T207 Tutorial 15th and 22nd March

T207 Tutorial 15th and 22nd March
4 TMAs done, well done! TMA05 due 30th March TMA06 due 4th May TMA17 due 11th April (Friday)

T207 – Preparing for TMA 05 using the appropriate galvanic series to determine the likelihood of corrosion occurring for dissimilar metals in a particular environment; explaining the use of anti-corrosion measures such as coating, galvanic protection and impressed current; describing the basic atomic structure of different types of materials; explaining that the plastic deformation of metals is a results of presence of dislocations in the crystal structure and their mobility; performing calculations using basic fracture mechanics concepts given appropriate data; using and interpreting creep curves and apply them to practical examples; understanding why creep occurs and identify various methods of limiting creep.

Routes to failure

Routes to failure In order to understand what is happening during these processes we need to think a bit about the microstructure of our materials

Corrosion What is corrosion?

Corrosion What is corrosion? Specific chemical reactions
Can be accelerated or retarded by both the environment and other metals in contact.

Corrosion What is corrosion?

Corrosion Metal bridges over railway tracks – significant corrosion and lamination over the area where the steam was most concentrated, especially if the engine was pulling uphill

Periodic table Microstructure: What is the periodic table?

Periodic table Most people understand that an atom is a very small particle of substance that can no longer be subdivided.

Periodic table What does an atom “look” like?

Periodic table What does an atom “look” like?
Protons and neutrons in the centre Elecrons orbit the nucleus in “shells” Each “shell” has a number of spaces available

Periodic table The rings around an atom can have “gaps”, and the atom would like to fill these

Periodic table Potassium 19 electrons 2 in shell 1 8 in shell 2

Periodic table More complex as you get more protons / neutons and electrons

Periodic table Mercury
Its atomic number is 80, which means it has 80 protons (positively charged particles in the nucleus of atoms) and 80 electrons (negatively charged probability clouds that show the probability of the particle being in a given space at any given time). According to the ‍inaccurate‍ Bohr Model*, of the 80 electrons, 2 are in the first “shell”, 8 are in the second “shell”, 18 are in the third “shell”, 32 are in the fourth “shell”, 18 are in the fifth “shell”, and 2 are in the sixth “shell”. In the periodic table, mercury occupies period 6, group 12, or II-B. The fact that it is in period 6 shows that there are 6 “electron shells”. The fact that it is in group 12, or II-B, means that it has 2 valence electrons (electrons in its outermost “shell”), and therefore reacts strongly with elements from group 16, or VI-A, like oxygen and sulphur

Periodic table We can arrange the elements into a table which gives us some idea of the properties. The periodic table has taken many forms over time, but this is the current, basic form

Periodic table Some atoms have “spare” electrons
When an atom with a “gap” gets together with an atom that has a “spare”, this makes a very strong unit called a molecule

Periodic table Hydrogen has one gap

Periodic Table

Periodic Table Carbon dioxide
By sharing the four electrons where the shells touch each oxygen and carbon atom can count 8 electrons in its outer shell. These full outer shells with their shared electrons are now stable, and the CO2 molecule will not react further with other oxygen or carbon atoms.

Corrosion Corrosion occurs when a refined metal converts to its more stable oxide

Corrosion Corrosion occurs when a refined metal converts to its more stable oxide This can be accelerated by the presence of gases such as oxygen, or compounds such as water

Corrosion If you have two metals in contact with an electrolyte, they will form a cell If two dissimilar metals are present, and they are in contact in a corrosive environment, one will corrode in preference to another

Corrosion We can exploit these facts to make batteries, for example

Corrosion Glavanic series

Corrosion Glavanic series Which is MOST reactive?

Which is LEAST reactive?
Corrosion Glavanic series Which is LEAST reactive?

If two are in contact, which will corrode?
Corrosion Glavanic series If two are in contact, which will corrode?

Corrosion Glavanic series If two are in contact, which will corrode?
The one furthest down the table

Example: If Lead and tin are in contact, which will corrode?
Corrosion Glavanic series Example: If Lead and tin are in contact, which will corrode?

Example: If Lead and tin are in contact, which will corrode?
Corrosion Glavanic series Example: If Lead and tin are in contact, which will corrode? Tin

Example: If silver and iron are in contact, which will corrode?
Corrosion Glavanic series Example: If silver and iron are in contact, which will corrode?

Example: If silver and iron are in contact, which will corrode?
Corrosion Glavanic series Example: If silver and iron are in contact, which will corrode? Iron

Corrosion Now that you know what causes corrosion, what do you think we can do to prevent it? Exploit some of the knowledge we have

Corrosion Somehow prevent that cell from forming, block the flow of electrons Coatings, galvanic protection and impressed current

Corrosion Coatings could be paint What are the possible issues here?
How do you carry out galvanic protection? How do you use impressed current? What are the possible issues?

Corrosion Coatings could be paint What are the possible issues here?
Movement of the structure causing cracks in the paint Breakdown of the structure of the paint (may be affected by pigments) Badly prepared surfaces

Corrosion How do you carry out galvanic protection?
What are the possible issues here? Use sacrificial anodes How do you use impressed current? What are the possible issues? Takes power May become more usable with solar power, for example maintenance

A bit more about microstruture
Three different classes of materials which have very different microstructures

Metals Ceramics Polymers

Metals Pure metals are an element Simple crystal structures Plastic deformation occurs due to imperfections in the structure

Metals These deformations are known as dislocations, and they can move through the crystal lattice This is what gives metals their ductility. The easier it is for these dislocations to move, the more ductile a metal will be

Ceramics Covalent bonds, very strong Limited movement of the crystal lattice Ceramics are therefore very brittle But can also very strong, although not tough (can’t withstand damge very well)

Polymers Long chains of atoms bonded in complex arrangements Chains are very strong Van der Waal’s forces between chains

Polymers The structure can be amorphous, tangled and chaotic, like a tangle of yarn The chains can slip past each other, which is why polymers can be ductile

Materials microstructure summary
Metals Ceramics Polymers Microstructure Regular crystal lattice of identical atoms for a pure metal Molecules Chains Bonding type Inter-atomic Covalent bonding Covalent for the chains Van der Waal’s forces between chains Ductility Dislocation slip limited Chain slip Corrosion? Yes – particularly for pure metals possible Fatigue? Yes Rare Possible This is really important for understanding material properties, as it underlies the key differences between classes of material and their response to stress and temperature

Materials microstructure summary
Why do I say “polymers” instead of “plastics”?

A bit about strengthening metals
How do we exploit our knowledge of the microstructure of metals to strengthen them?

We know that the microstructure of a pure metal is a crystal lattice We can add other substances to change the properties of that metal, this is known as alloying Disrupts the movement of the dislocations We can also change the ease of plastic flow by work hardening, grain size strengthening, age or precipitation hardening, and solution hardening

Hall-Petch Grain boundaries are good at inhibiting dislocation movement So if we can make the grain size small, we can strengthen the material

Strength is inversely proportional to the square root of the grain size Hall-Petch σ 𝑡 = σ 𝑜 +𝑘 𝑑 − 1 2 For this, σ 𝑜 and k are constants for a given metal σ 𝑡 is the strength d is the grain size Empirical equation What does this mean in reality?

Ex 3.1 σ 𝑡 = σ 𝑜 +𝑘 𝑑 − 1 2 σ 𝑜 =90MN/m2 σ 𝑡 =113MN/m2 d=0.4mm What grain size is required for a proof stress of 180MN/m2

Ex 3.1 σ 𝑡 = σ 𝑜 +𝑘 𝑑 − 1 2 σ 𝑜 =90MN/m2 σ 𝑡 =113MN/m2 d=0.4mm First step, find k σ 𝑡 = σ 𝑜 +𝑘 𝑑 − 1 2 113× 10 6 =90× 𝑘 (113−90)× 10 6 = 𝑘 23× 10 6 × =𝑘 k=460000Nm-3/2

Ex 3.1 σ 𝑡 = σ 𝑜 +𝑘 𝑑 − 1 2 σ 𝑜 =90MN/m2 σ 𝑡 =113MN/m2 d=0.4mm Next step, find d σ 𝑡 = σ 𝑜 +𝑘 𝑑 − 1 2 180× 10 6 =90× 𝑑 (180−90)× 10 6 = 𝑑 90× 10 6 × 𝑑 =460000 d= × =26.1× 10 −6 m

A bit about Fracture Mechanics
What is fracture mechanics?

A bit about Fracture Mechanics
Again, we need to think about the microstructure of the material For metals there are a number of different ways that the material can fail The ductility of the material is a key issue; will it fail in ductile (plastic) or brittle failure mode?

A bit about Fracture Mechanics: Ductile failure

A bit about Fracture Mechanics: Brittle failure

A bit about Fracture Mechanics: Fatigue
Loading must be cyclic Some tensile stresses present Stresses are lower than the yield stress of the material Material must have some ductility

If you enjoy this topic area, then think about taking T357

What drives a crack? Geometry Material properties Stress applied

Fracture Mechanics The key equation; 𝐾 𝐼 =𝑌σ π𝑎 Where
𝐾 𝐼 =𝑌σ π𝑎 Where 𝐾 𝐼 - crack tip characterising parameter 𝐾 𝐼𝐶 - a material property, the toughness of the material (the greater this value, the harder it will be to fracture the material) Y – a geometric property a- measure of the crack length

Fracture Mechanics How do we use this? Example (P52):
Wall thickness 100mm Crack length 5mm Tensile stress 450MN/m2 Fracture toughness, KIC=90MN m-3/2 100mm Model as a flat plate 5mm

We want to know what stress would cause the crack to grow by fast brittle failure. In order to do that we set KI=KIC So we are saying that the stress that is driving the crack tip is equal to the toughness 100mm Model as a flat plate 5mm

We can calculate Y using the geometry 100mm Model as a flat plate 5mm

So using the fracture mechanics equation we get 𝐾 𝐼 =𝑌σ π𝑎 90× 10 6 =1.13σ π0.005 Rearranging for σ gives 90× × π =σ σ=635MN/m2 100mm Model as a flat plate 5mm

So using the fracture mechanics equation we get 𝐾 𝐼 =𝑌σ π𝑎 90× 10 6 =1.13σ π0.005 Rearranging for σ gives 90× × π =σ σ=635MN/m2 What does this mean? it means that at this stress the crack will start to grow rapidly 100mm Model as a flat plate 5mm

Creep What is creep?

Creep Slow failure over time Usually under a low, but constant load
Ductile Temperature dependant, higher temperatures may increase the rate dramatically

Creep Key equation 𝑇 𝐻 = 𝑇 𝑇 𝑚
𝑇 𝐻 = 𝑇 𝑇 𝑚 This is the ratio of the oerating temperature to the melting point, and is known as the homologous temperature Needs to be kept below 0.4 to prevent creep

Creep Creep curves

T207 Tutorial 15th and 22nd March

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T207 Tutorial 15th and 22nd March

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