DEVELOPMENT OF TEST GUIDANCE FOR COMPACT TENSION FRACTURE TOUGHNESS SPECIMENS CONTAINING NOTCHES INSTEAD OF FATIGUE PRE-CRACKS

摘要

Structural integrity assessment codes such as R6 and BS7910 provide guidance on the assessment of flaws that are assumed to be infinitely sharp. In many cases, such as fatigue cracks, this assumption is appropriate; however it can be pessimistic for flaws that do not have sharp tips such as lack of fusion, porosity or mechanical damage. Several methods have been proposed in the literature to quantify the additional margins that may be present for non-sharp defects compared to the margins that would be calculated if the defect were assumed to be a sharp crack A common feature of these methods is the need to understand how the effective toughness, characterised using the J-integral for a notch, varies with notch acuity. No comprehensive guidance currently exists for obtaining J experimentally from specimens containing notches; hence the typical approach is to use equations intended for pre-cracked specimens to calculate J for notched specimens. This paper presents a comprehensive set of test guidance for calculating J from Compact Tension (CT) fracture toughness specimens containing notches instead of fatigue pie-cracks. This has been achieved using 3D Finite Element Analyses to quantify the accuracy of formulae intended for pre-cracked specimens in fracture toughness testing standards ASTM E1820, BS7448-1 and ESIS P2-92 when applied to specimens containing notches. The paper quantifies the accuracy of these equations for notched CT specimens and identifies the conditions under which the equations can lead to inaccurate measurement of J for notched specimens.