Failure analysis of one main wheel hub bolt fracture of A320 aircraft

When an aircraft was performing flight tasks, it was found that one connecting bolt of No. 3 main wheel was missing during the short stop inspection. The bolt uses 148 ups and downs this time, and the use time is significantly lower than normal. It is inferred that the high probability of its fracture is not caused by long-term use, and it is speculated that the fracture reason is related to the quality of the bolt itself. Material analysis showed that there were no obvious abnormalities in the bolt material, and slight scratches were found near the bolt. The bolt is fatigue fracture and the fracture position is located at the transition arc position. Microscopic observation shows that there is a V-shaped groove on the side of the crack source area, but because the gasket is no longer in the installation position, it cannot directly determine the installation problem of the gasket. It is recommended to optimize the installation process and NDT (No Destructive Testing, NDT) detection process in the future.


Introduction
Screws and bolts are the most common types for industrial system, and the difference between them is only one of intended use [1].On May 1, 2023, one aircraft of China some Airlines performed flight mission, and found that one of the connecting bolts of the No. 3 main wheel was missing during a short stop inspection.About three months ago, the wheel hub of the 3# main wheel of the aircraft was removed from another aircraft due to tire wear to the mark.And two mouth ago, the main wheel's outer tire renovation, valve core replacement and NDT (No Destructive Testing, NDT) inspection of bolts were completed in the overhaul department of one Base.The wheel was installed on the aircraft about three weeks, and was replaced due to a broken hub bolt.The bolt at position No. 14 that is broken has a total usage cycle of 1687 take-off and landing from installation to fracture, and 148 take-off and landing are used in this installation.According to the time of four major repairs (1500 take-off and landing), its service life is about 28% (1687/(4×1500)), and it is inferred that the high probability of its fracture is not caused by long-term use.It is speculated that the reason of the fracture is related to the quality of the bolt itself.
In response to similar problems, domestic and foreign users and scholars have carried out extensive analysis and research [2,3], mainly focusing on the following three aspects, the first is the NDT detection research, the second is the structural mechanics analysis, and the last is the user reliability finite element analysis.For NDT detection research, Huang collected the data of Boeing 737 aircraft hub bolts, and found that even the existed that the NDT methods cannot found all the cracks around the bolts, the data of failure hub bolts increasing with longer of cycle of NDT testing [3].So, reduce the cycle will provide more reliable security.T.N.Chakherlou [4] using an experimental method found that, Increasing the tightening torque or clamping force on the double shear lap joints, leads to improved fatigue life.Junsheng Qu [5] The maximum load of bolt appears in the condition with an initial braking speed of 280 km/h.For the mechanical structure and materials research, Takao [6] investigated one Hawker 125-800XP jet, to determine the root cause of the failure of wheel, and found that intergranular region surrounded by a ductile field on the surface of fracture.And cracked nut that may have been embrittled by hydrogen during cadmium plating.Using finite element analysis [7], Yang Xianxue [7] found that, if the final tightening torque of the connecting bolt and nut is greater than 10% of the specified value [8], the connecting bolt and nut should be scrapped.In order to improve the accuracy of contact element simulation, Angelos [9] proposed an indirect boundary element technique to help investigators analyze the contact stress between bolts and wheel hubs.Robert [10] found that for high preloads the cracks arise in the contact zone between the nut and the sheet material.Additionally, the number of load cycles to failure increases with the preload.
In this case, the surface morphology of the fracture was mainly observed, and it was found that there were obvious fatigue damage characteristics on the surface of the fracture, the fatigue bands were relatively fine, and the crack originated from the transition arc position of the bolt nut.Because there is a certain stress concentration in this position, fatigue cracks often occur.However, the root cause of fatigue crack is still unclear.In order to find the root cause of failure, the fracture side, bolt material and metallographic structure are analysed.The results show that the bolt material has no obvious defect characteristics, but V-shaped scratches are found on the side of the main crack.According to the fracture characteristics and process characteristics, the V-shaped scratches may be formed when the bolt breaks.Therefore, the bolt fracture may be a high cycle fatigue fracture.

Installation structure and characteristics
The schematic diagram of the installation position of wheel hub bolts is shown in Fig. 1.

Macroscopic morphology analysis
The overall macroscopic morphology of the fracture of No. 14 wheel hub bolt is shown in Fig. 2. The section is V-shaped, the middle is concave, and both sides are protruding.In Figure 2, the area of the red rectangle box is relatively flat, and the area on the opposite side is relatively undulating.According to the above morphological characteristics, the upper protruding area in Fig. 2 May be a transient breaking area, and the position of the red rectangle box may be a crack early expanding area.The macroscopic morphology of the local location is shown in Fig. 2, from which it can be found that there is an obvious bright reflective region (bright region) in the early crack propagation region.The bright area of the crack early expansion area is due to the reflection caused by the local relatively flat, probably because in the early stage of the formation of the crack source, the crack growth rate is slow, and the upper and lower sections continue to have relative friction in the cracked area, forming a local flat wear surface.
The macro morphology of the side of the early extension area of the fracture is shown in Fig. 3.The contours along the fracture edge are basically consistent with the external transition arc, and there are a large number of V-row steps near the suspected source area, whose micro morphology needs to be further analysed and determined.The macro morphologies of other bolts at the same position are investigated too.Slight scratches were found at the root of nut of nearby bolt No. 15, as shown by the red arrow in the figure.On the adjacent 13 bolt, no scratch marks were found on the nut base.

Microscopic morphology analysis
The microscopic morphology of the early crack growth area in the section of bolt No. 14 is shown in Fig. 4A.The overall surface is relatively flat, and there are a large number of beach-like lines in the section, which is characteristic of fatigue crack growth.The microscopic appearance of the suspected crack initiation area is shown in Fig. 4B, in which the radial distribution streaks are faintly visible, and the radial distribution streaks converge to the edge of the bolt in reverse.The above morphological characteristics indicate that the fatigue crack originated from the edge position of the screw in Fig. 4B.However, due to the serious friction between the two sides in the early crack formation and propagation process, the microscopic morphology of the crack origin was almost completely worn, and the initial microscopic morphology could not be identified.The microscopic morphology of the section near the crack source area is shown in Fig. 5.In which, the red dotted line is the dividing line between the fracture surface and the original fracture area.It can be seen from the microscopic morphology that the scratches on the wear area are relatively thick and the wear plane is relatively flat.The micromorphology of the original fracture area is relatively fine.Fig. 6 is the original fracture topography photo at this location, from which it can be found that the striations with step-like distribution are fatigue bands, indicating that the early fracture form of the fracture is fatigue fracture.Fig. 6 shows the microscopic morphology of other locations in the crack source region (near the center of the fracture), in which step-like distribution of fatigue fringes can also be observed, indicating that the fatigue crack begins to expand from the initiation position in Fig. 6, and the expansion area extends to the midline of the fracture, that is, the expansion area of the fatigue crack accounts for more than 50% of fracture.The gap of early fatigue bands is small and the arrangement is dense.The microscopic morphology of the hub bolt in the transient fracture zone is investigated too, and the overall morphology is observation.The section fluctuation is large, and there are a large number of white edges and pits in the fracture.The above features are typical characteristics of dimple fracture.The characteristics indicate that the region involved that which is a transient fracture caused by stress overload.A large number of V-shaped grooves can be observed on the side of the bolt fracture.The V-shaped grooves are located near the crack source of the fracture.The topography of V-shaped grooves is shown in Figure 7. From the topography, V-shaped grooves may be friction or impact marks.In position, the V-shaped groove is located just near the transition arc position of the bolt.The cause of the formation of V-shaped grooves remains to be further analysis.

Metallographic analysis
The metallographic structure of the bolt metal material is shown in Fig. 8.The metallographic structure of the wheel hub bolt is flake or acicular martensitic structure with a small amount of ferrite structure.The size of acicular martensite is about 10 microns, and the structure is uniform.The boundary outline of the original austenitic structure is faintly visible, and the metallographic structure has no obvious segregation or impurities, and no obvious abnormalities.

The characteristics and root cause analysis of fracture location
The No. 14 bolt break occurs at the position of the transition between the nut and the screw, which has an R chamfer, and the break is in the chamfer position.This location has two characteristics.First, the stress distribution in this location forms a local stress concentration area, which is easy to initiate fatigue cracks.However, from the point of view of prevention, the position is covered behind the bolt, and ordinary visual inspection cannot find early fatigue cracks in this position.Secondly, historical failure cases show that if the gasket is installed incorrectly, the location is easy to form local damage points due to the intrusion of local sharp objects, inducing fatigue cracks.However, in this case, the gasket has been detached and lost, and it is not easy to establish a direct correlation between the initiation of fatigue cracks in the gasket.According to the microscopic features of the fracture side, before the fracture, the transition arc area may be due to the relative extrusion of other objects and bolts, forming local V-shaped grooves.Fatigue cracks start in the transition arc contour line and the edge of the V-shaped grooves, and gradually form penetrating cracks.Therefore, the formation of local V-shaped grooves is the cause of fatigue cracks induced in local areas, but is the formation of V-shaped grooves caused by gasket installation problems?This possibility exists and which was illustrated by Fig. 9, but other foreign bodies and dust stuck in this position may also form such local damage characteristics [6], so the current experimental data cannot directly form the above conclusions.There may be suspicion that the gasket alignment is reversed, but there are other possibilities that may cause V-shaped grooves in the transition arc position.

Correlation analysis between NDT inspection and crack prevention of wheel hub bolts
The maintenance records of the No. 14 bolt show that 148 lifts have been used since the NDT inspection.The data is obviously lower than the normal state.At present, the more effective preventive measure is NDT inspection.The methods that can find local cracks in wheel hub bolts include fluorescence detection, magnetic particle detection and eddy current detection.Due to the structural characteristics and stress state of the hub bolt and nut root, this position is the key inspection area.However, the fatigue crack initiation period of high strength materials is long and the occurrence stage is not obvious.There are some traces of use on the surface of the bolt, and there may be a possibility of missing detection of small defects, but the current experimental evidence cannot establish a correlation between the two.However, it is suggested that NDT should pay attention to the small cracks in the transition arc region of the nut position, focusing on the small cracks in the transition arc region parallel to the outer contour line.

Analysis of breakage of relevant wheel hub bolts
As the hub bolt is a high-strength material, affected by material technology, processing methods and other factors, its service process may be affected by impact, speeding, heavy landing and other factors [9].Therefore, at present, in the domestic fleet, the related wheel bolt breakage events occur.At present, some airlines will make decisions based on the decoded heavy landing data and tire overspeed data, combined with the wheel fracture data.

Conclusions
An airline A320 had a hub bolt break, but the hub bolt renewal time was about 1/4 of the refurbishment cycle.The bolt has been tested by NDT, and there is no abnormality.Obvious characteristics of fatigue MATMA-2023 Journal of Physics: Conference Series 2691 (2024) 012027 IOP Publishing doi:10.1088/1742-6596/2691/1/0120277 bands were found on the fracture surface, and the origin of the crack was located at the transition arc position of the nut, where fatigue crack initiation was possible due to local stress.Metallographic analysis showed that there was no obvious abnormality in the material of the bolt.A large number of Vshaped scratches were found on the side of the fracture, but the location of the scratches was different from the origin of the cracks.The analysis believes that it may be due to the friction or impact between the fracture edge and the wheel hub during the fracture process, resulting in local V-shaped grooves.The fracture of bolt No. 14 is a fatigue fracture, and the crack originates from the transition arc position of the nut root.

Recommendations
It is recommended that the relevant maintenance units strictly manage the quality and pay attention to the installation requirements of bolt gaskets.It is suggested that the aviation department further cooperate with relevant scientific research institutes to carry out relevant prevention and reliability analysis.

Figure 1 .
Figure 1.Wheel hub bolt mounting structure.Wheel hub bolts are used to connect the outer wheel hub and the inner wheel hub.There are gaskets in the position of bolt head and nut.Between the bolt nut and the screw, there is a transition arc to avoid stress concentration at the joint position of the nut and the screw.The breaking bolt is numbered as bolt

Figure 2 .
Figure 2. The macroscopic morphology of bolt fracture.

Figure 3 .
Figure 3. Macroscopic morphology of the fracture side.

Figure 4 .
Figure 4.The micromorphology of suspected crack source region.

Figure 5 .
Figure 5. Wear marks and fatigue bands near the crack source area.

Figure 6 .Figure 7 .
Figure 6.Local fatigue bands in the middle of the fracture.

Figure 9 .
Figure 9. Cause analysis of local V-shaped grooves in transition arc region.