Strength of Materials
This section explores the strength of materials, focusing on tensile strength, and defines key properties like brittleness, toughness, malleability, and ductility.
Is it strong enough?
What is Strength of Materials?
- Definition: Strength is a material’s ability to resist forces such as compression, torsion, bending, shear, and tension.
- Explanation: In engineering, materials are chosen based on their strength to ensure they can withstand forces without breaking or deforming. This chapter focuses on tensile strength, which is the ability to resist stretching forces.
- Importance: Knowing a material’s strength helps engineers design safe structures and products, like bridges or car parts.
Properties of Materials
- Brittle: A material that is hard and strong but breaks easily when hit or dropped.
Example: Glass is brittle; it shatters if dropped. A steel file may crack if dropped on a hard floor. - Tough: A material that is not brittle and can withstand impact or bending without breaking.
Example: Polyethylene tubing used for water pipes is tough and can be hit or bent without breaking. - Malleable: A material that can be shaped by hammering or pressing without breaking.
Example: Sheet metal is malleable and can be pressed into car body panels. - Ductile: A material that can be stretched into a wire or extruded through a die to form shapes.
Example: Copper and low-carbon steel are ductile and used to make wires. Aluminium is extruded into window frame strips.
Tensile Strength and Testing
Definition: Tensile strength is a material’s ability to resist breaking under stretching forces.
Testing Process:
- In engineering labs, a test specimen (a sample of the material) is stretched in a machine with jaws that clamp and pull it apart slowly.
- A force gauge measures the stretching force, and distance gauges measure how much the specimen stretches.
- As the force increases, the specimen may form a narrow “neck” where it stretches most, eventually breaking at that point.
Graphing Results:
- A graph plots stress (force per unit area) against strain (amount of stretch).
- Elastic Region: At low forces (e.g., from point 0 to 1 on the graph), the material stretches slightly and returns to its original shape when the force is removed, like a rubber band.
- Plastic Region: At higher forces (e.g., from point 1 to 2), the material deforms permanently, getting narrower and longer as atoms slide past each other.
- Failure Point: At a critical force (point 2), the material breaks as atoms can no longer hold together, often with a loud bang.
Ductility in Testing: If a material stretches significantly before breaking (between points 1 and 2), it is ductile, meaning it can change shape without fracturing.
Example: Steel is ductile, forming a neck and stretching before breaking, while paper is not ductile and breaks without much stretching.
Importance of Materials Testing
- Explanation: All materials will break if the stress is too high, so engineers test materials to know their maximum tensile strength.
- Applications: Accurate testing ensures safety in buildings, vehicles, and other structures, as people’s lives depend on materials not failing under stress.
Density of Materials
This section explains density as a property of materials and how to calculate it.
What is Density?
- Definition: Density is a material’s mass per unit volume, often described as its “heaviness-for-size.”
- Explanation: Density indicates how heavy a material is for its size. Materials with the same volume can have different masses due to differing densities.
- Example: A cement brick and a polystyrene block of the same size look similar, but the brick is much heavier because its density is about 2 g/cm³, while polystyrene is about 0.024 g/cm³ (100 times less dense).
Characteristics of Density
- Consistency: Density is a property of the material, not the object’s size. A large block of steel has the same density as a small block of steel.
- Applications: Density affects material choice. For example, aluminium ladders (density ~2.7 g/cm³) are lighter than steel ladders (density ~7.8 g/cm³), making them easier to carry.
Calculating Density
Formula: Density = mass / volume
In symbols: D = m / V
- D: Density (grams per cubic centimeter, g/cm³)
- m: Mass (grams, g)
- V: Volume (cubic centimeters, cm³)
Rearranged Formulas:
- Mass: m = D × V
- Volume: V = m / D
Examples:
- A 300 cm³ aluminium block with a mass of 822 g:
D = 822 / 300 = 2.74 g/cm³ - 5 liters (5000 cm³) of petrol with a density of 0.72 g/cm³:
m = 0.72 × 5000 = 3600 g
Magnetic and Non-Magnetic Materials
This section revises magnetism and classifies materials based on their magnetic properties.
What are Magnetic Materials?
- Definition: Magnetic materials are substances strongly attracted by a magnet, experiencing a force even without direct contact.
- Explanation: Magnets have a north-seeking pole (N) and a south-seeking pole (S), with the strongest force at the poles. Opposite poles attract (N to S), while like poles repel (N to N or S to S).
- Examples: Pins and nails (made of steel) are attracted to magnets because they are magnetic materials.
Magnetic vs. Non-Magnetic Materials
- Magnetic Materials: Only certain metals—iron, nickel, and cobalt—are strongly attracted to magnets.
Example: Steel (mostly iron) is magnetic, so paper clips and cold-drink can sides are attracted to magnets. - Non-Magnetic Materials: Most materials, including non-metals (e.g., plastic, glass, paper) and most metals (e.g., copper, aluminium), are either not attracted or respond so weakly to magnets that they are considered non-magnetic.
Example: Stainless steel sinks, often made of a non-magnetic alloy, are not attracted to magnets.
Note: Objects like coins or brass screws may appear non-magnetic but are often made of magnetic materials (e.g., steel) coated with non-magnetic substances (e.g., brass or plastic).
Uses of Permanent Magnets
Definition: Permanent magnets retain their magnetic properties and are used in various applications.
Types:
- Ferrite Ceramic: Made from iron oxide and cobalt carbonate, resembling black pottery. Used in loudspeakers and microwave ovens.
- Neodymium Ceramic: Made from iron, boron, and neodymium, coated with nickel for protection. Used in computer hard drives and electric car motors.
Applications:
- Fridge and cupboard door magnets.
- Neodymium magnets in earrings, electric car motors (e.g., 1 kg in a Prius), and wind-powered generators (up to 600 kg).
- Ferrite ring magnets in loudspeakers and microwave ovens.
Melting and Boiling Points
This section discusses the states of matter and the temperatures at which materials melt and boil.
States of Matter
Definition: Matter exists in three states: solid (hard, keeps shape), liquid (flows, takes container’s shape), and gas (flows, fills container, and escapes upward).
Explanation: The state of matter depends on temperature. Heating or cooling can change a material’s state.
Examples:
- Margarine is solid in a fridge but becomes liquid on a hot day.
- A rubber hose is flexible when warm but may crack if bent in cold temperatures.
- Molten rock (lava) flows from volcanoes, showing that even rock can melt at high temperatures.
Melting and Boiling Points
Definition: The melting point is the temperature at which a solid becomes a liquid. The boiling point is the temperature at which a liquid becomes a gas.
Explanation:
- Pure substances melt at a specific temperature, while mixtures (e.g., alloys) melt over a range of temperatures.
- Most substances can melt, boil, evaporate, or condense, depending on temperature.
Examples:
- Ice melts at 0°C, and water is often said to boil at 100°C (though this varies by location).
- Solder (an alloy of tin and lead) for electronics melts at 183°C, while other solders have different melting points.
- Wax melts at a low temperature, safe to touch, while molten solder is much hotter and dangerous.
- Zinc coatings on steel washers melt when heated, and iron can boil at extremely high temperatures.
Melting Points of Metals and Alloys
- Pure Metals: Have specific melting points.
Example: Lead, tin, silver, zinc, and tungsten have distinct melting points (found in Resource Pages). - Alloys: Often have lower melting points than their constituent pure metals.
Example: Solder for electronics (tin and lead alloy) melts at a lower temperature than pure tin or lead.
Applications: Engineers use melting point data to select materials for specific purposes, like low-melting solder for electronics or high-melting tungsten for light bulb filaments.
Boiling Points and Observations
- Water’s Boiling Point: Commonly said to be 100°C, but it may be lower depending on location (explained in Chapter 11).
- Other Materials: Liquids like solder or iron can boil at very high temperatures, though these are rarely observed in everyday conditions.
- Temperature Behavior: When water is heated, its temperature rises steadily until it reaches the boiling point, where it remains constant despite continued heating, as energy is used to change the state from liquid to gas.
Points to Remember
- Strength is a material’s ability to resist forces like compression, torsion, bending, shear, and tension, with tensile strength being the resistance to stretching.
- Brittle materials (e.g., glass) break easily when hit, while tough materials (e.g., polyethylene) resist breaking. Malleable materials (e.g., sheet metal) can be shaped, and ductile materials (e.g., copper) can be stretched into wires.
- Tensile strength tests measure how much a material can stretch before breaking, showing elastic (reversible) and plastic (permanent) deformation regions.
- Density is mass per unit volume (D = m / V), measured in g/cm³, and remains constant regardless of an object’s size.
- Magnetic materials (iron, nickel, cobalt) are strongly attracted to magnets, while most materials, including non-metals and many metals, are non-magnetic.
- Permanent magnets, like ferrite and neodymium ceramics, are used in fridges, loudspeakers, electric motors, and wind generators.
- Matter exists as solid, liquid, or gas, with state changes driven by temperature (e.g., melting at 0°C for ice, boiling near 100°C for water).
- Pure substances have specific melting points, while alloys melt over a range. Boiling points vary, and water’s boiling point may be below 100°C depending on location.
- Engineers use material property data (strength, density, magnetism, melting points) to design safe and effective products.
- Materials testing is critical to ensure safety, as all materials can fail under excessive stress.
Difficult Words
- Strength: A material’s ability to resist forces without breaking or deforming.
- Tensile Strength: The ability of a material to resist breaking when stretched.
- Brittle: A property of materials that are hard but break easily when hit or dropped.
- Tough: A property of materials that can withstand impact or bending without breaking.
- Malleable: A property of materials that can be shaped by hammering or pressing without breaking.
- Ductile: A property of materials that can be stretched into wires or extruded into shapes.
- Elastic: A property where a material returns to its original shape after being stretched or compressed.
- Density: The mass of a material per unit volume, measured in g/cm³.
- Magnetic Materials: Materials (e.g., iron, nickel, cobalt) strongly attracted by magnets.
- Non-Magnetic Materials: Materials not significantly attracted by magnets, including most metals and non-metals.
- Melting Point: The temperature at which a solid becomes a liquid.
- Boiling Point: The temperature at which a liquid becomes a gas.
- Alloy: A mixture of two or more metals, often with a lower melting point than the pure metals.
Summary
This chapter examines the properties of materials critical for engineering and technology. Strength, measured by a material’s ability to resist forces like tension, is tested through tensile strength experiments, revealing properties like brittleness (e.g., glass), toughness (e.g., polyethylene), malleability (e.g., sheet metal), and ductility (e.g., copper). Density, defined as mass per unit volume (D = m / V), determines a material’s heaviness-for-size, influencing choices like aluminium over steel for ladders. Magnetic materials (iron, nickel, cobalt) are attracted to magnets, unlike most non-magnetic materials, and permanent magnets (ferrite and neodymium) are used in applications from fridges to wind generators. Matter exists as solid, liquid, or gas, with melting and boiling points indicating state changes; pure substances have specific melting points, while alloys melt over a range, and water’s boiling point varies by location. These properties guide engineers in selecting materials to ensure safety and functionality in designs.