Brittle – ductile transition and scatter in fracture toughness of ferritic steels

A B S T R A C T Whether flaws in structures containing residual (secondary) stresses will extend under particular operational (primary) loads depends on the extent to which the residual stress field affects: (a) the nature and distribution of initiators; (b) the combined (primary + secondary) stresses and strains experienced by potential initiators. This paper compares fractographic data from specimens loaded by only a primary stress with data from specimens also containing a tensile residual stress field. Three-dimensional elastic-plastic finite element calculations are used to characterize the stress-strain conditions at the initiation sites at the onset of brittle fracture. The introduction of a residual stress changes the dominant stage in fracture nucleation from microcrack extension to particle cracking. This offsets some of the decrease in fracture toughness expected when the residual stress field increases specimen constraint.CT = compact tension specimen E = Young's modulus EDX = energy-dispersive X-ray analysis of composition FE = finite element HT = heat treatment J = contour integral J c = value of J at cleavage K = stress intensity K Jc = stress intensity at specimen failure MVC = ductile microvoid coalescence PCT = pre-loaded compact tension specimen SEM = scanning electron microscope X c = separation of initiation site from pre-crack front (in direction of macroscopic crack growth) Z c = separation from initiation site from mid-line of specimen, in plane normal to loading direction ε 22elastic = elastic portion of ε 22,total ε 22,total = total (elastic+plastic) strain in loading direction, acting on plane normal to loading direction ν = Poisson's σ 22 = principal tensile stress, in loading direction, acting on plane normal to loading direction

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“…the fractured particles). 26 The data then show no correlation between X c and K Jc , as in Figs 3a and 5a.…”

Section: Discussionmentioning

confidence: 81%

“…When fracture is microcrack extension controlled (stress‐controlled 24,25 ), the initiation sites lie in lower‐strain regions, and the scatter in K Jc is determined by scatter in the sizes of the microcracks (i.e. the fractured particles) 26 . The data then show no correlation between X c and K Jc , as in Figs 3a and 5a.…”

Section: Discussionmentioning

confidence: 93%