The Technological World

Manufacturing Technical ObjectsChapter 12

MaterialsTechnical Objects: Objects humans

have created and designed for a specific purpose

• Their use exposes them to different types of stress which can cause the materials they are made of to deform.

• Knowing what type of stress a technical object will be subjected to will allow manufacturers to determine which material is most appropriate for the creation of the object.

Materials• All technical objects are made up of one or

more kinds of materials. • Each type of material has its own set of

properties.

Ex: A bridge can be made out of steel and concrete. Ex: A road can be made out of asphalt.

What is a Constraint?• A constraint describes the effect of an

external force on a material.

**It is the resulting action of what happens to a material when it is exposed to an external force**

Ex: You step on a can and crush it. The resulting action of the can after it has been exposed to the external force (your foot crushing it) is compression (a type of constraint).

Type of Constraint Description Symbol Examples

Compression Crushing of material due to external forces

-hands squeezing a wet sponge-a foot crushing a can

Tension Stretching of material due to external forces

-copper stretched into a wire-rope in a tug of war

Torsion Twisting of material due to external forces

-an earthquake twisting a bridge-hands wringing a wet towel

Deflection Bending of material due to external forces

-a fish bending a fishing rod-clothes weighing down a clothes line

Shearing Cutting of material due to external forces

-scissors cutting paper-metal cutters trimming shapes from sheets of metal

When a material has undergone a constraint, it becomes deformed.

Type of Deformation Description

Elastic Temporary change in shape or dimensions of the material. When constraint is removed, material returns to its original form.

Plastic Permanent change in the shape or dimensions of material. Even when the constraint is removed, the material remains deformed.

Fracture The constraint it so intense that the material breaks.

Example

A mountain climber is secured with a safety rope that holds her weight.

Constraint: Tension

Deformation: Elastic (rope will return to its original form once tension is removed)

Example (continued)

If the rope began to fray while holding the mountain climber, it would no longer return to its original shape once the tension was removed.

Deformation in this case would be Plastic.

Example (continued)

If the rope broke while the tension was applied to it, the type of deformation would be Fracture because the material has broken.

Mechanical Properties of MaterialsMechanical Property Definition

Hardness Ability to resist indentation

Elasticity Ability to return to their original shape after undergoing a constraint.

Resilience Ability to resist shocks without breaking

Ductility Ability to be stretched without breaking

Malleability Ability to be flattened or bent without breaking

Stiffness Resistance of an elastic material to deflection

Other Properties

Property Definition

Resistance to corrosion Ability to resist the formation of rust or degradation by acids, salt, water, etc.

Electrical Conductivity Ability to carry and electrical current

Thermal Conductivity Ability to transmit heat

Try This!Which mechanical property is sought in each of the

following examples?

1. A plastic that keeps its shape even when twisted.

2. Wooden flooring that resists indentation by pointed objects, such as shoe heels.

3. A metal that stretches well to make wire.

4. A boat hull that resists shocks caused by running into shoals.

5. A material that bends easily, without breaking, to make gutters beneath a roof.

Degradation and Protection• Degradation: Decline in a material’s properties due to

the effects of the surrounding environment.

• Protection of a material: Application of a substance to a material to prevent or delay its degradation.

Categories of MaterialsThere are 2 categories of wood:1)Hardwood Deciduous trees2)Softwood Conifers

• Hardness, resilience, elastic• Low thermal and electrical conductivity• Light weight

Modified Wood

• Wood mixed with other substances (glue, plastics, preservatives)

Problems with Wood

• Decomposition (rotting)• Insect, fungi and microorganism infestations

These degradations reduce wood’s mechanical properties

Solutions:1) Varnishing, painting, protective coatings2) “Treated Wood” (more expensive)3) Can be coated with a basic substance (turns wood green)

CeramicsSolid material obtained from heating inorganic

matter containing oxides (compound containing oxygen).

*Often made from sand or clay*

• Low electrical conductivity• Hard• Resist heat• Fragile

Problems with Ceramics:

1) Acids and bases can degrade ceramics2) A sudden change in temperature can deteriorate the properties of ceramic

Metals and AlloysAlloys are metals mixed with other metals or non-metals

to make them more useful.

• Ductile• Malleable• Good conductors of heat and energy

Problems with metals and alloys:1) Rust

Solutions:- Coating with paint, grease, enamel...- Coating with other metals such as gold, chrome, zinc, etc.- “Quench hardening” and “Tempering” rapidly heating and

cooling them

PlasticsPlastics are made from fossil fuels.-Other substances can be added to plastics to obtain

specific properties-Poor conductors of heat and electricity

Thermoplastics• Become soft when heated and hard when cooled (can be recycled)

Thermosetting Plastics• Remains permanently hard (more resilient)

Problems and Solutions for Plastic Degradation:

1)Liquid absorption (use a waterproof coating)

2)Oxidation (add antioxidant materials to plastic)

3)UV rays (add pigment that absorbs UV rays)

CompositesA combination of materials from different categories Contain 2 parts:1. Matrix (skeleton, gives shape)2. Reinforcement (fill the matrix to give strength)

Matrix Reinforcement Properties

Plastic X Durable, light weight, resilient, inexpensive

Metal X Ductile, good conductor, stiff

Ceramic X Durable, heat resistant

X Fibreglass Stiff, corrosion resistant

X Kevlar Low density, resilient

X Carbon Stiff, low density, electrical conductor

Protection:1) Both the matrix and the reinforcement need to be

protected (they can fracture)2) Strong adherence between the two parts is

important

Uses:-Airplane wings, engines, brakes-sporting equipment (hockey sticks, helmets, skis)-bullet proof vests