Flashcards
Materials
Flashcards do conjunto (21)
- 1What is a crystalline material?Crystalline materials have a crystal structure. A crystal is a regular 3D arrangement of atoms, ions, or molecules. All crystals are made from identical repeating units called cells. They tend to be ductile and malleable. In metals, the lattice particles are positive ions, held together by a sea of delocalised electrons. Polycrystalline materials consist of many small crystals called grains.
- 2What is an amorphous material? Give some examples.Amorphous solids have no definite or regular structure. They are brittle, totally elastic, and generally Hookean. They tend to be covalently bonded. Unlike crystals, they melt over a range of temperatures.
Examples: glass, ceramics, wax, ice - 3What is a polymeric material?Polymeric materials consist of polymers, which are long chain molecules that are made up of repeating units called monomers. They are made through a process of polymerisation, where a monomer double bond opens out into two side bonds, and many such monomers link up. Mostly covalent bonding. Low thermal and electrical conductivity.
Rubber is polymerised isoprene (polyisoprene). - 4Describe the features of a force-extension (or stress-strain) graph for a metal such as copper.1. Hooke's law obeyed up to the limit of proportionality.
2. Elastic deformation up to the elastic limit. Beyond that, plastic deformation.
3. At the yield point, small increases in stress produce large increases in strain.
4. Ultimate tensile stress is the maximum stress before material breaks
5. Breaking stress is the stress at point of rupture. - 5What are dislocations?A dislocation is a defect or irregularity within a crystal structure. Dislocations behave as individual particles—they can move around, but maintain their identity. There are different types: a point defect is where a single lattice ion is missing, or a foreign atom or additional ion is present; edge dislocations occur when a half-plane of atoms is inserted into the crystal structure, distorting nearby lattice planes. There are also screw dislocations.
- 6What are the effects of dislocations?The movement of large numbers of dislocations is responsible for plastic strain. This motion is called slip. Plastic deformation only occurs in materials where the energy required to move dislocations is smaller than the energy required to fracture the material. During plastic deformation, the number of dislocations increases dramatically: dislocations spawn from existing dislocations, from grain boundaries, from defects, surface irregularities.
- 7How can metals be strengthened?All methods of strengthening for metals are trying to achieve the same thing: obstacles to dislocation movement. There are many methods available:
1) Grain size reduction
2) Adding foreign atoms to the lattice
3) Strain hardening
4) Cold working (work hardening)
5) Tempering - 8Explain how the following methods of metal strengthening work: 1) Increasing the number of grains 2) Introducing foreign atoms into lattice 3) Work hardening1) Dislocations cannot easily pass through grain boundaries, especially if they are very misaligned. Increasing the number of grains therefore restricts their movement.
2) This causes strains that can anchor the dislocation—these strains cancel out with the strains caused by the dislocation, achieving a state of low PE.
3) Repeatedly hammering, beating, or bending the metal increases the number of dislocations, and these dislocations lock each other in place. The metal therefore becomes harder, stiffer, and more brittle. - 9What is tempering, and how does it work?The metal is heated to red hot, and then allowed to cool to a certain temperature before being quenched by plunging it into cold water, oil, or brine.
Quenching freezes in the grain structure. If the metal is quenched at higher temperatures, the grain size will be smaller. The material properties of the metal can therefore be controlled by this method. - 10What is necking?The fracture of a rod or wire is preceded by local thinning which increases the stress. This is necking, and is the characteristic fracture process of a ductile material.
- 11Describe the features of the force-extension (or stress-strain) graph for a brittle material such as glass.No plastic deformation. Brittle materials obey Hooke's law up to the breaking point, and only undergo elastic strain.
- 12Describe the process of brittle fracture.Brittle fracture occurs via crack propagation. A crack introduces a stress concentration. Under recurrent loading, there is a slow and stable crack propagation, and then a sudden rapid failure when the crack reaches a critical crack length. During rapid failure, the crack propagates at the speed of sound resulting in catastrophic failure.
- 13Why does brittle fracture via crack propagation lead to sudden rapid failure (catastrophic failure)?Interatomic forces cannot cross the gap of the crack, and so stresses must be transmitted around the crack tip. This leads to a greatly magnified stress at this point.
- 14What is "fracture"?A form of failure where the material separates into pieces due to applied stresses at well below the melting point. Fractures can be brittle or ductile.
- 15Explain why surface imperfections reduce the breaking stress of brittle materials?Fracture occurs due to stress concentrations at surface imperfections.
- 16How can pre-stressed concrete withstand greater stresses? (i.e. why is breaking stress increased by pre-stressing?)Concrete can be compressively pre-stressed as follows:
1. A tendon, situated at the base of a mould, is placed under tension.
2. Concrete to be cast is poured into the mould, and allowed to set.
3. The tendon is then cut and contracts, putting the base of the concrete under compressive stress.
If the concrete beam is subsequently used to support loads, the lower surface of the beam will not be under tension unless a very large load is applied. - 17How can glass be compressively pre-stressed?Top and bottom surfaces of a softened (i.e. hot) glass are rapidly cooled with air jets. The surface cools before the bulk melt. When the bulk melt cools, it pulls the surface inwards, resulting in an internal compressive stress at the surface of the glass. Such a slab can withstand much higher tensions.
- 18Describe the features of a force-extension (or stress-strain) graph for rubber.The graph is non-linear: stiff then less stiff then very stiff (just imagine blowing up a balloon). There is elastic hysteresis—the unloading curve is beneath the loading curve. Exhibits large strains (strain up to 5, i.e. 6x unstrained length); low ultimate tensile strength.
Note: the volume stays roughly constant despite the huge extensions. Therefore, it is very important to distinguish the engineering stress from the true stress. - 19What meaning does the Young's modulus have for rubber?Depending on the context:
1. gradient of stress-strain graph at origin
2. value of stress/strain for a particular stress
Note: NOT the derivative - 20Explain the tensile behaviour of rubber.The rubber polymer can freely rotate about each of its C-C single bonds. So the natural shape of rubber molecules is to be randomly tangled. When rubber is placed under stress, the polymer molecules respond by straightening out. This is what allows rubber to achieve such large strains, and also why the stress needed is so much smaller—bonds aren't being stretched, only rotated. The thermal motions of the atoms in the polymers tend to randomise their shape, and so provide the opposition to the straining (the stiffness). When the tension is removed, random thermal motion of the atoms re-randomises the shape of the polymers, so the rubber contracts. Less work is done contracting, leading to the hysteresis effect.
- 21What accounts for the shape of the stress-strain curve for rubber?Initial stiffness is due to the need to overcome the Van Der Waals forces between the tangled polymer molecules. Once the molecules are separated and aligned, rubber becomes less stiff because strain is now due to unraveling of molecules. Once they are fully unraveled, further strain is resisted by covalent bonds, and the rubber becomes very stiff again.
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- A level Physics (A level) (Paper 2) FlashCards sobre Materials, criado por muon neutrino em 06-06-2018.
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- muon neutrino · jun 2018
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- 6 jun 2018
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