Fatigue failure

Definition

Fatigue failure refers to the progressive structural damage that occurs when a material is subjected to repeated loading and unloading cycles, often leading to unexpected fracture. This phenomenon is critical in understanding how materials behave under various types of stresses, particularly where cyclic loading is prevalent. It involves the initiation and growth of cracks that can result in catastrophic failure long before the material reaches its ultimate strength.

5 Must Know Facts For Your Next Test

1. Fatigue failure can occur at stress levels lower than the material's yield strength, highlighting the importance of considering cyclic loading in design.
2. The initiation of fatigue cracks typically occurs at locations of stress concentration, such as notches or surface defects.
3. Fatigue life can be affected by environmental factors, such as temperature and humidity, which can accelerate crack growth.
4. Material treatments, like shot peening or surface hardening, can improve resistance to fatigue failure by reducing surface defects and increasing compressive residual stresses.
5. Understanding fatigue failure is crucial for designing components in applications like automotive, aerospace, and structural engineering where safety is paramount.

Review Questions

• How does fatigue failure differ from other forms of material failure, and what factors contribute to its occurrence?
  • Fatigue failure is unique because it results from repeated loading cycles rather than a single overload event. Factors contributing to fatigue include the magnitude and frequency of the applied load, the presence of stress concentrators like notches, and environmental conditions. Unlike ductile failure, which often gives warning signs like deformation before fracture, fatigue failure can occur suddenly with little or no prior indication.
• In what ways do axial, bending, and torsional stresses influence the likelihood of fatigue failure in mechanical components?
  • Axial, bending, and torsional stresses can create different patterns of stress distribution within a component. Axial stresses typically produce uniform stress conditions; however, bending can introduce tensile and compressive stresses that vary across the cross-section. Torsional stresses create shear stresses that can also lead to fatigue failures. Understanding these stress types allows engineers to predict where fatigue cracks might initiate and develop strategies to enhance component longevity.
• Evaluate the implications of fatigue failure in the design process for rolling element bearings and journal bearings.
  • In designing rolling element bearings and journal bearings, engineers must consider fatigue failure because these components are frequently subjected to cyclic loads during operation. The implications include selecting appropriate materials with high endurance limits and applying surface treatments to enhance fatigue resistance. Additionally, factors such as lubrication and operating conditions must be carefully evaluated to prevent premature fatigue crack initiation. This comprehensive approach ensures reliability and safety in applications where these bearings operate under repetitive loads.

Related terms

Endurance Limit:

The maximum stress level a material can withstand for an infinite number of cycles without failing due to fatigue.

S-N Curve: A graphical representation of the relationship between the cyclic stress (S) applied to a material and the number of cycles to failure (N), which is essential in fatigue analysis.

Crack Propagation: The process by which a crack grows in a material under stress, influenced by factors such as load amplitude, environment, and material properties.