1.1 material selection

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  • Created by: helizm69
  • Created on: 11-04-18 16:31

Factors affecting choice of material?

Functional Requirements

  • hardness and rigidity
  • weight and stability
  • flexibility and buoyancy
  • texture and colour
  • environment

Economic Requirements

  • wastage and electricity
  • labour costs and new training
  • repair and servicing machinery
  • hard/easy to work with material(gold easy to work with and easy polished as it is soft)

Material Avaliability

  • most products come in stand forms, shape and sizes.
  • wood-sheet, plank, box section, tube, coils of wire
  • metal-sheet, plate, box section, tube, rod, coils of wire etc.
  • plastic-sheet, rod, tube, block etc.  
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JOINING permanent

welding-heat, molten metals joined with joining material.

brazing-copper adn steel joined under heat, with rod made of copper and zinc alloy.

soldering-lower temperature than brazing.

riveting-metal pins thorough sheets or plates and hammered.

adhesives-with glue eg.PVA, contact adhesive, epoxy resin

frame joints-**** joints, halving joints, mortice tenon, dovetail joints

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JOINING semi-permanent

can be disassembled

screws,nuts,bolts and washers

snap joints-moulded plastic components

knockdown fittings-screws, corner blocks, self assembly fittings

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Material Choice

Density

  • amount fo matter(mass) in an object
  • mahogany denser than pine
  • dense material, less buoyancy

Electrical conductivity

  • movement of electricity through a material
  • good conductors-gold, silver,copper, some gases
  • insulators-woods, ceramics, glass, plastics eg.PVC, acrylic

some semiconductive eg.silicon and germanium

Thermal Conductivity

  • movement of heat through a material
  • asbestos great for keeping heat in and silica tiles also used
  • cooking utensils prevent burning
  • double glazed windows-glass and air to create barrier and polysterene pumped into cavaties
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Mechanical Properties

mechanical properties of a material are related to how the material reacts to various forces that it may be subject to.  

deformation is when the force is to significant for the material to endure.  There are two types of deformation being temporary(elastic) and permanent(plastic).

STRENGTH

the materials ability to withstand force without it breaking or permanently bending.  

tensile strength-ability to resist pull forces and used for cables, chains and ropes.

compressive strength-ability to withstand push forces that try to crush or shorten it.

bending strength-force applied to a material in an attempt to bend it.  

shear force-force that subjects materials to sliding action across their surfaces.  eg.hinges, scissors, secateurs, hedge clippers or drill bits.  

torsional force-force that is applied to a material to try to twist it.  also known as torque.

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Mechanical Properties

Elasticity

Ability of a material to flex, bend and be deformed and return to its original shape when forces are removed eg.elastic band.

plasticity

ability of a material to permanently change shape when forces are applied without cracking or breaking.

malleability

degree to which materials can be deformed in all directions by a compression force such as hammering, pressing or rolling without cracking.  

ductility

ability of a material to be drawn out longer and thinner eg.wire, by a tensile force.   all ductile materials are malleable but not all malleable materials are ductile.  

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Mechanical Properties

toughness

ability to withstand sudden shocks or blows.  tested using a large pendulum which swings to hit a notch in the material.  the tougher the material the more impact it can take.  

Hardness

Ability to resist indentation.eg.with centrepunch.  

scratch tests-minerologists rank scratch resistance of materials.  diamond is the hardest material known to man-used in drill bits.  

indentation tests-forcing a hard object intot eh surface of a material.  the result affected by SA. 

brinell test-hard steel ball like shot put

vickers test-pyramid shaped diamond tool

rockwell test-steel ball or diamond cone

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Mechanical Properties

durabiliity

capability of withstanding wear and teat and deterioration as a result of weathering.  eg.meatls prone to corrosion.  

brittleness

opposite of plasticity and is like a human bone breaking.  materials that are brittle break or shatter eg.castiron ro glass. brittle materials possess high compressive properties but low tensile properties. eg.castiron is not used in bridge building.  

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