PSI - Issue 7

Igor Varfolomeev et al. / Procedia Structural Integrity 7 (2017) 359–367 Igor Varfolomeev et Al./ Structural Integrity Procedia 00 (2017) 000–000

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1. Introduction Since forging defects cannot be excluded during the fabrication of rotor discs, the components are examined with respect to the presence of flaws by ultrasonic testing (UT) before their commissioning. The acceptance of flaws detected by UT is then assessed by means of fracture mechanics using appropriate flaw characterization rules and, if applicable, proximity rules for groups of defects, see e.g. BS 7910 (2013), FITNET (2008), FKM (2009). Thereby the flaws are conservatively considered as cracks, while their propagation is estimated using fatigue crack growth curves derived in tests on standard fracture mechanics specimens, ASTM E647 (2015). This approach is overall conservative, since i) the initial flaw geometry is usually determined as an envelope of the UT indication, and ii) the flaw is immediately assumed as a sharp crack. In particular, the latter aspect seems to introduce a considerable level of conservatism, as neither the stage of crack initiation from volumetric defects nor formation of an envelope crack due to the coalescence of multiple microcracks are considered in the integrity assessment. This study aims at investigating the initiation and propagation of fabrication flaws in a high-performance rotor steel under fatigue loading. For this purpose, material blocks containing fabrication defects are examined by ultrasonic testing to determine the size and location of the flaws. Subsequently, test specimens are extracted from the material blocks in such a way that the UT indications are located in the middle part of the cross-section. The specimens are then tested under tensile cyclic loading with stress amplitudes representative of service conditions. To trace back the crack propagation, beach marks are produced by altering the stress ratio and/or stress magnitude. This test procedure together with fractographic examinations of the fracture surfaces allow for estimating the number of cycles and the respective stress magnitude required for a sharp crack to initiate from a defect field. Subsequently, a numerical framework is derived for predicting the crack initiation phase. It employs static and cyclic material properties of the matrix (i.e. defect-free) material, statistical distributions for the defect size and distance between neighboring defects, as well as a material model which describes both cyclic hardening and fatigue damage evolution in the matrix material. Nomenclature 2 minor axis of an ellipse describing the defect shape, mm 2 major axis of an ellipse describing the defect shape, mm diameter of a circular-shaped defect, mm

damage parameter, − elastic modulus, MPa 0 yield strength, MPa EFBH equivalent flat bottom hole LCF low cycle fatigue UT ultrasonic testing

2. Experimental program 2.1. Overview

For this study, three material blocks containing natural forging defects have been provided. The defects have been first indicated by means of ultrasonic testing and characterized in terms of the size of an equivalent flat bottom hole (EFBH) which usually correlates with a crack size adopted in fracture mechanics calculations, see e.g. FKM (2009). Further UT measurements using the phased array technique were then performed at the Fraunhofer IZFP, Saarbrücken, to more precisely estimate the defect size, orientation and location. Fig. 1 shows the UT indications for each material block. It can be seen that the defects in the blocks 1 and 3 are concentrated within a compact area, whereas the block 2 contains a defect field with several non-resolvable indications. In all cases, the defects are located close to the center of the respective material block.