Chapter 5 Flashcards
What can slow cooling form?
Slow, controlled cooling can produce a single crystal, which is a highly ordered array of atoms. An example is a single crystal of silicon,used for making microchips.
What effect does rapid cooling have?
Rapid cooling tends to trap particles in an amorphous state,which resembles the disordered arrangement in a liquid. Glass is an example of an amorphous material.
What is a polycrystalline material?
As a liquid cools, crystals start to form at different points within it.Each crystal grows out into the liquid, until it runs into its neighbours. The result is a patchwork of tiny crystals or grains. Where the grains meet, the interface is known as the grain boundary. Therefore a polycrystalline material consists of a number of grains, within which the structure is ordered.
What is the eye lens composed of?
The eye lens is composed of a gel containing proteins, called crystallins, arranged in a regular array.
Why do people’s eye sight deteriorate?
The protein molecules are susceptible to oxidation damage as people age. This tends to decrease the solubility of the crystallins and degrades the crystalline structure of the lens. The result is loss in transparency.
Why can’t we see clearly through frosted glass?
Frosted glass is translucent, it lets most light through. However, the frost has a rough surface and so causes the light to refract at all angles.
Why does a shattered car window appear white and opaque?
Is you think as the shattered glass as many little pieces of glass, where light beams into them it gets refracted several times and several directions.
Can polymers be opaque or transparent?
They can be both.
Why is polythene opaque?
In polythene, light is reflected and refracted at boundaries between crystalline and amorphous regions, which have different refractive indices causing it to become opaque.
Why is Perspex transparent?
Perspex, which is all amorphous is transparent because the whole think is the same refractive index.
What makes things translucent or opaque?
Frequent reflections and refraction so make things translucent or opaque.
Why is glass much weaker than theoretically calculated?
The presence of cracks and flaws act as stress raisers, in other words, the stress around a crack can bee up to a thousand times greater than the actual applied stress. Bending the glass slightly opens up a crack which spreads throughout the material.
Why is some glass coloured?
Glass is not completely colourless. If you look through a sheet of glass edge-on, it looks green or blue. Metal ions in the glass absorb light of certain wavelengths,colouring the glass.
Why are metals shiny?
Light penetrates almost no distance in metals. I bounces back from the surface. Free electrons in the metal absorb the light energy, and then immediately re-emit it. Metals are on sis hint for the same reason that they conduct electricity, they contain free electrons.
How can dislocations in metals make it stronger?
In a metal, dislocations in the ordered crystalline structure can help to even out high stress concentrations around a crack.
Explain the whole process from when stress is applied to glass to when the glass shatters.
1) When you bend glass, it becomes strained elastically. It stores strain energy.
2) At the tip of here crack, two neighbouring atoms are pulled apart, work is done in breaking this bond.
3) The next two atoms are pulled apart and so on, the crack moves through the glass like a zip being undone. The energy required to do this is known as the fracture energy.
4) Once the glass is broken it is no longer strained so it no longer stores energy. The entry has been used to break bonds and vibrate atoms.
What is toughness?
The toughness of a material is measured by the energy needed to deepen and extend cracks, creating new fractured surface in the cracks. If the energy available from the stresses in the material is larger than the energy to extend the crack, the crack will propagate and the material will fail. This is why glass can break almost explosively.
Why are the figures very approximate for the measuring of fracture energy?
The energy used in fracturing a specimen varies a good deal, the new surface may vary in roughness, the number of small flying pieces can alter, and so on.
Explain where the stress is on material with a crack.
Cracks deflect tensile stress and causes the stress to be concentrated on the crack. The area is very small at the tip of a crack causing the stress to be very large there. This causes the bonds between the atoms at the bottom of crack to be highly stressed and may cause them to break.
Under tensile stress what happens to cracks?
Cracks propagate through materials.
Why do metals resist crack propagation?
Metals resist cracking because they are ductile. Cracks are broadened and blunted. They do not propagate.
Metals are tough because they are ductile.
Why are fibre reinforced materials tough?
If you think as the fibre-reinforced material as a. Soft matrix with strong thin fibres inside it. When stress is applied and a fibre breaks the stress is taken up by the other fibres. Fibre- reinforced materials use the matrix to share stress amongst many strong fibres. The matrix also protects the fibres from cracks forming. Fibre-reinforced materials are tough because cracks can’t propagate through the soft matrix.
