Technical Windshield Facts You Should Know

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Focused View Of Windshield Layering, Stresses On An Edge Crack

Understanding Windshield Tempering For Professional Windshield Repair

Understanding Windshield Sheet Of Glass Tension Stress For Professional Windshield Repair

When a stone impact penetrates the surface compressive depth, entering into the center tension stress area in the middle of the glass it makes a stone-break. When it does not penetrate the 21% depth it makes a surface pit. The thinner the glass the thinner the 21% compressive depth.

The inner and outer windshield layers are float glass, which is a sheet of glass made by floating molten glass on a bed of molten metal, typically tin. This method gives the sheet uniform thickness and very flat surfaces. Modern windshields are made from float glass. Most float glass is soda-lime glass. The float glass process is also known as the Pilkington process, named after the British glass manufacturer Pilkington, which pioneered the technique (invented by Sir Alastair Pilkington) in the 1950s. The lower side of glass in the float process, i. e. the side that is in contact with the pool of molten tin is called the tin-side; the upper side of glass is called the air-side.

PVB is the most commonly used interlayer material for laminated glass. Polyvinyl butyral is a viscoelastic resin that is made from vinyl acetate monomer as the main raw material. It provides strong binding, optical clarity, adhesion to many surfaces, toughness and flexibility.

The Frit - Enameled glass - ceramic pigments or frits are rolled, poured or screen printed over one side of glass and are baked onto the glass during heat treatment for a permanent bond. Enameling reduces the bending strengthen of the glass by about 40%. This would be the innerlite frit area which is also where the windshield is bonded to the pinchweld. This cosmetic addition lowers penetration resistance, roof crush and airbag support.

Glass will break when subjected to tension stress. The normal strength of float processed annealed windshield glass is approximately 9,400 psi. However a stone-break or crack can reduce the strength to as little as 800-1500 psi. That means a loss of about 8,000 psi of support for the airbag, roof crush in a rollover or debris from the road impact penetrating the windshield. Restoring this strength is the main reason you repair a windshield. Preventing the damage from spreading is a byproduct of restoring the strength.

The Edge Crack Area, Windshield Professional Crack Repair

The two-inch edge area of the entire perimeter of the windshield is a different animal than the middle area. The middle area has surface compression strength and depth, no applied surface stresses and a 3,000 psi PVB bond that prevents a stone-break from cracking out. The glass of the first 2-inches of the edge area around the perimeter of the windshield on both the inner and outerlites have multiple manufacturing, thermal and applied stresses.

The strength of glass is its surface compressive strength/depth.

The approximate first two-inch edge area of the outer light is vulnerable to chips/fracture dues to low surface compression stress. Surface compression depth is less than 21% in this area. This in turn means that the tension stresses at the edge area are at or near the surface.

During the manufacturing bending and annealing (molding) process a flat cut sheet of glass goes onto the mold for the specific vehicle. It then goes into an oven or leir wherein the glass is heated to just over 1,000 degrees F. The glass goes soft and sags or gravity bends into the mold. After the bending it then goes to the cooling or annealing cycle. This is where the stresses of the glass go array. The varying and uneven cooling temperatures at the edge area after that windshield comes out of the bending furnace causes a decrease in the compressive stress and an increase of the tension stress. The middle and surface are cooling at an even rate but the hot iron mold near the edge under the glass is still generating heat causing temperature variances and uneven cooling at the edge area. With glass the last area to cool goes into tension; that would be this area where the mold is which results in a tension bulge, also referred to as the “weak spot”. This will cause this area to chip and fracture easier than the rest of the windshield. Minimal surface damage such as the common miniscule pin-head size surface pit in this area will form a long edge crack from the applied surface stress coming from the gluing of the windshield to the vehicle, called installation stress or induced stress. The edge crack will continue until the PVB bond becomes greater than the installation stress, usually at 8-10 inches. The edge crack will remain at this length until thermal stress or another applied stress causes it to go further.

Stresses at the edge area that cause edge cracks:

(1). Residual Surface Stress – stress in the glass surface after it is manufactured. This weakness in the structure (glass) is caused by and during the molding process from uneven chilling (annealing).

(2). Surface Stress – total of residual stress and applied stress on the surface.

(3). Tension Bulge –starts at the mold contact and lowers the compression depth to zero or near zero.

(4). Thermal Stress – heat and varying temperatures caused by the black color of the frit.

(5). Installation Stress – applied stress which causes a surface pit, or chip in the edge area to crack-out.

There are two ways to eliminate this defect: one is to slow the annealing process down to increase the compressive depth which would significantly increase the manufacturing time and BTUs and subsequently the cost of each windshield. The other less costly and simpler alternative is to coat the edge area. Neither of these methods are used by vehicle or windshield manufacturers.

References: Guardian, Pilkington, Matthias Haldimann, Saint Gobain, Martina Eliášová, Wikipedia, Ultra Bond, Richard Campfield, Your Mechanic Jason Unrau.

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