Q1: Failure of a material is attributed to __________ if it fails below its yield point.

ANS:B - fatigue

Failure of a material below its yield point is typically attributed to fatigue rather than impact loading. Let's delve into fatigue failure in more detail: Fatigue:

• Fatigue failure occurs when a material fails under repeated cyclic loading, even if the stress levels are below the material's yield strength.
• It is a progressive and localized structural damage that occurs when a material is subjected to repeated loading and unloading cycles.
• The repeated cyclic loading causes microscopic cracks to initiate and grow, eventually leading to catastrophic failure, often without any significant prior warning.
• Fatigue failure is particularly common in materials subjected to dynamic or fluctuating loads, such as vibrations, alternating stresses, or cyclic loading conditions.

Mechanism of Fatigue Failure:

1. Initiation: Fatigue failure begins with the initiation of microcracks at stress concentrations, surface defects, or material discontinuities.
2. Propagation: These microcracks gradually propagate and extend with each loading cycle, driven by the alternating stresses applied to the material.
3. Final Fracture: Eventually, the cracks reach a critical size where the remaining cross-sectional area of the material becomes insufficient to support the applied load, resulting in sudden fracture or failure.

Factors Influencing Fatigue Failure:

• Material Properties: Factors such as material strength, ductility, and microstructure significantly influence a material's resistance to fatigue.
• Stress Concentrations: Sharp changes in geometry or surface irregularities can act as stress concentrators, accelerating crack initiation.
• Loading Conditions: The magnitude, frequency, and type of loading (tensile, compressive, bending, etc.) play a crucial role in determining fatigue life.
• Environment: Factors such as temperature, humidity, and corrosive environments can also affect fatigue behavior.

Prevention of Fatigue Failure:

• Improving material properties through alloying, heat treatment, or surface modification.
• Designing components to minimize stress concentrations and avoiding sharp corners or abrupt changes in geometry.
• Applying surface treatments such as shot peening or residual stress induction to introduce compressive stresses that resist crack initiation and propagation.
• Regular inspection, monitoring, and maintenance to detect early signs of fatigue damage and prevent catastrophic failure.

In summary, fatigue failure is a common mode of failure in materials subjected to repeated cyclic loading, characterized by the initiation and propagation of microcracks leading to eventual fracture. Understanding and mitigating fatigue is crucial in ensuring the reliability and longevity of engineering components and structures.