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

Two procedures 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.