Abstract
Keywords - Glazing, Polymer, Directional Failure, Impact Resistance, Interfacial Strength
Abstract - For many years, glass has been the only choice of material available for vehicle glazing systems. However polymer materials have recently been promoted as viable alternatives to glass, the most successful of these being polycarbonate. There are a number of observable benefits associated with using polymers including styling freedom, weight reduction and, particularly with polycarbonate, enhanced intruder resistance.
However there is a recurring issue being raised by automotive consumers who are concerned with escape from a vehicle fitted with seemingly indestructible glazing in an emergency.
Warwick Manufacturing Group has developed a laminate system that permits directional impact failure to be induced in polycarbonate glazing components. Test components exhibit near-full impact resistance when impacted from one side, but fail at much lower energy levels when impacted from the other.
In practical terms this means that any glazing component made from this material system will exhibit excellent intruder resistance from the outside, whilst emergency escape becomes much easier from the inside. It should be noted that the failure mechanism is only initiated when a sharp localised impact is made upon the inside, e.g. from a key or stiletto heel, meaning that occupant retention is maintained in the event of impact from a head or elbow.
The system is made up of a combination of polymer layers with the polycarbonate on the inside of the vehicle and a second polymer laminate on the outside. This means that extra features can be incorporated into the polymer laminate such as extra UV and chemical protection areas where polycarbonate is less resilient than other materials.
The mechanism of directional failure relies upon the fact that the outside laminate contains a series of small flaws, generated during the manufacturing process, that close up when impacted from the outside. However when impacted from the inside the outer laminate is placed into tension and these flaws initiate cracks, which propagate rapidly backwards through the component inducing premature failure.
The level of inside impact resistance can be varied by altering the processing conditions under which a component is made so that the interfacial strength between the polycarbonate and laminate is weakened. A weaker interface means that crack propagation is less dramatic as the laminate de-bonds from the polycarbonate matrix and the crack tip is blunted. This means that greater energy is required at the crack tip so that the crack breaches the interface and propagation continues back into the polycarbonate matrix.