Failure analysis of right-front wheel hub rim of nose gear of a cargo aircraft

Following a post-flight inspection of a cargo aircraft upon landing, it was discovered that the right front wheel hub rim of the nose gear had experienced a fracture. In order to determine the cause behind this fracture, a comprehensive analysis was conducted on the hub, encompassing macro-micro-morphological observation, energy spectrum analysis, microstructure examination, and mechanical testing. The findings revealed the presence of a lengthy oxide defect at a stress concentration point located on the inner corner of the hub rim. Fatigue cracks emerged at this defect site, exhibiting signs of fatigue growth, ultimately leading to high-cycle fatigue fracturing at the hub rim position. It is suggested that quality control should be strengthened during the production process of wheel hubs to avoid defects forming. Further inspection of the transition position of the wheel flange during maintenance and repair is needed to find the crack as soon as possible.


Introduction
The aircraft wheel assembly serves as the main load-bearing component during crucial phases such as taxiing, take-off, and landing.Among these, the landing process subjects the wheel assembly to immense impact forces and thermal stresses generated by the brakes.The hub, being a vital element of the wheel assembly, plays a pivotal role in ensuring aircraft safety [1,2] .
Scholars at domestic and abroad have carried out in-depth and extensive research on the fracture of the aircraft hub.Nauman A used finite elemental analysis to identify the stress concentration regions nearly at the same positions of the wheel hub from where it failed in reality.Overloading was a cause of the final failure with visible features of chevron marks pointing back in the direction of the crack originating site [3] .According to the stress state of the wheel hub, multi-axial stress analysis [4] and finite element method [5] are common technical means in recent years.
The external assembly and materials manufacture is also one of the important reasons that can easily lead to failure.B. Kosec [6] revealed a non-destructive examination of the Slovenian air carrier Adria Airways during regular control by the maintenance unit.It was found that the crack on the rim investigated was a typical fatigue crack.It was branched with a size lower than the critical size for fast fracture.Takao Kobayashi et al. [7] investigated a failed wheel assembly from a Hawker 125-800XP corporate passenger jet. the root cause of the wheel assembly failure was a cracked nut that may have been embrittled by hydrogen during cadmium plating.Finally, Non-Destructive Techniques (NDT) were an efficient method to pre-found the failed points of the wheel hub.Recommendations were issued to improve the Non-Destructive Techniques (NDT) used to monitor the wheel's structural integrity.Laboratory tests showed that ultrasounds in this case would be far more efficient than the prescribed eddy currents [8] .
In this case, following the landing of a cargo aircraft, a thorough inspection revealed that half of the hubs in the right front wheel assembly had experienced rim fractures and separation.Further examination of the right-front wheel assembly showed that, apart from the rim fractures, the remaining structure and components remained intact.The tire air pressure was within the normal range, and no abnormalities were detected in the wheelset bolts, bearings, or other components.Upon reviewing the usage records, it was determined that the right front wheel assembly had accumulated a total of 84.14 flight hours/59 landings since its last repair.To determine the nature of the hub rim fracture and identify the underlying cause of the failure, this study carried out a comprehensive analysis including fracture macro-micro-morphological observation, energy spectrum analysis, microstructure examination, hardness testing, and other experimental analysis on the hub.

Appearance of failed components
Fig. 1 illustrates the overall appearance of the failed aircraft tire assembly.The assembly comprises a hub and tire with two matching halves of the hub connected by bolts.The tire is positioned in the outer circle of the hub.Once the tire is inflated, the tire bead comes into contact with the inner side of the hub rim, which serves as a fixation limit for the tire and bears the axial stress.Upon tire removal, the overall appearance of the hub is shown in Fig. 2. By observing the hub, it becomes evident that half of it is fractured at the corner transition point between the rim and the main body.The fracture predominantly extends along the corner transition position of the rim, resulting in a long arc-shaped fractured section measuring approximately 30 cm.The fracture surface appears generally flat, exhibiting a smooth, dark grey coloration with a slightly old appearance.Notably, clear fatigue arcs are visible on the fracture surface, symmetrically present on both the left and right sides.At the two ends of the fracture, relatively rough edges can be observed and characterized by a bright silver-grey color and distinct signs of rapidly expanding tearing ridges, as depicted in Fig. 3.
The fracture was further examined by using an optical microscope (ZEISS SteREO Discovery.V20).It can be observed that the center of the fatigue arc is located in the black area at the lower edge of the fracture's middle section.The color of this area distinctly differs from the gray color of the remaining fracture surface, indicating that it serves as the fracture source area.According to the morphology of the hub, the fracture source area is identified as the corner transition position on the inner side of the hub.Specifically, the side comes into contact with the tire bead, as illustrated in Fig. 4.

Microstructure of the fracture surface
The fracture was further investigated by using scanning electron microscopy (SEM) (TESCAN VEGA3).As depicted in Fig. 4, the fracture source area exhibits prominent ridges radiating upward (in the hub's wall thickness direction) and extending to the left and right (in the circumferential direction of the hub).This further reinforces that the fracture indeed originates from this specific position, as shown in Fig. 5 (a).The crack source is characterized by a small linear segment with a length of about 1.3 mm.Under high-powered microscope observation, the surface of the crack source displays a granular and honeycomb-like structure, which starkly contrasts with the fracture separation characteristics typically observed in the metallic matrix, as demonstrated in Figs. 5 (b) and (c).On the side surface of the crack source, processing traces of the material show interruptions, indicating a discontinuity between the material in the source area and the surrounding matrix, as shown in Fig. 5 (d).Microscopic examination reveals that the fatigue source area represents a heterogeneous material region, displaying distinct differences from the metallic matrix.
Once the fracture originates, fatigue propagates to both the opposite side as well as the left and right sides, resulting in significant fatigue striations across the entire fracture.These fatigue striations exhibit a fine and dense pattern with minimal spacing, indicating substantial fatigue growth.The fatigue zone encompasses more than 90% of the total fracture area, as depicted in Fig. 6.The fracture zone located at the left and right edges of the wheel hub represents rapid and transient fractures characterized by dimples.These transient fracture zones comprise a relatively small combined area, accounting for no more than 10% of the total fracture area.
Upon microscopic examination, the fracture reveals heterogeneous material solely in the fatigue source area, while the remaining sections of the fracture exhibit no similar heterogeneous material or other defects.

Section of the source area
The source area is bisected perpendicular to the fracture direction, allowing for subsequent observation and analysis of both the source area and the matrix following grinding and polishing.By employing SEM, notable disparities in sectional morphology between the source area and the matrix are discernible, revealing evident discontinuities, as shown in Fig. 7.The energy-dispersive X-ray spectroscopy (EDS) spectra of the matrix and source area are illustrated in Figs. 8 (a) and (b) respectively.Table 1 summarizes the results of the elemental content analysis.Remarkably, the source area comprises alloying elements such as Al, Mg, Cu, and others.However, it exhibits a notably higher concentration of oxygen (O) element compared to the matrix.This discrepancy strongly suggests that the source area originated from the oxidization of Al alloy.

Metallographic structure and hardness
Samples were collected in a manner parallel to the hub fracture, aiming to assess their characteristics using a LEICA DM4000M metallographic microscope and a hardness gauge (Qness Q60A+).The grains within the samples exhibit prominent elongation and deformation, indicating the utilization of a forging process during the manufacturing of the hub.Notably, the metallographic structure appears to be uniformly distributed without any abnormalities or irregularities.Additionally, the samples demonstrate consistent hardness values throughout with an average microhardness of about 154 (Hv0.1).

fracture mode
The macroscopic and optical observation analysis results show that the fracture between the rim and the hub body originates from the inner side of the corner transition position, which is characterized by a single source.The fracture surface is flat and smooth.Obvious fatigue arcs can be found.The microscopic observation results indicate that the fatigue crack propagates towards the opposite side and the left and right sides from the source area due to cyclic working stress.Obvious fatigue striations that are fine and closely spaced can be found at the fatigue propagation zone.The fatigue fracture surface propagates sufficiently and the fatigue zone area exceeds 90% of the total fracture area.These characteristics indicate that the fracture mode of the aircraft hub is typical high cycle fatigue fracture [9] .

Fracture reason
The experimental observation results indicate that the surface of the source area is honeycomb-like, which is different from the fracture separation morphology from the base metal.The machining marks of the lateral surface are discontinuous between the fatigue source area and the hub matrix.Except for containing the same Al, Mg, and Cu elements, the content of the O element in the source area is much higher than the hub matrix.These characteristics indicate that the source area of the hub fracture surface is an oxide defect of the matrix material.According to its composition and morphology, the oxide defect is probably oxide skin which is not completely removed in the manufacturing and processing of the wheel hub.During the later forging process, the oxide skin deforms and extends along the circumference of the wheel hub, forming a long linear defect.
When the aircraft tire contacts with the runway at work, it is compressed and deformed.cyclic working stress is applied to the wheel hub flange.At this time, the transition position with linear oxide defect between the wheel flange and the hub body is exactly the highest stress area.Then the crack initiates from the oxide defect and propagates along the corner transition zone by the working cyclic stress.When the crack propagates to a certain length, the remaining part cannot bear the stress and a rapid fracture occurs.
According to the above analysis, it can be concluded the reason for the high cycle fatigue fracture of the wheel hub flange is the existence of a linear oxide defect at the corner transition position between the wheel flange and the hub body.

Suggestion
In engineering practice, such linear oxide defect is difficult to detect in eddy current testing [10,11] .So we suggest that quality control should be strengthened during the production process of wheel hubs to avoid defects forming.Further inspection of the transition position of the wheel flange during maintenance and repair should be carried out to find the crack as soon as possible and take measures to avoid other serious consequences.

Conclusions
The fracture occurring at the hub rim is attributed to high-cycle fatigue fracture.The reason for the high cycle fatigue fracture of the wheel hub flange is the existence of a linear oxide defect at the corner transition position between the wheel flange and the hub body.

Figure 1 .
Figure 1.Appearance of the failed tire assembly.Figure 2. Appearance of the failed wheel hub.

Figure 2 .
Figure 1.Appearance of the failed tire assembly.Figure 2. Appearance of the failed wheel hub.

Figure 3 .
Figure 3.The fractured and separated part of the hub.

Figure 4 .
Figure 4. Optical micrograph showing the morphology of the fracture source area.

Figure 5 .Figure 6 .
Figure 5. Morphology of the fracture source area: (a)Radiating ridge features, (b)Morphology of the crack source, (c) Morphology of the fracture section, (d) Morphology of the side surface.

Figure 7 .
Figure 7. SEM image of sections of the source area and the matrix.

Figure 8 .
Figure 8. EDS spectra of the material in (a) the matrix, and (b) the source area.

Table 1 .
Chemical composition analysis of materials in the source area and the matrix (wt%).