Properties of Materials

Nature of Design

• The rapid pace of scientific discovery and new technologies has had a major impact on material science, giving designers many more materials from which to choose for their products.
• Choosing the right material is a complex and difficult task with physical, aesthetic, mechanical, and appropriate properties to consider.

Physical Properties

• A physical property is any property that is measurable that describes a state of a material.

Matter Takes Up Space

All matter is made up of four general properties:

Mass

• The amount of matter in an object.

Weight

• Weight is the measure of the pull of gravity on an object.
• In most cases it is proportional to mass, but the strength of gravity can change on different planets and places on Earth.

Volume

• Volume is the amount of space an object or liquid takes up.

Density

• Density is the mass per unit volume; D=M/V.

Physical Properties of Materials Under Certain Conditions

Electrical resistivity

• Electrical resistivity is the measure of a material’s ability to conduct or resist electricity.
• Lower electrical resistivity means it conducts electric current better.
• Materials are classified into conductors and insulators based on their resistivity.

Thermal conductivity

• The measure of how fast heat is conducted through a slab of material with a given temperature difference across the slab.

Thermal expansion (expansivity)

• A measure of the degree of increase in dimensions when an object is heated.

Hardness

• Hardness is the resistance a material offers to penetration or scratching.

Mechanical Properties

• Properties of a material that involve the relationship between stress and strain or a reaction to an applied force.

Tensile strength

• The ability of a material to withstand pulling forces.

Compressive strength

• The ability of a material to withstand being pushed or squashed.

Stiffness

• The resistance of an elastic body to deflection by an applied force.

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.

Graphing Material Properties

Stress/Strain Graphs

• We can identify and collect considerable amounts of information from a stress-strain graph.
• It is important when selecting materials.
• Every material will perform differently under the application of stress and strain and therefore each material’s graph will be different.

• A stress/strain graph maps how the stress applied to a material (y-axis) affects the strain on the material (x-axis).

• The elastic region is the region in which a material will return to its original shape after being deformed.
• This is generally a straight line.

• At the limit of it’s elastic region (the elastic limit / Yield point) it will hit the plastic region; the material deforms plastically and does not return to its original length or shape.
• The line of the graph will change to a curved line (plastic region).

• The strain continues to increase, the material will eventually reach it's ultimate strength point.
• After the ultimate strength point, the material starts necking, where it stretches out to be thinner and thinner as it is pulled apart.
• When the line ends; it means that the material will fracture at that point. The breaking point / fracture point.

• Different materials have differently-shaped stress/strain graphs.

Material Selection Charts

• A chart used to identify appropriate materials based on the desired properties.
• One material property is placed in the x-axis and another in the y-axis.
• Different groups of materials are organized into "blobs", with larger "blobs" indicating a larger variation of the properties within the material group.

Aesthetic Characteristics

• Aesthetic characteristics can define the appeal of a product beyond its function.
• A product that is attractive to look at, or pleasing to experience, determines its aesthetic appeal.
• Aesthetic characteristics include:

1. Smell: the ability to perceive odours such as sweet, acrid or fragrant.
2. Taste: sour, sweet, spicy.
3. Sound: related to pitches but also the sound of a crisp apple or a chip/crisp bag that makes a crinkling sound.
4. Texture: it is how something feels or looks, it can be rough or smooth.
5. Appearance: favorable in terms of appearance. The design of appearance in a product must be aesthetically pleasing for it’s target audience (user) to attract the customer.
6. Color: can be warm (e.g. browns) or cool (eg blues) can have psychological effects (e.g. greens are calming).
7. Shape: geometric or organic.

• Some aesthetic characteristics are only relevant to food, while others can be applied to more than one material group.
• Although these properties activate people’s senses, responses to them vary from one individual to another, and they are difficult to quantify scientifically, unlike the other properties.

Smart Materials

• Materials that have been designed to have one or more properties that can be modified when subject to an external stimuli in a way that the output can be controlled.

Piezoelectricity

• When a piezoelectric material is deformed, it gives off a small electrical discharge.
• When an electric current is passed through it, it increases in size (up to a 4% change in volume).

Shape memory alloys (SMA)

• Shape memory alloys are metals that when deformed, can spring back into its original shape once released
• Shape memory alloys are often returned to their original shape after being heated or cooled.

Photochromicity

• A photochromic material changes color in response to an increase in light.
• When the light source is removed, it returns to its original color.
• Photochromic materials are most often used in windows and glasses to block out glare.

Electro-rheostatic (ER) and Magneto-rheostatic (MR)

• Fluids that can undergo dramatic changes in their viscosity.
• They can change from a thick fluid to a solid in a fraction of a second when exposed to a electric field (ER) of magnetic field (MR), and the effect is reversed when the field is removed.
• The viscosity of the material can be adjusted based on the field applied, making it easy to fine-tune changes.

Thermoelectricity

• This refers to a smart material that when heated can produce an electric current.
• A thermoelectric material is comprised of two dissimilar conductors.

Sources

https://civilsguide.com/what-is-the-stress-strain-curve/

https://flybynightgraphics.com/article/best/926

http://www-g.eng.cam.ac.uk/125/now/mfs/tutorial/non_IE/charts.html