List at least two different forms of failure and explain the circumstances under which a material will experience that type of failure. Also what are the three steps of heat treatment (annealing)?
The correct answer and explanation is :
Two Forms of Failure in Materials
- Fracture Failure:
Fracture failure occurs when a material breaks or cracks due to stress. This form of failure typically happens when a material experiences excessive force, causing it to exceed its tensile strength. Fracture can occur in two main forms: ductile fracture and brittle fracture.
- Ductile Fracture: This occurs when the material deforms plastically (i.e., it stretches or distorts significantly) before breaking. Materials like metals (e.g., steel, aluminum) exhibit ductile fracture when they experience tensile stress that surpasses the yield strength of the material, especially when subjected to slow and steady loading conditions. The fracture zone will typically appear with noticeable elongation and necking.
- Brittle Fracture: This happens when the material fails with little or no prior deformation. It generally occurs rapidly, and the fracture surfaces are flat, showing little sign of plastic deformation. Brittle fracture usually occurs in materials that are brittle, such as cast iron or ceramics, particularly when subjected to impact loading or at low temperatures.
- Fatigue Failure:
Fatigue failure is a result of repeated or cyclic loading that causes a material to crack and eventually fail. Even if the applied load is lower than the material’s ultimate tensile strength, repeated cycles of stress can cause microscopic cracks to form and propagate over time. Eventually, these cracks will grow large enough to cause catastrophic failure. Fatigue failure is commonly seen in parts like gears, turbine blades, and aircraft wings, where cyclic loading is inevitable. Factors such as high-stress concentration, surface defects, and corrosive environments can accelerate fatigue failure. The material does not need to be exposed to a single excessive load but to many cycles of lower stress that cause gradual degradation.
Three Steps of Heat Treatment (Annealing)
- Heating: In the first step, the material (often metal) is heated to a specific temperature above its recrystallization point. This temperature range allows the material’s internal structure (grain structure) to become more flexible, facilitating changes in the crystalline arrangement. The temperature and time are carefully controlled to ensure uniform heating without exceeding the material’s thermal limits.
- Soaking: The material is then held at the target temperature for a set period to ensure uniform temperature distribution throughout the material. This soaking phase allows the material to reach a state where internal stresses are minimized, and the crystalline structure can re-align or soften. The duration of soaking can vary based on the material and the desired outcome.
- Cooling: Finally, the material is slowly cooled at a controlled rate, often by allowing it to cool in the furnace (furnace cooling), or by air cooling. The cooling rate is crucial, as rapid cooling can lead to undesirable properties like brittleness. Slow cooling promotes the formation of a more stable and uniform microstructure, leading to improved ductility and toughness.
In summary, heat treatment like annealing aims to alter the microstructure of a material, improving its mechanical properties, such as strength, hardness, or ductility, depending on the desired outcome.