3.1 Mount fitting of the top rudder drive
Fracture origins No. 1 and No. 2 are located at the distance of about 1 mm from the edge of the bolt hole, while fracture origins No. 3 and No. 4 are located on the edges of the holes (Fig. 4). At an insignificant distance from the fracture origins in lug sections No. 1 and No. 2 the products of fretting process are observed on fracture surfaces. The products are formed as a result of contact interaction of the propagating crack edges. Therefore, the crack opening along the lug sections was insignificant, and the crack growth rate was low and corresponded to the first stage of the kinetic diagram – Paris curve. According to the existing concepts of the fatigue crack kinetics, at this stage the crack growth rate is less than 40 nm/cycle.
Therefore, the load sequence and interaction under variable amplitude loading conditions has a significant effect on the crack retardation [17]. It belongs to the short crack development process, that is the longest part of the total lifetime of structural component spent for nucleation and the stage of initial fracture. As in the considered case the fracture relief elements were not formed with any regularities, the duration of fracture process development discussed below excludes the stage of short-crack propagation corresponding to the initial crack length of 1 mm.
On fracture surface No. 1, the meso-beach-marks (MBM) were formed along the entire direction of crack growth from the hole towards the opposite surface of the lug, while in all other sections the blocks of fatigue striations were revealed (Fig. 5).
Therefore, the spacing of the MBM “h ” on fracture surface No. 1 and the fatigue striation spacing “δ” on the fracture surfaces No. 2-4 were measured, respectively (Fig. 6).
After comparing the step of the loading blocks per flight identified by the MBM spacing (fracture surface No. 1) with the fatigue striation spacing discovered in other sections of the lugs it was established that fracture surface No. 1 was formed first, but not through total section, when cracks in the remaining sections began to propagate. However, an insignificant portion of the fast bracket fracture in all lug sections indicates that, as the crack propagated, the load was redistributed and the bracket was finally fractured at a low-stress level. It is worth noting that when the crack length reaches about 8 mm in lug section No. 3, the crack growth rate is decreased abruptly as compared to the lug section No. 4. Such behavior of the material may be resulted from a change in the loading intensity of the bracket because of to the redistribution of loads due to the fact that the crack in lug section No. 4 began to propagate later than in lug section No. 3. This fact is confirmed by the performed estimation of the crack increment per a unit loading cycle or block as a function of the crack length for both fracture surfaces No. 3 and No. 4 (see Fig. 6).
The assessment of the crack growth duration showed that the number of blocks “n f” during fracture of section No. 1 was about 4000, and through sections No. 2-4 the numbers of unit cycles “N f” were 50000, 97000 and 29000, respectively (Fig. 7).