We summarize a good part of everything learned in this "Trip to the center of a windshield impact”, The first part of which focused on how glass damage occurs. In this second installment we will address what happens after the impact and what factors determine that the glass ends up cracking completely.
WHAT WE KNOW ABOUT GLASS RESISTANCE
AA Griffith published in 1921 the first theory to explain the mechanism of glass fracture and why the actual strength of this material is much lower than its theoretical strength. In his research, he determined that the stress required for an impact to enlarge is inversely proportional to the size of the impact. That is, the greater the damage, the less force will be required to increase its size. On the other hand, as the size of the impact increases, the resistance of the windshield as a whole decreases. S. Wiederhorn delved into this theory, and in 1967 published a study showing that the growth of an impact on glass also depends on the humidity of the environment.
THE DIFFERENT AND VARIOUS SOURCES OF STRESS RECEIVED BY A WINDSCREEN
Windshields are constantly subjected to stresses caused by a number of material and physical factors that work in combination. This stress comes, first of all, from the windshield's own manufacturing and design. The manufacturing process involves cutting the glass and bending the two parts, using a mold to create the required curvature; and the insertion between them a layer of plastic to laminate the glass so that it does not break into a thousand pieces when it is cracked. This process can leave a residual stress level 'frozen', the degree of which can vary from batch to batch, and even from part of the windshield to another. In addition, the windshield has a particular size and shape, with complex curvatures. Its own orientation, inclination and distribution of masses, also generates tensions.
Second, the windshields are attached to the vehicle with polyurethane adhesives, the hardening of which can be accompanied by a certain degree of volumetric shrinkage that can exert additional residual stress. Furthermore, two identical vehicles may be slightly different from each other (due to their dimensional tolerances) and the adhesive used may also vary slightly in properties from batch to batch; so each windshield will have a different tension pattern.
Third, the windshield is designed to act as a structural component that provides rigidity to the body of a vehicle, as well as resistance to crushing of the roof. As part of the structure, the windshield receives pressures from the G forces that twist the body in the accelerations, decelerations and curves that occur in the movement of the vehicle.
And that's not all ... The body of a vehicle expands and contracts with changes in temperature. Being a complex volumetric structure, the nature of the expansion or contraction can vary considerably from one model to another. As glass and steel or aluminum have different coefficients of expansion (and different rates of expansion or contraction), changes in temperature can cause alterations in the bodywork that are transmitted to the windshield, altering its stress patterns. And the glass itself also suffers when withstanding the difference in temperature between the exterior and the interior of the vehicle, or sudden changes, such as when we try to defrost the windshield with hot water or we plug in the cold air from the air conditioning in summer.
Lastly, cyclical vibrations come to the windshield from potholes in the pavement and from the road itself when driving; and wind load, depending on the size, shape and angle of inclination of the glass. For example, going through a speed bump at 30 km / h generates up to 5G of acceleration on the windshield.
HOW AND WHY IS AN IMPACT ENLARGED?
AA Griffith already demonstrated in 1921 a damaged piece of glass is much weaker than an undamaged part and that the greater the impact, the less stress (or force) is needed to make it grow larger. Wiederhorn certified in 1967 that, even with extremely low stresses and in the presence of moisture, an impact grows continuously, perhaps only molecule by molecule, but without stopping. And studies by Belron Technical have shown that temperature and its variations also affect the growth of windshield damage.
In this way, the speed of growth will depend on the specific unit of the windshield, the size of the impact, the stresses that the glass receives, changes in temperature and the humidity of the environment. When the combination of all these factors - which sooner or later will happen with everyday use - causes a necessary threshold to be exceeded, the windshield will collapse.
In the end, all the investigations carried out reach the same conclusions. The first, that a glass with an impact always ends up breaking, it is only a matter of time before it does. The second, that a repaired glass recovers its original resistance, so if we suffer an impact it is vitally important to go and repair it before it reaches a size that makes it irreversible.
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