Semester 1 Flashcards
List 5 ways a metal can be processed
Rolled, Forged, Extruded, Cast, Produced from powder
What type of microstructure does a metal have?
polycrystalline
What type of microstructure do ceramics have?
polycrystalline
List 3 typical properties of metals
electrically conducting, high thermal conductivity, ductile, malleable, strong, stiff, non-transparent.
List 3 typical properties of ceramics
stiff, hard, brittle, low thermal conductivity
Describe a covalent bond
involves sharing of electrons with with adjacent atom. These are difficult to rearrange resulting in a stiff, hard material. e.g diamond
Describe an ionic bond
involves the complete transfer of electrons between atoms to form charges ions. e.g common salt
What type of microstructure is glass?
Amorphous
List 3 typical properties of Glasses
stiff. hard, brittle, transparent, electrically insulating, low thermal conductivity.
What does a polymer structure consist of?
long organic, carbon based chains
Describe the difference in microstructure between Thermoplastics and Thermosets
Thermoplastics have non-cross-linked chains so soften on heated. Thermosets initially soften but then set. when heated sufficiently they will decompose.
list 3 typical properties of polymers
low density, not stiff, not strong, ductile, good insulators
what kind of bond keeps the polymer molecules together
Covalent
Name 5 classes of materials
Metals and alloys, polymers, ceramics and glass, composites, natural materials
Name 5 metals
Iron and steels, Aluminium, Copper, nickel, titanium
Names 5 polymers
polyethene (PE), polymethacrylate (acrylic), nylon, polystyrene (PS), polyurethane (PU), polyvinylchloride (PVC), epoxies (EP), elastomers (NR)
Name 5 ceramics and glasses
aluminia, magnesia, silica, silicon carbide, cement and concrete
Name 5 composites
fibre glass (GFRP), carbon fibre (CFRP), filled polymers, cermets, reinforced concrete
Name 4 natural materials
wood, leather, cotton/wool, bone
How do you calculate normal stress?
Force/Area
How do you calculate strain?
change in length/starting length
How do you calculate young’s modulus?
E= stress/strain
How do you calculate Poisson’s ratio?
V= -transverse strain/longitudinal strain
How do you calculate the shear modulus?
G= shear stress(t)/shear strain =E/2(1+v)
How do you calculate bulk modulus?
B= pressure/volumetric strain =E/3(1-2v)
What is an isotropic material?
uniform properties in all directions. only need two moduli to relate stress to strain. i.e any two of E,V,G,B
What is a transversely isotropic material?
unidirectional continuous fibres, both longitudinal and perpendicularly transverse. 3 perpendicular axes of symmetry will give 7 elastic constants, of which 5 are independent: 2 tensile moduli, 1 shear modulus, 2 poisson’s ratios
What is an orthotropic material?
laminated structures such as plywood. 3 perpendicular directions of symmetry, hence 9 independent components: 3 tensile, 3 shear, 3 poissons
What assumptions must be made when calculating true stresses and strains?
for shear stresses, clockwise is positive. All stresses are static and are in equilibrium. materials are homogenous, isotropic and linear elastic. deflections are small.
What does the Rankine criteria refer to?
the maximum yield stresses for materials featuring an elastic limit
What does the Von Mises critera refer to?
it is the maximum distortion energy, plastic flow occurs when the shear deformation energy in the complex state of stress = shear deformation energy in uniaxial stress
What does the Tresca criteria refer to?
the maximum shear stress, which can be derived from Mohr’s circle
In a metal structure, what are grains?
regions of the same crystal orientation, each with a 1D structure
what is the most efficient arrangement of grains in a metallic structure
hexagonal
What type of cubic structure creates a pyramid arrangement in a metal?
FCC
Name 5 metals with an FCC structure?
aliminium, silver, gold, copper, lead, platinum,
How many atoms occupy an fcc unit cell?
4
What is the maximum efficiency packing for an fcc unit cell?
74%
How many crystal system structures are there?
7
Name 3 crystal system structures
FCC, HCP, BCC
What are phases?
distinguishably different regions of composition and structure. within a phase, all physical properties are the same.
How does the structure of iron change with temperature?
Alpha-iron: bcc<912’c. Gamma-iron: 912’c
What is a phase diagram?
maps of the most stable phases and will show form given enough time based on ratio of mixtures and temperature.
Define elastic deformation
when loads are applied, the atomic bonds stretch then return to normal when load is removed
Define plastic deformation
when a load is applied the atomic bonds slide over each other and remain in the new position
What are dislocations?
atoms create rucks which are linear defects moving through the material. This is how the metal flows allowing ductility and toughness.
Name 4 strengthening mechanisms
1.atoms in solution, 2.second phases, 3.other dislocations 4.boundaries
How can alloys increase the strength of a material?
foreign atoms of different size sit in the structure and slow down dislocations. Interstitual atoms occupy the space in the atomic lattice creating distortions which prevent layers from sliding.
How can second phases effect strength?
barriers between phases block movement of dislocation. this is known as dispersion strengthening
How can work hardening improve strength?
working a metal creates dislocations. creating enough, they will become tangled up and block each other
How can grain boundaries effect strength?
dislocations cannot get through boundaries at misorientations. Reducing the grain size increases material strength. this can be achieved by deformation and heat treatment.
What can effect the properties of iron and steel?
amount of carbon present, other alloying elements (Mn, Si), processing
Name the 5 stages of processing steel
iron making, steel making, secondary steel making, casting, primary forming
Describe the process of iron making
feed iron ore, coke and limestone into the top of a blast furnace and hot air is blown at the base, 800’c top and 1500’c bottom. Coke acts as a heat source and reducing agent. Liquid metal and waste is removed every few hours
Describe the process of steel making
use oxygen injection to reduce the carbon content and impurities, either basic or electric arc.
Describe the process of secondary steel making
involves making further compositional adjustments, creating alloys. Continuous casting is the most commonly used method. Can alter properties such as thickness. shape and finishes
Name 4 methods for shaping steel
casting, rolling, forging, extrusion
Name the 4 equilibrium different phases of iron and steel
Austenite, Ferrite, Cementite, Pearlite
Describe the Austenite phase
typically high temperature but present at room temp in some stainless steels. FCC structure
Describe the Ferrite phase
high purity phase and is present in most steels. BCC structure
Describe the Cementite phase
ceramic like material (intermetallic) and has a fixed composition
Describe the Pearlite phase
mixture of ferrite and cementite. Can be achieved by annealing at 900’c then slowly cooling.
Describe the Martensite phase
not an equilibrium phase so doesn’t appear on the phase diagram. Can be produced by fast cooling. Very hard and brittle
What types of ceramics are used in structures?
generally oxides, typically several crystal phases will be present (complex micro-structure).
What is flat glass?
non-crystalline solid that retains the atomic structure of a liquid. Bulk glass is typically soda-lime-silica composition. formed using the float process
Describe the float process
1.produce melt in a furnace (1600’c), 2.melt passes over the top surface of a bath of molten tin (1100’c) cooling so it solidifies. using liquid creates a flat surface, 3.passes through he annealing Lehr (600’c) to remove residual stresses that would cause fracture, 4.glass is cut and stacked then moved into a warehouse.
How can you strengthen glass?
thermal tempering, chemical tempering, laminating
Describe the process of thermal tempering
1.heat the glass back above the transition temperature, 2.rapidly cool using jets of cold air so that the surface becomes rigid so the exterior is in compression and interior is in tension, 3.if the glass breaks tensile stresses will cause multiple fragmentation (safety glass)
Describe the process of chemical tempering
produced using diffusional ion exchange process to create compression stresses. this method is slow and expensive, used in smart phone screens.
What is fibre glass used for?
reinforcing elements in polymer matrix composites. Also used for thermal insulation in glass wool and stone wool for greater fire resistance.
Why are cements important?
structures, CO2 emission reduction, nuclear waste disposal.
What is portland cement?
consists mostly of hydraulic calcium silicates, usually some calcium sulfate. Reacts with water and hardens once set.
list 3 types of cements
portland cement, speciality cement, medical cement
What are speciality cements?
rapid-hardening, low CO2, other technical properties. used for immobilisation of radioactive waste.
What are medical cements?
used for tooth and bone replacements
Define paste, mortar, concrete
paste= cement+water. mortar=paste+sand(fine aggregate). concrete=paste+sand+course aggregate
What are aggregates?
sand, gravel and rocks are much cheaper than cement. cement acts as a binder. Ideally use non-reactive aggregates such as quartz, basalt, granite. sulfates react with the cement, expanding and causing failure.
How can you reinforce concrete?
Pouring concrete around a steel framework (rebar) creates a stronger composite. steel is good in tension and concrete is good in compression. Almost all structural concrete is reinforces with steel.
Describe the importance of water in concrete
concrete is cast so needs to be able to flow. mixing trucks keep rotating to avoid setting. concrete is self compacting with no vibrations or pressure. Too much water can result in reduction in durability, strength, and delay hardening. Normal ratio is 0.5+/-0.2
How can you improve concrete durability?
main cause of failure is due to steel expanding due to rust. Rusting can happen due to contamination or if the concrete is immersed in water. Effects can be reduced by reducing permeability and water/cement ratio with superplasticisers.
What causes concrete to degrade?
steel rust, corrosive solutions, expansion of aggregate, sulphate attack, frost-salt cycling, poor workmanship.
How do you make portland cement?
Mix limestone powder with clay/shale in rotary kiln (1400’c). CaO combines with the clay/shale/sand to form clinker phases. The clinker is grounded down into powder and forms cement.
What is Tricalcium silicate also known as?
Alite (C3S)
What is Dicalcium silicate silicate also known as?
Belite (C2S)
What is is Tricalcium Aluminate also known as?
Aluminate (C3A)
What is Cement clinker made up of?
alite 60%(45-75), belite 20%(7-32), aluminate 10%(0-13), tetracalcium aluminoferrite 10%(0-13), gypsum 5%(0-5)
What is white portland cement?
contains no tetracalcium aluminoferrite. More expensive has it needs to be 200’c hotter in the kiln
What are supplementary cementitious materials (SCM)?
A powder which is not itself a cement, but which reacts and contributes to material performance when blended with portland cement and water. e.g. pozzolans, ground granulated furnace slag, limestone.
Name the 3 types of Addition polymerisation
Initiation, Propogation, Termination
Define thermoplastic polymer
not chemically cross-linked, linear or branched chains. Soften when heated, chains straighten out.
Define thermosetting polyer
chemically cross-linked, do not soften, generally harder. more brittle and dimensionally stable.
Describe the structure of polymers
crystalline regions, chains aligned. Amorphous regions, not aligned. The more crystalline, better mechanical properties.
Names 5 polymer additives
fillers, plasticizers, stabilisers, colourants, flame retardants.
Name 5 thermoplastics
Polyethene (PE), polypropylene(PP), polyvinylchloride (PVC), polycarbonate (PC), polymethylmethacrylate (PMMA), polystyrene, polyamides (PA), bitumen
Name 5 thermosets
epoxies, unsaturated polyester, vinyl ester, polyurethane, phenolic foam.
What are the two fabrication stages of thermosets?
Initially a linear polymer, typically a liquid with relative low molecular weight. Secondly, curing occurs. This reaction is irreversible. Cross-linking occurs making the material dimensionally stable.
What is an elastomer?
show rubber like elasticity, will return to its original shape after forced deformation. Cross-links present.
Name 5 elastomers
natural rubber, butyl rubbers, ethylene vinyl acetate (EVA), polychloroprene (neoprene), silicone rubber.
Define a composite
combination of two or more materials chemically bonded, made up of a matrix phase (larger part) and a reinforcement phase. Aim of a composite is to have the best properties of each material with few/no disadvantages.
What is pre-stressed concrete?
steel helps to resist fracture, but concrete can crack with only small extensions. stretching steel elastically during processing keeps the concrete under compression.
What is post-tensioned concrete?
once concrete is set, steel tendons are applied with hydraulic jacks. Tendons are locked off at anchorages and jacks are removed resulting in tendons trying to shorten. causing compression in the concrete.
Define softwood
coniferous trees, don’t lose leaves in winter. Generally cheaper and often used in construction. examples include: spruce, douglas fir, larch.
Define hardwood
broadleaved tree. Generally slow growing and more expensive, generally more prone to warp, swell and split. Examples include: beech, oak, ash, mahogany.
What is Anisotropy?
Properties of a material vary depending on the axes and direction. A timber has its greatest properties along the grain and lowest perpendicular. fibres run along the grain.
What factor needs to be considered when cutting wood?
the anisotropic structure needs to be considered in order to avoid uneven properties and warping. Quarter saw can be used to get the best properties. also the appearance can change depending on orientation e.g plane sawn
What are the effects of drying wood?
Drying in air or a kiln results in shrinkage and propety changes
What preservatives can prevent degradation of wood?
Without treatment wood will degrade in 0-25 years. Preservatives can be oil based (creosote) or waterborne (fixed or unfixed). light organic solvent (LOSP) is a combination of fungicides and insecticides.
What is a wood product?
man made timber material created to overcome the problem of orientation of wood being a limiting factor with shape and size.
Name 4 wood products
plywood, chip board, fibre board, glulam
How can you join metals?
fasteners, brazing and soldering, welding.
What are the defects of welding?
porosity, incomplete fusion, lack of penetration, overlap, inclusions, cracking
