MATERIALS

Cohesive wear

asperities of the harder material plough through the softer one, grinding off wear particles

Assumptions made in calculating atomic packing factor

atoms are rigid spheres, radius is maximum value such that atoms don't overlap

What test is used to determine the strength of ceramics?

bend test

How to stop galvanic corrosion?

block the electron path (i.e. insulator between the two metals), block the ion path (paint the metals, dehumidifying the environment)

Problems with protective metal oxide layer

some (e.g. FeO) has a higher ion and electron diffusivity so is less stable than the metal, metal oxide layer may not adhere well to the metal

Rust generated by corrosion of reinforced steel bars, builds up and causes...

spalling and delamination - allows for more water to reach the steel, allowing for even more corrosion

Equation for stress

stress = force/area

Fatigue limit

stress below this value does not cause any crack growth (materials with no fatigue limit will show crack growth at all cyclic stresses)

Low cycle fatigue

stresses are above the yield strength but below tensile strength of the material e.g. components subject to occasional overload (hip joints)

High cycle fatigue

stresses are below yield strength e.g. rotating systems

Types of solid solutions

substitutional and interstitial

Wear rate depends on...

surface roughness, load, hardness

General corrosion

takes place over most of the metal surface

Amonton's Law of Friction

the force of friction is independent of the apparent area of contact the force of friction is directly proportional to the applied load

Wear

the progressive loss of material

Coefficient of friction

the ratio of the force of friction to the normal force acting between two objects

Hell-petch relationship

there is an inverse relationship between yield strength and grain size to some power (the smaller the grain size, the smaller the repulsion stress felt by a grain boundary dislocation and the higher the applied stress needs to propagate dislocations through the material)

When is carbonation a problem?

when there is a thin layer of concrete covering the reinforcing layer, the concrete is porous and cracked.

shot peeing

work hardening and compressive stresses in surfaces

Equation for deflection

y = (F x l^3)/ E x I x 3

What's the problem with the bend test?

you can't predict where the fracture will occur (premature failure) - instead you can use a 4 point bend

Crystalline imperfections: self-interstitials

"extra" atoms positioned between atomic sites

Limiting oxygen index

% of oxygen in the atmosphere needed for combustion (i.e. polymer with low LOI will burn easily)

Equation for material selection for floor beams

(shape factor * E)^1/2 / density

Intergranular corrosion

- Attack on or adjacent to grain boundary in metal - Occurs due to changes in metal chemistry at grain boundaries - often results from heat treatment 'sensitising' grain boundaries

Applications that need high toughness

- Container for liquid nitrogen (needs good toughness at low temperatures, good strength/ corrosion resistance) - ski rescue sled (needs good toughness against possible stone strike, high stiffness and strength, good wear resistance, low density)

Problems with work hardening

- bone plates (work hardened by trying to fit body, makes it more susceptible to stress corrosion cracking) - bicycle frames (increases susceptibility to cracking)

Stress corrosion cracking

- combined action of mechanical stress and corrosive environment - re-passivation at crack tip prevented by applied stress - cracks grow perpendicular to applied stress direction - intergranular corrosion can cause stress corrosion because the corrion sites along grain boundaries are very sharp making cracking easier

Applications of work hardening

- copper and water pipes - car body panel (yield strength increases from work hardening, gives panel greater dent resistance)

Critical crack lengths

- cracks grow slowly until they reach critical crack length - critical crack lengths are a measure of the damage tolerance of a materials - tough materials are able to contain large cracks but still yield in a predictable, ductile manner

Failure in ceramics

- in tension, the "worst flaw" propagates to failure - caused by porosity. - in compression, cracks can still extend but in a stable manner (ceramics are much stronger (10 times) in compression)

How to measure hardness

- indent made into sample - indent measured across the diagonal - indent measured across the second diagonal hardness value calculated (simple, inexpensive and non-destructive)

Crystalline imperfections: dislocations

- linear defects leading to atom misalignments - causes slip between crystal plane when they move

Corrosion of mild steel

- mild steel is protected from corrosion in wet environments by the presence of a thin "passive" oxide film - oxide is stable in alkaline environments - oxide dissolved in acidic environments leading to the formation of rust

Galvanic corrosion

- occurs where dissimilar metals are in contact - one material acts as an anode, the other as a cathode - anode is more chemically reactive - anode will suffer material loss (corrosion)

Charpy impact toughness test

- sample hit with a hammer - measure energy absorbed (polymer will absorb less energy on fracture, use a smaller hammer)

Methods to improve fatigue life

- surface harden component - reduce applied tensile stresses - improve surface finish - avoid stress concentrations - refine microstructures

How to prevent pitting?

- using alloys with low impurity levels to stop pits from forming - adding alloy elements - having smooth, clean surfaces

work hardening

Also known as strain hardening or cold working, this is the process of toughening a metal through plastic deformation. - FCC metals work harden more than BCC and HCP materials - increases dislocation densities - makes it more difficult for dislocation movement

FCC metals include

Aluminium, copper and nickel

Toughness

Amount of energy absorbed by a material as it fractures

Grain

An individual crystal in a polycrystalline metal or ceramic.

For a smooth sample, how can toughness be measured from the tensile test?

Area under the stress-strain curve

Parabolic oxidation growth curve

As oxide layer gets thicker, it is more difficult for ions to diffuse through - see a parabolic oxidation growth rate

Properties of ceramics

Brittle, abrasion resistant, stiff, hard, good high temperature strength

Significance of fatigue crack growth

Can predict growth rate and hence number of cycles until failure then can implement suitable inspection and maintenance programme

Non-crystalline or amorphous polymer

Carbon chains are arranged randomly e.g. natural rubbers

Semi-crystalline polymer

Carbon chains folded carefully e.g. polyethylene

How to determine the fatigue life of a smooth component?

Carry out tests at a given stress amplitude and measure number of cycles to failure. Plot data on an S-N curve where S is stress and N is number of cycles to failure.

Why can't the tensile stress be used to measure the strength of ceramics?

Ceramics are brittle - difficult to prepare samples with the required geometry - difficult to grip samples without fracturing them

For a notched sample (notched toughness), how can toughness be measured?

Charpy impact test *** notch toughness depends on temperature

Differential aeration corrosion

Corrosion of mild steel takes place in the middle of the droplet because the cathodic reaction takes place at the edge of the droplet as there is easy access of oxygen from the air

Protection methods for metals against general corrosion

Corrosion resistant alloys, barrier coating, sacrificial coating (galvanising)

Problems with the corrosion of rebars (other than spalling and delamination)

Decreases the cross section, causing higher stresses making them more susceptible to fracture

Bulk mechanical properties

Density, modulus, yield and tensile strength, hardness, impact resistance

Equation for young's modulus, for materials that obey Hooke's law (only in tension)

E = stress / strain

Production properties

Ease of manufacture, fabrication, joining, finishing

Properties of metals

Electrically and thermally conductive, prone to corrosion and fatigue, medium/high modulus, range of meting points

Structure: ductility

FCC > BCC > HCP

Different conditions for fatigue

Fatigue of cracked structures - component contains cracks - cracks propagate during service life until detected and removed or until failure Fatigue of uncracked structures - failure is initiation controlled

HCP metals include

Magnesium, zinc and titanium

True stress

Force over instantaneous area (up to strain where necking begins, specimen deforms with constant volume) - equal to or larger than engineering stress

Engineering stress

Force over original area (up to strain where necking begins, specimen deforms with constant volume)

Examples of ceramics

Glass, pottery, cements, concretes

Structure: strength

HCP > BCC > FCC

Solid Solution Strengthening

Hardening and strengthening of metals that result from alloying in which a solid solution is formed. The presence of impurity atoms restricts dislocation mobility.

Effect of temperature on strength for polymers

Increasing temperature causes: - a decrease in elastic modulus - a reduction in tensile strength - an increase in ductility

Methods to improve high temperature oxidation

Instead of using plain steel - add chromium to produce protective Cr2O3 film (used in medical implants) For nickel based superalloys (in turbine blades) add Cr for more protective oxide and add hafnium which 'keys in' oxide to surface to prevent spalling

BCC metals include

Iron, chromium and vanadium

Properties of polymers

Light, low density, easily shaped, high strength/weight ratio, lack stiffness.

Hardness

Measure of a material's resistance to localised plastic deformation

Strain rate

Measure of how fast you deform a material

Corrosion

Metals are protected by a very thin passive film, when the film becomes unstable, corrosion reactions take place

The three classifications of engineering materials.

Metals, polymers and ceramics

What affects toughness?

Presence of a notch, strain rate, environmental conditions

Necking in polymers

Secondary (intermolecular) bonds between polymer chains are broken allowing the sliding of chains. Once the length of the polymer chains are aligned, the bonds between the carbon atoms in the chains need to be broken for further extension.

Grain size control

Smaller grain sizes result in increases strength levels

Examples of metals

Steels, copper alloys, nickel alloys, titanium alloys

Flexural strength

Stress at fracture from a bend (or flexure) test.

Types of stress

Tensile, compressive, shear, biaxial tension, hydrostatic pressure

Crystalline imperfections: vacancies

The absence of an atom or molecule from a point it would normally occupy

Grain size

The average grain diameter as determined from a random cross section.

Adhesive wear

adhesion at work hardened junctions cause shear-off materials when surfaces slide

Atomic packing factor

The fraction of volume in a crystal structure that is occupied by constituent particles.

Grain boundary

The interface separating two adjoining grains having different crystallographic orientations.

Aesthetic properties

appearance, texture, feel

Bulk non-mechanical properties

Thermal properties (heat capacity, conductivity, diffusitivity), magnetic and electrical properties (dielectric constant, 'hard' or 'soft' magnet)

Examples of polymers

Thermoplastics, resins, elastomers, woods

For samples with a crack (fracture toughness), how can toughness be measured?

Use a critical load for failure

Phase

a homogeneous portion of a system that has uniform physical and chemical characteristics and is physically distinct from other parts of the system

Crystalline imperfections: grain boundaries

boundaries between crystals have a change in orientation across them

What is carbonation?

carbon dioxide in the environment reacts with water to form carbonic acid and lowers the pH.

How is grain size altered?

control during processing

Ductile-brittle transition

depends on the composition and microstructure of the steel

Carbon exhibits polymorphism between... due to...

diamond and graphite due to temperature and pressure changes.

precipitate strengthening

distribution of large numbers of fine precipitates prevents dislocations from moving past them

Lubrication allows for...

easy shear between the surfaces and reduced coefficient of friction

Three types of close packed structure

face centred cubic (FCC), hexagonal close packed (HCP) and body centred cubic (BCC)

Fatigue

failure under applied cyclic stress

Iron exhibits polymorphism between...

ferrite (BCC), austenite (FCC)

Characteristic appearance of fatigue cracks

flat, originating from a single point

Fracture test (Kic) equation

for an edge crack = a for an internal crack = 2a

Crystalline imperfection: substitutional impurity atoms

foreign atoms in a lattice

5 types of localised corrosion

galvanic corrosion, pitting corrosion, crevice corrosion, intergranular corrosion, stress corrosion cracking

Oxidation: metals

generally not stable (except gold), metals oxidise in air but metal oxide may be stable and protective

Strength and hardness relationship

high strength = high hardness

Increasing the strain rate...

increases the strength of a polymer (polymers are most sensitive to changing strain rate)

The behaviour of a polymer depends on ...

its temperature relative to the materials glass transition temperature (Tg)

Equation for stiffness

load / deflection

Energy usage depends on which phase of life

manufacture, use or end of life

Life cycle analysis looks at...

material, manufacture, transport, use, disposal

Localised corrosion

most of the metal surface structure is protected by a passive film, but a local area starts to corrode

Oxidation: ceramics

mostly oxides, as they already involve oxygen are stable at high temperatures (therefore use as refractory linings (in furnaces etc.))

Pitting corrosion

needs the presence of a chloride (e.g. in salt water), usually occur at inclusions in metals. - the pit is the anode (where the metal dissolves) and the cathodic reaction takes place on the passive metal surface. - the pit keeps growing because of the anodic reaction in the pit leads to a lower pH which stops the passive film from forming

Oxidation: polymers

not stable to oxidation and tend to burn if ignited, properties degrade at hight temperatures (PTFE is an exception)

Crevice corrosion

occurs when two metal surfaces are pressed together with moisture between them, the anode develops between the metal surfaces and the cathode is the outside, the crevice continues to grow because the anodic reaction leads to a lower pH which prevents growth of the passive film

Surface properties

oxidation and corrosion resistance, friction, abrasion and wear

Define the modulus of elasticity

resistance of a material to elastic deformation (doesn't vary depending on size, determines natural vibrational frequency)

Define component stiffness

resistance of the component to elastic deformation (depends on the elastic modulus and the design of a component)