Engineering Materials: Metals and Alloys

Introduction to Engineering Materials

• Engineering materials are used in mechanical and structural elements.
• Materials fall into categories based on fundamental science.
  • Metals
  • Ceramics
  • Composites
  • Natural Materials

Metals

• Metals are effective conductors of electricity and heat.
• When polished, metals exhibit a lustrous appearance.
• Metals form the majority of elements and appear on the left side of the periodic table.
• Common metals include iron, gold, nickel, silver, and magnesium.

Material Properties of Metals

• Metals are lustrous or shiny when polished.
• Metals are good conductors of electricity and heat.
• Metals have relatively high melting points.
• Metals are malleable, allowing them to be hammered into thin sheets.
• Metals are ductile, allowing them to be drawn into wires.
• Metals have good mechanical strength.

Alloy

• An alloy is a metal combined with other elements to achieve desired characteristics.
• A pure metal is not mixed with any other material.
• Most pure metals are either too soft, brittle, or chemically reactive for practical applications.

Examples of Alloys

• Steel is a mixture of iron, carbon, and other alloying elements.
• Brass is a mixture of copper and zinc.
• Bronze is a mixture of copper and tin.
• Nichrome is a mixture of nickel, chromium, and iron.

Properties of Alloys

• The material properties of alloys vary considerably depending on the parent metal and additions.
• Steel is an example.
  • Carbon increases the strength and hardness of steel.
  • Chromium increases the corrosion resistance of steel.
    • Stainless steel contains a minimum of 10.5% chromium by mass.
  • Tungsten increases the heat resistance of steel.
  • Tungsten has the highest melting point of any known material, 3422 °C.
  • Nickel makes steel non-magnetic.

Ceramics

• Ceramics are solid inorganic materials.
• Ceramics consist of metals, non-metals, and metalloid atoms primarily held in ionic or covalent bonds.
• Common ceramics include porcelain, brick, glass, and terracotta.

Material Properties of Ceramics

• Ceramics are poor conductors of electricity and heat, making them insulators.
• Ceramics have very high melting points and are heat resistant.
• Ceramics have high hardness and strength.
• Ceramics are brittle.
• Ceramics are chemically unreactive.

Composite Materials

• Composite materials, are made from two or more constituent materials with separate physical and chemical properties.
• Combined properties lead to improved or different characteristics compared to individual constituents.

Examples of Composite Materials

• Reinforced concrete is concrete reinforced with steel bars to improve its tensile and flexural strength.
• Composite wood (e.g., plywood) involves gluing together layers of wood, with adjacent layers having their wood grain at 90 degrees to improve stiffness and consistent strength.
• Reinforced plastics (e.g., fiberglass) consist of a plastic matrix reinforced with glass fibers, resulting in a higher strength-to-weight ratio than metals, and can be molded into complex shapes.

Natural Materials

• A natural material is any product or physical matter that comes from plants, animals, or the ground.
• Often used in buildings and clothing and possess a broad range of unique properties.
• Properties are hard to imitate with synthetic materials.
• Examples include:
  • Wood: rattan, bamboo, bark, etc.
  • Natural Fibres: silk, wool, cotton, etc.
  • Stone: marble, granite, obsidian, sandstone, etc.
  • Native Metals :copper, iron, gold, silver, tin, etc. that are found naturally in their pure form.

Hardness

• Hardness describes the resistance of a given material to indentation or abrasion.
• Soft materials are easy to scratch and deform.
• Hard materials are more difficult to scratch or indent.
• The Vickers Hardness Test measures relative hardness by sinking an indenter into a material sample.
• The hardness value is calculated as the ratio of indentation load to the surface area of the impression.
• Vickers Hardness values (kg/mm²) for common materials include:
  • Gold: 30
  • Plain carbon steel: 200
  • Concrete: 650
  • Brick: 1120
  • Diamond: 10,000

Stress vs. Strain

• Normal stress is the measure of the total force acting over an object per unit of area .

Engineering Strain

• 'Engineering strain' measures how much a material deforms under tensile or compressive force.
• Mathematically, it is the ratio of the change in length to the original length.

Tensile Testing

• Tensile Testing is a fundamental experiment in materials science.
• It subjects a material sample to controlled tension, increasing until the sample ruptures.
• Tests determine stress-strain properties, especially Ultimate Tensile Strength (UTS).
• The breaking point and deformation before fracture depend on whether the material is ductile or brittle.

Ductile vs. Brittle Materials

• Ductile materials (metals) undergo plastic deformation before rupturing.
• A metal sample in a tensile testing machine typically elongates before breaking.
• Brittle materials (ceramics) fracture with little prior plastic deformation.
• Fractures in brittle materials are sudden and accompanied by a snapping sound.

Plastic vs. Elastic

• Plastic deformation results in a permanent change to the object's size or shape, which is not reversible.
• Elastic deformation is reversible, allowing an object to return to its original size or shape when the applied force is removed.

Stress-Strain Graphs

• Stress-Strain Graphs illustrate deformation behavior of different materials, metals and ceramics notably.
• The shape of the curve varies for ductile versus brittle materials.

Stiffness & Young's Modulus

• Stiffness describes how well an object resists deformation in response to force.
• Greater stiffness means greater resistance to deformation.
• Young’s Modulus is a mechanical property measuring material stiffness.
• It is equal to the gradient of the elastic region of the stress-strain curve.

Toughness

• Toughness describes a material's ability to absorb energy and plastically deform without fracturing.
• It is a total amount of energy per unit of volume that a material can absorb before rupturing.
• Toughness is determined by finding the total area under the stress-strain curve, leading up to the point of rupture.
• Toughest materials have both good strength and ductility, like low-carbon steel.

Resilience

• Resilience indicates a material's ability to absorb energy during elastic deformation.
• Resilience is measured in MJ/m³.
• Equals the area under the stress-strain curve up to the yield point only, i.e. area of the elastic deformation region.
• Resilient materials have a high yield stress and can elongate considerably prior to yielding.
• Examples: rubber, certain plastics, and high-carbon steels.

Proportional Limit / Elastic Limit

• "Proportional Limit" refers to the maximum stress a material can withstand before plastic deformation begins.
• Up to this point, the material exhibits elastic behavior.
• Stress is directly proportional to strain, forming a straight line.
• Proportional limit is also characterized as the yield stress of the material.