By Philippa Moore and Wee Liam Khor
Fracture toughness of ductile materials is characterised by a tearing resistance curve, or R-curve, of J or CTOD against crack extension. Current Standards vary in the CTOD R-curves resulting from the same test, and differ with the strain hardening properties of the alloy. CTOD R-curves determined to different Standards have been compared with silicone replicas taken from SENB specimens, and an improved CTOD R-curve formula is introduced for inclusion in future Standards.
- CTOD R-curve methods based on CTOD from J in ISO 12135 and ASTM E1820 gave lower and less accurate R-curves than the rigid rotation methods in BS 7448-4 and JWES, compared with measurements from silicone replicas.
- Accuracy of the BS 7448 Parts 1 & 4 formulae varied for the different materials, and was non-conservative for low tensile ratio steel.
- Inaccuracies in the BS 7448-1/ISO 12135 formula for CTOD in SENB specimens with nominal a0/W ratio of 0.5 over a range of tensile ratio steels can be mitigated by substituting the rotational factor of 0.4 with a value equal to 0.4668 (σys/σuts) + 0.0996.
- The best approach to determine CTOD for a wide range of alloys was offered by using this rotational function in combination with the JWES ‘m’ function in the calculation of the elastic component of CTOD.
- This approach can also improve the accuracy of the CTOD R-curve equation in BS 7448-4.
Definition of CTOD at the tip of an initial fatigue pre-crack in an SENB specimen deformed after testing