标准编号：ISO 22889:2007
中文名称：金属材料 用低约束样品测定抗稳定裂纹扩展的试验方法
英文名称：Metallic materials — Method of test for the determination of resistance to stable crack extension using specimens of low constraint
发布日期：2007-12
标准范围
This International Standard specifies methods for determining the resistance to stable crack extension interms of crack opening displacement, δ5, and critical crack tip opening angle, ψc, for homogeneous metallicmaterials 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 slowlyincreasing displacement.This International Standard describes methods covering tests on specimens not satisfying requirements forsize-insensitive fracture properties; namely, compact specimens and middle-cracked tension specimens inrelatively thin gauges.Methods are given for determining the crack extension resistance curve (R-curve). Point values of fracturetoughness for compact specimens are determined according to ISO 12135. Methods for determining pointvalues of fracture toughness for the middle-cracked tension specimen are given in Annex D.Crack extension resistance is determined using either the multiple-specimen or single-specimen method. Themultiple-specimen method requires that each of several nominally identical specimens be loaded to aspecified level of displacement. The extent of ductile crack extension is marked and the specimens are thenbroken open to allow measurement of crack extension. Single-specimen methods based on either unloadingcompliance or potential drop techniques can be used to measure crack extension, provided they meetspecified accuracy requirements. Recommendations for single-specimen techniques are described inISO 12135. Using either technique, the objective is to determine a sufficient number of data points toadequately 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 aresistance curve, which has been shown to be unique within specified limits of crack extension. Beyond thoselimits, δ5 R-curves for compact specimens show a strong specimen dependency on specimen width, whereasthe 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 constantvalue achieved after a certain amount of crack extension. The CTOA concept has been shown to apply to verylarge 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 wellestablished and can be applied to structural integrity problems by means of simple crack driving forceformulae from existing assessment procedures.The CTOA concept is generally more accurate. Its structural application requires numerical methods, i.e. finiteelement analysis.Investigations have shown a very close relation between the concept of constant CTOA and a unique R-curvefor both compact and middle-cracked tension specimens up to maximum load. Further study is required toestablish analytical or numerical relationships between the δ5 R-curve and the critical CTOA values.
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