Читать книгу Engineering Physics of High-Temperature Materials - Nirmal K. Sinha - Страница 59

Naturally occurring flint is a form of silica or silicon dioxide (SiO₂), like sand or quartz. Arrowheads were often made during the Stone Age by fracturing flint stones. Chemically, flint is related to common soda‐lime window glasses composed of more than 70% silica. Thus, glass is one of the oldest man‐made materials that found widespread use since the Stone Age. Today, the skyline of big cities around the world is highlighted with towering glass‐clad skyscrapers.

Freshly drawn glass fibers are stronger than metals and metallic alloys by orders of magnitude. Glass plates with pristine surfaces are intrinsically one of the strongest engineering materials at ordinary temperatures. However, their practical strengths are lowered significantly due to the development of microscopic surface cracks produced by abrasives and chemicals. Griffith (1921) tackled this issue and explained the low strength of glass objects.

The glass that we see through from the windows of automobiles and trains, watch and admire in shimmering skyscrapers, and consider the pride of our cities is not a simple transparent material. It is a unique amorphous material, toughened and/or laminated with layers of polymers. The mechanical and optical properties of glass depend not only on their chemical compositions, but also on structural transformation induced during processing at high temperatures and subsequent thermal treatments. To understand the thermal tempering or toughening process, a detailed knowledge of the rheological properties of glass is required. This will be treated in Chapter 5, which deals with the fundamentals of creep. It is appropriate to mention here that the creep and failure models for polycrystalline materials, to be presented later in this book, have actually been developed on the basis of the primary author's investigations on the rheo‐optical behavior of a common window glass. There is, therefore, a need to describe the basic features of the structure of real glass, the science of the glassy state, and industrial processes.