Materials Materials constraints and deformations Parts of objects

Materials

Materials : constraints and deformations � 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.

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

Compression: forces that tend to crush Other types of compression: Compression shorts for sports Squeezing a wet sponge Crushing a pop can

Tension: forces that tend to stretch Other examples: Copper stretched into a wire Two teams of tug-of-war

Compression vs tension

Torsion: forces that tend to twist Examples: An earthquake twisting a bridge Hands wringing a wet towel

Deflection: forces that tend to bend What other examples can you think of?

Shearing: forces that tend to cut Scissors and metal cutters

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

Elastic deformation

Plastic deformation of piece of railway track

Fracture deformation before and after…

Correct activity 48

Properties of materials � 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 Hardness is the ability to resist indentation or abrasion

elasticity 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 � Malleability is the ability to be flattened or bent without breaking

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

Other properties � Resistance � � to corrosion electrical conductivity thermal conductivity

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

Electrical conductivity � Ability to carry an electric current

Thermal conductivity � Ability to transmit heat

Degradation and protection � 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?

How can we counteract degradation � We can protect material by treating the material � Rust-proofing of cars – 2 ways

degradation

Categories of materials � Wood � Ceramics � Metals and alloys � Plastics � Composites

Wood and modified wood � 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)

Properties of wood Considerations: � Species of tree used � Speed of growth and injuries � Water content

Other properties to consider � Hardness, elasticity, resilience and toughness � Low thermal conductivity

Modified wood � 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”?

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

particleboard 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

fibreboard 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

Degradation and protection of wood and modified wood � 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

Treated wood Dipped wood for deck building Heat treated wood for flooring

Ceramics � Made from heating oxides like Si. O 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

What goes ‘in’ to ceramics � Traditionally clay and sand

Properties of ceramics � 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

Degradation of ceramics � 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

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

Metals and alloys � METALS are made from mineral ore, shiny � Good thermal and electrical conductivity � Some are ductile and malleable � 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

Alloys – ferrous and nonferrous � Ferrous means with iron, examples are cast iron and steel � Nonferrous without iron, examples are aluminum alloys, brass, bronze

Degradation and protection of metals and alloys � 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)

steel

steel � An alloy made mostly from iron and carbon � Heat treated

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

thermoplastics � 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)

Thermoplastics What else can you think of?

Thermosetting plastics � 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!

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

composites

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

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

Examples of composites � 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

Used in many products and sectors � Aerospace � Sports � arts/music instruments � Engines and braking systems for high performance � Military and police – bullet proofing

Degradation and protection of composites � 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