T207 Tutorial block 4 Please check your headphone

T207  Tutorial block 4 Please check your headphone

T207 Tutorial block 4 Please check your headphone and microphones and adjust them for sound levels Check your audio settings by running the Audio Setup Wizard: either press the icon in the 'Audio and Video' panel, or

navigate to Tools > Audio > Audio Setup Wizard and follow the on-screen T207 Tutorial 05 Parts 1 - 3 Corrosion Material Properties

Corrosion List the different types of corrosion. Suggest ways of preventing the

types of corrosion. Corrosion What is corrosion? What causes corrosion? What is necessary for corrosion to occur?

Corrosion Corrosion is an electro chemical process that removes electrons from one material and passes them to another. Corrosion is caused by having two dis-similar metals

in contact through an electrolyte and results in an electric current passing between them. Corrosion There are FOUR basic requirements in order for corrosion to occur

An ANODE A CATHODE An ELECTROLYTE (a solution that will allow electrons to flow An ELECTRIC CURRENT (or flow of electrons)

Anode Loss of electron in oxidation Anode &atCathode the anode

Oxidation always occurs Cathode Gain of electron in reduction e

d o h at c e th

at rs u c c o s

y a lw Reduction a Corrosion The chemical process is dependent upon the

Galvanic series and Electro-potential These series show the electrical potentials (as measured against a datum, such as hydrogen) that are usually listed in order of their NOBILITY (or likelihood to corrode)

Corrosion Other corrosion mechanisms See section 4 of Block 5 Part 1 Pitting corrosion (doesnt require 2 dissimilar metals usually a scratch in a protective surface)

Corrosion Other corrosion mechanisms See section 4 of Block 5 Part 1 Pitting corrosion (doesnt require 2 dissimilar metals usually a scratch in a protective surface)

Corrosion Other corrosion mechanisms See section 4 of Block 5 Part 1 Stress-corrosion cracking (combination of a tensile stress, chemical environment and a material susceptible

to chemical attack Corrosion Other corrosion mechanisms Crevice corrosion (occurs where conditions suggest corrosion shouldnt take place; a crevice where moisture can

collect and stagnate) Corrosion Other corrosion mechanisms Erosion and cavitation corrosion (fast flowing fluids where low pressures cause air bubbles to collapse)

Preventing corrosion You need to remove at least one of the 4 requirements for corrosion to occur This can be in the form of a layer of protective

material, sacrificial metal to corrode etc. Preventing corrosion Use the same metals (not dis-similar) Avoid crevices where moisture can collect and stagnate

Avoid water accumulation by sealing joints etc. Joint design Welding design (double V joints smoothed over) Coatings, painting etc. Sacrificial metals Polymers

Polymers can degrade due to 1. Elevated temperatures for prolonged periods 2. UV degradation 3. Chemical degradation UV light and polymers UV light can attack the long molecular chains,

breaking them down. Chemical degradation of polymers Usually associated with elevated temperatures and an

acidic or alkaline environment Material Properties How does the atomic structure of metals differ from ceramics? How does this affect their mechanical properties?

Metals Metals are made of a crystalline structure that is fairly simple and repetitive. They have been investigated through X ray diffraction techniques and the images analysed to

ascertain the underlying crystalline structure Basic crystalline structures Basic crystalline structures Face centred cubic

Usually more ductile than other structures Basic crystalline structures Body centred cubic Less tightly packed (more free space)

Basic crystalline structures Hexagonal close packing Each atom is surrounded by 6 other atoms

Slip planes Atoms can slip (move) in the crystalline structure Slip planes Atoms can slip (move) in the crystalline structure Slip planes

The slip planes are associated with the easiest path for atoms to move past each other. Close packed structures have several slip planes and so they are more malleable and ductile than other packing structures

2D slip planes Dislocations Dislocations are imperfections in the metallic crystalline structure which make it easier for atoms to

move and thus increase the ductility of metals. This ductility is also associated with the extended plastic behaviour of metals (deformation after the elastic limit has been achieved). Plasticity requires the presence of dislocations and

the ability of them to move through the crystalline structure Structure of ceramics Ceramic materials are crystalline (as metals) but are usually compounds of two or more elements

The arrangement of the atoms is specific (not random) and this affects the packing structure. Size of atoms affects the structure (large and small ions may not form a rectangular structure)

Sodium chloride (salt) Barium Titanate Ba Ti O4 Amorphous ceramics Usually oxides, these have no close

packed structure and hence have no crystalline structure as such Amorphous ceramics Differences between metals and ceramics

Metal bonding allows the atoms to move past each other (slip planes) which give srise to ductility, malleability and plasticity Ceramic atoms are chemically bonded to each other, so there are NO slip planes for the atoms to move past

each other Force on a ceramic will lead to fracture of the bonds, and hence the material (but at high temperatures there may be limited slip) Structure of polymers

Polymers are made up of long strings of atoms, which are built up of regular or irregular sub strings Bonding in polymers Individual chains are strongly chemically bonded, but

links with other chains is from weaker inter-atomic Van der Vaal's forces How polymer chains appear Transition temperatures From block 2, the ductile brittle temperature is the

temperature where the material will suffer a ductile failure rather than brittle Melting temperature This is the temperature when the substance changes

phase from solid to liquid (Tm) Melting temperature This is the temperature when the substance changes phase from solid to liquid (Tm)

Glass temperature For an amorphous ceramic, it is the temperature when the material will become less brittle and less stiff Stress strain relationships for polymers

Stress strain relationships for polymers Stress strain relationships for polymers

Viscoelasticity Where a polymer exhibits both viscous and elastic behaviour Viscous is where polymer chains slip pass each other Elastic is where the chains are subjected to tensile

forces Tine dependent (hysteresis behaviour on laoding/unloading) Increasing strength of metals Controlling the strength of metals is all about

controlling how the atoms slip past each other and impeding the passage of dislocations. Impeding dislocation movement Plastically deforming the material will cause

dislocations to interfere, entangle and impede the motion of other WORK HARDENING As crystal boundaries impede the motion of the dislocations, reducing the size of the crystal grain size also impedes the movement of dislocations GRAIN

SIZE STRENGTHENING Impeding dislocation movement Changing the size of the atoms (alloying with other atoms) this addition of alloying elements to make a

solid solution is called SOLUTION HARDENING Discrete particles of different composition of the second phas can present obstacles to dislocation movement AGE HARDENING or PRECIPITION HARDENING

Hall-Petch Equation SAQ 3.1 Page 123 Block 5 Part 3 The 0.1% proof stress of an aluminium alloy was measured for different grain diameters, d, produced by annealing at various temperatures. At the largest grain diameter of 0.5 mm the 0.1% proof stress was 110 MN m-2 and the value of k was found to be 0.45 MN m-3/2. Estimate the 0.1% proof stress with a grain

diameter of 0.04 mm. d = 0.5 mm = 0.0005 m t = 110 MN m-2 = 110x106 N m-2 and k = 0.45 MN m-3/2 = 0.45x106 N m-3/2 Using the Hall-Petch equation t = 0 + kd0.5 Hall-Petch Equation

SAQ 3.1 Page 123 Block 5 Part 3 d = 0.5 mm = 0.0005 m t = 110 MN m-2 = 110x106 N m-2 and k = 0.45 MN m-3/2 = 0.45x106 N m-2 Using the Hall-Petch equation t = 0 + kd0.5 Know d, t and k Need to find 0 0 = t - kd0.5 0 = 110x106 - 0.45x106 x 0.0005-0.5

So 0 = 89875388 N m-2 = 90 MN m-2 Hall-Petch Equation SAQ 3.1 Page 123 Block 5 Part 3 d = 0.5 mm = 0.0005 m t = 110 MN m-2 = 110x106 N m-2 and k = 0.45 MN m-3/2 = 0.45x106 N m-2 Using the Hall-Petch equation

t = 0 + kd0.5 Know d, t and k Need to find 0 0 = t - kd0.5 0 = 110x106 - 0.45x106 x 0.0005-0.5 So 0 = 89875388 N m-2 = 90 MN m-2 t = 90 + 0.45d0.5 With a grain diameter, d = 0.04 mm = 0.00004 m t = 90106 + (0.45106 0.000040.5)

t = 161151247 N m-2 = 161 MN m2 Hall-Petch Equation I was asked to explain the transposition of a formula in Exercise 3.1 of Block 5 Part 3 to mke k the subject. t = 0 + kd0.5 Know t and 0 need to find k t 0 = kd0.5

(t 0) / d-0.5 = k or k = (t 0) / d-0.5 A negative index means the inverse so 1 / d-0.5 = d0.5 So k = (t 0) d0.5 0 = 90x106 N m-2 t = 113x106 N m-2 d= 0.0004 m k = (113x106 90x106) x 0.00040.5 k = 460000 = 460x103 N m-3/2

Strengthening polymers Change the chain structure by copolymerisation or modification of the repeated monomer unit Alignment of the chains in the direction of loading Fabrication of composites such as the Mosquito

aircraft in WW2 made by cabinet makers with wood and glue. Now the Boeing Dreamliner continues the work!

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