Mechanism of factory-roof formation

Abstract

ABSTRACT. The investigated factory-roof (F-R

Near‐threshold behaviour of shear‐mode fatigue cracks in metallic materials

“…The related numerical analysis 36 revealed that such elements are located at crack front sites predominantly loaded in mode II. The related numerical analysis 36 revealed that such elements are located at crack front sites predominantly loaded in mode II.…”

“…The extrinsic (shielding) mechanisms occur in the surrounding of the crack tip as contacts of fracture surfaces during the cyclic loading, usually called crack closure. [24][25][26] For pure geometrical reasons, therefore, the size of crack wake asperities in form of branches or factory-roofs reduces and completely vanishes in the very low-cycle fatigue regime. Hence, the resistance to fatigue crack growth consists of two components: effective and crack closure induced.…”

“…In aluminium alloys, Merati et al 71 determined ΔK III,th ≈ 6.1 MPa m 1/2 for the 7075-T6 alloy and ΔK III,th ≈ 3 MPa m 1/2 for the 2014-T6 alloy. 25 In the stainless steel, Pokluda et al 74,75 has shown that the sharp factory-roof morphology and the related clamping of crack wake asperities caused lower crack growth rate under the mode III loading than that under the mode II one, where the cracks propagated along smooth mode I branches. Murakami et al 28 measured ΔK III,th = 10.6 MPa m 1/2 (=ΔK II,th ) in the 0.47% carbon steel (JISS45C) by observing the sizes of semi-elliptical mode II + III cracks initiating the factory-roof morphology.…”

“…At the same time, the density of crack initiation sites along the precrack front becomes higher. [24][25][26] For pure geometrical reasons, therefore, the size of crack wake asperities in form of branches or factory-roofs reduces and completely vanishes in the very low-cycle fatigue regime. Moreover, the shearmode cyclic plastic zone is widely extended along the maximum shear plane thus embracing many characteristic microstructural elements and promoting the damage mechanism based on the coalescence of the main crack with microcracks nucleated inside the plastic zone.…”

“…(5) for Δk IIeff,d = 0). Consequently, a simple necessary condition for growth of a mode I branch at the crack front loaded in mode II was formulated as follows: 3,35,36 the stress intensity range Δk Id for the branched crack must be greater than the mode I threshold ΔK I,th , that is, Δk Id = 1.15 ΔK II > ΔK I,th . It should be noted that, in this criterion, the effective values ΔK IIeff and ΔK Ieff,th should be used rather than the remote ones.…”