IB Design Technology - Topic 4: Material Properties and Timber

IB Design Technology - Topic 4: Material Properties and Timber

Physical Properties

• A physical property is a characteristic of a material identified through an experiment
• Mass - The amount of matter in a unit of material
• Weight - Relates mass with the force of gravity
• Volume - Amount of space a unit of material takes up
• Density - Mass / Volume
• Electrical Resistance - How well a material resists the flow of electricity
• Thermal Conductivity  - How well a material allows the flow of heat
• Thermal Expansion - How much a material increases in volume when heated
• Hardness - A material’s resistance to small permanent deformation (scratching)
• Tensile Strength - How well a material can withstand a pulling force
• Compressive Strength - How well a material can withstand a crushing force
• Stiffness - How much a material bends
• Toughness - The ability to absorb energy without fracturing
• Plasticity - How easily something can permanently change in shape

Smart Materials

• Smart Mat - A material that has properties that can change with external stimuli
• Photo-Chromisity - A change of color caused by light (e.g. transition sunglasses)
• Thermoelectricity - A property of a material that converts a temperature difference into electric current
• Magneto/Electro-Rheostatic - Property that allows liquids to change in viscosity when introduced to a magnetic or electric field
• Piezo-Electricity - Electricity generated when a material is deformed, often a crystal

• Can be used to generate tones, used in simple buzzers

• Shape-Memory - A property that exhibits pseudo-elasticity, an applied load causes molecular rearrangement which is reversed when the load is taken off

Timber

• Timber is a category of materials that covers woods from trees
• Contrary to what their names imply, there is no distinct difference in hardness between hardwoods and softwoods
• Hardwood - A wood that comes from coniferous trees
• Softwood - A wood that comes from deciduous trees
• Both hard and softwoods have a wide variety of properties that makes classification by properties difficult

• There are some hardwoods that are softer than some softwoods and vice versa

• In general, woods tend to be moderately hard materials and very dense when in their pure form
• Medium-Density Fiberboard - Material made by gluing together wood fibers, can be fire/water-resistant and is a homogeneous product (looking at it one can barely notice a grain)
• Particleboard - Made from gluing wood chips, usually veneered or plastic laminated to hide the ugly interior, heterogeneous, NOT WATER RESISTANT
• Plywood - Layers of veneered wood glued with the grains perpendicular to each other
• Treatments - Preservatives can be added to wood products to improve their resistance and durability
• Things that people want to protect wood from are:

• Wood destroying fungi
• Wood destroying insects
• Seasoning (release of sap)

• Air-drying - A technique that cuts the wood into strips and leaves them out to dry for a long time, this reduces its water content and prepares for production
• Kiln-drying - Similar to air-drying but wood is dried in a kiln instead of in the air, dries quicker but runs the risk of splitting
• Creosote - A material that penetrates wood fibers and protects them from fungi and insects, however, this material is also a carcinogen
• Stain Preserves - Soaks into fiber providing it with a new tone and protection
• Timber Finishes  - Similar to stain preserves but has the added benefit of improving the natural beauty
• Varnish - Gives wood a hard surface
• Oils and Wax - Give a dull shine, improves color, and protects