Fatigue Performance of Materials

1. Fatigue Phenomenon

Fatigue and fracture are major causes of failure in engineering structures. Under repeated loading, structural materials can experience brittle failure below their static load strength, necessitating consideration of fatigue strength in design. Since the 19th century, extensive research on fatigue failure has led to rich knowledge regarding fatigue observation, life prediction, and design. The development of fracture mechanics in the 1950s further advanced the understanding of fatigue crack propagation and failure control.

2. Severity of Fatigue Fracture

  • In 1936, a fully welded bridge over the Albert Canal in Belgium suffered brittle fracture due to unreasonable design, severe stress concentration, and poor construction quality at -20°C.
  • In 1951, a Canadian steel bridge with spans of 55m and 45.8m experienced cracking and partial repairs, leading to multiple fractures at -35°C.
  • In 1962, an Australian steel beam bridge with high carbon content and poor weldability saw brittle fractures from stress concentration areas.
  • In 1965, a drilling rig in the North Sea experienced crack propagation at connection rods with low impact testing values, fracturing at 3°C.
  • In 1967, the Pleasanton cable-stayed bridge in the USA collapsed within 60 seconds after a crack developed and propagated in a hanger rod.

3. Definition of Fatigue

According to the American Society for Testing and Materials (ASTM), fatigue is the process of localized, permanent structural changes in a material due to the application of disturbing stress at one or more points, resulting in the formation of cracks or complete fracture after sufficient cycles of disturbance. This definition highlights several characteristics of fatigue:

  1. Disturbing Stress: Fatigue occurs only under the influence of disturbing stress, which can vary over time and be defined as disturbing load (force, stress, displacement, strain, etc.).
  2. Local Origin: Unlike static load failure, fatigue failure initiates from localized areas of high stress or strain, often at stress concentration points.
  3. Cyclic Nature: Fatigue failure occurs after many cycles of disturbing load, beginning with crack initiation and followed by crack propagation until critical dimensions are reached.
  4. Development Process: Fatigue is a developmental process where damage accumulates over time under cyclic loading.

4. Mechanism of Fatigue Fracture and Fracture Features

Fatigue cracks typically display:

  1. A source of cracks, an area of crack propagation, and a final fracture area.
  2. A smooth and flat surface in the crack propagation area, often with “beach marks” and corrosion traces.
  3. Cracks generally initiate in areas of high stress or material defects.
  4. Minimal plastic deformation, even in ductile materials.

5. Comparison of Fatigue and Static Load Failure

  • Fatigue Failure: Results from localized damage accumulation; fractures are smooth and exhibit fatigue features.
  • Static Load Failure: Occurs instantaneously; fractures are rough and show no signs of wear.

6. Mechanism of Crack Initiation

Fatigue crack initiation, or nucleation, occurs at high-stress points, often due to:

  1. Stress Concentrations: Defects, inclusions, holes, or welds create stress concentrations.
  2. Surface Features: Surface machining marks and environmental corrosion contribute to crack initiation.

7. Mechanism of Crack Propagation

Under cyclic loading, micro-cracks formed by slip bands extend along the maximum shear stress plane, coalescing into a main crack that then transitions to propagate perpendicularly to the maximum tensile stress direction.

8. Material Properties Affecting Fatigue Performance

Fatigue performance is closely linked to the material’s characteristics, which can be classified as:

  1. Macroscopically Isotropic Materials
  2. Macroscopically Anisotropic Materials

9. Stress-Strain Characteristics of Materials

  • Engineering Stress (S) and Engineering Strain (e)
  • True Stress (σ) and True Strain (ε)

10. Cyclic Stress-Strain Behavior

The stress-strain curves under cyclic loading differ from those under monotonic loading, playing a crucial role in describing a structure’s state under cyclic loads.

11. Cyclic Hardening and Softening

Repeated cyclic stress leads to changes in material properties due to plastic deformation, resulting in either increased (hardening) or decreased (softening) resistance to deformation.

12. Bauschinger Effect

Following plastic deformation, materials exhibit lower yield strength upon reverse loading compared to continuous deformation yield strength.

13. Steady-State Cyclic Stress-Strain Curve

The steady-state cyclic stress-strain curve describes the stress-strain relationship when the material’s transient behavior stabilizes, and most materials reach stability after 20-50% of their fatigue life.

14. Memory Characteristics of Materials

Materials exhibit memory characteristics, enabling them to “remember” previous deformations under cyclic loading, forming closed loops in the stress-strain response.

15. Stress-Strain Response Under Variable Amplitude Cycles

Under variable amplitude loading, the stress-strain response can be illustrated, demonstrating the material’s behavior in response to cyclic loads.