Fracture mechanics testing on specimens with low constraint––standardisation activities within ISO and ASTM

Fracture mechanics tests are traditionally designed to measure material resistance to stable or unstable crack extension using specimens that are highly constrained to plastic deformation. For a variety of reasons, structural members may be made of thin gage-materials with inherently low constraint to plastic deformation. There is currently little guidance for measuring crack extension resistance under such conditions. The international standards organisations ISO and ASTM are responding to that need, and this paper describes one aspect of their current activity. ocedures are being developed; one based on the δ5 crack opening displacement parameter, the other on the constant value of the crack tip opening angle, ψc. The measurement of δ5 is well established and relatively simple, whereas ψc is more difficult to determine experimentally. Evaluations of ψc from finite-element analyses are currently the most accurate approach, since measurements can only be made on the exterior surfaces similar to δ5. Questions naturally arise regarding the correspondence of surface indication with full-thickness response in the laboratory experience. Both measures of crack extension resistance are suitable for structural assessment. The δ5 concept is applied by means of crack driving force formulae from existing assessment procedures and hence relatively easy to use. On the other hand, the CTOA concept is potentially more accurate and can be applied to cases of multiple cracks and complex structures. But its structural application requires numerical methods, which have been successful in predicting the failure of large-scale cracked structural components.