Mechanical Properties of Materials
Mechanical Properties
Mechanical properties are the characteristics of a material that describe how it reacts to forces such as stretching, compressing, or bending, and how it behaves under stress and strain.
Mechanical properties are intrinsic to a material, meaning they are inherent and do not change unless the material's composition or structure is altered.
Key Mechanical Properties:
- Tensile strength
- Compressive strength
- Stiffness
- Toughness
- Hardness
- Malleability
- Elasticity
- Plasticity
- Ductility
Tensile Strength
Tensile Strength
Tensile strength is the ability of a material to withstand pulling forces without breaking or deforming.
- Measured in pascals (Pa) or megapascals (MPa).
- Critical for materials used in cables, ropes, and structural beams.
Universal Testing Machine (UTM)
- The UTM is used to test the tensile strength of a material
- It has two clamps (called grips) that hold the material sample.
- One grip is fixed, and the other moves slowly upward or downward, pulling the material.
- As the machine pulls, it measures:
- The force applied (in newtons)
- The amount the material stretches (elongation)
- The test continues until the material breaks.
Compressive Strength
Compressive Strength
The ability of a material to withstand forces that try to squash or compress it without breaking or deforming.
- Measured in pascals (Pa) or megapascals (MPa).
- Important for materials used in columns, foundations, and pavements.
Concrete has a high compressive strength, making it suitable for building foundations.
Stiffness
Stiffness
The ability of a material to resist bending or deflection when a force is applied, helping it maintain its shape.
- Not the same as strength: A material can be strong but still flexible (like steel wire).
- Measured by how much a material bends under a load.
- High stiffness = very little bending (e.g., glass, steel).
- Low stiffness = easily bends (e.g., rubber, plastic).
- Related to Young’s Modulus, a measure of stiffness in materials science.
- Important in structural design, where shape and stability must be maintained under load.
Carbon fiber is stiff, making it ideal for aerospace and automotive applications where weight and rigidityare critical.
Toughness
Toughness
The ability of a material to absorb energy and resist the spread of cracks, especially when subjected to a sudden impact or shock load.
- Measured by the area under the stress-strain curve (Young's Modulus Graph)
- High toughness materials can withstand sudden impacts or shocks.
- Important in materials used for impact resistance, like car bumpers, helmets, and tools.
- Brittle materials like glass or ceramics have low toughness, they crack easily under stress.
Rubber is tough, making it suitable for tires and shock absorbers.
Don't confuse toughness with hardness. Toughness is about absorbing energy, while hardness is about resisting surface deformation.
Hardness
Hardness
The ability of a material to resist scratching, indentation, or penetration.
- Important for cutting tools and wear-resistant surfaces.
- Common testing methods:
- Mohs Scale: ranks materials from 1 to 10.
- Brinell, Rockwell, and Vickers tests: use force and indenters to measure resistance to penetration.
- High hardness = difficult to scratch or dent (e.g., diamond, tool steel).
- Low hardness = easily marked or indented (e.g., lead, soft plastics).
Cutting Tools (e.g., drill bits, saw blades)
- Made from hardened steel or tungsten carbide.
- Need high hardness to cut through other materials without wearing down.
Smartphone Screens
- Often made from Gorilla Glass or sapphire glass.
- High hardness helps resist scratches from keys, coins, or drops.
Tempered glass window is hard (resists scratches) but has low toughness, meaning it can fracture easily under sudden impact.
Malleability
Malleability
The ability of a material to be permanently deformed (flattened or shaped) by compressive forces without cracking or breaking.
- Important for metals used in sheet and foil production.
- Involves plastic deformation: the material changes shape and doesn’t return to its original form.
- Tested by hammering, rolling, or pressing the material.
- High malleability = can be shaped into thin sheets (e.g., gold, aluminum).
- Low malleability = cracks or breaks when compressed (e.g., cast iron, glass).
Gold is highly malleable, allowing it to be formed into thin sheets for jewelry and electronics.
Elasticity
Elasticity
The ability of a material to return to its original shape after being stretched, compressed, or deformed, once the force is removed.
- Measured by Young’s Modulus: higher values mean stiffer and more elastic materials.
- Rubber bands are highly elastic, clay is not.
- Important in springs, seals, sports equipment, and wearable tech.
- If a material is stretched beyond its elastic limit, it will not return to its original shape.
Rubber bands are elastic, stretching under tension and returning to their original shape when the force is removed.
- Elasticity is temporary deformation.
- Once the force is removed, the material returns to its original shape.
Plasticity
Plasticity
The ability of a material to be permanently deformed without breaking when a force is applied.
- Involves permanent shape change: the material does not return to its original shape after the force is removed.
- Happens when a material is stressed beyond its elastic limit.
- Important in metal forming, clay modeling, and plastic manufacturing.
- High plasticity = easily reshaped (e.g., clay, copper).
- Low plasticity = breaks or cracks instead of deforming (e.g., glass).
Ductility
Ductility
The ability of a material to be stretched, drawn, or extruded into a long, thin shape such as a wire, without breaking.
- Involves plastic deformation under tensile (pulling) forces.
- High ductility = can be drawn into wires (e.g., copper, aluminum, gold).
- Low ductility = breaks or cracks when pulled (e.g., cast iron, glass).
- Important in electrical wiring, metal forming, and structural components.
Think of ductility like stretching a piece of chewing gum into a long strand without it breaking.
Related to malleability, but ductility is about stretching (tensile forces), while malleability is about compressing.
Choose an everyday product from the list below and complete the prompts.
- Smartphone
- Water bottle
- Luggage case
- What are the most important material properties for this product?
- Why are these properties important for the function and user experience?
- Was there likely a trade-off between two or more properties?
For example, is it durable but heavy? Lightweight but with reduced strength?
Apply the theory of Young’s modulus to explain why this cantilevered balcony is stiff enough to safely support people standing on its outer edge. [3]
Solution
Answers may include:
- Young’s modulus quantifies stiffness of a material by relating stress to strain (E = stress/strain). [Award 1]
- High Young’s modulus of reinforced concrete means it undergoes very small elastic deformation under the bending and shear stresses imposed by a person’s weight. [Award 1]
- Minimal deflection under load ensures the balcony remains level and does not crack or feel “springy,” keeping it safe to walk on. [Award 1]
Award [1] for each relevant principle or theory up to [3 max].
