numero26

S. Foletti et alii, Frattura ed Integrità Strutturale, 26 (2013) 123-131; DOI: 10.3221/IGF-ESIS.26.12 124 European FITNET procedure [3]. Each of these procedures uses a combination of continuum mechanics and fracture mechanics concepts to make a creep loading assessment. The information required for the assessment is: - the operating conditions; - the geometry of the defect; - material data; - structural analysis in order to transfer material data to the behaviour of complex structures. When such procedures are used at the design stage, the size of postulated defects is determined by the use of non- destructive inspection methods. Otherwise, the information may be used to assess whether a defect of a given size will grow to an unacceptable size under a given exposure time to high temperature. For the evaluation of the crack initiation the use of Two Criteria Diagram (2CD) promised to be the appropriate way to treat the problem of different types of damage behaviour in components [4]. Inside the no damage zone of Fig. 1 highlighted in green, cracks do not initiate and propagate while inside the damage zone highlighted in red, crack initiation and propagation is possible. In the 2CD [5] for creep crack initiation the nominal stress  n,pl considers the stress situation in the ligament while the ideal elastic stress intensity factor K Iid , describes the condition at the crack tip, at a time zero. The loading parameters (  n,pl , K Iid ) are normalized by the respective time and temperature dependent data, which indicate the strength of the material against crack initiation.The normalized parameters are, for the stress in the ligament, the stress ratio:     n pl R R (1) and, for the crack tip, the stress intensity ratio  I id Ii K R K K (2) The value  R is the creep rupture strength of the material while the parameter K Ii characterizes the creep crack initiation of the material. These parameters shall be determined from creep rupture tests on cylindrical specimens and from Creep Crack Growth (CCG) tests on CT specimens respectively. While R k considers the presence of a defect in the component, R σ just take into account the creep effects on smooth specimens away from possible defects. The 2CD defines three mechanisms of damage responsible for crack initiation. A low value of the ratio K Iid /  npl indicates the beginning of the propagation due to the widespread damage in the resistant section behind the crack tip, a high ratio K Iid /  npl indicates the initiation of the propagation due to localized damage of the crack tip, while between these extremes a transition zone indicates initiation due to a mixed damage mode. The two criteria method described has been developed as a way of practice to transfer creep crack initiation data, from specimens with different size to larger components with similar characteristics and crack tip situation, and its applicability has been well proved [6]. Figure 1 : Prediction of creep crack initiation time by using 2CD.