materials test 2

3 Stages of Fatigue

1. Initiation

2. Propagation

3. Final Fracture

Cyclic Loading

Application of repeated or fluctuating stresses and strains to a component

Fatigue Limit

the stress amplitude below which fatigue failure will not occur

Fatigue Strength

The stress amplitude value for failure to occur in a specified number of cycles

Striations

micro-scale features indicating crack origin and growth characteristics

Beachmarks

macro-scale features visible by the eye, usually associated with periodic start-up and shut-down operation (one line per start-up and shut down). Does NOT reflect load cycles

Tensile stress is needed for crack growth

FALSE. Cracks will not grow if only compressive stresses are present.

3 Factors affecting Fatigue Life

1. Mean Stress

2. Surface Treatments

3. Design Factors (reduce stress concentration areas)

Surface Treatments

• Case Hardening

• Shot Peening

Case Hardening

Material maintains toughness but has a harder (wear resistant) surface

Shot Peening

Compressive surface stresses to delay crack initiation

Creep

time dependent plastic deformation at high homologous temperatures

Creep Strain

1. Primary

Strain which occurs after the first instantaneous change ( ɛ = ε_c + ε_E+ε_P)