When glass breaks unexpectedly, understanding the cause can often be perplexing. Paul Warren, principal technologist at Pilkington UK, describes what to look for when determining the cause of a fracture.
It sounds like a technique that might be used in an American television crime drama, but fractography is the study of fractures or cracks in a material such as glass or metal. Fractographic examination methods are routinely used to determine the cause of failure in engineering structures, especially in product failure, by studying the characteristics of the fractured pieces and, especially, the fracture surfaces.
Fractography can be employed during the glass manufacturing process to identify the origin of the failure when glass breaks. Broken glass can form jigsaw-like pieces and by patching together these fragments, technologists are able to establish the cause of the break. They can then use this knowledge to counteract recurring problems, which helps to make the glass more robust. The ability to understand how and why glass may break can help those who frequently work with the material to ensure that it’s safely handled, specified and stored.
Identifying the origin of the fracture
Glass can be a very strong material, evidenced by its use in large-scale construction projects such as skyscrapers. At times, however, its strength can be reduced due to the presence of invisible defects on the surface, which cause stress concentrations and allow cracks to spread. Strength may also be reduced further by larger, visible defects. On the surface of a pane of toughened glass, the origin of a fracture can usually be found by identifying two characteristically-shaped pieces of glass – called ‘the cat’s eyes’. At times however, the cat’s eyes may not be completely formed, but by following the branching of the cracks it is possible to trace the failure back to the origin. On the surface of annealed glass, no cat’s eyes are formed – but, again, by looking at the branching of the cracks the origin may be found. It is important to note that the effect of applied stress can provide important clues in determining the cause of the fracture, as the extent of branching worsens as the level of applied stress increases.
Understanding the features of a fracture
There are two main types of fracture surface features; rib marks and hackle. Rib marks are curved lines, concave in the direction from which the crack is running, and they convey information on the history of the fracture. If fracture occurs at a slow speed, or if the glass is weak, there may be no markings on the cracked surface at all. Meanwhile, hackle is a series of lines usually running in the local direction of cracking and it conveys information about crack velocity, shifting stress fields and structural anomalies. A common example of hackle is mist hackle, which with increasing speed becomes rough, fibrous and elongated in the direction of cracking, and stiffens to the stage at which the crack splits.
Causes of breakages
Some of the potential causes of unexpected glass breakages include edge damage, poor glazing installation and thermal stress.
Edge damage: When glass is being moved or fitted into a frame it’s possible for those handling the material to accidentally chip the edge of the pane. Parts of the frame can also nick the glass edges if incorrectly installed. Although these chips may not result in immediate breakage, over time, as the glass expands and contracts with changes in temperature, microscopic cracks can develop and ultimately lead to the glass breaking.
Poor glazing: Insufficient clearance between the glass edge and frame will cause the glass to come into contact with the frame as it expands and contracts. If no space is provided at the perimeter of the glass, the glass will bind against the frame, causing internal stresses to develop which can exceed the strength of glass, increasing the likelihood of breakage. This is particularly common if it comes into contact with other parts of the frame such as a fixing screw. Before glazing, the size of the glass pane should be checked against its frame. Glass that is sized incorrectly should not be reduced by nipping as this could damage the edge of the glass.
Thermal stress: Excessive thermally-induced stress can result in breakage caused by a difference in temperature between two areas of a pane of glass. For instance, in hot weather, the centre of the glass warms up faster than the edge, as the edge is often insulated within the frame. Assuming the area of glass within the frame is insignificant compared with that exposed to solar radiation, as the centre of the pane expands due to the increase in temperature, the cooler edge will be forced to expand by a similar amount creating tensile stress.
Thermal breaks typically run from an edge and are usually the easiest type of fracture to spot. Provided there is access to the edge, for example by removing beads or deglazing, a thermal breakage can be identified by the fact that it initially runs at 90° to the edge of the glass pane. To prevent thermal fracture, before it is installed, glass should be stored on blocks of wood or felt, in dry, ventilated conditions, and out of direct sunlight.
Preventing glass failure
Glass can be an incredibly strong material and in modem building design it’s increasingly used by architects as a `go-to’ material, thanks to its range of added performance benefits. As a result, it’s becoming ever more important to understand what can cause glass to break.
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