Int J Fract (2013) 181:147–154 DOI 10.1007/s10704-013-9827-2 © Springer Science+Business Media Dordrecht 2013 LETTERS IN FRACTURE AND MICROMECHANICS 123 FRACTURE TOUGHNESS DETERMINATION FOR ALUMINIUM ALLOY 2011-T6 USING TENSILE NOTCHED ROUND BAR (NRB) TEST PIECES G.G. Vanian, A.K. Hellier*, K. Zarrabi and B.G. Prusty School of Mechanical and Manufacturing Engineering, The University of New South Wales, Sydney, NSW 2052, Australia * e-mail: a.hellier@unsw.edu.au, Corresponding Author Abstract. While the use of notched round bar (NRB) test pieces to determine plane- strain fracture toughness K Ic is not novel, relatively few authors have so far attempted this. This letter serves to report a series of such tests conducted on Al 2011-T6, a high mechanical strength, free-machining aluminium alloy. A total of 9 specimens was used, comprising 3 specimens each of 3 different notch root radii, ρ. No fatigue pre- cracking was carried out. A graph of apparent fracture toughness K ρ versus ρ / D was extrapolated to zero, corresponding to a fatigue pre-cracked configuration. The resulting K Ic for this alloy tested using the NRB specimens was thus found to be 27.9 MPa√m. This is a valid K Ic value, as there is only a 6.8% difference between it and the median value (26.0 MPa√m) determined previously from 67 tests using the existing ASTM E399 standard. A new geometric correlation based on ρ, D and d is proposed to linearly extrapolate K ρ values, measured on NRB specimens with arbitrary geometries (but assuming plane-strain conditions hold), back to a single value of K Ic . Keywords: Plane-strain fracture toughness K Ic , stress concentration factor (SCF) K t , aluminium alloy 2011-T6, notched round bar (NRB) test piece. 1. Introduction. Solutions to the stress concentration problem of a cylindrical bar with a circumferential V-shaped groove are mainly used in practice for the design of shafts. SCFs and SIFs are also important for test specimens used to investigate the fatigue strength of a metal (Nisitani and Noda, 1984; Noda and Takase, 2003). Following the 1984 paper, a circumferentially notched tension (CNT) test piece was developed by Stark and Ibrahim (1986) as a smaller plane-strain fracture toughness (K Ic ) test piece than is allowed by the current standards, ASTM E399 (2009) and E1820 (2011), which make use of either compact tension (CT) or single-edge-notched bend (SENB) test pieces. These standard tests are elaborate, complex and costly. Finite element (FE) studies on fatigue pre-cracked round bar specimens confirmed that the CNT specimen geometry, even if of a very small size, often ensures plane- strain conditions for a fracture toughness test. These FE studies indicated that the fatigue crack should be at least twice the Irwin plastic zone correction factor in depth; if shallower than that, the result tended towards the higher plane-stress result. In order that general yielding of the final ligament at fracture be avoided, the average stress across the ligament at fracture should not exceed 2.5 times the yield strength of the material. In the event that this limit is exceeded, then a larger specimen diameter would be required (Ibrahim and Stark, 1987). However, the stress concentration factor (designated as SCF or K t ) of the notch is not of critical importance, as it is only there to facilitate the growth of a uniform fatigue pre-crack under rotating-bending loading. Such specimens are subsequently tested in tension and have been used, for example, to investigate sustained-load fracture at stress intensity factors (SIFs) K I less than K Ic in the neck of aluminium 6061 and 6351 gas cylinders (Stark and Ibrahim, 1992). Londe et al. (2010) have used a very similar test piece, which they have dubbed the circumferentially cracked round bar (CCRB), to obtain a valid K Ic for Al 6082-T6.