Failure Mechanisms

· Most experimental data lies between the Goodman & Gerber models. The Goodman model is more conservative and is a good guide for design decisions.
· In the Goodman expression: σ_a = σ_N(1 - {σ_m /σ_UTS} ) , σ_α is the stress amplitude that will give a fatigue life of N cycles when a mean stress, σ_m, acts on the sample. σ_N is the periodic stress that will give a fatigue life of N cycles when the mean stress is zero, and σ_UTS is the ultimate tensile stress of the sample in uniaxial tension.