3.2 Structural components of the aircraft wing
The studied fragments of four cracks were numbered by No. 1 (crack length of 21 mm), No. 2 (crack length of 96 mm), No. 3 (crack length of 23 mm) and No. 4 (crack length of 74 mm) (Fig. 8). On three fragments of fracture surfaces (No. 2, 3, and 4), the fracture origins are located on the inner surface of the holes at a distance of about 1 mm from their edges; on the fragment of fracture surface No. 1, the origin is located at the edge of the hole.
Origins of cracks No. 3 and 4 have pronounced features of the effect of contact loads on the hole surface that led to the formation of intense fretting products and a partial mechanical damaging of the crack initiation zones. The cracks were initiated by the fretting corrosion process. The occurrence of cracks in both holes was caused by the high-stress concentration, which was associated with a high level of contact loads (for cracks No. 1 and 2) and with a fretting corrosion process over the hole surface (for cracks No. 3 and 4).
Crack propagation had a fatigue nature and belonged to the high-cycle fatigue regime. A characteristic feature of crack propagation across all four fracture surfaces was the formation of MBM and fatigue striations with different spacing at different stages of growth (Fig. 9). The revealed features of fracture relief formation were used to estimate the duration of crack growth in terms of the number of blocks “n f” for each cracks studied (Fig. 10).
From the results of evaluating the crack growth duration, it follows that the formation of cracks in the stiffened panel in the area of the bolt hole for the root bracket of the engine pylon is primary in regard to cracks formed in the zone of the bolt hole for the middle bracket of the engine pylon.
The obtained estimates of the crack growth duration in all studied lug sections are demonstrated in Table 2.
Therefore, it can be concluded that loading blocks (almost half of 20487 blocks) were associated with the successive nucleation of fatigue cracks in different areas of the structure and their propagation for at least 10000 blocks.