 Parts of objects can be subjected to one or more external FORCES  Remember forces? An action that can change the motion of an object, or deform the object, by pushing or pulling on it.

 Materials tend to be deformed by forces  A constraint describes the effect of external forces on a material  Compression, tension, torsion, deflection, shearing

Other types of compression: Compression shorts for sports Squeezing a wet sponge Crushing a pop can

Other examples: Copper stretched into a wire Two teams of tug-of-war

Examples: An earthquake twisting a bridge Hands wringing a wet towel

What other examples can you think of?

Scissors and metal cutters

 Three types: elastic, plastic, fracture  ELASTIC: temporary change in shape  PLASTIC: permanent change  FRACTURE: breakage

Plastic deformation of piece of railway track

before and after…

 The mechanical properties of a material describe how it reacts when subjected to one or more constraints  Hardness, elasticity, resilience, ductility, malleability and stiffness

Hardness is the ability to resist indentation or abrasion

Elasticity is the ability to return to their original shapes after undergoing a constraint

Resilience is the ability to resist shocks without breaking

Ductility is the ability to be stretched without breaking The opposite of ductile is brittle

 Malleability is the ability to be flattened or bent without breaking

Stiffness is the ability to retain shape when subjected to various constraints

 Resistance to corrosion  electrical conductivity  thermal conductivity

 Ability to resist corrosive substances (water, salt, fumes) which can cause damage like rust

 Ability to carry an electric current

 Ability to transmit heat

 Degradation of a material is the decline in some of its properties  This is due to the effects of the surrounding environments – climate, humidity, chemical  Examples : old photographs, rust on cars  What else degrades over time?

 We can protect material by treating the material  Rust-proofing of cars – 2 ways

 Wood  Ceramics  Metals and alloys  Plastics  Composites

 Wood comes from trees!  There are 2 types of wood : hardwood and softwood  Categorized based on their hardness  Hardwood : maple, oak (deciduous trees)  Softwood : pine or spruce (conifers)

Considerations:  Species of tree used  Speed of growth and injuries  Water content

 Hardness, elasticity, resilience and toughness  Low thermal conductivity

 Made from wood, glue, plastics and preservatives  Modified wood is treated wood or a material made from wood mixed with other substances  Plywood, particleboard and fibreboard  Why do we need modified wood? Aren’t the trees “good enough”?

Resistance to cracking, shrinking and twisting/warping, it is very strong Layers are glued together, grain opposite (perpendicular grain)

Made from wood particles like wood chips and shavings and sawdust Pressed together with resin Cheap to make, very dense, not as strong as plywood

sometimes called MDF for medium density fibreboard Made from wood fibers by breaking down the hardwood or softwood and mixing it with wax and resin It is denser than plywood Used in manufacturing of furniture More dense than particle board

 Wood can degrade quickly because it is organic  We treat wood to prevent it from rotting  The wood can be dipped in an alkaline solution containing copper : greenish wood  Also can heat wood to a very high temp, expensive treatment

Dipped wood for deck building Heat treated wood for flooring

 Made from heating oxides like SiO 2  When the raw material is heated, the water evaporates and the bonds between the compounds are rearranged  Ceramics are always solid at room temperature

 Traditionally clay and sand

 Low electrical conductivity, used as insulators  High degree of hardness used for bricks & building materials like tiles and also as cutting tools  Heat resistant and low thermal conductivity so used a lot in the kitchen, dishes, ovens  Resistant to corrosion  Fragile, can break easily unless treated

 Quite durable, think archaeological digs  Strong acids and strong bases can degrade ceramics significantly (‘Breaking Bad’ season 1 bathtub scene?)  Can deteriorate with sudden thermal shock  Glass is a type of ceramic

Pot-in-pot refrigerator Zeer fridge Easy to make, very efficient, cheap!

 METALS  METALS are made from mineral ore, shiny  Good thermal and electrical conductivity  Some are ductile and malleable  ALLOYS  ALLOYS are a mixture of a metal with one or more other substances which may be metallic or non-metallic  Metallic materials are rarely pure metal

 Ferrous means with iron, examples are cast iron and steel  Nonferrous without iron, examples are aluminum alloys, brass, bronze

 Main degradation is oxidation which causes corrosion  They are treated with coatings  Coating can be metallic (zinc, chrome, gold, silver, nickel, aluminum, lead)  Coating can be non-metallic (paint, enamel, grease, resin)

 An alloy made mostly from iron and carbon  Heat treated

 Made from petroleum and natural gas  Made of polymers and other substances  Two types THERMOPLASTICS and THERMOSETTING PLASTICS

 Plastic that becomes soft enough when heated to be molded or remolded and that hardens when cooled to hold its shape  ¾ of plastics are thermoplastics  Containers  Recyclable (codes 1-6)

What else can you think of?

 These plastics remain hard even when heated  If heated to its decomposition temp (max temp) then it will decompose  Melamine, polyester  Harder and more resilient than thermoplastics  Not recyclable  Kayaks, cafeteria trays

Famous Kingston Penn escape done with cafeteria trays!

 Cracks can form over time along with colour fading  Slow degradation but irreversible  Different methods of protection include waterproofing, antioxidant additions, pigmentation

composites

 Formed by combining materials from different categories to obtain material with enhanced properties  2 main components : matrix and reinforcement

 No – not the movie  The matrix is the skeleton of the material and gives it shape  It surrounds and supports the reinforcements  The reinforcements are inserted into the matrix to strengthen it

 Plastic reinforced with fiberglass is used in airplanes: plastic is the matrix and fiberglass is the reinforcement  The properties of the two are combined to obtain a material that is stiff and resilient

 Aerospace  Sports  arts/music instruments  Engines and braking systems for high performance  Military and police – bullet proofing

 Deformation or fracture of matrix  Loss of adherence between matrix and reinforcement  Protection by testing for correct material mix and testing for adherence in conditions present

worksheet