Analysis of turbine blade breakages in an engine starting motor

\ PERGAMON

Engineering Failure Analysis 5 "0888# 134Ð140

Analysis of turbine blade breakages in an engine starting motor C[H[ Tao\ N[S[ Xi\ M[L[ Xie\ P[D[ Zhong\ Y[ Zhang AVIC Failure Analysis Center\ Beijing 099984\ China Received 1 June 0887^ accepted 03 September 0887

Abstract This paper presents an analysis of turbine blade breakage in an engine starting motor[ Fractographic analysis\ _nite element analysis and fractographic quantitative calculation of beach marks have been per! formed[ The analysis shows that the fractures are due to _rst order bending vibration fatigue damage[ No defects were found at the fractured section of blade[ The e}ective preventive measure is the removal of the {{dead crank|| standard that is seldom used in real service[ Þ 0888 Elsevier Science Ltd[ All rights reserved[ Keywords] Turbine blade^ Vibration^ Fatigue

0[ Introduction A power turbine in an engine starting motor has 26 blades as shown in Fig[ 0[ It was fabricated by casting as a whole and K307B nickel based superalloy was used[ The turbine blade breakage took place 3 times in the course of trial runs[ The starting lives were generally 081Ð084 starts\ however\ no blade cracking occurred in the power turbine after 599 starts in actual service[ The trial run test is composed of 79) {{normal starts|| and 19) {{dead cranks|| which are respectively shown in Figs 1"a# and "b#[ The objective of this investigation is to undertake a failure analysis to determine and describe the factors responsible for the failures of the power turbine blades\ as well as to show that the proper application of failure analysis techniques can produce a valuable feedback to design and testing procedure improvements[ 1[ Experimental results and analysis 1[0[ Macro!damage characterization of turbine Only one blade was broken at 3Ð4 mm from the root for each failure as shown in Fig[ 0[ Although macro!cracks were not found for the other blades\ deformation lines were found at the  Corresponding author[ 0249!5296:88:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reserved[ PII] S 0 2 4 9 ! 5 2 9 6 " 8 7 # 9 9 9 2 4 ! 0

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C[H[ Tao et al[ : En`ineerin` Failure Analysis 5 "0888# 134Ð140

Fig[ 0[ Power turbine in an engine starting motor[

Fig[ 1[ Scheme of "a# {{normal start|| and "b# {{dead crank||[

same position for unfractured blades[ The deformation lines observed by means of scanning electron microscopy "SEM# are shown in Fig[ 2[ 1[1[ Fracture morphology analysis of turbine blade The fracture section has the typical macro!features of fatigue damage as shown in Fig[ 3[ The main crack initiated at the edge of the admission side\ shown in Fig[ 4[ There were small

C[H[ Tao et al[ : En`ineerin` Failure Analysis 5 "0888# 134Ð140

136

Fig[ 2[ Deformation lines between 3Ð4 mm of the blade root "SEM#[

Fig[ 3[ Macro!morphology of fractured blade\ arrow shows main source and ABC is demarcation line between fatigue propagation and rapid fracture[

dimples in the local region^ besides\ there were some secondary fatigue sources in the tub side[ The crack initiated at the surface of blade and no defects\ rough machining or other damage were found in the blade surface near the fracture site[

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C[H[ Tao et al[ : En`ineerin` Failure Analysis 5 "0888# 134Ð140

Fig[ 4[ Main source morphology[

Fig[ 5[ River patterns[

Many fracture facets with di}erent orientations exist on the fracture surface[ These facets are microscopically of the characteristics of river patterns "shown in Fig[ 5# or parallel sawtooth patterns "shown in Fig[ 6#[ These cleavage!like patterns are the typical characteristics of the _rst propagation stage of fatigue cracks for nickel!based superalloys[ Typical beach marks can be seen

C[H[ Tao et al[ : En`ineerin` Failure Analysis 5 "0888# 134Ð140

138

Fig[ 6[ Parallel sawtooth patterns[

Fig[ 7[ Fatigue beach marks in the propagation region[

in the crack propagation region and the distance between neighboring beach marks is large as shown in Fig[ 7[ The crack propagated from the admission side towards the exhaust side and from the blade tub side towards the rear[ The rapid fracture region is situated at the exhaust side and the area is about 0:2 of that of the general fracture section[

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Table 0 Measurements of residual stress Measurement position

0

1

2

3

4

Residual stress\ MPa

−242

−762

−468

−242

−213

Fig[ 8[ Scheme of measurement[

1[2[ Measurement of residual stress Residual stresses have been measured by X!ray stress analysis for the blade next to the fractured one[ The residual stresses at the rear are shown in Table 0 and the scheme of measurement is shown in Fig[ 8[ The stress situated at the deformation lines is the largest[ 2[ Analysis Each fractured blade has the typical macro! and micro!characteristics of fatigue fracture\ mul! tiple sources and small dimples near the main source[ No defects\ rough machining marks or other traces were found in the crack initiation region[ Thus\ the possibility that fracture is related to defects can be ruled out[ There are fracture facets with di}erent orientations which are the typical characteristics of _rst stage fatigue crack propagation[ The fracture site\ which is 3Ð4 mm from the blade root\ is in general at the joint line of _rst order bending vibration of the blade[ The main source of failure is at the edge of the admission side where the bending vibration stress is the largest[ This strongly suggests that the fracture of blade was induced by bending vibration stress[ The deformation lines shown in Fig[ 2 also imply that the stresses at this position are a maximum[ In order to investigate the possibility of bending vibrations of the blade\ a _nite element analysis has been performed and the calculated results show that the _rst bending intrinsic vibration frequency of the blade is about 7999 Hz "37\999 rpm#[ Obviously\ in normal conditions\ vibration resonance has little possibility of happening[ However\ considering the total number of blades 18\ the rotation speed for resonance is about "7999:18#×59  05\449 rpm[ The double resonance is

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C[H[ Tao et al[ : En`ineerin` Failure Analysis 5 "0888# 134Ð140 Table 1 Relationship of beach marks and crack length Distance from the main source\ mm

9[2Ð9[85

9[85Ð1

1Ð1[6

1[6Ð1[8

1[8Ð2[74

Number of beach marks

29

29

19

04

20

22\099 rpm[ According to Fig[ 1"b#\ it is known that the turbine blade stayed for a longer time at this speed and hence the bending vibration resonance is liable to take place[ The vibration measurement in the simulated test indicates also that there indeed exists great vibration at this rotation for the blade[ The relationship between the beach marks and the start lives has been investigated by means of the fractographic quantitative analysis technique[ The evaluation of the beach marks as a function of the crack length from the main source is shown in Table 1[ Table 1 shows that there exist 015 beach marks in the whole fracture surface\ indicating low cycle fatigue[ This demonstrates the fatigue crack initiated at the early period of the trial run^ the initiation lives are about 081−015  55 starts[ The stress ss on the blade can be expressed as follows] ss  sv¦sa where sa is normal stress\ a function of working state\ and sv is vibration stress caused by resonance[ If the following condition is satis_ed] s s − sf where sf is the yield stress of materials\ then the crack initiated in the blade[ Once the crack initiated\ the vibration stress decreased and the working state changed[ However\ the structure contained a crack\ which propagated and induced the structure failure[ Therefore\ one should change the {{dead crank|| test standard because this starting mode is seldom used in real service[ 3[ Conclusion Fracture of a power turbine blade in an engine starting motor is due to _rst order bending vibration fatigue[ No defects were found at the fractured section of blade[ The e}ective preventive measure is the removal of the {{dead crank|| standard that is seldom used in real service[