It is also influenced by the stress ratio R = σ min/σ max. In general the crack propagation rate of a given crack subjected to a constant amplitude loading depends primarily on the range of stresses in the fatigue cycle ( σ max, σ min), and on the crack length. Since the stress-strain field near a crack tip is uniquely determined by the stress intensity factor, fatigue crack growth rates can be correlated to ΔK and Fig.5.1 shows a typical plot which can be divided into three zones threshold, stable crack growth and instability.įig.5.1 A typical fatigue crack growth curve The reason for this is simple: the material near the crack tip is under severe plastic deformation (see Chapter 3). In fact, the range of the stress intensity factor ΔK, where ΔK = K max − K min in a cycle may also be well below the materials fracture toughness K IC. When a constant range of cyclic stress, Δσ ( = σ max - σ min), is applied to a cracked structure, stable fatigue crack growth can occur at stress levels well below the yield stress of the material. Fatigue and Life Prediction 5.1 Fatigue Crack Growth Equations
