Toughness, Hardness, And Strength

What is Toughness?

Toughness is the capacity of a material to absorb energy and undergo plastic deformation without fracturing. It represents a combination of strength and ductility, i.e. a tough material can endure high stresses and significant deformations prior to breaking. Toughness is typically quantified by the area under the stress–strain curve obtained from a tensile test. It is measured in Joules (J) or Pound-force inches (lbf-in).

Hardness vs. Toughness

Although both hardness and toughness relate to a material’s resistance to deformation, they describe distinct properties:

- Hardness is the ability of a material to resist localized plastic deformation, typically via indentation or scratching. Hard materials resist surface wear and abrasion.

- Toughness refers to the ability of a material to absorb impact energy and deform plastically without fracturing. A material that is tough is not necessarily hard, and a hard material is not automatically tough.

For example, ceramics are often very hard but lack toughness because they are brittle and fracture easily upon impact. Metals, such as steel, are usually tougher than ceramics, thereby withstanding both load and impact forces before failure.

Toughness and Strength

Strength describes a material’s ability to resist an applied force without breaking or undergoing permanent deformation. Toughness, on the other hand, indicates the amount of energy a material absorbs during deformation before fracturing. A material may exhibit high strength but low toughness, as in some brittle materials like cast iron, or it may show high toughness with lower strength, as in certain ductile metals.

Example:

• Steel: Certain grades of steel are engineered for high strength and toughness, and they are used in construction and the automotive industry.
• Cast Iron: Cast iron possesses high compressive strength; however, its brittleness yields low toughness, so it fractures easily under tensile or impact loads.

Factors Influencing the Toughness of Metals

1. Temperature:

• At low temperatures, many metals become brittle and lose toughness, thereby increasing the risk of fracture. Consequently, materials used in cold climates, such as steel for pipelines or aircraft, are often treated to improve low-temperature toughness.
• High temperatures also affect toughness. Under high temperatures, metals become more ductile and are less prone to fracture.

2. Grain Structure:

• Materials with a fine-grained microstructure generally exhibit higher toughness. Smaller grains provide additional barriers to dislocation motion, thereby allowing the material to absorb more energy before fracturing.

3. Alloying Elements:

• The addition of alloying elements such as carbon, nickel and chromium can enhance a material’s toughness. For example, adding nickel to steel increases its toughness, particularly at low temperatures.

4. Heat Treatment:

• Heat treatment techniques, including quenching and tempering, can improve toughness by altering the metal’s microstructure. For instance, tempered martensitic steel demonstrates a better balance between toughness and strength than untreated martensite.

5. Strain Rate:

• High strain rates (rapid loading) reduce toughness, leading to fracture under impact. Materials subjected to slow, gradual loading generally display higher toughness.

Applications Requiring High Toughness

Materials with high toughness are essential in industries where failure due to impact or stress can have severe consequences. Key applications include:

• Aerospace: Materials for aircraft must withstand substantial loads and impact forces without fracturing.
• Automotive Industry: Components such as bumpers, frames and suspension parts must absorb impact energy during collisions.
• Construction: Construction steels used in buildings and bridges must tolerate dynamic loads, including wind and seismic forces.
• Sports Equipment: Helmets, protective gear and other sports apparatus are designed to absorb impact energy and thus protect the user.
• Military: Armour and vehicle structures require high toughness to withstand extreme impact forces.

Toughness and Hardness in Common Metals

Further information is available at Stanford Advanced Materials (SAM).

Frequently Asked Questions

What is the difference between toughness and ductility?

Ductility refers to a material’s capacity to undergo deformation under tensile stress, while toughness is the ability to absorb energy and deform plastically without fracture. Ductility contributes to toughness but they are not equivalent properties.

Can hardness affect toughness?

Yes, increased hardness often results in reduced toughness. Hard materials, such as ceramics or hardened steel, tend to exhibit crack formation under sudden loading and are therefore less tough.

Is high toughness always desirable?

High toughness is essential in applications requiring the absorption of impact or extreme loads, for example in aerospace and the automotive sectors. In some cases, such as in cutting tools, hardness is prioritised over toughness.

How does temperature affect toughness?

At low temperatures most metals become brittle, thereby reducing toughness. High temperatures can influence toughness by increasing ductility or, in some cases, promoting softening.

Why is toughness important in construction?

Toughness is critical in construction because it ensures that materials can absorb dynamic loads from seismic activity, wind or heavy machinery without failing catastrophically.