Issue 35

G. Dhondt et alii, Frattura ed Integrità Strutturale, 35 (2016) 108-113; DOI: 10.3221/IGF-ESIS.35.13 110 Figure 2 : LCF-Mission augmented by HCF-cycles The first one is for the mission augmented by the eigenmodes at the specified locations (Fig. 2). This corresponds to a LCF calculation for an extended mission. The usual procedure is followed involving the determination of a dominant loading point to determine the crack propagation direction, the reduction of the mixed-mode K-values to an equivalent K-factor, calculation of the crack propagation of each loading point separately, cycle extraction on the resulting curve and evaluation of the crack propagation of each extracted cycle based on the maximum cycle temperature (for details the reader is referred to [8]). The crack propagation increments from each extracted cycle are summed. Including HCF will frequently lead to more crack propagation, since a HCF cycle at a maximum point of a mission will increase the equivalent K-factor. The second and third calculation concerns the eigenmode itself, centered at the appropriate loading point (Fig. 3). Also here, the usual cycle extraction routines are applied to short missions consisting of the loading point plus the eigenmode and the loading point minus the eigenmodes. Since the calculations are linear the K-factors can be summed appropriately. For the pure HCF-evaluation the criterion is that no propagation should occur. Indeed, the HCF frequency is usually so high that propagation results in immediate failure. Figure 3 : HCF cycles. E XAMPLE simple example is presented in the form of an imaginary blade (Fig. 4) subject to centrifugal loading. Only two loading points are considered, full power and zero loading. An initial quarter circular crack of with radius 0.4 mm is inserted at the location of maximum principal stress about 25 mm above the disk. The orientation of the crack plane was orthogonal to the maximum principal stress, acting in radial direction. At first a calculation consisting of 50 iterations of CRACKTRACER3D was performed without HCF. The mesh in the last calculation is shown in Fig. 5. One can clearly see the domain in which the mesh was modified in order to accommodate the crack. At the crack tip a focused A