4.1.2 Principles of Material Properties Notes

Properties of Materials

Imagine you're tasked with designing a new product, perhaps a smartphone, a bridge, or even a pair of running shoes. What factors would you consider when selecting the materials? You might immediately think about strength, weight, or appearance. But there’s much more to it! Materials possess a range of properties like physical, mechanical, aesthetic, and even "smart" properties, that determine their behavior and suitability for specific applications. In this section, we’ll explore these categories of material properties in detail, helping you understand how they influence design decisions.

Physical Properties: The Fundamentals

Physical properties describe characteristics that can be observed or measured without altering the material’s identity. These properties often dictate how a material interacts with energy, such as heat or electricity, or how it performs under specific conditions.

Mass and Weight: What’s the Difference?

Mass is the amount of matter in an object and remains constant regardless of location. Its SI unit is the kilogram (kg). Weight, however, is the force exerted by gravity on that mass and depends on the gravitational field strength, which varies depending on location (e.g., Earth vs. the Moon).

The relationship is expressed as:

$$
\text{Weight} = \text{Mass} \times g
$$

Where $g$ is the acceleration due to gravity (approximately $9.8 , \text{m/s}^2$ on Earth).

Density: How Compact is the Material?

Density ($\rho$) refers to the mass of a material per unit volume. It is calculated using the formula:

$$
\rho = \frac{\text{Mass}}{\text{Volume}}
$$

High-density materials, like metals, are often used where strength and compactness are essential, such as in construction. Conversely, low-density materials, like plastics, are ideal for lightweight applications.

Electrical Resistivity and Thermal Conductivity

• Electrical Resistivity ((ρ)): This measures how strongly a material resists the flow of electric current. Metals like copper and silver have low resistivity, making them excellent conductors, whereas materials like rubber and glass have high resistivity and act as insulators.

$$
\sigma = \frac{1}{\rho}
$$

Where $\sigma$ is electrical conductivity.

• Thermal Conductivity ((k)): This property determines how efficiently heat flows through a material. Metals, with high thermal conductivity, are used in heat sinks, while materials like wood and polyurethane, with low thermal conductivity, are used as insulators.

Thermal Expansion and Hardness

• Thermal Expansion: Most materials expand when heated due to increased atomic vibrations. This is quantified by the coefficient of linear thermal expansion ($\alpha$):

$$
\Delta L = \alpha \cdot L \cdot \Delta T
$$

Where $\Delta L$ is the change in length, $L$ is the original length, and $\Delta T$ is the temperature change.

• Hardness: This measures a material's resistance to scratching or indentation. For example, diamond, the hardest natural material, is widely used in cutting tools.

Mechanical Properties: Strength and Flexibility in Action

Mechanical properties describe how materials respond to forces. These properties are critical for ensuring that a product can withstand the stresses it will encounter during use.

Tensile and Compressive Strength

• Tensile Strength: The maximum stress a material can endure while being stretched. It’s essential for materials used in ropes, cables, and structural beams.
• Compressive Strength: The ability of a material to resist being compressed or squashed. Concrete, for example, has high compressive strength.