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The appraisal of high temperature structural integrity and design and life extension of engineering components, using fracture mechanics methodologies, is dependent on available material properties derived from short term tests. The scatter attributed to fabrication, geometry, testing, material creep properties could vary by up-to a factor of ±2.5. The parameter C* adopted in ‘The standard test method for measurement of creep crack growth rates in metals’ (ASTM STP E1457-92) and also in the R5 [1–2] and A16 [3–4] crack growth assessment procedures, can vary by a further factor of ±3.3 using differing experimental methods for its estimation. The NSW [5–6] model which describes cracking behaviour over the plane stress/strain range identifies this variability in terms of crack tip constraint. It is concluded from the NSW model (Nikbin. Smith and Webster [5,6]) that for creep ductile materials a factor of 25 in crack growth rates would bound the creep cracking data and an additional factor of up-to 10 would be added when transient and initiation behaviour prior steady crack growth were to be included in the life predictions.
Fracture Mechanics, Creep, Crack Growth, Constraint, Life Assessment, Scatter, Structural Integrity, K, C, *
Imperial College, London,