![]() ![]() Fracture also creates similar crack patterns in both glasses and polymers. For example, the fracture mirror of glass is called the smooth region in polymers. One consequence is the development of similar topographical features on the fracture surfaces of both polymers and glasses. The consequence of this high level of surface sensitivity is that glass objects readily develop surface flaws and then can be easily broken under a wide variety of conditions and at different states of stress.įracture toughness values of glass are comparable to those of many polymers, or plastics. Glass is sensitive to the creation of surface flaws which are the precursors to most glass failures. This very low fracture toughness makes glass highly susceptible to various forms of surface damage. Glass is brittle, considerably less tough than even the most brittle of metals, such as the cast irons. ![]() Its fracture toughness, K IC value, is only ~1 MPa Glass is not highly resistant to fracture. The crack extension, even though only breaking one bond at a time, typically occurs so fast during a glass fracture event that it appears to be instantaneous to the observers. However, glass fracture events consist of the extensions of individual cracks that rupture a single atomic bond at a time. When glass containers break they often seem to violently explode in all directions. Glass usually appears to fracture instantly and dramatically, as when a thrown rock or other projectile hits a glass window. However, the topographical surface features of the broken glass and the resulting crack patterns of the reconstructed glass object will accurately, objectively, and faithfully document the history of the glass fracture event. Eyewitness accounts are not always reliable as the anxiety of the moment may create confusion among observers. From a more practical perspective, there is the assignment of responsibility for glass failures in litigation when there are personal injuries. It also includes understanding how and why cracks in glass extend or grow in the myriad of ways that they do. Paramount, from an academic point of view, is the continuing quest for a greater fundamental understanding of glass strength. The fracture surface topography and the crack patterns that develop when any glass object breaks are of interest for many reasons. Other glass crack patterns such as those from impact and thermal stress are also considered. The patterns of crack branching or forking, the branching angles and the crack length prior to forking, are also discussed. Characteristic cracking patterns are reviewed for several common glass fractures including those for pressure breaks, both bottle explosions and flat glass window failures from wind pressure whose basic similarities are described. Dicing fragmentation of high-strength tempered glass and the long sword-like shards of low-strength annealed glass fracture are contrasted through their strain energies. Distinctive crack patterns that evolve during fracture, that is the traces of the cracks intersecting the glass free surfaces, are described. Formation of the mist and hackle surface regions are also fundamentally addressed, as is crack branching. The concept of a fracture mirror constant is related to the strength. ![]() Quantitative aspects of the fracture process such as the stress level at fracture for a glass object are directly related to the size of the fracture mirror. The surface features known as the mirror, the mist, and the hackle are illustrated and addressed through the principles of fracture mechanics and associated energy criteria. Strength is central to the fracture surface features for it determines the strain energy release rate and the dynamics of crack extension. They are considered after a brief discussion of glass strength. The topographical features which appear on the fracture surfaces of broken glass objects and the resulting crack patterns which develop are Nature’s documentation of the fracture event.
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