Experiments on a free–machining steel have been undertaken to elucidate the role of non–metallic inclusions in fatigue crack growth. In the mid–to–high stress intensity range ∆K of the crack growth date da/dN versus ∆K curve inclusions are found to influence fatigue crack growth rates by plastic strain intensification. High crack growth rates are associated with testing orientations which exhibit low initiation values of fracture toughness. In the low growth rate regime, orientations which exhibit low values of fracture toughness give high threshold values of ∆K. Crack growth behaviour in this regime is strongly influenced by crack closure phenomena.
MST/318
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