Issue 24

A. A. Shanyavskiy, Frattura ed Integrità Strutturale, 24 (2013) 13-25; DOI: 10.3221/IGF-ESIS.24.03 17 section of a disk. As the crack propagated, plastic deformation of the disk occurred to cause a 35-mm crack-opening displacement, next to which final fracture occurred over the two different radial sections. Finding the material to fit assigned mechanical properties indicated that the LCF cracks developed in the discs stressed as high as to exceed their working capacity. Therefore, the problem arose to make use of the actual durability with the still impossible failure of the disk due to the growth of LCF cracks. In fact, this was the problem of running engines with the disks tolerably damaged. We solved successfully this problem owing to the complex investigations that included quantitative fractography as a way of estimating the growth period of fatigue cracks. We examined visually the fracture surface of the opened cracks to find them initiated in the disk-hub body at the surface of one or several fastening holes on the central-hole side. The cracks propagated along the disk radius toward the central disk hole, Fig. 4. Once a crack grew through, it began to grow toward the disk rim with a danger of disk fracture along its radius. The fractures colored golden formed between the fasting holes, used to bolt the disk to the turbine shaft, and the central disk hole. The fracture morphology indicated that the crack grew transgranularly, typical of superalloys fatigue cracking. Each of the disks had most strongly oxidized zones, arranged closer to the end of the hub. This zone was indicative of quite a long propagation period of the initially non-through semi-elliptic crack (see Fig. 4b). The web of the disks, broken in service, showed signs of transgranular cracking of a cyclic nature over the region 63 to 65 mm long between the bolt-joint hole and the rim. These cycles left behind a sequence of fracture zones oxidized to different degrees. Elliptic fatigue lines so-called beach marks (BM) bordered these four zones (see Fig. 4c). Similar trends were revealed by the fracture surface of Stage-I high-pressure-compressor disk, made of titanium alloy VT3-1, in a D-30 engine: here zones of different roughness formed in the disk web [12]. A distance between two neighboring zones (each identified with BM) of different roughness answered to crack propagation during one flight. So in the above turbine-disk case, we considered the whole pattern of crack-propagation from the bolt-joint hole to disk rim as formed during five flights, with the last executive start of the engine in that number. In both turbine disks, the two remaining fractures were of a mixed type (intergranular plus transgranular) with the signs of macroscopic plastic deformation all over the fracture boundaries; such a fracture pattern is typical of superalloys subjected to short-term static loading. An important finding was that, in both disks, each of the short-term fractures revealed a portion of fatigue cracking, which, similar to the general long-term fatigue fracture, began by a bolt-joint hole. It was indicative of a multiple fatigue cracking from those holes in both of the in-service failed disks. Those initial portions of fracture were also transgranular and oxidized to golden-gray color. Their semi-elliptic boundary with the area of final shirt-term fracture was distinct, typical of the case that an abrupt increase of applied load occurred as soon as the general fatigue crack grew to its critical size. The general fatigue cracks grew to the dimensions of 2c = 6.5 and 2c = 1.2 mm at the disk hole surface and a = 3.0 and a=0.3 mm in depth direction for the P-1 and P-2 disks, respectively. The crack grew to sequentially form four fracture zones of respectively changed dominating fracture mechanism that was indicated by the difference in fracture surface patterns. Zone I (see Fig. 4) reveals gradual growth of the crack from the disk hole, predominantly by forming fatigue striations, Fig. 5. The striation spacing increases along the minor ellipse axis, toward disk-body depth. The smallest striation spacing was 0.5 and 0.3  m for discs P-1 and P-2, respectively. Both values of spacing indicate to low-cycle fatigue conditions of the crack initiation, i.e., to a stress level as high as or higher than the yield strength of the material. A special feature is that the striation spacing increased linearly with crack growth to achieve 1.2  m by the crack-penetration depth of about 1.2 mm, where the second fracture zone began in both broken disks. In the open fractures, striation spacing amounted to 0.7 and 1.2  m for the greatest crack depths of 0.3 and 1.2 mm, respectively. All cracks revealed similar laws of increasing the striation spacing to obviously indicate that both disks experienced an about equally high level of stressed state around the holes. Designing disks in terms of durability takes into account effect of protractedly holding them mechanically loaded when starting and shutting down the engine. With such holding under LCF-conditions, thermally activated plastic deformation and fracture increase the probability that slowly-going damage of grain boundaries and subboundaries by grain-boundary sliding and vacancy fluxes comes to a completion. Consequently, fracture becomes a mixed type (transgranular plus intergranular) of predominantly intergranular. The fracture surface revealed fatigue striations to confirm that, with a moderate thermal tension of the disks, the intergranular mode of fracture was suppressed by the transgranular mode and slip. Transgranular slip brings about vigorous cracking of the material and, thereby, hinders from the formation of fatigue striations. Consequently, fatigue striations coexist with cracked regions of the fracture surface. Zone II of further crack propagation (see Fig. 5) predominantly reveal fracture facets with the steps of intensive transgranular slip and hardly visible dimpled regions. Besides, the dimples look quire shallow. Such an abrupt transition to