Matériaux métalliques — Méthode d'essai pour la détermination de la résistance à la propagation stable de fissures au moyen d'éprouvettes à faible taux de triaxialité des contraintes

ISO 22889:2013 specifies methods for determining the resistance to stable crack extension in terms of crack opening displacement, δ₅, and critical crack tip opening angle, ψ_c, for homogeneous metallic materials by the quasistatic loading of cracked specimens that exhibit low constraint to plastic deformation. Compact and middle-cracked tension specimens are notched, precracked by fatigue, and tested under slowly increasing displacement.

ISO 22889:2013 describes methods covering tests on specimens not satisfying requirements for size-insensitive fracture properties; namely, compact specimens and middle-cracked tension specimens in relatively thin gauges.

Methods are given for determining the crack extension resistance curve (R-curve). Point values of fracture toughness for compact specimens are determined according to ISO 12135. Methods for determining point values of fracture toughness for the middle-cracked tension specimen are given.

Crack extension resistance is determined using either the multiple-specimen or single-specimen method. The multiple-specimen method requires that each of several nominally identical specimens be loaded to a specified level of displacement. The extent of ductile crack extension is marked and the specimens are then broken open to allow measurement of crack extension. Single-specimen methods based on either unloading compliance or potential drop techniques can be used to measure crack extension, provided they meet specified accuracy requirements. Recommendations for single-specimen techniques are described in ISO 12135. Using either technique, the objective is to determine a sufficient number of data points to adequately describe the crack extension resistance behaviour of a material.

The measurement of δ₅ is relatively simple and well established. The δ₅ results are expressed in terms of a resistance curve, which has been shown to be unique within specified limits of crack extension. Beyond those limits, δ₅ R-curves for compact specimens show a strong specimen dependency on specimen width, whereas the δ₅ R-curves for middle-cracked tension specimens show a weak dependency.

CTOA is more difficult to determine experimentally. The critical CTOA is expressed in terms of a constant value achieved after a certain amount of crack extension. The CTOA concept has been shown to apply to very large amounts of crack extension and can be applied beyond the current limits of δ₅ applications.

Both measures of crack extension resistance are suitable for structural assessment. The δ₅ concept is well established and can be applied to structural integrity problems by means of simple crack driving force formulae from existing assessment procedures.

The CTOA concept is generally more accurate. Its structural application requires numerical methods, i.e. finite element analysis.

Investigations have shown a very close relation between the concept of constant CTOA and a unique R-curve for both compact and middle-cracked tension specimens up to maximum load. Further study is required to establish analytical or numerical relationships between the δ₅ R-curve and the critical CTOA values.