Short Answer:

Toughness is the property of a material that shows its ability to absorb energy and resist fracture when subjected to shock or sudden impact. It measures how much energy a material can take before it breaks.

In simple words, a tough material can bend or deform without breaking when a sudden load or impact is applied. Materials like mild steel and copper are considered tough, while brittle materials like glass or cast iron are not tough. Toughness combines both strength and ductility in a single property.

Detailed Explanation :

Toughness

Toughness is one of the most important mechanical properties of engineering materials. It is defined as the ability of a material to absorb energy before failure or fracture occurs. Toughness represents the combined effect of strength and ductility — meaning that a tough material can deform plastically and also resist fracture under impact or shock loads.

In other words, toughness measures how much energy a material can store and absorb before it finally breaks. The more energy a material can absorb, the tougher it is. It is an essential property in materials used for parts that experience impact, vibration, or shock, such as automotive components, shafts, and machine tools.

1. Meaning and Importance of Toughness

Toughness is a measure of the material’s ability to resist failure under impact or dynamic loading conditions. It is not the same as strength — a material can be strong but not tough. For example, glass is strong in compression but brittle, meaning it breaks easily under shock. On the other hand, mild steel is both strong and ductile, making it tough.

Toughness ensures that a component can absorb energy without sudden failure. It is important for mechanical parts like springs, gears, and axles that experience fluctuating or impact loads. In structures such as bridges, cranes, and vehicles, toughness ensures safety by preventing catastrophic failure.

2. Measurement of Toughness

Toughness is generally measured by impact tests. These tests determine how much energy a material can absorb before it fractures when subjected to a sudden load. The two most common tests are:

1. a) Charpy Impact Test:
   In this test, a notched specimen is supported at both ends, and a pendulum hammer strikes it in the middle. The energy absorbed to break the specimen is measured. The higher the absorbed energy, the tougher the material.
2. b) Izod Impact Test:
   Here, the specimen is held vertically and struck by a pendulum hammer on the notched side. Similar to the Charpy test, the absorbed energy is recorded to indicate toughness.

The energy absorbed in these tests is usually expressed in joules, representing the amount of energy required to cause fracture.

3. Relation of Toughness with Strength and Ductility

Toughness is directly related to both strength and ductility.

• Strength means the ability of a material to resist stress.
• Ductility means the ability to deform without breaking.

A tough material needs both these properties. If a material is strong but not ductile, it will break suddenly (like glass). Similarly, if it is ductile but weak, it will deform easily but cannot bear high loads (like lead). Therefore, toughness is achieved by having a balance between strength and ductility.

4. Factors Affecting Toughness

Several factors influence the toughness of a material:

• Temperature: At low temperatures, materials tend to become brittle and lose toughness.
• Heat Treatment: Processes like annealing can improve ductility and toughness, while hardening can reduce it.
• Strain Rate: Rapid loading reduces toughness since materials do not get enough time to deform plastically.
• Impurities and Defects: Cracks, voids, and impurities can reduce toughness by making the material brittle.
• Composition: Alloying elements like nickel and chromium can improve toughness in steel.

5. Types of Toughness

Toughness can be categorized as:

1. a) Impact Toughness:
   It measures a material’s ability to resist sudden impact loads. This is what Charpy and Izod tests determine.
2. b) Fracture Toughness:
   It is the ability of a material to resist the propagation of cracks. It is very important in materials used in aircraft, pressure vessels, and pipelines where failure due to cracks can be dangerous.
3. c) Modulus of Toughness:
   It is the total area under the stress-strain curve, representing the total energy per unit volume that a material can absorb before fracture.
4. Applications of Tough Materials

Tough materials are used in applications where strength and resistance to impact are both required. Examples include:

• Automobile bodies and chassis
• Railway components
• Gears, shafts, and springs
• Pressure vessels and pipelines
• Structural steel in buildings and bridges

7. Examples of Tough and Brittle Materials

• Tough materials: Mild steel, wrought iron, copper, and bronze.
• Brittle materials: Cast iron, glass, ceramics, and concrete.

Tough materials are capable of bending and deforming before breaking, while brittle materials crack or shatter suddenly.

Conclusion:

Toughness is a key mechanical property that represents a material’s ability to absorb energy and resist fracture when subjected to sudden or repeated loads. It depends on both strength and ductility. Materials with high toughness ensure safety, durability, and reliability in mechanical and structural applications. Impact and fracture toughness tests help engineers select suitable materials for parts that must withstand shock or dynamic loads. Therefore, toughness is essential in the design and selection of materials used in engineering and manufacturing.