1P2 Materials Flashcards
(171 cards)
1
Q
What are interstitial solid solutions? and what elements can do it?
A
A solution where small atoms fit inbetween the gaps. Carbon and hydrogen usually can.
2
Q
What bonds are directional?
A
Ionic and metal bonds are non directional, metallic bonds are directional.
3
Q
Examples of covalently bonded structures:
A
Diamond (regular lattice), networks (glasses), long-chain molecules (polymers)
4
Q
What is the dissociation seperation?
A
The distance between two atoms where they are seperated from one another.
The max force on the force distance graph between two atoms.
5
Q
How do substances melt?
A
They can be modelled as stiff springs vibrating with kinetic energy kT (k=botzlman constant)
All bonds break down when kT exceeds bond energy, then the substance melts.
6
Q
What causes secondary bonds?
A
Dipoles, centre of positive charge and negative charge are not in the same place.
7
Q
How many close packed directions are there in a close packed lattice?
A
3 in a close-packed plane
8
Q
What structure is ABC stacking of close packed atomic planes?
A
FCC - Face Centred Cubis
9
Q
What structure is ABABA stacking of close packed atomic planes?
A
CPH - Close Packed Hexagonal
10
Q
Characteristics of FCC stacking:
A
Very ductile when pure
Work harden rapidly
Generally tough (High Kic)
Retain ductility and toughness to absolute zero.
11
Q
What are the two constants of the CPH unit cell?
A
a, the side length of the hexagonal base
c, the height of the prism
12
Q
Characteristics of CPH:
A
Reasonable ductile, but less than FCC
Structure makes them more anisotropic
13
Q
Characteristics of BCC
A
Not close packed
14
Q
Describe FCC unit cell.
A
Cube with atoms in all corners and the centres of all faces
15
Q
Describe the BCC unit cell
A
Cube with atoms at all corners and one in the centre.
16
Q
Describe the CPH unit cell.
A
Two hexagonal planes with atoms at corners and one in the centre. A triangualr plane inbetween the two planes.
17
Q
Describe the location of the tetrahedral and octahedral hole in the FCC atomic structure.
A
Tetrahedral hole in all the corners of the unit cell. Octahedral hole in the centre of the unit cell.
18
Q
Describe the location of the octahedral hole in CPH.
A
Between the sides of two unit cells.
19
Q
How do you visualise the structure of ceramic crystals?
A
CPH, FCC, BCC lattices with atoms in the insterstitial holes.
20
Q
How are glasses formed?
A
By the amorphous structure of silica (SiO2).
Amorphous structure gives transparency.
21
Q
Define a thermoplastic.
A
A polymer without crosslinks between polymer chains. Can be either amorpous or semi-crystalline.
22
Q
Define elastomers and thermosets.
A
Elastomers and thermosets have crosslinks (thermosets have a lot more)
23
Q
Tensile strength vs yield strength
A
Maxmimum stress before fracture (tensile)
Maximum stress before permanent deformation (yiedl)
24
Q
MEasure of ductility.
A
The strain to failure (=
25
Definition of toughness
Plastic work per unit volume, area under the curve
26
Definition of resilience
Elastic work stored per unit volume
27
What materials have high toughness and which ones have low toughness?
Metals have high, polymers and ceramics have low
28
Advantages of tensile test, bending test and natural frequency for measuring YM.
Tensile test, only small deflections, difficult to measure precisely, must allow for flexure of machine (could use strain gauge)
Bending test, dimensions must be measured accurately, but much more deflection for a given load.
Natural frequency gives accurate value for E.
29
Define poisson's ratio, v
v = - lateral strain/tensile strain
30
Typical values of poisson's ratio for metals, porous solids, and elastomeric polymers
metal (and other crystalline materials) = 0.2-0.33
porous solids = 0
elastometric polymers = 0.5
31
Define dilation and give its expression in terms of strains.
change in volume/original volume = e1+e2+e3 (strain in three d)
32
Define bulk modulus.
pressure = K x dilation
33
What is the difference between shear and normal stress?
Shear stress is parallel to a plane, and normal stress is perpendicular to a plane.
34
What is shear strain?
gamma = w/l0
gamma is the angle of the distorted cuboiad, w is the horizontal, l0 is the vertical.
35
Define shear modulus
shear stress/shear strain
36
During a tensile test, when does it retain a constant volume?
During yielding
37
Expression for elastic energy stored per unit volume
yield stress^2/(2*Young's Modulus)
38
Expression for plastic work per unit volume
integral(nominal stress d nominal strain)
39
What's the difference between annealed and drawn?
Annealed -> softened by heat treatment
Drawn -> previously hardened, by stretching
40
How do ceramics respond to compression and tension?
Tensile strength is controlled by the growth of the worst flaw
Compressive strength controlled by "Crushing"
40
Define hardness.
H=F/A
41
What's hardness relation to yield stress?
H=3 x yield stress
42
Define true strain
de = dl/l
43
How do true stress strain curves compare in compression and tension?
They're identical in compression and tension
44
How do you make an amorphous metal?
By cooling it very fast.
45
What are the two key temperatures for polumers?
Glassy transition temperature Tg where secondary bonds are overcome.
The melting point where primary bonds are overcome.
46
What happens to thermoplastics when heated?
The amorphous portions melt into a viscous liquid at Tg. Crystalline region survives to a higher melting point, Tm.
47
What happens to elastomers or thermosets when heated?
Cross-links do not break, so the polymer doesn't melt but decomposes or burns.
48
What has an effect on the elasticity of polymers?
Temperatures and rate of loading.
49
What causes elasticity above Tg in thermoplastics?
Reptation, molecules sliding past one another. Entanglement creates stiffness. No secondary bonds.
50
What happens above Tg in elastomers?
Large recoverable strains, chains are unravelling, pulled back by crosslinks
51
What is the young's modulus of a foam.
ratio of YM = ratio of densities ^2
52
What are the three types of composities?
Particulate (metal/polymer and ceramic), fibres and laminates
53
How do you estimate the ideal strength of a material.
Atoms must slide over one another and therefore overcome a periodic energy ponetital.
Force is the rate ofchange of this.
Max shear stress required using this force.
Differentiate with respect to shear strain to get the shear modulus.
Rearrange for G
Ideal strength = E/15
54
What is the tangent vector and burgers vector for dislocations?
Tangent vector points along the line of a dislocation.
Burgers vector describes the closure failure of a circuit through the crystal.
55
Define an edge dislocation.
Shear stress and burgers vector both at right angles to the dislocation.
Dislocation moves in the direction of the stress.
56
Define an edge dislocation.
Shear stress and burgers vector are both parallel to the shear stress.
Dislocation moves at right angles to the stress.
57
Why is a constant volume maintained with plastic deformation?
Dislocations means blocks of material slide past one another without affecting crystal packing.
58
How to find resistance to the motion of a dislocation?
Find the force applied by shear stress, and its work done (using burgeers vector)
Find the resistance force (f per unit length) and its work done.
Equate the two to find:
shear stress x burgers vector = force per unit length
59
What is the energy per unit length or "line tension" of a dislocation? What causes it?
T = Gb^2/2, Atoms are displaced from their equilibrium positions and thus have a higher energy.
60
How do you determine the shear stress to overcome obsticales.
Force = shear stres x burgers vector x length between obstacles
Force = c x line tension
Where C is a constant depending on how strong the obstacles are (c = 2 for strong obstacles)
increase in shear stress = a x Gb/L
61
What can be used to pin dislocations?
Work hardening (other dislocations pin dislocations), solid solution hardening (solute atoms provide weak obstacles), precipiation hardening (solid particles provide strong obstacles)
62
How do you determine the shear stress needed to slip a plane?
shear stress = instrinsic resistance + sshear stress needed to overcome obstacles
63
What is the remote shear stress given multiple grains of metal?
remote shear stress = 3/2 shear stress needed to move dislocations
64
How does yield stress relate to shear stress needed to move dislocations parallel to a slip plane?
shear stress = 3 * shear stress needed to move dislocations
65
What is a jog?
The pinning of a dislocation where a dislocation is no longer one strait line, reduces its mobility.
66
How does dislocation pinning contribute to yield stress?
Yield stress is proportional to (Gb/L) where L is the distance between dislocations L = 1/sqrt(dislocation density)
67
What are the challenges with precipitation
hardening?
Difficult to manufacture particles so small, therefore heat treatment is used to form precipitates.
68
What is grain boundary hardening? What is the Hall-Petch Relationship?
Having more grain boundarys can halt dislocations as they cannot slip from grain to grain, creates additioanll stress.
contribution to yield stress is proportiaonl to 1/sqrt(d) where d is the grain size.
69
What effect does glass transition have on the stress-strain response of thermoplastics?
Below Tg, they are elastic-Brittle
Above Tg, they are elastic-plastic
70
What is crazing?
When microcracks open in tension, bridged by stiff fibres of materials preventing fracture
71
What are shear bands?
When molecules align along shear planes giving greater ductility.
72
What is cold drawing?
Molecules align in neck of material and strengthen it, neck spreads accross material.
73
What happens to elastomers above Tg?
They undergo very large elastic strains with no ductility, all strain is recovered. This is non linear. Due to cross linkage.
74
What happens to thermosets above Tg?
There is limited shear yielding.
75
What is the stress concentration factor?
The ratio of stress at the crack tip to the remote applied stress, it works for blunt features.
=1+2sqrt(a/r)
2a is the flaw length
r is the radius of curvature at the tip
76
What is the strain eenrgy release rate?
The work done by the growth of a crack per unit crack area.
77
How do you derive strain energy release rate?
Find the work done in creating a larger stress-free region (model as a circle).
Find the increased area of the crack (model as a rectangle with thickness t and width 2a)
78
What is stress intensity factor?
A way of measure the stress at a finite distance from an infinitely sharp crack tip.
stress = K/sqrt(2pi x distance from crack tip)
K = Y stress x sqrt(pixa)
Y = 1.12 for an edge crack, K = 1 for a centre crack
79
What happens at a crack tip when plasticity is involved?
It is blunted such that the max stres = yield stress.
Increases the radius of curvature at the crack tip
80
How do you determine the size of the process zone?
r = Kic^2/(pi x yield stress^2)
81
What causes ductile fracture?
Voids are nucleated by inclusions. The grow, coalese and cause failure.
82
What causes failure of brittle materials such as ceramics?
The growth of the worst flaw.
83
What causes compressive failure in brittle materials?
Mode II failure, along line of maximum shear stress (45o to loading axis).
Usually 10-15 times greater than tensile strength.
84
What absorbs energy in compisites?
Fibres being pulled out of their sockets during crack advance.
85
What is the weakest link theory?
The strength of the whole sample = strength of weakest flaw.
86
What is m in the survival probability equation?
The Weibull modulus. Measures the variability of failure stress.
87
What is the stress range?
Max stress - min stress
88
What law for low cycle fatigue?
The Coffin-Manson law, based on strain (usually plastic)
88
What law for high cycle fatigue?
Basquin's law, based on stress
89
What Paris' law?
Rate of crack grwoth = A(change in K)^n. For steady state crack propogatio.
89
What assumptions are made in Miner's rule of cumulative damage?
Ignore sequence of loads, only applies for high cycle fatigue (under elastic deformation)
90
What creates stress concentrations?
variations in surfaces, and materials inhomogeneities. (machining, marks, flaws, notches, corrosion pits)
91
How can you delay crack initiation?
Shot peening - work hardening + residual compressive stress
Surface hardening - hardness
92
What's the difference between clamshell marks and striations?
Striations on the microscopic scale, clamshell marks visible to the eye. Both due to crack propogation under fatigue.
93
What is the leak before break critereon?
Critical flaw size should be larger than the wall thickness. Gas will leak out before fast fracture.
94
What other requirements (apart from leak before break) should be applied to thin walled pressure vessels?
Ensure the yield stress is not reached, using KIC.
95
How do you proof test a pressure vessel?
Fill the pressure to beyond planned working pressure, sets an upper bound on crack size. Use water, as this will not explode in the event of failure. Check there is no leakage or deformation.
96
Where does contact occur between two materials?
At Asperities.
97
What is the true area of contact?
At = W/H
98
What is frictional force?
Shear strength x True Area of contact
99
What is the effect of oxides in tribology?
The oxide layer on top of the metals accounts for the shear strength. All metals except gold forms a layer of oxides.
100
What happens to metals without oxide layers?
There are very large frictional forces because once sliding occurs, the true area of contact becomes very large. Large plastic deformation.
101
What is sticking friction?
When the true area of contact approaches the nominal area of contact and the frictional shear stress tends to the yield shear stress.
102
What happens between two surfaces of different hardness when they come into contact?
Abrasive contact occurs, the asperetieis of the hard material will penetrate the soft material. Causes a large dissapation of energy.
103
What is wear rate?
The volume of material removed/sliding distance
104
What is the wear coefficient
Wear rate = wear coeffecient x true area of contact
Depends on how much material is pulled off at the contacts.
105
What is the difference between adhesive and abrasive wear?
Adhesive - small bits shear off every time yield occurs.
Abrasive - materials comes off as hard asperities plow through soft material.
106
What does lubrication do?
Reduces wear and decreases energy dissapation. Has a very low coefficient of friction.
107
What happens in a journal bearing.
At high velocities, the oil phase supports the shaft, causing hydrodynamic lubrication.
When the shaft is in contact with the bearing there is boundary lubrication.
108
What does the Stribeck number indicate?
The optimum velocity for least energy dissapation.
viscosity x velocity / pressure due to shaft loading
109
How do polycrystalline materials form from a liquid?
crystals grow from small nuclei, to form grains.
110
Define ceramics.
Compounds of metals or silicon with non-metals.
Such as alumina, silicon carbine, glasses (made from silica), and porous ceramics (brick, concrete)
111
What explains Young's modulus on a microstructural scale?>
Bond stiffness of primary bonds, which is approximately linear at similar displacements .
Primary bonds behave as stiff elastic springs, which extends to the macrostructural scale.
112
What is the young's modulus of a solid solution?
Mixture of bond stiffness, somewhere between the YM of pure one.
113
What does density depend on on a microstructural scale?
Density depends on atomic packing and atomic mass.W
114
What properties are microstructure-insensitive?
YM and density, just to do with the atmoic packing and atomic mass,
115
How are amorphous metlas made?
By cooling them very fast.
116
What are the properties of amorphous metals?
Mechanically hard, very low damping
117
What are the tow methods of alloying polymers?
Copolymers, more than one monomer polymerised together.
Polymer blends, molecular scale mictures of two polymer chains, no cross linking.
118
What defines the melting point and glass transition temp of polymers?
Breaking of primary bonds defines the melting point.
Breaking of secondary bonds defines the glass transistion temperature.
119
What bonds are strongest (i.e. highest Tm)?
Covalent - 3800 degrees
ionic - 2000 degrees
Metallic - 1500 degrees
120
What happens above Tg?
Amorphous thermoplastics melt to a viscous liquid (molecules can slide over one another)
Semi-crytalline thermoplastics : amorphous regions melt, crystalline regions survive to a higher Tm.
Elastomers and thermosets - secondary bonds melt at Tg, but cross-links do not, polymer decomposes or burns.
121
What polymers can be reused?
Thermoplastics, because can make them viscous liquid, whereas, elastomers can only be moudled once.
122
What effects the elastic response of polymers?
Temperature, rate of loading, crystallinity, cross-lionking.
123
What is a typical value for Tg for amorphous thermoplastics?
80 degrees > room temperature.
124
Why does the modulus fall gradually during glass transition and melting?
Range of bond lengths in amorphous microstructure.
125
Between Tg and Tm in amorphous thermoplastics, what is the properties of polymers?
Very low residual stiffness, due to molecular chain entanglement, above Tm all chains slip by viscous flow.
126
Why is polymer elasticity affected by loading rate?
Molecular sliding is very sensitive to rate of deformation. Tg must be defined at a reference loading rate.
127
What does a represent for crack propogation?
Half the length of an internal crack.
The whole length of an external crack.
128
What is the process zone in terms of crack propgation?
The area of plastic deformation around a crack tip.
129
When is using K and stress intensity factor valid?
When the process zone is 50 times smaller than the length of the crack.
129
What effect does higher strength have on fracture toughness?
Usually decreases fracture toughness.
130
What is the difference in the E against temperature curves, for crystalline thermoplastic compared to amorphous thermoplastics?
Crystalline have much lower Tg (below room temp). But have a much higher YM above Tg compared to amorphous.
131
What is the Young's modulus against temperature response for a thermoset?
YM decreases very little over Tg. Stiff due to high-cross linking.
Higher YM than elastomer.
132
What is the Young's modulus - Temperature response of elastomers.
Very stiff below Tg.
Above Tg becomes rubbery, with large recoverable strains. Chains unravel extensively but pulled back by cross links.
133
What dominates the elastic response of foam?
The bending of solid ligaments.
134
How is the longitudinal and transverse modulus determined?
Longditudinal modulus is based on uniform strain.
Transverse is based on uniform stress.
135
What is an example of a laminate?
plywood, 'GLARE' (Al-GFRP)
136
Define strength.
The nominal stress at the elastic limit
137
What happens at he elastic limit for brittle materials under tension and compression?
Tension, fracture occurs (tensile strength)
Crushing occurs, (compressive strength)
138
What does elastic and plastic deformation represent in terms of atoms?
Elastic deformation displaces atoms by a fraction of their equilibrium spacing.
Plastic deformation involves relative movement of material of large multiples of atomic spacing.
139
Why is the real strength of a material much lower than the ideal strength?
Becauase not all atoms are displaced at once, rather one by one through the movement of dislocations.
140
What is the slip step in materials?
The passage of one dislocation, the burgers vector.
141
What is the difference between an edge and slip dislocation?
Shear stress and burgers vector are at right angles to the dislocations for edge dislocations. Dislocations move in direction of the stress.
Whereas, screw dislocations, shear stress and burgers vector are parallel to the dislocation.
Dislocation moves at right anlges to the stress.
142
Where does the instrinsic lattice resistance come from?
Bond stretching as the dislocation moves each Burgers vector step.
143
how does the intrinsic lattic resistance to dislocation motion vary with material?
Diamond/ceramics (covalent/ionic bonds) leads to a very high resistance, so is hard.
Metals have weaker metallic bonds leading to lower resistance. Annealed pure metlas are soft.
144
Why do dislocations try to be as short as possible?
To minimize the potential energy sotred.
145
What happens when a gliding dislocation meets an obstacel?
It is pinned by the obstacle and bows out between them, increasing resistance per unit length, more shear stress is needed.
146
What is the yield stress needed to move dislocations parallel to a slip plane?
τ = τo + (Δτ)
intrinsic resistance + contribution to to yield stress due to dislocation pinning.
147
What is recrystallisation?
Grains re-from in solid-state by heat treatment.
148
Under a remote stress, what dislocation move first?
The didlocations will move first in grains which are orientated with the shear stress.
149
What is the shear yield stress?
The remote shear stress required to move dislocations. = 3/2 x shear stress parallel to dislocations required for them to move.
150
How does stress vary due to dislocation density?
proportional to 1/L, which is the spacing between between dislocations.
151
How can you recover ductility?
By heat treating (annealing) to decrease the dislocation density.
152
How are metals work hardened?
By deformation processing (rolling, wire drawing) to increase dislocation density whilst shaping the product.
153
How does solid solution hardening work?
Mixture of metal and other elements, different size and local bonding, which roughens the slip plane. Provides a weak obstacle.
154
What is interstitial solid solution hardening?
Smaller atoms are in interstitial holes, which also displaces host atoms from their equilibrium positions, roughening the slip plane.
155
What is the strenght contribution of solid solution hardening?
C^1/2 Where C is the solute concentration. Since stress from dislocaion pinning is proportional to 1/L.
156
How is soild solution hardening done?
By using mixtures in casting.
157
How does precipitation hardening work?
Particles provides strong obstacles.
158
What is the yield strength contribution of precipitation hardening?
3Gb/L
Where L is the particle spacing.
159
How is particle spacing determined for precipitation hardening?
can be determined by their size and volume fraction. MOdel as a cubic array of particles of radius R, centre to centre spacing D.
160
How is precipitation hardening done?
By using heat treatment in a solid state, forming precipitates.
161
What is GBH and how useful is it?
Grain boundary hardening, relatively weak mechanism.
Can be used for pure metals or dilute alloys.
Dislocations cannot slip from grain to grain, causing pile-ups at boundaries. stress from pile-up nucleates dislocations in the adjoining grain.
Finer grain size more often boundaries obstruct dislocations.
proportional to 1/(sqrtd) where d is grain size.
162
What determines the strength of polymers?
The ability of the chain molecules to unravel and slide.
The temperature, strain rate
Crystallinity, and cross linking
163
How does polymer strengths compare to their YM? and why?
Of a similar order of magnitude (0.01-10 xE)
Stiffness dominated by secondary bonds.
Strenght involves breaking of primary bonds
164
How do thermoplastics fail?
Below Tg, they fail brittle, chain sliding limited, fail due to flaws with limited ductility.
Above Tg, chain mobility increase as van der waals bonds melt.
yielding takes place by crazing shear yielding or cold drawing.
165
What is the difference between thermosets and elastomers in their loading responses above Tg?
Thermosets have limited shear yielding.
Elastomers are non-linear elastic, very large elastic strains to fialure with little ductility, elastic strain is recovered.
166
What is the difference between the fracture surface of ductile and brittle materials?
Brittle materials tend to have smooth fracture surfaces as they fail by cleavage.
bcc and cph materials can become brittle at low temperatures.
Ductile materials, form inclusions and large plastic flow.
167
How do you delay crack initiation?
shot peening (work hardening, residual compressive stress)
surface hardening (work hardens)
