks. Most rock chips and small cracks can be effectively repaired and structural integrity will be restored, a small percentage will crack out, those most likely to break out are rock chips or other impacts close to the edge of the windshield or those that neglect repair allowing moisture and dirt to enter the break along with stresses and temperature swings are most vulnerable.
What should you do if you get a rock chip? Cover the break with clear tape to keep water and dirt which can enter the break out, but don't impair your vision with the tape for safety reasons. If possible Keep the automobile at a temperature between 60 and 90 degrees in a dry place and call an automobile windshield chip or crack repair technician ASAP and have them repair that rock chip or small crack before summer's heat takes its toll on your windshield and cracks it out.