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Coupled mechanical, metallurgical and FEM based failure investigation of steam turbine blade
Accepted Manuscript Coupled mechanical, metallurgical and FEM based failure investigation of steam turbine blade Sanjeev Saxena, J.P. Pandey, Ranjit Singh Solanki, Gaurav K Gupta, O.P. Modi PII: DOI: Reference:
S1350-6307(15)00068-0 http://dx.doi.org/10.1016/j.engfailanal.2015.02.012 EFA 2512
To appear in:
Engineering Failure Analysis
Received Date: Revised Date: Accepted Date:
31 October 2014 19 January 2015 27 February 2015
Please cite this article as: Saxena, S., Pandey, J.P., Solanki, R.S., Gupta, G.K., Modi, O.P., Coupled mechanical, metallurgical and FEM based failure investigation of steam turbine blade, Engineering Failure Analysis (2015), doi: http://dx.doi.org/10.1016/j.engfailanal.2015.02.012
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Coupled mechanical, metallurgical and FEM based failure investigation of steam turbine blade Sanjeev Saxena1, J. P. Pandey, Ranjit Singh Solanki, Gaurav K Gupta, O. P. Modi CSIR- Advanced Materials and Processes Research Institute, Hoshangabad Road, Bhopal, India.
ABSTRACT Steam turbine blades are the critical component in power plant, specifically low pressure blades are generally found to be more susceptible to failure. A mechanical, metallurgical and FEM based coupled methodology is used in the present failure investigation of low pressure steam turbine blade. The results of each investigation of turbine blade failure were then interpreted that leads to find the location of primary failure, sequence of failure and the root cause of its failure. All the three aspects of failure investigation are important in answering the questions raised for the failure and to avoid any future miss-happening. Keywords: Failure investigation Steam turbine blade, , FEM, mechanical testing, metallurgical evaluation
1. Introduction Thermal Power Plants contributes nearly 57% of India’s installed electricity capacity. Steam turbine blades are the critical component in power plant which converts the linear motion of high temperature and high pressure steam flowing down a pressure gradient into a rotary motion of the turbine shaft. The low-Pressure(LP) turbines blades, design to extract the final remnant of energy from the passing steam flow, are relatively large scale rotating airfoils due to the significant centrifugal forces experienced during normal operation. Researches have shown that the LP blades are generally found to be more susceptible to failure than intermediate and high pressure blades. The literature survey revealed that the major cause of failure of turbine blade is known to be fatigue [1, 2]. Excessive mechanical vibration, resulting in large alternating stresses
1
Corresponding author. Tel.: 91+245760 (Ext. 1245); fax: 91+2457042 E-mail address: [email protected] (S. Saxena).
1
in conjunction with the steady state centrifugal and gas bending stresses are the reason for such fatigue failure [3, 4]. Generally such failure investigations have been done using only either, fractography, EDS and microstructure [3,6-8] or only done using numerical technique like FEM [1, 5, 9] has been used. The possible questions which are required to be answer in any such failure investigation are: Where are the critical locations of primary failure which needs more precautions to avoid such failure in future and the first crack initiation point?, what is the sequence of failure? and what is root cause of failure material or design?. Thus there has to be a holistic approach to carry out any such type of failure investigation. In the presented failure investigation work a mechanical, metallurgical and FEM based coupled methodology was used to find the answers of the questions raised above. In the present work a failure investigation was carried out in a more than 30 years old LMW design thermal power plant. The present turbine blade failure was of Low Pressure (LP) Turbine which has 8 stages (24-31). The one blade of stage 26th got fractured from the assembly of 92 blades. These blades were relatively new and replaced nearly three-four months back. It was reported that the LP turbine unit was shut down after a severe vibration was noticed immediately when the plant comes in operation, for half an hour, after a weeks time of plant repair. After opening the turbine casing it was found that one blade No.18 of stage 26 got fractured and in consequence several other blades were also found damaged in the near-by area. The present investigation started with the study of plant's record and the proper assessment of service exposed conditions. As the referred blades are relatively new, the possibility of blade failure due to fatigue is minimum. In the present investigation the essential experiments were then planned to characterize the material of failed and un-failed blades. The following essential examinations were carried out: visual examination, non-destructive examination, chemical analysis, physical analysis, hardness testing, tensile testing, impact testing, fractographic examination, metallographic analysis and elemental analysis (through EDS). An in-depth review of service exposed condition was carried out by creating solid modeling of turbine blade and analyzed it using finite element method (FEM). The results of each investigation were then interpreted that leads to find the location of primary failure of the turbine blade, sequence and the root cause of its failure
2
2. EXPERIMENTATION 2.1 Visual examination The failed blade (No.18) along with two adjacent blades (i.e blade No.17 and No.19) were brought to the institute for laboratory investigation. The blades were measured and found to be averaging 0.848 meter in length (Fig. 1(a)). Fig. 1 showed the un-failed and failed turbine blades and locations of failure in blade No. 18. Failure in blade No. 18 occurs from three major places, one near the boss, second at the root of the blade and the third from the pin location (Fig. 1(a & b)). During the visual examination, no corrosion or pitting marks were observed on the failed or un-failed blades. It is also observed that the failure is brittle in nature which can be seen from the fractured surface of failed blade No. 18 near the boss section (Fig. 1(c)). 2.2 Liquid penetrant test Non-destructive examination of fractured surfaces of failed and un-failed blades were done using Liquid Penetrant Test (LPT) in the fluorescent mode. It was performed to find the post failure surface defects (Fig. 1(d)). The locations of cracks were confirmed through the LPT tests that were observed in visual examination. Figure 1(e) clearly shows the location of crack initiation in LPT test (fluorescent mode) and the cross section thinning near the boss portion of blade in un-failed blade No. 19. 2.3 Chemical composition Emission Spectrophotometer is used to find the chemical composition of blade materials. An average measurements of chemical composition for all the three blades has been found to be nearly same as given in Table 1. It can be seen in Table 1 that the chemical composition values obtained for all the three blades material nearly resembles to ASTM410 grade (martensitic stainless steel) 10]. 2.4 Density The density of turbine blade samples were measured by water displacement technique with the help of Mettle Microbalance [11]. An average measurements of density has been reported in Table 2. It can be seen that there is also not much variation in the density of three blades material. 3
2.5 Hardness Using universal hardness tester the Vickers hardness measurements were carried out for all the three blades material and its average values are listed in Table 2. The hardness testes were performed at an applied load of 20 kg. The hardness values of un-failed blades No. 17 and No. 19 were observed to be within the standard range and also confirms to proper tempering treatment. Whereas in failed blade No. 18, it is showing almost double hardness value that could be possible due to improper tempering. The higher hardness of the failed blade corresponds to martensitic structure. 2.6 Tensile testing The tensile test specimens were prepared using the material taken from root of each blade as per ASTM E8 code [12]. Three tensile specimens were prepared from the two un-failed blades (blade No. 19 and blade No. 17), whereas one tensile specimen could be prepared from failed blade (blade No. 18) because of test material shortage in this case (Fig. 2). These specimens were tested at room temperature at a strain rate of 0.01. The tensile specimens were having 8mm gauge diameter and a gauge length of 36mm. The important tensile property data obtained by testing tensile specimens prepared from three blade material is given in Table 2. It can be seen in Table 2, that though the ultimate tensile strength (UTS) value of failed blade No. 18 is higher than the other two un-failed blades but there is a significant reduction in percentage elongation and cross sectional area reduction before the failure of tensile specimen. In case of failed blade No. 18, the failure of tensile specimen is sudden and brittle without much reduction in cross section area (2.45 % only). Whereas in the other two un-failed blade tensile test results, the failure is ductile and with significant reduction in cross-sectional area (60-65%). Failed blade No. 18 also has reduced energy dissipation under quasi-static condition (145 Joule) as compared to other two un-failed blades (Table 2). The energy dissipation under quasi-static in failed blade No. 18 material is 87-88% lesser than unfailed blade cases. The significant reduction in it may be the cause of failure of Blade No. 18. 2.7 Impact testing
4
Charpy Impact test of specimen were carried on using Instron high energy impact tester. The machine has arm angle of 143 degree and the weight of hammer is 43 kg. The specimen were prepared using ASTM standard E23 [13]. The average reading has been reported in case of two un-failed blade and only one specimen prepared and tested in case of failed blade No. 18 due to shortage of material. The impact tested sample for blade No. 18 is illustrated in Fig. 3. In Table 2 it can be seen that failed Blade No. 18 has reduced impact resistance (5.68 Joule only) as compared to other two un-failed blades. The impact resistance of failed Blade No. 18 material is 87-88% lesser than the blade material of un-failed blades. The reduction in impact resistance may be the cause of failure of blade No. 18.
3. FEM based assessment of failure Due to the damage of blade No. 18, the geometry of Blade No. 17 is used to generate the solid model of turbine blade. The variations of cross-section and the angle of rotation with the change in length of blade are taken careoff to generate the correct geometry of the blade. There is a difference in pin position in alternate blades, therefore for Blade No. 17 and Blade No. 19, pin location model exactly in these blades as shown in Fig. 4. The true-stress strain curves used in the FEM analyses are obtained by conducting tensile tests of all the three blade materials. The material Young’s Modulus (E ) = 203GPa and poison’s ratio = 0.3 is used in the analysis The FEM analyses are carried out using commercially available ABAQUS software. The FEM mesh used in the analysis of blade is shown in Fig. 5. The mesh is having 24,225 elements and 26,487 number of nodes. Eight nodded hexahedral elements are used in most of the places, except at some less significant location where tetrahedral elements have been used. The explicit solver with reduced integration technique is used in the analysis. The steam turbine is subjected to several loads, but centrifugal force is major among all of them. The two applied forces considered to act on the blade are centrifugal force and steam pressure. The data used to estimate the two applied loads are: angular velocity of the blade = 3000rpm, material density (as determined in the present study) = 7699 Kg/m3, volume of blade ( as determined numerically) = 9.5176 x 10-4 m3, length of the blade = 0.848m, distance of blade from the centre of rotation = 0.250m. The present case of LP turbine operates at designed inlet steam pressure of 1.38 kg/cm2 and the exhaust steam pressure of 0.1042 kg/cm2. Using the stated 5
data the estimated centrifugal force is 794.285kN and load due to stream pressure is 153.177kN, which was assumed to act on a projected area of 122,389mm2. These two forces are applied on the turbine blade using the couplings as available in ABAQUS code. It can be seen in Table 2 that in case of Blade No. 18, the energy dissipated under quasi-static condition is higher than the fracture energy capacity of the material. Whereas in the other two un-failed blade, the energy dissipated due to applied loads is less than the fracture energy capacity of these blade material, therefore no-failure occurs in Blade No. 17 and Blade No. 19. In case of Blade No. 18. the FEM results also confirm the location of first crack initiation which corresponds to the maximum energy dissipation as shown in Fig. 6. In FEM analysis results, it can be seen that maximum energy dissipates (crack initiation occurs) near the upper edge of Boss and towards the inner side (i.e. concave side) of the blade. At this location, there is a reduction in cross section of blade resulting in stress concentration and as a result the crack initiation and failure occurs from this portion. Finally the upper portion got detached at the boss location. The evidence of FEM predicted location of crack initiation is also confirmed while performing Liquid Penetration Test (LPT) in florescent mode, in which a clear crack initiation and thinning of blade had been seen, which fully support the FEM prediction (Fig. 1(e)). Assuming that top portion of the Blade No. 18 failed (marked as location 1 in Fig. 1(a & b) and got separated at boss location and strike the remaining portion of the blade and a dynamic event took place. Therefore, in the FEM results of Blade No. 18, demarking the region where the energy exceeds the impact energy (5.68 Joule) obtained in impact Charpy test (Table 2), for Blade No. 18 case. Figure 7 shows the regions where the energy dissipation is greater than the impact energy of the Blade No. 18 material. These regions approximately define the locations where crack formation and growth possibility is high. These demarcated resembles well with the actually failed three regions obtained in Blade No. 18 as shown in Fig. 1. It can also be seen in Fig. 7, there is variation of energy across the bottom regions of blade. This might have twisted the bottom region of the failed blade, which can be seen in Fig. 1(b). The other possibilities of failure in blade No. 18 at second and third locations have been also investigated in the details. under these conditions, the dissipation of energy is lesser than the capacity of the material under quasi-static condition therefore the failure at other two location (location 2 & 3) can only be possible under dynamic conditions. 6
It is very interesting to examine in two un-failed blade that the crack initiation occurs only in un-failed Blade No. 19 and not in un-failed Blade No. 17. This is quite evident that due to rotation of the blade, impact of failed blade is only on one blade which is following the failed Blade No. 18 and not on both the adjacent blades. Hence, this also validated our understanding of this failure. The other possibilities of failure of Blade No. 18 are also investigated quite carefully, but non holds good for the formation of cracks at three specified locations. 4. Fractographic and microstructural examination For micro-structural examinations, the specimens of desired dimension were cut from three turbine blades roots and observe in transverse and longitudinal directions. Specimens for micro structural examination were prepared by standard metallographic practices. The process involves specimen polishing with different grades of emery papers and fine polishing using colloidal alumina suspensions on Sylvie clothes. Metallographical polished specimen was etched with picral. The etched specimens were examined under FESEM (FEI Nova Nano SEM 430) Netherlands. The un-etched metallographically polished specimen was examined for nonmetallic inclusion in an advanced motorized optical microscope of LEICA make, Germany. The fractured surface of the failed turbine blade was extracted from the region of failure for fractographic study. These were subjected to ultrasonic cleaning in an organic solvent to remove all loosely adhering external material. 4.1 Fractographic analysis The fractographic study conducted on SEM revealed inter-grannular crack at several locations and in only few areas trans-granular cracks (Fig. 8 (a & b)) were observed. Figure 8(c) is showing small amount of corrosion product found at some location due to the depletion of Chromium. 4.2 Microstructural analysis 4.2.1 un-etched un-etched specimens prepared from the three blade materials were observed for microstructures. In general, the microstructure consisted of mangnese sulphide (MnS) particles which are introduced to improve machinability of martensitic stainless steel. The specimens from un7
failed blade showed MnS particles having size of 15-25 microns, whereas in failed turbine Blade No. 18 it is in the range of 5-10 microns (Fig. 9 a-c)). The presence of smaller size MnS particle in case of failed Blade No.18 resulted in poor machinability that found during the preparation for various test specimens [14]. 4.2.2 Etched The etched microstructure of both the un-failed blades (No. 17 and No. 19) exhibited uniform tempered martensitic structure without the presence of grain boundary thereby indicating that the M3C type carbide fully dissolved in the matrix for the formation of M 22C6 type carbide (Fig. 10 (a & b)). The microstructure of etched specimen of failed blade No. 18 also revealed the presence of tempered martensitic structure with prominent grain boundary containing carbides (mostly iron carbide and not the mixed carbide). These carbides can be seen as the white lines at grain boundaries (Fig. 10(c)). These continuous network of non-metallic carbides at the grain boundary significantly reduces metal to metal bonding and the ductility as well as toughness of the material. Such type of carbides facilitate crack initiation and propagation leading to the reduced life of a component. This has also been confirmed by EDS analysis (Fig. 11(a)). In addition, in ASTM 410 material, tempering in 425-600oC range produces a change from M3C type carbide to M7C3 and M22C6 type carbides in the matrix [15]. The tempering in this range accompanied by a modular degree of secondary hardening lead to notable loss of toughness and an increase in the corrosion rates. The appearance of M7C3 carbide is accompanied by an increase in Cr content of the carbide and thereby results in some localized Cr depletion. In case of failed blade No. 18 material properties showed a very high hardness and UTS with a low toughness. This is also confirmed through micro-structural examination exhibiting carbide formation at the grain boundary in case of failed turbine Blade No. 18. The presence of carbide at the grain boundary in the case of failed turbine blade specimen has been observed to be Iron Carbide and not the mixed carbide, which is undesirable. This showed the possibility of tempering of this blade material took place between 425 oC and 600oC, resulting in development of temper embrittlement. With tempering treatment higher than 600 oC, resulting in a progressive decrease in the proportion of M7C3 carbide and an increase in proportion of M22C6 carbide. This results in a 8
rapid loss of hardness and increase in the toughness values. In failed blade microstructure Cr-free carbide at grain boundary are seen. As a result Cr depleted localized regions generated, which might have resulted in corrosion product as shown in Fig. 10(d). This has also been confirmed by EDS analysis (Fig. 11(b)).
5. Conclusions The following conclusions of turbine blade failure are drawn in the present investigation:
The first location of crack initiation in failed blade No. 18 is found above the Boss and on the upper side towards concave face (Fig. 1 (a) and (b) for location).
Under quasi-static condition: The FEM predicted energy dissipation for failed Blade No. 18 is higher than the capacity of the blade material (Table 2). The energy dissipated is just sufficient to initiate the crack but it took time (nearly three-four months or inservice life of this blade) for this crack to cross the critical limit and damage the blade near the boss (1st location in Fig. 1(a& b)). In case of un-failed blade No. 17 and 19 the energy dissipation is lower than the capacity of these blade materials. The energy dissipated is not sufficient to initiate the crack in these blades.
Under dynamic condition: The development of cracks at other two locations (No. 2 & 3), as shown in Fig. 1(a &b) in failed Blade No. 18, can only be possible under a dynamic condition created when the blade failed across the boss strikes the remaining blade. In case of blade No. 17 and 19, crack initiation ahead Boss portion is only predicted under dynamic condition. But crack initiation is examined (Fig. 1(e)) near the boss only in Blade No. 19 due to the continuous rotation of the turbine blade which corroborates well with the assumed hypothesis.
The un-etched specimen exhibited manganese sulphide (MnS) particles in all the three blades. Blade No. 17 and 19 have large size of MnS particles as compared to blade No. 18, which has resulted in poor machinability in blade No. 18.
Variation of material properties among the failed and un-failed blades and also the microstructural observation supported with EDS analysis showed enough evidences that the tempering in failed blade No. 18 may have been done in the range of 450 o C to 600o C, as compared to tempering at above 600oC, may have been done in case of un-failed 9
blades No. 17 and No. 18. This resulted in higher hardness, low toughness, low resistance to corrosion and Cr depleted region in the matrix and Cr free carbides at the grain boundaries in case of blade No 18, which may be the cause of failure in blade No. 18 (Fig. 11(a)).
In consideration to the various findings obtained by coupling the mechanical, microstructural and FEM based evaluation of the steam turbine failure investigation, the root cause for the failure of turbine blade No. 18 appears to be the inferior mechanical and micro-structural properties, which might be due to improper secondary processing, in general and the tempering operation, in particular.
Acknowledgment The authors are thankful to N. Prashant, Anup Kumar and Mohamed Shafiq for their assistance.
References
[1] P. Mˇeˇst’´anek. Low cycle fatigue analysis of a last stage steam turbine blade, Applied and Computational Mechanics 2 (2008), pp. 71–82. [2] G. D. Robinson, Mipenz. Review of fatigue failure in spindle blades of a 120 MW steam turbine, IPENZ Transactions, 2000, Vol. 27, No. 1/Gen, pp. 25-30. [3] Paul S. Prevéy, Dr. Nayarananan Jayaraman, Ravi Ravindranath. Fatigue life extension of steam turbine alloys using low plasticity burnishing (LPB), Proceedings of ASME Turbo Expo 2010: Power for Land, Sea and Air, June 14-18, 2010, Glasgow, UK. [4] Chi-Hshiung. Prediction of corrosion fatigue damages for turbine blades subjecting to randomly distributed power system unbalance, JSME Int. Jrl., Vol. 47(1), A, 2004. [5] Zdzislaw Mazur, Rafael Garcia-Illescas, Jorge Aguirre-Romano, Norberto Perez-Rodriguez. Steam turbine blade failure analysis, Engineering Failure Analysis 15 (2008), pp. 129–141. [6] J. Kubiak Sz., G. Urquiza B., J. Garcı´a C., F. Sierra E. Failure analysis of steam turbine last stage blade tenon and shroud, Engg Failure Analysis 14 (2007), pp. 1476 -1487.
10
[7] Hassan Farhangi and Ali Asghar Fouladi Moghadam. Fractographic investigation of the failure of second stage gas turbine blades, Proceedings of 8th Int. Frac. Conf. 7 - 9 Nov. 2007, Istanbul/Turkey. [8] Wei-Ze Wang , Fu-Zhen Xuan, Kui-Long Zhu, Shan-Tung Tu. Failure analysis of the final stage blade in steam turbine, Engineering Failure Analysis 14 (2007), pp. 632–641. [9] J. Kubiak Sz , J.A. Segura, G. Gonzalez R, J.C. García, F. Sierra E, J. Nebradt G, J.A. Rodriguez. Failure analysis of the 350MW steam turbine blade root, Engg Failure Analysis, 16 (2009), pp. 1270–1281. [10] Metals Handbook, Howard E. Boyer and Timothy L. Gall, Eds., American Society for Metals, Materials Park, OH, 1985. [11] ASTM B962-14 Standard Test Methods for Density of Compacted or Sintered Powder Metallurgy (PM) Products Using Archimedes’ Principle, ASTM International, PA 194282959, United States. [12] ASTM standard E 8M-04. Standard Test Methods for Tension Testing of Metallic Materials. ASTM International, PA 19428-2959, United States. [13] ASTM standard E23- 12C. Standard Test Methods for Notched Bar Impact Testing of Metallic Materials, ASTM International, PA 19428-2959, United States. [14] Hiroshi Yaguchi, Effect of MnS inclusion size on machinabilty of low-carbon, leaded, resulfurized free-machining steel, Journal of Applied Metalworking, July 1986, Volume 4, Issue 3, pp 214-225. [15] Advances in the technology of stainless steels and related alloys, ASTM special technical publication, No, 369, by American Society for Testing, Materials, Philadelphia, ASTM, 1965.
11
Table 1 Chemical composition of turbine blade materials Blade No.
C
Mn
Cr
Mo
Ni
P
S
Si
Fe
17
0.18
0.50
13.54
0.07
0.44
0.018
0.0068
0.196
Rem
18
0.15
0.45
13.27
0.05
0.45
0.02
0.0076
0.31
Rem
19
0.18
0.45
13.39
0.047
0.54
0.020
0.0074
0.28
Rem
<0.15
<1.0
11.5-13.5
---
<0.04
<0.03
<1.0
Rem
ASTM: 410 Steel
12
Upto 0.75
Table 2 Tested and FEM predicted properties of blade materials
FEM
Experimentally Tested
Property
Blade No. 17
Blade No. 18
Blade No. 19
3
Density (g/cm )
7.682
7.699
7.681
Hardness (HV) at 20 Kg load
241±3
532±4
247±2
Charpy Impact Energy (Joule)
45.4
5.68
45
Ultimate tensile strength (MPa)
827
1620
802
% Elongation
23.6%
14.7%
26.4%
% area in reduction
59.6%
2.45%
64.9%
Energy Dissipation (Joule)
1095
145
1237
284
155
627
Predicted energy dissipation at applied loading condition (Joule)
13
3
1
1
2
(a)
23
(b)
Crack
Thinning
(c)
(d)
(e)
Fig. 1. Blade Examination: (a) Un-broken blade (blade No. 17); (b) Broken blade piece (blade No. 18); (c) fractured surface ahead of boss portion; (d) LPT fluorescent mode (e) Thinning of blade and location of crack initiation
14
Fig. 2. Tested tensile samples for turbine blade
15
Fig. 3. Tested Charpy impact samples for turbine blade
16
Fig. 4. Solid model created for blade
17
Fig. 5. FEM mesh used for steam turbine blade
18
Fig. 6. Location of crack initiation in blade No. 18 using tensile test fracture energy
19
Possible Crack Location
Possible Crack Location
Possible Crack Location
Fig. 7. Region demarcation with Charpy impact energy in failed blade No. 18
20
(a)
(b)
(c)
Fig. 8. Fractographs of Blade No. 18: (a) Inter-Granullar Crack; (b) Trans-Grannular Crack; (c) Corrosion Product 21
(a)
(b)
(c) Fig. 9. Un-etched specimens: (a) Blade No. 17; (b) Blade No. 19 and (c) Blade No. 18
22
(a)
(b)
Edge
(c)
(d)
Fig. 10. Photomicrograph of: (a) Blade No. 17; (b) Blade No. 19; (c) Blade No. 18; and (d) Blade No. 18 near the edge
23
(a)
24
(b) Fig. 11. EDS analysis results for Blade No. 18: (a) On the grain boundary; (a) Near the edge
25
Highlights
Failure investigation of low pressure steam turbine blade Identification of root cause of failure, crack initiation location and sequence of failure Illustration of use of mechanical, microscopic and FEM tools in failure investigation A rational approach is presented to do any such failure investigation, which validates well with the evidences of blade failure
26
S1350-6307(15)00068-0 http://dx.doi.org/10.1016/j.engfailanal.2015.02.012 EFA 2512
To appear in:
Engineering Failure Analysis
Received Date: Revised Date: Accepted Date:
31 October 2014 19 January 2015 27 February 2015
Please cite this article as: Saxena, S., Pandey, J.P., Solanki, R.S., Gupta, G.K., Modi, O.P., Coupled mechanical, metallurgical and FEM based failure investigation of steam turbine blade, Engineering Failure Analysis (2015), doi: http://dx.doi.org/10.1016/j.engfailanal.2015.02.012
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Coupled mechanical, metallurgical and FEM based failure investigation of steam turbine blade Sanjeev Saxena1, J. P. Pandey, Ranjit Singh Solanki, Gaurav K Gupta, O. P. Modi CSIR- Advanced Materials and Processes Research Institute, Hoshangabad Road, Bhopal, India.
ABSTRACT Steam turbine blades are the critical component in power plant, specifically low pressure blades are generally found to be more susceptible to failure. A mechanical, metallurgical and FEM based coupled methodology is used in the present failure investigation of low pressure steam turbine blade. The results of each investigation of turbine blade failure were then interpreted that leads to find the location of primary failure, sequence of failure and the root cause of its failure. All the three aspects of failure investigation are important in answering the questions raised for the failure and to avoid any future miss-happening. Keywords: Failure investigation Steam turbine blade, , FEM, mechanical testing, metallurgical evaluation
1. Introduction Thermal Power Plants contributes nearly 57% of India’s installed electricity capacity. Steam turbine blades are the critical component in power plant which converts the linear motion of high temperature and high pressure steam flowing down a pressure gradient into a rotary motion of the turbine shaft. The low-Pressure(LP) turbines blades, design to extract the final remnant of energy from the passing steam flow, are relatively large scale rotating airfoils due to the significant centrifugal forces experienced during normal operation. Researches have shown that the LP blades are generally found to be more susceptible to failure than intermediate and high pressure blades. The literature survey revealed that the major cause of failure of turbine blade is known to be fatigue [1, 2]. Excessive mechanical vibration, resulting in large alternating stresses
1
Corresponding author. Tel.: 91+245760 (Ext. 1245); fax: 91+2457042 E-mail address: [email protected] (S. Saxena).
1
in conjunction with the steady state centrifugal and gas bending stresses are the reason for such fatigue failure [3, 4]. Generally such failure investigations have been done using only either, fractography, EDS and microstructure [3,6-8] or only done using numerical technique like FEM [1, 5, 9] has been used. The possible questions which are required to be answer in any such failure investigation are: Where are the critical locations of primary failure which needs more precautions to avoid such failure in future and the first crack initiation point?, what is the sequence of failure? and what is root cause of failure material or design?. Thus there has to be a holistic approach to carry out any such type of failure investigation. In the presented failure investigation work a mechanical, metallurgical and FEM based coupled methodology was used to find the answers of the questions raised above. In the present work a failure investigation was carried out in a more than 30 years old LMW design thermal power plant. The present turbine blade failure was of Low Pressure (LP) Turbine which has 8 stages (24-31). The one blade of stage 26th got fractured from the assembly of 92 blades. These blades were relatively new and replaced nearly three-four months back. It was reported that the LP turbine unit was shut down after a severe vibration was noticed immediately when the plant comes in operation, for half an hour, after a weeks time of plant repair. After opening the turbine casing it was found that one blade No.18 of stage 26 got fractured and in consequence several other blades were also found damaged in the near-by area. The present investigation started with the study of plant's record and the proper assessment of service exposed conditions. As the referred blades are relatively new, the possibility of blade failure due to fatigue is minimum. In the present investigation the essential experiments were then planned to characterize the material of failed and un-failed blades. The following essential examinations were carried out: visual examination, non-destructive examination, chemical analysis, physical analysis, hardness testing, tensile testing, impact testing, fractographic examination, metallographic analysis and elemental analysis (through EDS). An in-depth review of service exposed condition was carried out by creating solid modeling of turbine blade and analyzed it using finite element method (FEM). The results of each investigation were then interpreted that leads to find the location of primary failure of the turbine blade, sequence and the root cause of its failure
2
2. EXPERIMENTATION 2.1 Visual examination The failed blade (No.18) along with two adjacent blades (i.e blade No.17 and No.19) were brought to the institute for laboratory investigation. The blades were measured and found to be averaging 0.848 meter in length (Fig. 1(a)). Fig. 1 showed the un-failed and failed turbine blades and locations of failure in blade No. 18. Failure in blade No. 18 occurs from three major places, one near the boss, second at the root of the blade and the third from the pin location (Fig. 1(a & b)). During the visual examination, no corrosion or pitting marks were observed on the failed or un-failed blades. It is also observed that the failure is brittle in nature which can be seen from the fractured surface of failed blade No. 18 near the boss section (Fig. 1(c)). 2.2 Liquid penetrant test Non-destructive examination of fractured surfaces of failed and un-failed blades were done using Liquid Penetrant Test (LPT) in the fluorescent mode. It was performed to find the post failure surface defects (Fig. 1(d)). The locations of cracks were confirmed through the LPT tests that were observed in visual examination. Figure 1(e) clearly shows the location of crack initiation in LPT test (fluorescent mode) and the cross section thinning near the boss portion of blade in un-failed blade No. 19. 2.3 Chemical composition Emission Spectrophotometer is used to find the chemical composition of blade materials. An average measurements of chemical composition for all the three blades has been found to be nearly same as given in Table 1. It can be seen in Table 1 that the chemical composition values obtained for all the three blades material nearly resembles to ASTM410 grade (martensitic stainless steel) 10]. 2.4 Density The density of turbine blade samples were measured by water displacement technique with the help of Mettle Microbalance [11]. An average measurements of density has been reported in Table 2. It can be seen that there is also not much variation in the density of three blades material. 3
2.5 Hardness Using universal hardness tester the Vickers hardness measurements were carried out for all the three blades material and its average values are listed in Table 2. The hardness testes were performed at an applied load of 20 kg. The hardness values of un-failed blades No. 17 and No. 19 were observed to be within the standard range and also confirms to proper tempering treatment. Whereas in failed blade No. 18, it is showing almost double hardness value that could be possible due to improper tempering. The higher hardness of the failed blade corresponds to martensitic structure. 2.6 Tensile testing The tensile test specimens were prepared using the material taken from root of each blade as per ASTM E8 code [12]. Three tensile specimens were prepared from the two un-failed blades (blade No. 19 and blade No. 17), whereas one tensile specimen could be prepared from failed blade (blade No. 18) because of test material shortage in this case (Fig. 2). These specimens were tested at room temperature at a strain rate of 0.01. The tensile specimens were having 8mm gauge diameter and a gauge length of 36mm. The important tensile property data obtained by testing tensile specimens prepared from three blade material is given in Table 2. It can be seen in Table 2, that though the ultimate tensile strength (UTS) value of failed blade No. 18 is higher than the other two un-failed blades but there is a significant reduction in percentage elongation and cross sectional area reduction before the failure of tensile specimen. In case of failed blade No. 18, the failure of tensile specimen is sudden and brittle without much reduction in cross section area (2.45 % only). Whereas in the other two un-failed blade tensile test results, the failure is ductile and with significant reduction in cross-sectional area (60-65%). Failed blade No. 18 also has reduced energy dissipation under quasi-static condition (145 Joule) as compared to other two un-failed blades (Table 2). The energy dissipation under quasi-static in failed blade No. 18 material is 87-88% lesser than unfailed blade cases. The significant reduction in it may be the cause of failure of Blade No. 18. 2.7 Impact testing
4
Charpy Impact test of specimen were carried on using Instron high energy impact tester. The machine has arm angle of 143 degree and the weight of hammer is 43 kg. The specimen were prepared using ASTM standard E23 [13]. The average reading has been reported in case of two un-failed blade and only one specimen prepared and tested in case of failed blade No. 18 due to shortage of material. The impact tested sample for blade No. 18 is illustrated in Fig. 3. In Table 2 it can be seen that failed Blade No. 18 has reduced impact resistance (5.68 Joule only) as compared to other two un-failed blades. The impact resistance of failed Blade No. 18 material is 87-88% lesser than the blade material of un-failed blades. The reduction in impact resistance may be the cause of failure of blade No. 18.
3. FEM based assessment of failure Due to the damage of blade No. 18, the geometry of Blade No. 17 is used to generate the solid model of turbine blade. The variations of cross-section and the angle of rotation with the change in length of blade are taken careoff to generate the correct geometry of the blade. There is a difference in pin position in alternate blades, therefore for Blade No. 17 and Blade No. 19, pin location model exactly in these blades as shown in Fig. 4. The true-stress strain curves used in the FEM analyses are obtained by conducting tensile tests of all the three blade materials. The material Young’s Modulus (E ) = 203GPa and poison’s ratio = 0.3 is used in the analysis The FEM analyses are carried out using commercially available ABAQUS software. The FEM mesh used in the analysis of blade is shown in Fig. 5. The mesh is having 24,225 elements and 26,487 number of nodes. Eight nodded hexahedral elements are used in most of the places, except at some less significant location where tetrahedral elements have been used. The explicit solver with reduced integration technique is used in the analysis. The steam turbine is subjected to several loads, but centrifugal force is major among all of them. The two applied forces considered to act on the blade are centrifugal force and steam pressure. The data used to estimate the two applied loads are: angular velocity of the blade = 3000rpm, material density (as determined in the present study) = 7699 Kg/m3, volume of blade ( as determined numerically) = 9.5176 x 10-4 m3, length of the blade = 0.848m, distance of blade from the centre of rotation = 0.250m. The present case of LP turbine operates at designed inlet steam pressure of 1.38 kg/cm2 and the exhaust steam pressure of 0.1042 kg/cm2. Using the stated 5
data the estimated centrifugal force is 794.285kN and load due to stream pressure is 153.177kN, which was assumed to act on a projected area of 122,389mm2. These two forces are applied on the turbine blade using the couplings as available in ABAQUS code. It can be seen in Table 2 that in case of Blade No. 18, the energy dissipated under quasi-static condition is higher than the fracture energy capacity of the material. Whereas in the other two un-failed blade, the energy dissipated due to applied loads is less than the fracture energy capacity of these blade material, therefore no-failure occurs in Blade No. 17 and Blade No. 19. In case of Blade No. 18. the FEM results also confirm the location of first crack initiation which corresponds to the maximum energy dissipation as shown in Fig. 6. In FEM analysis results, it can be seen that maximum energy dissipates (crack initiation occurs) near the upper edge of Boss and towards the inner side (i.e. concave side) of the blade. At this location, there is a reduction in cross section of blade resulting in stress concentration and as a result the crack initiation and failure occurs from this portion. Finally the upper portion got detached at the boss location. The evidence of FEM predicted location of crack initiation is also confirmed while performing Liquid Penetration Test (LPT) in florescent mode, in which a clear crack initiation and thinning of blade had been seen, which fully support the FEM prediction (Fig. 1(e)). Assuming that top portion of the Blade No. 18 failed (marked as location 1 in Fig. 1(a & b) and got separated at boss location and strike the remaining portion of the blade and a dynamic event took place. Therefore, in the FEM results of Blade No. 18, demarking the region where the energy exceeds the impact energy (5.68 Joule) obtained in impact Charpy test (Table 2), for Blade No. 18 case. Figure 7 shows the regions where the energy dissipation is greater than the impact energy of the Blade No. 18 material. These regions approximately define the locations where crack formation and growth possibility is high. These demarcated resembles well with the actually failed three regions obtained in Blade No. 18 as shown in Fig. 1. It can also be seen in Fig. 7, there is variation of energy across the bottom regions of blade. This might have twisted the bottom region of the failed blade, which can be seen in Fig. 1(b). The other possibilities of failure in blade No. 18 at second and third locations have been also investigated in the details. under these conditions, the dissipation of energy is lesser than the capacity of the material under quasi-static condition therefore the failure at other two location (location 2 & 3) can only be possible under dynamic conditions. 6
It is very interesting to examine in two un-failed blade that the crack initiation occurs only in un-failed Blade No. 19 and not in un-failed Blade No. 17. This is quite evident that due to rotation of the blade, impact of failed blade is only on one blade which is following the failed Blade No. 18 and not on both the adjacent blades. Hence, this also validated our understanding of this failure. The other possibilities of failure of Blade No. 18 are also investigated quite carefully, but non holds good for the formation of cracks at three specified locations. 4. Fractographic and microstructural examination For micro-structural examinations, the specimens of desired dimension were cut from three turbine blades roots and observe in transverse and longitudinal directions. Specimens for micro structural examination were prepared by standard metallographic practices. The process involves specimen polishing with different grades of emery papers and fine polishing using colloidal alumina suspensions on Sylvie clothes. Metallographical polished specimen was etched with picral. The etched specimens were examined under FESEM (FEI Nova Nano SEM 430) Netherlands. The un-etched metallographically polished specimen was examined for nonmetallic inclusion in an advanced motorized optical microscope of LEICA make, Germany. The fractured surface of the failed turbine blade was extracted from the region of failure for fractographic study. These were subjected to ultrasonic cleaning in an organic solvent to remove all loosely adhering external material. 4.1 Fractographic analysis The fractographic study conducted on SEM revealed inter-grannular crack at several locations and in only few areas trans-granular cracks (Fig. 8 (a & b)) were observed. Figure 8(c) is showing small amount of corrosion product found at some location due to the depletion of Chromium. 4.2 Microstructural analysis 4.2.1 un-etched un-etched specimens prepared from the three blade materials were observed for microstructures. In general, the microstructure consisted of mangnese sulphide (MnS) particles which are introduced to improve machinability of martensitic stainless steel. The specimens from un7
failed blade showed MnS particles having size of 15-25 microns, whereas in failed turbine Blade No. 18 it is in the range of 5-10 microns (Fig. 9 a-c)). The presence of smaller size MnS particle in case of failed Blade No.18 resulted in poor machinability that found during the preparation for various test specimens [14]. 4.2.2 Etched The etched microstructure of both the un-failed blades (No. 17 and No. 19) exhibited uniform tempered martensitic structure without the presence of grain boundary thereby indicating that the M3C type carbide fully dissolved in the matrix for the formation of M 22C6 type carbide (Fig. 10 (a & b)). The microstructure of etched specimen of failed blade No. 18 also revealed the presence of tempered martensitic structure with prominent grain boundary containing carbides (mostly iron carbide and not the mixed carbide). These carbides can be seen as the white lines at grain boundaries (Fig. 10(c)). These continuous network of non-metallic carbides at the grain boundary significantly reduces metal to metal bonding and the ductility as well as toughness of the material. Such type of carbides facilitate crack initiation and propagation leading to the reduced life of a component. This has also been confirmed by EDS analysis (Fig. 11(a)). In addition, in ASTM 410 material, tempering in 425-600oC range produces a change from M3C type carbide to M7C3 and M22C6 type carbides in the matrix [15]. The tempering in this range accompanied by a modular degree of secondary hardening lead to notable loss of toughness and an increase in the corrosion rates. The appearance of M7C3 carbide is accompanied by an increase in Cr content of the carbide and thereby results in some localized Cr depletion. In case of failed blade No. 18 material properties showed a very high hardness and UTS with a low toughness. This is also confirmed through micro-structural examination exhibiting carbide formation at the grain boundary in case of failed turbine Blade No. 18. The presence of carbide at the grain boundary in the case of failed turbine blade specimen has been observed to be Iron Carbide and not the mixed carbide, which is undesirable. This showed the possibility of tempering of this blade material took place between 425 oC and 600oC, resulting in development of temper embrittlement. With tempering treatment higher than 600 oC, resulting in a progressive decrease in the proportion of M7C3 carbide and an increase in proportion of M22C6 carbide. This results in a 8
rapid loss of hardness and increase in the toughness values. In failed blade microstructure Cr-free carbide at grain boundary are seen. As a result Cr depleted localized regions generated, which might have resulted in corrosion product as shown in Fig. 10(d). This has also been confirmed by EDS analysis (Fig. 11(b)).
5. Conclusions The following conclusions of turbine blade failure are drawn in the present investigation:
The first location of crack initiation in failed blade No. 18 is found above the Boss and on the upper side towards concave face (Fig. 1 (a) and (b) for location).
Under quasi-static condition: The FEM predicted energy dissipation for failed Blade No. 18 is higher than the capacity of the blade material (Table 2). The energy dissipated is just sufficient to initiate the crack but it took time (nearly three-four months or inservice life of this blade) for this crack to cross the critical limit and damage the blade near the boss (1st location in Fig. 1(a& b)). In case of un-failed blade No. 17 and 19 the energy dissipation is lower than the capacity of these blade materials. The energy dissipated is not sufficient to initiate the crack in these blades.
Under dynamic condition: The development of cracks at other two locations (No. 2 & 3), as shown in Fig. 1(a &b) in failed Blade No. 18, can only be possible under a dynamic condition created when the blade failed across the boss strikes the remaining blade. In case of blade No. 17 and 19, crack initiation ahead Boss portion is only predicted under dynamic condition. But crack initiation is examined (Fig. 1(e)) near the boss only in Blade No. 19 due to the continuous rotation of the turbine blade which corroborates well with the assumed hypothesis.
The un-etched specimen exhibited manganese sulphide (MnS) particles in all the three blades. Blade No. 17 and 19 have large size of MnS particles as compared to blade No. 18, which has resulted in poor machinability in blade No. 18.
Variation of material properties among the failed and un-failed blades and also the microstructural observation supported with EDS analysis showed enough evidences that the tempering in failed blade No. 18 may have been done in the range of 450 o C to 600o C, as compared to tempering at above 600oC, may have been done in case of un-failed 9
blades No. 17 and No. 18. This resulted in higher hardness, low toughness, low resistance to corrosion and Cr depleted region in the matrix and Cr free carbides at the grain boundaries in case of blade No 18, which may be the cause of failure in blade No. 18 (Fig. 11(a)).
In consideration to the various findings obtained by coupling the mechanical, microstructural and FEM based evaluation of the steam turbine failure investigation, the root cause for the failure of turbine blade No. 18 appears to be the inferior mechanical and micro-structural properties, which might be due to improper secondary processing, in general and the tempering operation, in particular.
Acknowledgment The authors are thankful to N. Prashant, Anup Kumar and Mohamed Shafiq for their assistance.
References
[1] P. Mˇeˇst’´anek. Low cycle fatigue analysis of a last stage steam turbine blade, Applied and Computational Mechanics 2 (2008), pp. 71–82. [2] G. D. Robinson, Mipenz. Review of fatigue failure in spindle blades of a 120 MW steam turbine, IPENZ Transactions, 2000, Vol. 27, No. 1/Gen, pp. 25-30. [3] Paul S. Prevéy, Dr. Nayarananan Jayaraman, Ravi Ravindranath. Fatigue life extension of steam turbine alloys using low plasticity burnishing (LPB), Proceedings of ASME Turbo Expo 2010: Power for Land, Sea and Air, June 14-18, 2010, Glasgow, UK. [4] Chi-Hshiung. Prediction of corrosion fatigue damages for turbine blades subjecting to randomly distributed power system unbalance, JSME Int. Jrl., Vol. 47(1), A, 2004. [5] Zdzislaw Mazur, Rafael Garcia-Illescas, Jorge Aguirre-Romano, Norberto Perez-Rodriguez. Steam turbine blade failure analysis, Engineering Failure Analysis 15 (2008), pp. 129–141. [6] J. Kubiak Sz., G. Urquiza B., J. Garcı´a C., F. Sierra E. Failure analysis of steam turbine last stage blade tenon and shroud, Engg Failure Analysis 14 (2007), pp. 1476 -1487.
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[7] Hassan Farhangi and Ali Asghar Fouladi Moghadam. Fractographic investigation of the failure of second stage gas turbine blades, Proceedings of 8th Int. Frac. Conf. 7 - 9 Nov. 2007, Istanbul/Turkey. [8] Wei-Ze Wang , Fu-Zhen Xuan, Kui-Long Zhu, Shan-Tung Tu. Failure analysis of the final stage blade in steam turbine, Engineering Failure Analysis 14 (2007), pp. 632–641. [9] J. Kubiak Sz , J.A. Segura, G. Gonzalez R, J.C. García, F. Sierra E, J. Nebradt G, J.A. Rodriguez. Failure analysis of the 350MW steam turbine blade root, Engg Failure Analysis, 16 (2009), pp. 1270–1281. [10] Metals Handbook, Howard E. Boyer and Timothy L. Gall, Eds., American Society for Metals, Materials Park, OH, 1985. [11] ASTM B962-14 Standard Test Methods for Density of Compacted or Sintered Powder Metallurgy (PM) Products Using Archimedes’ Principle, ASTM International, PA 194282959, United States. [12] ASTM standard E 8M-04. Standard Test Methods for Tension Testing of Metallic Materials. ASTM International, PA 19428-2959, United States. [13] ASTM standard E23- 12C. Standard Test Methods for Notched Bar Impact Testing of Metallic Materials, ASTM International, PA 19428-2959, United States. [14] Hiroshi Yaguchi, Effect of MnS inclusion size on machinabilty of low-carbon, leaded, resulfurized free-machining steel, Journal of Applied Metalworking, July 1986, Volume 4, Issue 3, pp 214-225. [15] Advances in the technology of stainless steels and related alloys, ASTM special technical publication, No, 369, by American Society for Testing, Materials, Philadelphia, ASTM, 1965.
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Table 1 Chemical composition of turbine blade materials Blade No.
C
Mn
Cr
Mo
Ni
P
S
Si
Fe
17
0.18
0.50
13.54
0.07
0.44
0.018
0.0068
0.196
Rem
18
0.15
0.45
13.27
0.05
0.45
0.02
0.0076
0.31
Rem
19
0.18
0.45
13.39
0.047
0.54
0.020
0.0074
0.28
Rem
<0.15
<1.0
11.5-13.5
---
<0.04
<0.03
<1.0
Rem
ASTM: 410 Steel
12
Upto 0.75
Table 2 Tested and FEM predicted properties of blade materials
FEM
Experimentally Tested
Property
Blade No. 17
Blade No. 18
Blade No. 19
3
Density (g/cm )
7.682
7.699
7.681
Hardness (HV) at 20 Kg load
241±3
532±4
247±2
Charpy Impact Energy (Joule)
45.4
5.68
45
Ultimate tensile strength (MPa)
827
1620
802
% Elongation
23.6%
14.7%
26.4%
% area in reduction
59.6%
2.45%
64.9%
Energy Dissipation (Joule)
1095
145
1237
284
155
627
Predicted energy dissipation at applied loading condition (Joule)
13
3
1
1
2
(a)
23
(b)
Crack
Thinning
(c)
(d)
(e)
Fig. 1. Blade Examination: (a) Un-broken blade (blade No. 17); (b) Broken blade piece (blade No. 18); (c) fractured surface ahead of boss portion; (d) LPT fluorescent mode (e) Thinning of blade and location of crack initiation
14
Fig. 2. Tested tensile samples for turbine blade
15
Fig. 3. Tested Charpy impact samples for turbine blade
16
Fig. 4. Solid model created for blade
17
Fig. 5. FEM mesh used for steam turbine blade
18
Fig. 6. Location of crack initiation in blade No. 18 using tensile test fracture energy
19
Possible Crack Location
Possible Crack Location
Possible Crack Location
Fig. 7. Region demarcation with Charpy impact energy in failed blade No. 18
20
(a)
(b)
(c)
Fig. 8. Fractographs of Blade No. 18: (a) Inter-Granullar Crack; (b) Trans-Grannular Crack; (c) Corrosion Product 21
(a)
(b)
(c) Fig. 9. Un-etched specimens: (a) Blade No. 17; (b) Blade No. 19 and (c) Blade No. 18
22
(a)
(b)
Edge
(c)
(d)
Fig. 10. Photomicrograph of: (a) Blade No. 17; (b) Blade No. 19; (c) Blade No. 18; and (d) Blade No. 18 near the edge
23
(a)
24
(b) Fig. 11. EDS analysis results for Blade No. 18: (a) On the grain boundary; (a) Near the edge
25
Highlights
Failure investigation of low pressure steam turbine blade Identification of root cause of failure, crack initiation location and sequence of failure Illustration of use of mechanical, microscopic and FEM tools in failure investigation A rational approach is presented to do any such failure investigation, which validates well with the evidences of blade failure
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