4.1 Properties of materials Notes

Properties of Materials

1. Designers must choose materials carefully to optimize their product's performance, durability, and usability.
2. The right material can enhance a product’s strength, weight, safety, comfort, and longevity.
3. Understanding material properties helps designers innovate and refine their products for real-world applications.

Physical Properties of Materials

Mass vs. Weight: Why They Matter in Design

Mass

1. What It Means:
   1. The amount of matter in an object, measured in kilograms (kg).
   2. Mass is constant regardless of location. A 2kg bicycle frame has the same mass on Earth as it does on Mars.
2. Design Relevance:
   1. Mass affects how a product handles and performs.
   2. Bicycles, drones, and racing cars must balance mass with stiffness and strength for optimal efficiency.

Weight

1. What It Means:
   1. Weight is a force that depends on gravity, measured in Newtons (N).
   2. A 2kg object on Earth weighs about 19.6N, but on Mars, where gravity is weaker, it weighs only 7.4N.
2. Design Relevance:
   1. Aircraft, rockets, and spacecraft must consider weight differences in different gravitational environments.
   2. Backpacks and luggage are designed to be lightweight while maximizing storage capacity.

Volume: Space Considerations in Design

1. What It Means:
   1. The amount of 3D space an object occupies (measured in liters or cubic meters).
2. Design Relevance:
   1. Volume is crucial for storage products like backpacks, suitcases, or food packaging.
3. Interior designers and architects optimize volume for space efficiency.
   1. In industrial design, fuel tanks, water bottles, and containers must be compact yet hold sufficient material.

Density: Balancing Mass and Volume

1. What It Means:
   1. The mass per unit of volume (kg/m³). A material with high density is heavy for its size, while a low-density material is lightweight.
2. Design Relevance:
   1. Protective equipment like bicycle helmets uses low-density foam to absorb impact without adding weight.
   2. Aerospace materials use lightweight, high-strength composites to reduce weight while maintaining durability.

Electrical Resistivity: Designing for Safety and Conductivity

1. What It Means:
   1. A material’s ability to conduct or resist electricity.
      1. Low resistivity = good conductor (e.g., copper).
      2. High resistivity = good insulator (e.g., rubber, fiberglass).
2. Design Relevance:
   1. Electrical components must use conductors for wiring and insulators for safety.
   2. Tools and equipment for electricians must be made from high-resistivity materials.

Thermal Conductivity: Heat Management in Design

1. What It Means:
   1. A material’s ability to transfer heat.
   2. Metals like aluminum and copper conduct heat quickly.
   3. Wood and plastic are poor conductors and act as insulators.
2. Design Relevance:
   1. Cooking utensils often combine materials to manage heat transfer.
   2. Thermal insulation in buildings prevents heat loss or excessive heating.

Thermal Expansion: Designing for Temperature Changes

1. What It Means:
   1. Materials expand when heated and contract when cooled.
   2. Different materials expand at different rates, which can cause structural failures if not accounted for.
2. Design Relevance:
   1. Bridges and railway tracks include expansion joints to prevent buckling in heat.
   2. Glass cookware like Pyrex is designed to resist sudden expansion, preventing cracks.

Hardness: Scratch and Wear Resistance in Materials

1. What It Means:
   1. Hardness is a material’s resistance to scratching, cutting, or denting.
   2. Harder materials resist wear and tear but may be brittle.
2. Design Relevance:
   1. Floor tiles in high-traffic areas must be highly scratch-resistant.
   2. Smartphone screens use hardened glass (e.g., Gorilla Glass) to prevent scratches and cracks.

Mechanical Properties in Design

1. Mechanical properties describe how materials respond to forces like pulling, pushing, bending, and impact.
2. Unlike physical properties, these are often measured using destructive testing, meaning the material is tested until it breaks or deforms.
3. Understanding these properties allows designers to choose the best material for durability, strength, flexibility, and safety in their products.

Tensile Strength: Resisting Pulling Forces

1. What It Means:
   1. A material’s ability to withstand pulling (tensile) forces without breaking.
2. Design Relevance:
   1. Suspension bridge cables must have high tensile strength to support the bridge’s weight.
   2. Elevator cables must hold heavy loads safely and reliably.

Compressive Strength: Resisting Pushing Forces

1. What It Means:
   1. A material’s ability to resist being squashed or compressed without failing.
2. Design Relevance:
   1. Concrete foundations in buildings must have high compressive strength to support weight.
   2. Skyscraper glass panels are designed to withstand compression while holding structural loads.

Stiffness: Maintaining Shape Under Force

1. What It Means:
   1. A material’s ability to resist bending or deformation when force is applied.
2. Design Relevance:
   1. Airplane wings need to be stiff enough to maintain aerodynamic shape while withstanding air pressure.
   2. Bicycle frames must be stiff to transfer pedaling energy efficiently while allowing some flex for comfort.

Toughness: Absorbing Impact Without Breaking

1. What It Means:
   1. A material’s ability to deform without cracking or shattering when impacted.
2. Design Relevance:
   1. Car bumpers need to absorb impact energy without breaking into pieces.
   2. Sports helmets must be tough enough to protect against repeated impacts.

Ductility: Stretching Into Wire-Like Shapes

1. What It Means:
   1. A material’s ability to be stretched into a wire-like form without breaking.
   2. Different from malleability, which refers to reshaping materials without cracking.
2. Design Relevance:
   1. Electrical wiring relies on ductile metals like copper, which can be stretched into long, thin wires.
   2. Extruded aluminum parts in construction benefit from aluminum’s ductility.

Elasticity: Bending and Returning to Shape

1. What It Means:
   1. A material’s ability to flex and then return to its original shape after the force is removed.
2. Design Relevance:
   1. Pole vaulting poles need to flex under force and snap back to propel the athlete over the bar.
   2. Car suspension springs absorb shocks and return to their original shape.

Plasticity: Permanently Changing Shape Without Breaking

1. What It Means:
   1. A material’s ability to be reshaped and hold its new form permanently.
   2. When stretched beyond its yield point, it does not return to its original shape.
2. Design Relevance:
   1. Metal casting and plastic molding rely on plasticity to shape materials into final products.
   2. Car body panels are stamped into shape due to metal’s plasticity.

Understanding Young’s Modulus, Stress, and Strain in Design

1. Engineers and designers use Young’s Modulus, stress, and strain to understand how materials respond to forces.
2. This helps in selecting the right materials for structural integrity, flexibility, and durability in a given design context.

Young’s Modulus: Measuring Stiffness

1. What It Means:
   1. Young’s Modulus is a measure of a material’s stiffness—how much it resists bending or stretching when a force is applied.
   2. It is the ratio of stress to strain, meaning it shows how much a material deforms for a given applied force.
2. Design Relevance:
   1. High Young’s Modulus = Very stiff material (e.g., steel, carbon fiber) → Used in structures that must not flex, like bridges or airplane wings.
   2. Low Young’s Modulus = Flexible material (e.g., rubber, plastics) → Used in products requiring elasticity, like shoe soles or elastic bands.

Stress and Strain: Understanding Material Deformation

1. What They Mean:
   1. Stress → The force applied to a material over a given area (measured in Pascals, Pa).
   2. Strain → The amount a material stretches or compresses relative to its original length (expressed as a percentage).
2. Design Relevance:
   1. High-stress materials are used in applications requiring strength and load-bearing capacity (e.g., suspension bridges, aerospace components).
   2. High-strain materials are used in products that must stretch or flex without breaking (e.g., rubber seals, flexible electronics).

The Stress-Strain Graph: The Journey of a Material Under Force

1. Think of the stress-strain graph as a material’s life cycle under force.
2. It shows how a material deforms as force increases and eventually fails (breaks).
3. It helps designers and engineers predict material behavior under tension, compression, and load-bearing applications.

Key Regions of the Stress-Strain Graph

1. O to A: Linear (Elastic) Region
   1. The material stretches proportionally to the applied force.
   2. If the force is removed, the material returns to its original shape (elastic behavior).
   3. Design Relevance:
      1. Important for springs, rubber bands, and shock absorbers, where flexibility and recovery are needed.
   4. Example:
      1. A diving board flexes when someone jumps but returns to its original position.
2. A to B: Proportional Limit and Yield Stress
   1. Proportional Limit (A): The last point where stress and strain are directly proportional (Hooke’s Law).
   2. Yield Stress (B): The material begins to deform permanently and won’t return to its original shape.
   3. Design Relevance:
      1. High-yield materials (like structural steel) resist permanent deformation, making them ideal for bridges and skyscrapers.
   4. Example:
      1. A bent paperclip no longer returns to its straight shape.
3. B to C: Perfect Plasticity (Plastic Deformation Begins)
   1. The material can deform without breaking but won’t return to its original shape.
   2. Useful in manufacturing (e.g., metal stamping, forging).
   3. Example:
      1. Aluminum sheets can be shaped into soda cans due to their plasticity.
4. C to D: Strain Hardening & Ultimate Tensile Strength (UTS)
   1. C to D: The material becomes stronger as it is stretched (strain hardening).
   2. D (Ultimate Stress Point): The material withstands its maximum load before weakening.
   3. Design Relevance:
      1. Strong, durable materials are used in high-performance applications like aerospace and automotive engineering.
   4. Example:
      1. Suspension bridge cables require materials with high ultimate tensile strength.
5. D to E: Necking & Failure
   1. Necking: The material begins to thin and weaken.
      1. E (Fracture Point): The material breaks completely under stress.
   2. Design Relevance:
      1. Materials that fail suddenly (like glass) must be used in controlled environments.
   3. Example:
      1. Ceramic plates shatter upon impact because they lack ductility.

Material Behavior Based on Graph Shape

1. Brittle Materials (Glass, Ceramics):
   1. Short elastic region, no plastic deformation.
   2. Breaks suddenly at Point B (little warning before failure).