It has been shown that when glass undergoes deflections that are in excess of 75 per cent of the panel thickness, the stresses in the glass due to lateral loads change from a bending stress profile to a membrane stress profile. 5. Alternately one should be prepared for such occurrences and for replacing NiS broken glasses. The entire community of designers, specifiers, manufacturers, fabricators and glaziers etc are now broadly aware of the problem. In applications, where tempered glass cannot be avoided due to load considerations or code requirements, it is advised to get it HST. Required fields are marked *, C-55, Okhla Industrial Area, Phase-I,New Delhi - 110020 (India), Copyright © 2020 McCoy Mart, all rights reserved. First, this is impact not a uniform load. To understand this phenomenon of spontaneous breakages due to nickel sulfide, we need to briefly understand the manufacturing process of float glass and its heat treatment namely tempering, heat strengthening and heat soak testing. Improper or uneven tempering can also cause a post installation breakage. Maintaining an elevated temperature facilitates faster conversion of any particle of NiS, if present, from alpha-phase to beta-phase, and therefore the idea is that any panel, which has a possibility of failure from NiS, should fail in the HST oven rather than on the building. Tempered glass with higher stresses, will have higher probability of NiS breakage as compared to tempered glass with lower stresses, just as a more pressurized balloon will need lesser provocative force to trigger its bursting. Tempered safety glass which has been laminated often does not fall out of its frame when it breaks – usually because an anti-splinter film has been applied on the glass, as seen in this grocery store meat case. And when toughened glass breaks, it does it in … Therefore, when glass is produced in the furnace, it is overwhelmingly likely that any NiS inclusions will be in the alpha-phase. If it is allowed to cool rapidly or in uneven manner, it will result into cracking while manufacturing or in service due to uneven stresses. For glass that is too thin to be heat tempered, JNS also has the ability to chemically strengthen glass. Because heat soaking can represent a significant cost, when presented with a proposal request for tempered glass, many glass manufacturers break out the additional cost impact to heat soak the glass. If there is a NiS particle sitting in the tensile zone or very close to it, the phase change from alpha to beta, can act as a trigger and cause the spontaneous breakage. It is also possible for a particularly bad batch of float glass to be produced, which would have a higher failure rate. There is a reasonable consensus in the industry as to how NiS is formed, from the compounds that are initially introduced into the glass. Heat-strengthened glass differs from tempered glass in surface compression; its mechanical strength is about 1.6-2 times that of annealed glass. There are costs associated to HST which are the processing costs, cost of glass breakages due to NiS in HST oven, and damage by broken glass to the neighboring panels in HST oven, potentially even causing them to break as well and propagating the problem. This type of crack—called a stress crack—can happen when a swing in temperatures causes windshield glass to expand (when heated) and shrink (when cooled) rapidly. Spontaneous breakage due to NiS is not due to any manufacturing defect but is an inherent risk or problem associated with use of tempered glass. While many people believe that the thermal expansion of the glass caused by the hot sun can cause it to crack and then shatter, it is nearly impossible for it … What Are The Different Types of Fasteners and Where to Use? “Heat-strengthened glass and tempered glass are very similar,” Baier says. Heat-treatment can generate subtle roller waves in glass, which are more likely to occur in tempered glass than in heat-strengthened glass. This can cause heat-treated glass to distort reflected images, a problem that may be exacerbated when it is used in glazing units with multiple lites or a laminated interlayer. Any of these can result in creating a concentrated point load on the edge or corner of the glass. The relationship between stress and diameter of NiS inclusion has been scientifically proven and field studies have also examples of such small inclusions causing the spontaneous breakages. When it does break, tempered/toughened glass is designed to shatter into small pieces that are much less likely to … Heat-strengthened glass has outstanding thermal stability, its flatness and light transmission is close to that of annealed glass and much better than that of tempered glass. Pre-existing cracks and scratches can often decrease the strength of the tempered glass. Although tempered glasses don’t easily break. Heat strengthened glass is a semi-tempered glass which has all properties same as annealed glass or float glass, except strength. It is important to note that sub-critical inclusions of size substantially lower than 0.05mm (say 0.02mm) are generally not capable of creating enough localized tensile stresses, to be able to break the glass, but these can cause a failure if the glass is placed under additional tensile stress due to bending or thermal loading. The cooled glass is cut into large sheets, and then sent to other processing locations for finishing, such as cutting to size, strengthening, or insulating. Edge quality is critical. There are other methods such as laser-imaging and ultra-sound which are non destructive but these methods will need a scan of each panel of glass which will show up all inclusions and not only NiS and thereafter, it will separately need an assessment of each inclusion in each panel to decide which ones are fatal types of NiS and located in the risk zone. It is then heated in a furnace to a temperature of 600-700 degrees C. This temperature is held until the glass softens slightly, at which time it is rapidly cooled through the use of air jets, a process called quenching. Any non-tempered glass will normally break as a single crack or multiple cracks, which develop or propagate from origin either spontaneously or gently. As this core cools, it induces compression in the already-cooled outer layers, and a balancing tension force is formed in the core of the panel. Track Order  |   Customer Support  |  About Us, Sell on McCoy Mart  |  Bulk Orders  |  Stories. Another study by a prominent Indian float manufacturer puts the no. In such cases, the point of origin of breakage will generally give a clue. Both of these processes have specification requirements, tolerances, and testing procedures which are defined by ASTM International document C-1048-04. None of the temperatures generally used in daily life cause glass to break. 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