Failure analysis of large and complex engine blades based on experimental research

On a certain day in December 2022, a GE90 engine equipped by an airline experienced a surge during flight. Ground inspection revealed broken blades of the 9-stage high-pressure compressor. To identify the cause of its fracture failure, the macroscopic fracture surface was first analyzed. Obvious fatigue crack propagation characteristics were found at the blade cross-section. Small pits formed by the impact were observed at the crack source location, indicating that the blade was first formed into a circular arc-shaped impact wound after being impacted by an external object. In the subsequent service process, under continuous vibration, fatigue cracks are formed at the impact point as the crack source and gradually propagate, ultimately leading to blade fracture and failure. To further explore the source of foreign objects causing impact damage, the surface scanning method using energy spectrum analysis was used. An enriched region of C element was found near the impact point. However, due to the lack of conditions for the formation of C combustion products at the compressor location, it can be considered that the foreign object may have come from external sources.


Introduction
Due to the long-term service in harsh working environments of aviation engine blades, damage and cracks may appear on the blades, which can easily cause fracture under high-speed rotation.The fractured blades will be thrown out with huge energy, which is highly likely to cause major safety accidents [1] .In 2022, a blade of a 9-stage high-pressure compressor in a GE90 engine experienced a fatigue fracture.The inspection found that all other blades of the engine were intact with only one blade experiencing fatigue fracture, but its fatigue life was significantly lower than the design life.
Experts and scholars at home and abroad have conducted extensive research on the fatigue damage and fracture of engine blades, which is mainly distributed in the following three aspects which are non-destructive testing research; theoretical and dynamic research; numerical simulation and experimental research.In terms of non-destructive testing research, Huang [2] conducted a comprehensive comparative study of non-destructive testing and online monitoring technologies for engine blades.It was found that the focus of the testing objects was on cracks and coating defects on the surface of the blades with less attention paid to the detection of common micro-cracks and volumetric corrosion defects at the root of the blades.At the same time, it heavily relies on point-bypoint exploration methods.There is currently no effective quantitative method for blade cracks.In terms of theoretical and dynamic research, Chen [1] conducted fracture mechanics analysis of highspeed rotating engine blade models and found that the depth and location of cracks had a significant impact on the stress intensity factor at the crack tip and the critical speed at which the blade flew off.
Salehnasab [3] proposed the mechanism and model of fatigue crack initiation and propagation in blade CM186LC directionally solidified material and calculated the crack propagation direction based on the maximum energy release rate criterion.Wang [4] proposed a high and low circumferential composite fatigue life prediction model based on the improved-Zhu model.Li [5] proposed an improved model and reliability evaluation method for multi-axis fatigue damage of aviation engine compressor blades, which has high prediction accuracy.In terms of numerical simulation and experimental research, Li [6] modeled and analyzed the propagation of a semi-elliptical crack on the suction surface of a blade and obtained the effects of parameters such as crack depth, location, and aspect ratio on the distribution of stress intensity factors at the front edge of the crack.Hall [7] used the finite element method to analyze the residual stress distribution caused by FOD and discussed the role of residual stress in characterizing fatigue crack propagation.Witek [8] conducted fatigue crack propagation tests on undamaged helicopter compressor blades under resonance conditions and used the nonlinear finite element method to analyze the stress state of the blades during the first-order lateral vibration process.The numerical analysis results showed that high stress in the resonant state of the blades was the main factor causing fatigue cracks in the blades.Witek [9] used a hybrid method of analytical fatigue analysis, numerical analysis and high cycle fatigue test analysis to dynamically predict blade crack propagation.
In this case, the overall morphology and fatigue fracture characteristics of the blade were first observed by using scanning electron microscopy and then the propagation direction of the crack was determined based on the microscopic morphology of the fracture surface crack.Secondly, the microscopic morphology characteristics of the crack source area were found through the crack tracing method.Obvious impact marks were found at the crack source area.To further determine the cause of initial damage, the surface scanning method of energy spectrum analysis was used to conduct coupling analysis on the element distribution pattern and impact point characteristics of the crack source area.The relevant analysis results provide a basis and method for preventing and judging internal or external damage to engine core components.

Installation structure and characteristics
Figure 1 provides a schematic diagram of the engine and its blade structure.

Experimental procedure
The macroscopic morphology of the damage was observed by using a Zeiss SteREO Discovery stereomicroscope.Then, the microstructure of the fracture was observed and analyzed by using the Czech TESCAN MIRA3 XM type field emission scanning electron microscope.To further identify the characteristics of foreign objects causing leaf damage, a British EDAX energy spectrometer was used to activate the surface scanning mode and conduct a comprehensive element scanning of the suspected impact location.The distribution image of chemical elements near the impact point was analyzed to explore the trace characteristics of foreign objects.

Macroscopic observation
The macroscopic morphology of the fracture is shown in Figure 2   The circumference of the semicircular arc impact pit is about 3.2 mm.Obvious fatigue beach lines can be seen on the outside of the impact pit, as shown in Figures 2 (b) and (c).From the characteristics of beach lines, fatigue cracks start from the edge of the impact pit and expand along the direction of the leaf basin.
So, from the perspective of macroscopic fracture, firstly this blade was impacted wound after being impacted by an external object, which was a penetration injury.During the subsequent service, under the condition of continuous vibration, the impact point was taken as the crack source to form fatigue cracks, which eventually led to blade fracture.

Analysis of fracture characteristics
The micromorphology of the side of the impact point is shown in Figure 3, which is the initial position of impact damage.However, no obvious impact residue materials were found except that a small amount of area coating fell off.The microstructure of the front of the impact point is shown in Figure 4 (a).There are obvious friction and slip marks (the yellow area in the figure) in the fracture, which run through the blade, indicating that the incoming direction of the impingent is at a certain angle to the leaf basin.The impingent penetrates the blade at a certain position.The scratches formed by the friction between the external object and the blade are shown in Figure 4 (b).No obvious marks of external objects embedded into the blade are found.Some bumps may be oxidation products or attached dust.

Analysis of foreign body on the fracture
The distribution of foreign chemical composition on the surface of the fracture is shown in Figure 6.The contents of Fe, Ni, Cr, and Co at the impact point of the fracture are significantly higher than those in the surrounding area.Therefore, elements Ni, Fe, Cr and Co may be matrix materials.However, there is an obvious enrichment of element C in the interior of the impact crater.There is no condition for the formation of combustion product C in the compressor position.So element C in this position may be from an external impactor.

Discussion
There are many sources of FOD such as birds in the air, hail, sand on the ground, screws, stones, etc.The forms and states of damage causes vary.Soft materials generally cause damage with a larger range but shallower depth.Hard materials generally cause damage with a smaller range but deeper depth.At the same time, analysis will also be conducted based on the energy, impact velocity, and damage location of FODs.For small mass, high velocity, and high hardness FODs, penetration damage is usually formed.For different positions, the position and airflow velocity of the fan blades are relatively low.When the fan blades come into contact with FOD, they will form large overturning deformation.The speed and size of the FOD in the core machine are high.It usually forms damage similar to the characteristics of partial penetration in this case.The damage at the impact point is relatively small.As the engine continues to operate, the blade forms crack propagation with the impact point as the fatigue source under the coupling effect of stress and temperature fields.

Conclusions
On a certain day in December 2022, a GE90 engine equipped by an airline experienced a surge during flight.Ground inspection revealed broken blades of the 9-stage high-pressure compressor.To identify the cause of its fracture failure, the macroscopic fracture surface was first analyzed.Obvious fatigue crack propagation characteristics were found at the blade cross-section.Small pits formed by impact were observed at the crack source location, indicating that the blade was first formed into a circular arc-shaped impact wound after being impacted by an external object.
From the perspective of macroscopic fracture, this blade was to form circular and arc impact wounds after being impacted by external objects.In the subsequent service, under the condition of continuous vibration, the impact point was taken as the crack source to form fatigue cracks, which eventually led to blade fracture.According to the microstructure of the fracture, foreign substances hit the blade edge at a high speed along a certain direction.Due to the high speed, foreign substances penetrate through the blade from the side position.There is obvious enrichment of the C element in the impact crater, while the compressor position has no condition to form a C combustion product.Thus, the C element in this position may come from an external impactor.

Figure 1 .
Figure 1.Engine and its blade structure.
(a) and (b).Under the position of Figure 2 (a), there is an arc-shaped impact trace.The external object impacts the blade at a high speed and passes through the blade from the side.

Figure 2 .
Figure 2. Photo of blade fatigue fracture (a) Morphology of fracture, (b) Source of fatigue crack, (c) Fatigue beach pattern.

Figure 3 .
Figure 3. Microstructure of the side of impact position (side of leaf basin).

Figure 4
Figure 4 SEM photos (a) Microstructure of impact point, (b) Penetration marks on the impact surface.

Figure 5
shows the schematic diagram of the collision process between the external object and the blade.The foreign material hits the blade edge at a high speed and along the dotted line in the figure.

Figure 5 .
Figure 5. Schematic diagram of the direction of the foreign body hitting the blade (Particles hit the blade along the blue dotted line).

Figure 6 .
Figure 6.Distribution of chemical elements on the impact surface of foreign bodies.