A B S T R A C T A new theoretical concept is introduced to describe the roughness-induced shielding effects in metallic materials. This approach is based on the statistics of the local ratio between the characteristic microstuctural distance and the plastic zone size. A general equation involving both the crack branching and the crack closure phenomena is derived in the frame of linear elastic fracture mechanics under the assumption of remote mode I loading. It enables the determination of the intrinsic values of both the fracture toughness and the fatigue crack growth threshold. Moreover, the roughness-induced component can be separated from other closure components, such as the plasticity or oxide-induced closure. In order to estimate the total roughness-induced shielding effect only standard materials data such as the yield stress, the mean grain size, the surface roughness and the fracture mode are necessary. Examples of applications concerning static fracture and fatigue are presented for selected metallic materials.A = coefficient in the Paris-Erdogan relation a = length of the fatigue pre-crack B = specimen thickness C pcl = closure ratio (other than roughness-induced) D = length of the kinked part of the crack path D dm = standard deviation of the mean grain size data D R = standard deviation of the surface roughness d c = critical (boundary) characteristic microstructural distance d m = mean characteristic microstructural distance da/dN = fatigue crack growth rate dW = elementary energy release (or fracture energy) d = characteristic microstructural distance g * = mean normalized local effective crack driving force g r eff = local effective crack driving force normalized to G G eff = remote effective crack driving force g eff = local effective crack driving forcē g * = normalized local effective crack driving force averaged along the fatigue crack path G I , G II , G III = remote crack driving forces (modes I, II, III) G u I = crack driving force related to the crack instability K = general denotation of the remote stress intensity factor k * = mean local effective stress intensity factor normalized to K I k 1 , k 2 , k 3 = local stress intensity factors (modes 1, 2, 3) K cl = remote closure stress intensity factor K eff,g = global effective stress intensity factor for brittle fracture

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