Failure Analysis on Foreign Object Damage of Aero-engine Compressor Blade

CHEN Rong, LV Feng-jun, Li Quan, Fu Guo-ru, Wang Hong-wei, Chai Zhi-gang, Xu Zhi-gang

Beijing Aeronautical Technology Research Center Beijing 100076, China

Abstract—Aero-engine blades always failure due to impact of foreign object damage in service. Damnification and consequent fracture failure of blade is one of the main failure modes for the compressor blades in aero-engine. Damage characteristic on blades caused by different kind of material are various. Observation on damaged surfaces, trace analysis and surface element analysis by means of SEM and X-ray energy spectrum are effective methods to estimate the attribute, component and outline of foreign object. In this paper, analysis on several cases of FOD are introduced. The features, principles and rules of several typical FOD modes are summarized through failure cases analysis.

Keywords-failure analysis; aero-engine blade; foreign object damage

1. INTRODUCTION

Any gas turbine engines ingest large amounts of air when in operation. As forepart of engine with high rotational speed, compressor blades could be subject to ingestion of small particles, leading to foreign object damage (acronym as FOD). FOD could cause blades fracture, even engine failure, and could lead to enormous economic lose for airlines company and airdrome[1]. According to the Boeing web page foreign object damage costs airlines, airports and airport tenants millions of dollars every year[2]. Statistic of American Federal Aviation Administration shows that there are totally 486 airplanes suffered “bird attack” in American since 2000.

The sources of FOD include many kinds. The characteristic and feature of FOD may vary obviously depending on the attribute and size of foreign object[3]. According to the nature of foreign object and potential damnification to engine, foreign object can be as diverse as sand particles, metal materials and soft materials, such as birds or verts.

In this paper, several typical failure cases are introduced, and characteristics, features of different kind of FOD and analysis means are summarized through failure cases analysis.

2. FAILURE CHARACTERISTICS of FOD

In this paper, foreign object means any materials except engine, including ingested sand, ice, bird, or metal part, as well as remainder and loosen bolt, gasket in the gas inlet chamber. Damage on engine caused by different kind of FOD is obviously unlikeness.

The following text will illustrate respectively the characteristics of FOD in terms of metal material, sand, soft material.

2.1 Metal object The degree of damnification of aero-engine caused by

metal foreign object is worst, and sometimes serious flight accident will be induced. The so-called metal foreign objects usually mean bolt, screw cap and the other metal parts. The metal foreign object has high intensity and is difficult to be broken into pieces when it impacts aero-engine. Aero-engine is often impacted for several times, until the metal foreign object turns into the next row of engine or the engine shut down. Therefore, metal foreign object usually can cause several blades at the same stage or several different stages to be damaged, see Fig.1. Some small size metal, such as small shim and safety pin, only can lead to damage of a few blades, see Fig.2. The damage impacted by metal object have such characters in general as follows:

a. The notches and bad scathes would be formed on the blade regardless of the size of metal object;

b. The damage mainly locates at leading side;

c. The shapes of damage traces are comparatively regular, and the pattern characters of metal foreign object can be left approximately;

d. The components of metal object also can be left on the cutting surface, which present an opportunity to confirm the metal components if the residual material are obvious different from the components of blade.

2.2 Sandstone Due to low intensity, the sandstone is easy to be smashed

after impacting and the damaged degree of blades is limited. The concave pit or partly distortion can be formed, sometimes the second stage blades can occasionally be damaged, but the amount of damaged blades is rather less. Comparing to damaged characters caused by metal foreign object, the damaged caused by sandstone have such characters as follows:

a. Small sand has no obvious direct harm for the blade, but when the sand size is larger than 5mm, the leading side of the first stage blade can be torn;

b. The damaged surface caused by sand object is rather rough, has no cutting traces.

978-1-4244-4905-7/09/$25.00©2009 IEEE 1085

c. Small sandstone particles can keep on the damaged surface, and the silicate element can be measured through energy spectrum analysis.

To judge the damage caused by sand whether or not, macroscopic characters are main bases, and the energy spectrum analysis only presents indirect evidence.

Fig1. the first stage blade damaged by copper parts

Fig2. blade damaged by very thin sheets

2.3 Soft object The soft objects are mainly included bird, vert, basketwork,

ice and et al. When the soft object attacks to aero-engine, the deformation on the leading side of the first stage compressor blade is main feature, but the notching and tearing are rare as well. If deformation were serious enough, the aero-engine would be in-flight shut down.

The damaged characters impacted by soft object are as follows:

a. The basketwork often damnify several adjacent blades simultaneously;

b. Fiber debris can be found on the damaged region;

c The ice often makes many blades bend;

d. The bird sometimes makes only few blades damage, at other times damnifys many blades. The feathers and blood traces often can be seen in the air intake and compressor chamber of engine when bird attacks engine, see Fig.3 and Fig.4.

Fig3. Aero-engine blade attacked by birds

Fig4. Blade deformation caused by bird attack

3. CASE ANALYSIS of FOD

The macroscopic characters of FOD are introduced in above paragraphs, which are foundations to analyze attributes of foreign objects. According to the integral damaged status of engine, the condition and quantity of the damaged blade, the attribute of foreign object can be initially confirmed. Then SEM and X-ray energy spectrum are used to detect the finest damaged facet, impacted traces and residues of foreign object. After finishing the macro and micro analysis, the foreign object attributes can be finally determined. Sometimes, the foreign object can be easily identified by means of the damaged trace examination, for example, when the bolt or screw cap attacked the engine, the outline of bolt or screw cap will be left on the blade or gas intake of airplane.

Case 1: In an accident, several first stage blades are damaged, the blades from second stage to sixth stage also are

1086

damaged in different degree. The blade is made of 1Cr11Ni2W2MoV stainless steel. There are obvious curling and notching in leading side of the damaged blade, and the crazed coat in the damaged places, see Fig5. Damaged degree of engine and characteristic of blades shows that the damage is caused by metal object. The SEM and x-ray energy spectrum are used to detect the damaged places and to identify the various phases in the finest facets. The results show that the residue of metal object exists on the cutting facets, and the residue mainly contains Fe, Zn and Cr element, which is different from the component of blade material. So, the foreign object is finally affirmed as galvanized steel parts.

Fig5. Curling and notching in leading side of blade

Case 2: When titanium alloy blade attacked by metal object, the cutting facets always attach some melted solid besides cracking of blade. Attachment is very useful to ensure the component of foreign object. Fig6 is the micrographic of foreign object damaged blade. x-ray energy spectrum analysis show that the components of melted solid mainly are Fe, Cr, Ni elements, and the weight percent of elements are very close to stainless steel, which indicates the blades are damaged by stainless steel parts.

Fig6. The melted solid on the cutting facets of titanium alloy blade

Case 3: There are three high-pressure compressor blades damaged in an engine. The three blades have same damaged characteristic, such as small circle dents and cracks on the edge of dent, see as Fig7 and Fig8. The diameter of two larger dents is about 1.4mm. The size of damaged places is small, but deformation and splitting is obvious, which indicate that the foreign object is metal material.

Case 4: Two blades of the first stage compressor blades are damaged at one time. The damaged deformation happen within about 10mm long area at the leading side, and the striated scratches are clearly found in the deformation area. On the other hand, the original surface of the blade are well preserved without cracks or notches, see Fig9 and Fig10. So, the foreign object can be judged as sandstone.

Fig7 Damaged high-pressure compressor blade

1087

Fig8 Micrographic of dents on the damaged blade

Fig9 Deformation of blade impacted by sand

Fig10 Micrographic of deformed places of blade

4. CONCLUSION

In the course of analyzing foreign object damage(FOD), the full-scale examination is the same important as partial inspection, macroscopic characteristic is same useful as microanalysis as well. The integral damaged status and macroscopic damaged characteristics are foundation to confirm the attribute of foreign objection, and the examination of morphology of damaged places and detection of residues components by means of SEM and X-ray energy spectrum are effective methods to accurately affirm the foreign object.

REFERENCES [1]. CHEN Lie,(1990). Analysis on Foreign Object Damaged of Engine,

Flight Accident and Failure Analysis, No1, pp.9-15 [2]. E. Silverira, G. Atxaga, A. M. Irisarri,(2008). Failure analysis of a set of

compressor blades, Engineering Failure Analysis, No15, pp666-674 [3]. FU Guo-ru, YU Zhe-ming, et al, (2006). Main Failure Attributes and

Rule of Compressor-blades in Aero-engine, Failure Analysis and Prevention, No1, pp18-24

About the author: CHEN Rong: Ph.D, graduated from Beijing University of Aeronautic and Astronautic, now engage in failure analysis of aviation equipment. Tel:010-66713280 Email:newcrong@tom.com Address: P.O.Box 9203-16,Beijing, 100076

1088