Materials

Properties of materials

Physical Properties Density Electrical resistivity Thermal conductivity Thermal expansion Hardness

Density: The mass per unit volume of a material. The physical properties of most interest to a designer is density and useful working temperature range. The density of a substance is its mass per unit volume. It is denoted as a p and is measured in kg/m 3. For a ships keel we would use a dense material such as lead, but for a lightweight camping frame we would use a material of low density such as aluminium.

Electrical resistivity (also known as specific electrical resistance) is a measure indicating how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electrical charge. The SI unit of electrical resistivity is the ohm metre. (SI: Système International d'Unités), The ohm (symbol: Ω) is the SI unit of electrical impedance or, in the direct current case, electrical resistance.

Thermal conductivity: A measure of how fast heat is conducted through a slab of material with a given temperature difference across the slab. Metals are generally good conductors of heat and materials such as copper and aluminium are therefore used when good heat transfer is essential. Nonmetals tend to be thermal insulators and are used to prevent heat transfer between different items. The low values of thermal conductivity (k) in brick, polystyrene, porcelain, PVC and rubber is why these materials are useful insulators. Air is a good insulator, which is why materials that trap air are used in products.

Conversion from toFormula CelsiusFahren heit °F = °C × FahrenheitCelsiu s °C = (°F – 32) / 1.8 CelsiuskelvinK = °C kelvinCelsiu s °C = K – Thermal conductivity is clearly an important quantity for construction and related fields. However, materials used in such trades are rarely subjected to chemical purity standards. Several construction materials' k values are listed left. These should be considered approximate due to the uncertainties related to material definitions

Nonmetals tend to be thermal insulators and are used to prevent heat transfer between different items. The low values of thermal conductivity (k) in brick, polystyrene, porcelain, PVC and rubber is why these materials are useful insulators. Air is a good insulator, which is why materials that trap air are used in products.

Thermal Expansion: A measure of the degree of increase in dimensions when an object is heated. This can be measured by an increase in length, area or volume. The expansivity can be measured as the fractional increase in dimension per Kelvin increase in temperature When things contract 'pulling' forces are created. When things expand 'pushing' forces are created. Most materials expand when heated and contract when cooled.

In materials engineering, the three primary types of materials have well defined rates of expansion. Polymers expand as much as 10 times more than metals, which expand more than ceramics. Expansion has to be taken into account for many things, such as when constructing railways. If there are not gaps for the line to expand, the line may buckle. Similar ideas are applied in building bridges, making water pipes and concrete slabs in roads. This phenomenon can be beneficial as well, and is used in techniques like shrink-fitting.

Hardness: is the characteristic of a solid material expressing its resistance to permanent deformation. Hardness can be measured on the Mohs scale or various other scales. (Mohs' scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created, in 1812, by the German mineralogist Friedrich Mohs and is one of several definitions of hardness in materials science).

There are three principal operational definitions of hardness: 1. Scratch hardness 2. Indentation hardness 3. Rebound, dynamic or absolute hardness

Scratch hardness In mineralogy, hardness commonly refers to a material's ability to penetrate softer materials. An object made of a hard material will scratch an object made of a softer material. Pure diamond is the hardest known natural mineral substance and will scratch any other material. Diamond is therefore used to cut other diamonds; in particular, higher-grade diamonds are used to cut lower-grade diamonds.

Indentation hardness Primarily used in engineering and metallurgy, indentation hardness seeks to characterise a material's hardness, i.e. its resistance to permanent, and in particular plastic, deformation. It is usually measured by loading an indenter of specified geometry onto the material and measuring the dimensions of the resulting indentation.

Rebound hardness Also known as dynamic or absolute hardness, rebound hardness measures the height of rebound of an indenter dropped onto a material using an instrument known as a scleroscope.

Mechanical Properties ► Tensile Strength: The ability of a material to withstand pulling forces. ► Toughness: The ability of a material to resist the propagation of cracks. ► Ductility: The ability of a material to be drawn or extruded into a wire or other extended shape.

Tensile Strength Tensile strength measures the force required to pull something such as rope, wire, or a structural beam to the point where it breaks. Specifically, the tensile strength of a material is the maximum amount of tensile stress that it can be subjected to before failure. The definition of failure can vary according to material type and design methodology. This is an important concept in engineering, especially in the fields of material science, mechanical engineering and structural engineering.

There are three typical definitions of tensile strength: 1. Yield Strength - The stress a material can withstand without permanent deformation. 2. Ultimate Strength - The maximum stress a material can withstand. 3. Breaking Strength - The stress coordinate on the Stress-strain curve at the point of rupture.

Steel has a very linear stress-strain relationship up to a sharply defined yield point. For stresses below this yield strength all deformation is recoverable, and the material will relax into its initial shape when the load is removed. For stresses above the yield point, a portion of the deformation is not recoverable, and the material will not relax into its initial shape. This unrecoverable deformation is known as plastic deformation. For many applications plastic deformation is unacceptable, and the yield strength is used as the design limitation.

In materials science and metallurgy, toughness is the resistance to fracture of a material when stressed. It is defined as the amount of energy that a material can absorb before rupturing. (Solids support both shear and normal stress, with brittle materials failing under normal stress, and plastic or ductile materials failing under shear stress). Toughness

Ductility Ductility is the physical property of being capable of sustaining large plastic deformations without fracture. A ductile material is any material that yields under shear stress (as opposed to brittle fracture, which yields under normal stress). Gold, copper, and aluminium are highly ductile metals. Ductility is related to malleability.

Malleability is a physical property of metals and metal alloys, or generally of any kind of matter. A malleable metal can easily be deformed, especially by hammering or rolling. Malleability is an important issue in, for example, stamping and form pressing materials such as metals and plastics. Malleability shows how mobile particles involved in metallic bonding can be pushed or pulled past each other, making metals malleable and ductile. Gold is the most malleable metal, followed by aluminium.

The IB Properties / Materials Matrix

TASK In groups of 4 you must research the material classification given to your group by your teacher. You must research the physical, mechanical and aesthetic properties and characteristics of your material and produce a 10 minute presentation to show to the class. You will be teaching the materials unit! Your lesson must include notes/handouts for future revision and could include tasks for students, homework, quizzes, etc. Deadline: Friday 1 st February