Issue 40

K. Kaklis et alii, Frattura ed Integrità Strutturale, 40 (2017) 1-17; DOI: 10.3221/IGF-ESIS.40.01 3 For crack lengths as presented above, high stress concentrations are generated at the crack tip which eventually lead to crack propagation and failure. At this point it should be stressed that cracks machined by rotating cutting saws are actually of a rectangular shape thus possessing four critical points, the corners of the rectangular slit, rather than two crack tips [13]. This fact leads to a more complicated failure mechanism in the closed vicinity of the crack crown. Here however, taking also into account the AE instruments available (namely the relatively big size of sensors used), interest is focused to a wider area considering a single dominating singularity, the crack tip. (a) (b) Figure 1 : Geometry of CCNBD specimens under (a) pure mode I loading and (b) mixed mode I-II loading. Fracture toughness calculation for CCNBD tests The stress intensity factors (SIF), K I and K II for the Cracked Straight-Through Brazilian Disc (CSTBD) specimen with a through notch length of 2α , can be expressed as [5]:       I Ι P K Ν π R B or     I Ι P K Y π R B (3a) where    I Ι Y Ν (3b)       II IΙ P K Ν π R B or     II IΙ P K Y π R B (4a) where    II ΙI Y Ν (4b) where P is the load applied in compression,  α / R a and Ι Ν , IΙ Ν are dimensionless coefficients that are functions of a and the notch inclination angle with respect to the loading direction, θ (Fig. 1b). Y I and Y II are the dimensionless stress intensity factors for CSTBD specimen. Fig. 2 compares the geometrical terms for CSTBD and CCNBD specimens. The intermediate crack length * α in CCNBD specimens varies between 0 α and 1 α and is regarded as equivalent of the crack length α in CSTBD specimens.