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BEHAVIOUR OF A 1Cr‐1Mo‐0.25V STEEL AFTER LONG‐TERM EXPOSURE‐II. CREEP CRACK INITIATION AND CREEP CRACK GROWTH
Author(s) -
Moliniéa E.,
Piques R.,
Pineau A.
Publication year - 1991
Publication title -
fatigue and fracture of engineering materials and structures
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.887
H-Index - 84
eISSN - 1460-2695
pISSN - 8756-758X
DOI - 10.1111/j.1460-2695.1991.tb00682.x
Subject(s) - creep , materials science , fracture mechanics , casing , growth rate , crack growth resistance curve , displacement (psychology) , crack closure , fracture (geology) , mechanics , turbine blade , composite material , structural engineering , turbine , thermodynamics , geology , geometry , mathematics , physics , engineering , psychology , psychotherapist , petroleum engineering
Both the initiation and the propagation of creep cracks have been studied in a 1Cr‐1Mo‐0.25V steel at 550°C using CT type specimens. The material taken from a large turbine casing was investigated in two conditions: (i) unaged and (ii) after a long exposure in‐service time of about 150,000 h at 540°C. In both cases the material was found to be creep ductile, which is justified in terms of fracture mechanics applied to creeping solids. It is shown that fracture mechanics is unable to provide unique correlations with global load‐geometry parameters, either K or C * for all the stages of both crack initiation and crack growth. However there exists a unique correlation between C * and the time to initiation, t i . This correlation does not depend on the initial conditions of the material. During crack growth two stages are defined. Stage I is a transient regime in which C * is almost constant, but the correspondence between the crack growth rate and C * is not unique since largely dependent on the initial loading applied to the specimens. It is shown that the apparent correlation between the crack propagation rate in stage II which corresponds to large crack growth rate is doubtful. A simplified method based on reference length and reference stress is used to calculate C * parameter and to simulate the load‐line displacement rate. The results obtained from this method are compared to those derived from finite element calculations published in the literature.