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PREDICTION OF FATIGUE CRACK INITIATION ON DEFECTS IN WELDED AND CAST COMPONENTS USED IN POWER PLANTS

Objective


Framatome and UNIREC have developed a methodology for predicting crack initiation from the stand point of linear elastic fracture mechanics. The criterion is the stress calculated at a characteristic distance d from the tip of the defects. The methodology has been successfully applied to the cases of defects like blow holes or shrinkages in austenitic or ferritic castings or weld discontinuities like inclusions, lack of fusion or cracks in welded components. It appears that the case of large defects could be treated by this methodology. The case of coalescence within a cluster of defects can also be analysed. LBF has developed a method where the defects are defined as notches and the local stress concentrations are related by a reduction factor of the fatigue resistance to S-N curves of the sound materials.

The method has also been successfully applied to casting and welding joints. This approach is suitable for both small defects and large defects. Both approaches are complementary and are able to provide fair predictions of fatigue crack initiation for a wide range of defect types and sizes.

A calculation method based on the theory of elasticity has been developed to predict fatigue life to crack initiation in welds and castings containing defects. Depending on the geometry of the actual defect an appropriate substitution geometry can be selected. Hence the stress concentration factor Kt and the fatigue notch factor Kf are calculated. The level of the S-N curve of the sound material state is then reduced by the factor Kf. Fatigue life to crack initiation can then be read from this new curve which represents the reference curve for thematerial with defect. As the level of the defect free material state was not known, for each specimen the actual nominal stress amplitude was transformed into a value representing the defect free material state by multiplying the nominal stress by Kt. The combination of all results after transformation will yield the defect free material properties, ready for future applications. For any defect the number of cycles to crack initiation can be obtained from a reference curve that is derived from the defect free material curve after a division by the fatigue notch factor Kf. Subsequently the remaining crack propagation life can be calculated using an appropriate crack propagation law.
AT THE PRESENT TIME FATIGUE LIFE ASSESSMENTS OF CRITICAL POWER STATION COMPONENTS WHICH CONTAIN DEFECTS OR SHARP GEOMETRICAL DISCONTINUITIES ONLY TAKE INTO ACCOUNT THE LIFETIME UNDER CRACK PROPAGATION CONDITIONS. HOWEVER, A SIGNIFICANT FRACTION OF OPERATIONAL LIFETIME IS SPENT UNDER CONDITIONS OF CRACK INITIATION.
THE PRINCIPAL OBJECTIVE OF THE PROJECT IS TO DEVELOP AN ENGINEERING METHOD CAPABLE OF ACCURATELY PREDICTING FATIGUE CRACK INITIATION FROM DEFECTS OR SHARP GEOMETRICAL DISCONTINUITIES IN WELDED AND CAST COMPONENTS USED IN POWER PLANTS.
THIS PREDICTIVE METHOD WILL ENABLE DECISIONS TO BE TAKEN DURING THE OPERATION OF A POWER PLANT ON WETHER OR NOT A DEFECT DISCOVERED IMMEDIATELY BEFORE OR DURING SERVICE REQUIRES REPAIRING AND, IF NOT, HOW MUCH REMANENT LIFETIME EXISTS BEFORE SUCH AN UNREPAIRED DEFECT WOULD BECOME CRITICAL.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

SOCIETE FRAMATOME
Address
Tour Framatome
92084 Paris La Defense
France

Participants (2)

FRAUNHOFER GESELLSCHAFT ZUR FORDERUNG DER ANGEWANDTEN FORSCHUNG E.V.*
Germany
Address
Bartningstrasse 47
64289 Darmstadt
Unirec
France
Address
9/4 Place De La Pyramide
92800 Puteaux