German Engineering Methods for Critical Crack Size Assessment in Ductile Regime Based on Envelop Flow Stress

Abstract

The purpose of this paper is to present engineering methods used in Germany for integrity assessment of structural components with a flaw in the ductile regime. The methods are validated by more than 1500 experiments and cover through wall crack as well as part-through wall cracks (in either axial or circumferential orientation) located in base material, heat affected zone, homogeneous or dissimilar metal welds. By fulfilling the toughness requirement, the structural integrity of a cracked component in the ductile regime can be assessed conservatively (safely) by considering simplified methods for which only strength and impact energies values are needed. The physical and mathematical background of the methods for circumferential cracks (flow stress concept FSC; plastic limit load PLL) and for axial cracks (Battelle approach BMI; Ruiz approach RUIZ), as well as the historical connection with the German basis safety concept will be reviewed. Similarity with other international methods (like R6, Section XI of ASME Code, SRP 3.6.3) will be emphasized. The range of validity resulting from experimental investigations (experimental database built from own research or from literature) is summarized and the validation procedure of the methods (FSC, PLL, BMI, RUIZ) is explained. The resulting correction factors on flow stress (envelop flow stress) to account for elastic-plastic fracture conditions and to result in conservative (safe) predictions of crack instability depending on the method (FSC, PLL, BMI, RUIZ), crack orientation (circumferential and axial) and type of material (ferritic and austenitic) is given. The ratio of the computed experimental to the theoretical stress gained by these several engineering methods (German flow stress based simplified methods and other international methods) will be presented on the basis of the experimental database (with a various combination of geometry, materials and loading combination) which covers also elbows and vessels. Recommendations on the good use of the methods for ductile integrity assessment are given including also dissimilar metal welds consideration.