City & Guilds 2850-20 in Engineering

Physical and Mechanical Properties of Materials

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Outcomes

• State the physical properties of materials.

• Define what is meant by mechanical properties of materials.

• State the mechanical properties of materials.

• Describe the mechanical properties of materials.

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Any Questions?

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Physical and Mechanical Properties

• It is the arrangement of atoms within a material that greatly influences that materials behaviour and its properties:

– Hardness.– Ductility.– Malleability.– Conductivity.– Thermal expansion.– Optical properties.– Magnetic properties.– Melting point.

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Hardness

• What is hardness?

– The ability of a material to

withstand impacts.

– This is defined by the

deformation when a

prescribed load is applied

to the surface of the material.

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Hardness

• Materials in order of hardness:

– Diamond.– Cubic boron nitride (ceramic).– Carbides.– Hardened steels.– Cast irons.– Copper.– Acrylic.– Aluminium.– Lead.

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Ductility

• Ductility is a mechanical property that describes the extent in which solid materials can be plastically deformed without fracture.

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Ductile Materials

• Materials in order of ductility:

• Gold.

• Silver.

• Aluminium.

• Copper.

• Steel.

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Malleability

• Malleability is a physical property of metals that defines the ability to be hammered, pressed or rolled into thin sheets without breaking.

• It is the property of a metal to deform under compression.

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Malleable

• Materials in order of malleability:

– Gold.– Silver.– Aluminium.– Copper.– Tin.– Lead.– Steel.

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Tensile Strength

• There are three typical definitions of tensile strength:

– Yield strength - The stress a material can withstand without permanent deformation. This is not a sharply defined point. Yield strength is the stress which will cause a permanent deformation of 0.2% of the original dimension.

– Ultimate strength - The maximum stress a material can withstand.

– Breaking strength - The stress coordinate on the stress-strain curve at the point of rupture.

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Stress/Strain

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Tensile Strength

• Materials in order of tensile strength:

– Steel.

– Copper.

– Aluminium.

– Zinc.

– Lead.

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Electrical Conductivity

• Electrical conductivity is the measure of a material's ability to accommodate the transport of an electric charge.

• A Conductor such as a metal has high conductivity, and an insulator like glass or a vacuum has low conductivity.

• A semiconductor has a conductivity that varies widely under different conditions.

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Electrical Conductivity

• Materials listed in order of conductivity:Copper.Cobalt.Iron.Zinc.Aluminum.Titanium.Magnesium.

Gold.Lead.Platinum.Mercury.Tin.Silver.Nickel.Silicon.

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Thermal Conductivity

• In heat transfer, the thermal conductivity of a substance is an intensive property that indicates its ability to conduct heat.

• Alloys will have variable thermal conductivities due to composition.

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Thermal Conductivity

• Materials in order of thermal conductivity:

Silver. Copper.Gold. Aluminium.Magnesium. Silicon.Zinc. Cobalt.Nickel. Iron.Platinum. Tin.Lead. Titanium.Mercury.

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Magnetic Properties

• While most materials can be influenced in some way by a magnetic field, the following materials are thousands of times more susceptible than other materials:

– Iron.– Nickel.– Cobalt.– Compounds containing these elements are also magnetic.

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Any Questions?

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Degradation of Materials

• Corrosion.– The deterioration of a material as a result of a reaction with its

environment, especially with oxygen (oxidation).

– Although the term is usually applied to metals, all materials, including wood, ceramics (in extreme conditions) and plastics, deteriorate at the surface to varying degrees when they are exposed to certain combinations of sunshine (UV light), liquids, gases or contact with other solids.

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Wood

• The environmental factors that affect degradation in wood are:

– Biological organisms – fungi and insects.

– Risk of wetting or permanent contact with water.

– Wood is susceptible to attack when the moisture content exceeds 20%.

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Wood

• Physical and mechanical effects of degradation in wood:

– Change in cross-sectional dimensions, swelling and shrinkage.

– Strength and stiffness decrease as moisture content increases.

– Durability is affected.

– Coatings can be compromised.

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Plastic

• It is widely accepted that plastics do not corrode.

• However, micro organisms that can decompose low-density polyethylene do exist.

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Plastic

• Elastomers can cause other plastics to corrode or melt due to prolonged contact (e.g. rubber left on a set square).

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Plastic

• UV light will weaken certain plastics and produce a chalky faded appearance on the exposed surface.

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Plastic

• Heat will weaken or melt certain plastics even at relatively low temperatures.

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Plastic

• Cold can cause some plastics to become brittle and fracture under pressure.

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Plastic

• Mould can grow on plastics in moist humid conditions.

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Plastic

• Bio-degradation – the chemical breakdown in the body of synthetic solid-phase polymers.

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Metal

• Most metals corrode because they react with oxygen in the atmosphere, particularly under moist conditions – this is called oxidation.

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Metal

• Ferrous metals such as steel are particularly susceptible to oxidation and require ongoing maintenance or they will suffer inevitable structural failure.

• Choice of metal, environmental location and design features must all be considered carefully.

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Metal

• Some non-ferrous metals are particularly resistant to corrosion (e.g. copper and zinc).

• They form strong oxides on their surfaces (as do aluminium and lead) and these protect the metal from further oxidation. Next

Metal

• Most corrosion of ferrous metals occur by electro-chemical reaction. This is also known as wet corrosion.

• Electro-chemical corrosion can occur when:– two different metals are involved.– there is an electrolyte present.– metals are separated on the Galvanic Table (potential difference

exists).– the metals are in contact.

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Metal

• When two dissimilar metals are placed in a jar of electrolyte (sea water), an electric current is produced.

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Metal

• When two dissimilar metals are placed in a jar of electrolyte (sea water), an electric current is produced.

• In actual two metal situations, designers must be aware of the Galvanic Series. The potential difference between the two metals determines which metal will corrode.

• In the environment, rainwater will also act as an electrolyte. One of the metals will be eaten away (the anode) if it is higher up on the Galvanic Table.

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Metal

• Protection and finishing:

• There are various protection and finishing treatments applied to metals, including:

– sacrificial protection.– design features.– anodising of aluminium.– protective coating (e.g. paint, plastic, metal).– electro plating.

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Metal

• Sacrificial (cathodic) protection.• This is where one metal is deliberately sacrificed to protect

another.

• Sea water attacks bronze propellers. A slab of magnesium, aluminium or zinc is attached to the wooden hull near the propeller. This becomes the anode and corrodes while the expensive propeller (cathode) is protected. The anode must be replaced regularly.

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Metal

• Design features:

• Avoid, or provide extra protection for stressed parts, elbows, folds and bends, etc.

• Avoid crevices or sumps that retain moisture.

• Reduce Galvanic effect by careful selection of metals or by design detailing.

• Select an appropriate alloy.

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Metal

• Anodising of aluminium:

– An electrolytic process that increases the thickness of aluminium's naturally occurring protective oxide film.

– Organic acid electrolytes will produce harder films and can incorporate dyes to give the coating an attractive colour.

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Metal

• Protective coating: paint• Paint is widely used particularly to protect steel. It is not effective

over time and under certain conditions and must be renewed regularly – often at considerable expense.

• The more effective paints contain lead,

zinc or aluminium in suspension.• Part of the protection they provide is

sacrificial.

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Metal

• Protective coating: plastic

• A variety of plastic coatings exist. • They include:

– brush-on coating.

– electrostatic spraying.

– hot dipping in fluidised tank.

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Metal

• Protective coating: metal

• Metal coatings give the best protection.• They include:

– hot dipping.– powder cementation.– metal spraying.– metal cladding.– electro-plating.

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Metal

• Protective coating: electro-plating• Uses the chemical effect of an electric current to provide

a decorative and/or protective metal coating to another metal object:

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Metals

• The effect of corrosion on mechanical and physical properties:– Reduction of metal thickness leading to loss of strength or

complete structural failure.– Localised corrosion leading to a ‘crack’ like structure. Produces

a disproportionate weakening in comparison to the amount of metal lost.

– Fatalities and injuries from structural failure, e.g. bridges, buildings, or aircraft.

– Damage to valves or pumps due to solid corrosion products.

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Metal

• Environmental considerations:

– Contamination of fluids/foodstuffs in pipes and containers.

– Leakage of potentially harmful pollutants and toxins into the environment.

– Increased production/design and ongoing maintenance costs. This results in greater use of scarce resources and the release of harmful CO² gasses into the environment.

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Any Questions?

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Modifying Properties of Materials

• Heat treating is a group of industrial and metalworking processes used to alter the physical, and sometimes chemical, properties of a material.

• Heat treatment techniques include:

– Annealing. – Case hardening.– Precipitation strengthening. – Tempering. – Quenching.

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Annealing

• A heat treatment that alters a material to increase its ductility and to make it more workable.

• It involves heating a material to above its critical temperature, maintaining a suitable temperature, and then cooling.

• Annealing can induce ductility, soften material, relieve internal stresses, refine the structure by making it homogeneous, and improve cold working properties.

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Case Hardening

• Case hardening is a process that is used to harden the outer layer of case hardening steel while maintaining a soft inner metal core.

• The case hardening process uses case hardening compounds for the carbon addition.

• Steel case hardening depth depends upon the application of case hardening depth.

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Case Hardening

• Case hardening is useful for objects that need to be hardened externally to endure wear and tear, but soft internally to withstand shock.

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Precipitation Strengthening

• A technique where heat is applied to a malleable material, such as a metal alloy, in order to strengthen it.

• The technique hardens the alloy by creating solid impurities, called precipitates, which stop the movement of dislocations in the crystal lattice structure.

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Precipitation Strengthening

• Dislocations are the primary cause of plasticity in a material.

• The absence of dislocations increases the material's yield strength.

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Tempering

• Tempering is a process of heat treating, which is used to increase the toughness of iron-based alloys.

• The exact temperature determines the amount of hardness removed.

• For example, very hard tools are often tempered at low temperatures, while springs are tempered to much higher temperatures.

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Quenching

• Quenching is an accelerated method of bringing a metal back to room temperature.

• Quenching can be performed with forced air convection, oil, fresh water, salt water and special purpose polymers.

• This produces a harder material by either surface hardening or through-hardening varying on the rate at which the material is cooled.

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Any Questions?

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Outcomes

• State the physical properties of materials.

• Define what is meant by mechanical properties of materials.

• State the mechanical properties of materials.

• Describe the mechanical properties of materials.

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