Out-of-plane constraint loss in three point bend specimens with notches

This paper presents an experimental and numerical study of the effect of specimen thickness on the effective notch toughness K_mat ^ρ for cleavage fracture measured using Single Edge Notch Bend (SENB) specimens containing a U-notch instead of a fatigue pre-crack. These specimens are typically used to measure a material's effective notch toughness K_mat ^ρ and to assess failure of a structure containing a non-sharp defect using the Notch Failure Assessment Diagram (NFAD). Both the experimental data and the Finite Element (FE) failure predictions show a significant influence of specimen thickness on K_mat ^ρ, over and above the microstructural weakest link effect arising from differences in the volume of the plastic zone. K_mat ^ρ is a function of not only the in-plane effect of the notch radius, but also an out-of-plane constraint loss which itself is enhanced by the presence of the notch radius. Significant out-of-plane constraint loss occurred for notched specimens with a ratio of thickness B to width W of 0.5, a geometry that if pre-cracked would have met the minimum thickness requirement mandated by ASTM E1820. Doubling the thickness to B/W = 1.0 was sufficient to eliminate the out-of-plane constraint loss. The use of experimentally measured K_mat ^ρ values in an NFAD assessment of a structure may therefore be non-conservative if B/W<1.0.