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AERO ENGINE INSPECTION

Non-destructive testing is extensively used on aero enginecomponents to ascertain their quality, integrity andreliability. Testing methods have been continuously improvedto yield quantitative data and are being effectively used notonly for conventional defect detection but also for materialcharacterisation, component life estimation studies, etc.Turbine rotor blades are life critical components in aeroengines and pose a technical challenge for inspectionowing to their complex design, manufacturing processand operating conditions. Interpretation, evaluation andquantication of the indications are difcult and aregenerally accomplished by complementary NDT methods.This paper gives an overview of the various NDT methodsapplied on a military aero engine turbine rotor blade, alongwith the results obtained, and also explores the possibilitiesof the application of emerging inspection technologies.

Keywords: Non-destructive testing, turbine rotor blade,

radiography, penetrant testing, eddy current testing,

ultrasonic testing.

1. Introduction

Non-destructive testing (NDT) methods are extensively

applied to ascertain the integrity and reliability of various

components used in aero engines[1]. Military aero engine

components are generally designed using a fail-safe approach.

The fail-safe approach provides that the component is free from

unacceptable defects; it is designed to remain intact for the

whole of its planned life[2]. These components are classified into

life critical, mission critical and non-critical components.

Producibility clearance ensures that the components can be inspectedby one or more NDT methods. Typical faults and defects targeted

in the NDT of gas turbine components include original defects

and deviations from manufacturing or repairs, as well as defects

in the coatings and base materials emerging and growing during

service. Apart from cracks or other discontinuities, the deviations

can also appear, for example, as wear, corrosion, excessive strain,

or blocking or inappropriate positioning of the cooling channels [3].

Defects in a coating on blade surfaces may be caused by hot

corrosion and oxidation, low resistance of the blade construction

to thermal fatigue, poor high-temperature strength and endurance

of the substrate, or by a combination of these factors [4]. Among the

critical components, blades from the compressor and turbine are

most vulnerable to rejection. Turbine rotor blades are life critical

components in an aero engine and are responsible for conversionof the thermodynamic energy into mechanical energy. Gas turbine

blades experience a complex thermal and mechanical loading

history during a typical cycle of operation, consisting of

start-up, steady-state operation and shutdown[5]. These blades are

subjected to alternating stresses, vibrations and extreme temperature

conditions and hence are likely to develop defects typical of

fatigue, creep and stress rupture[6]. This paper gives an overview of

the typical defects occurring on turbine rotor blades along with the

various NDT methods applied and also discusses the characterisation

and quantification of defects with relevant case studies. However,

the role of operator training and certification for the reliable

detection of these defects plays a major role, along with testing

equipment and organisational procedures[7]. The location of turbine

rotor blades in a typical twin-spool engine layout is illustrated in

Figure 1.

2. Turbine rotor blade

The turbine rotor blades discussed in this paper are made

from nickel-based superalloy Su247 by the investment casting

method. The castings are manufactured in ceramic shell moulds

by the directional solidication route under vacuum and are

further subjected to solution heat treatment and precipitation

hardening. The blades are then nish machined in the root

region. They are nally coated with oxidation and a corrosion-

resistant coating on the aerofoil by a diffusion pack aluminising

process. Figure 2(a) shows a photograph of the nished blade and

Figure 2(b) shows the schematic cross-section of the aerofoil of

the blade.

M R Vijaya Lakshmi* is working as Scientist D at GTRE, Bangalore. Shehas a BTech in Mechanical Engineering. She is certied as an ASNT NDT

Level III in RT, UT, MT and PT methods and has over 12 years of experience

in NDT. She has published ten conference papers and one journal paper.

She is a trained Lead Auditor in ISO 9001:2008. She is a life member of

ISNT and a member of ASNT.

A K Mondal is working as Technical Ofcer at GTRE, Bangalore. He

has a Diploma in Mechanical Engineering. He is certied as an ISNT NDT

Level II in UT, MT and PT methods and has over 21 years of experience in

NDT. He has published one conference paper. He is a life member of ISNT.

C K Jadhav is working as Scientist F at GTRE, Bangalore. He has an

MTech in Mechanical Engineering. He is Group Director of the Quality

Assurance Group and has over 30 years of experience in the elds of aero

engine assembly, NDT and engine testing. He has published one conference

paper.

B V Ravi Dutta is working as Scientist F at GTRE, Bangalore and heads

the Quality Assurance Group. He has a BE in Mechanical Engineering. He

is Group Director of the Quality Assurance Group and has over 25 years

of experience in the elds of metrology, aero engine assembly, NDT and

engine testing. He is a successful internal auditor and is the Management

Representative of GTRE. He has published one conference paper.

Sreelal Sreedhar is working as Scientist G at GTRE, Bangalore. He has an

ME in Mechanical Engineering. He is Associate Director of the Reliability

and Quality Assurance Group and has over 25 years of experience in the

elds of rotor dynamics, metrology, aero engine assembly, NDT and engine

testing. He has published four conference papers.

The authors are with the Quality Assurance Group, Gas Turbine Research

Establishment, Bangalore, India.

*Corresponding author. Tel: +91 080 2504 0743; Fax: +91 080 2524 1507;

Email: [email protected]

Overview of NDT methods applied on an aero engine

turbine rotor blade

M R Vijaya Lakshmi, A K Mondal, C K Jadhav, B V Ravi Dutta and S SreedharSubmitted 20.01.13Accepted 06.03.13

DOI: 10.1784/insi.2012.55.9.482

482 Insight Vol 55 No 9 September 2013


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Insight Vol 55 No 9 September 2013 483

3. NDT methods

3.1 Visual testing

Visual testing is the fundamental and preliminary NDT method

and usually precedes other methods. It can be accomplished

by the naked eye and magnication aids during inspection aftermanufacturing and engine disassembly.In-situinspection is carried

out in a test-bed by inserting a borescope or videoscope through

specially provided ports on the turbine casing. Figures 3(a), 3(b)

and 3(c) represent a few defects observed during visual testing of

turbine rotor blades.

3.2 Fluorescent penetrant testing

Penetrant testing is the NDT method most frequently used for

inspecting gas turbine blades and vanes. It is often recommended

to use uorescent dye penetrant (Type I) for inspection [1]. Type

II penetrant examination shall not be used for nal acceptance

examination of aerospace products[8]. An ultra-high sensitive

hydrophilic penetrant/emulsier system is used for inspection.

100% inspection is carried out on the cast blades preceding thecoating operation. After coating, the blades are nally inspected

for clearance for utilisation in the engine. Thereafter, in-service

inspection is carried out after every

disassembly to check for the initiation and

propagation of defects related to fatigue,

creep, oxidation, foreign/internal object

damage, nicks, etc. Figures 4(a), 4(b) and

4(c) show the typical defects detected on a

blade after removal from the engine.

Generally, defects such as cracks noticed

during inspection are not amenable for repair/

rework and the blades are rejected. However,indications due to nicks or foreign object

damage may be reworked by buffing and

subjected to re-inspection. While there are

no technological developments in the area

of penetrant testing, process improvements

are continuously in progress. Studies are also being carried out to

quantify the capability and reliability of fluorescent penetrant testing

systems intended for use on gas turbine engine components[9].

3.3 Ultrasonic testing

The blades under discussion are dual-walled components. These

blades are designed to ensure maximum conversion of energies

Figure 1. Schematic layout of engine showing location of turbine rotor blades

Figure 2. (a) Photograph of turbine rotor blade; (b) cross-sectionof the aerofoil region of blade

Figure 3. (a) Foreign object damage (yellow arrow) and resultantcrack (red arrow) noticed under microscope; (b) pitting noticedon trailing edge; (c) core shift in the blade


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with minimum losses and hence have a complex twisted prole

with continuously-varying wall thickness. Wall thickness is one

of the critical parameters and dictates the stress-rupture life of

the blade. The cooling passages within the blades are formed by

positioning ceramic cores within the shell mould. There is the

possibility of the shifting of cores during pouring of the molten

metal, resulting in a deviation of wall thickness from nominal

values. Ultrasonic measurement using a thickness gauge is the

preferred and most popular inspection method. The scheme for

wall thickness measurement is shown in Figure 5. The criticality of

measurement is the customised calibration, probe positioning andprecise identication of the backwall echo.

The characterisation of creep damage in blade material using

ultrasound imaging[10] and in-situ inspection in the rabbet (fir

tree) of turbine blades using creeping waves[11] are a few of the

innumerable research works being carried out for exploring the

potential of ultrasonic testing in application to aero engine turbine

blades.

3.4 Conventional, microfocal and neutron radiography

Conventional radiography is extensively used for the 100%

inspection of the blades for examining the presence of typical

casting defects, such as unfused chaplets, shrinkage, cracks,

porosity, inclusions, etc. Furthermore, microfocal radiography can

be used to enhance the sensitivity to detect microcracks and other

ne defects[12]. Figure 6(a) shows the radiograph of a blade with a

root crack. Figure 6(b) reveals ner microcracks during microfocalradiography of the blade. Neutron radiography has proven to be the

most efcient non-destructive method for the detection of residual

core material in air-cooled turbine blades[13].

The deployment of real-time radiography (both conventional

and microfocal) with a digital flat-panel will greatly enhance the

speed and reliability of the inspection of turbine blades.

3.5 Computed tomography (CT)

Computed tomography is nding increasing usage in the inspection

of blades as an effective inspection and analysis tool. Initially, a

digital radiograph (DR) of the turbine blade was obtained and then

CT slices were taken at selected locations from the DR image[14].

The images are shown in Figures 7(a) and 7(b). CT images provide

details of core shift, remnant core in cooling passages, if any, anddimensions such as wall thickness, chord radius, twist, etc.

CT data in conjunction with CAD model data will be extremely

helpful in obtaining the densitometry details of blades and will

ultimately lead to an improvement in the production quality of

blades.

3.6 Eddy current testing

Eddy current testing is highly favourable for detecting and sizing

surface cracks[15]. The eddy current system was calibrated using a

customised reference standard and was successfully applied for

detection and quantication of vertical cracks on turbine blades[5].

Figures 8(a) and 8(b) show the reference standard with known

defects and the signal obtained from them, respectively[5]. Figures

9(a) and 9(b) show the defective blade and signal obtained fromthe same[5].

Eddy current testing is also used for measuring coating

Figure 4. (a) Crack on the root region of blade; (b) crack on the

convex llet originating from coating and extending into basematerial; (c) crack originating from aligned porosity on trailingedge

Figure 5. Schematic for wall thickness measurement



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Insight Vol 55 No 9 September 2013 485

thickness and the development of customised probes will enhance

the inspection of leading and trailing edges and firtree location of

the blades. Furthermore, eddy current sensors are being effectively

utilised in the form of tip-timing systems for the health monitoring

of blades during engine operation.

4. Conclusions

Effort has been made to comprehensively present an overview of the

various NDT methods applied on aero engine turbine rotor blades.

Figure 8. (a) LPT rotor blade with reference defects; (b) displayscreen showing signals from 0.2 mm and 0.5 mm defects

Figure 9. (a) Crack along the length of the blade; (b) screendisplay and analysed signal window from cracked region

Figure 6. (a) Conventional radiograph (arrows point to thecrack in the root of the blade); (b) microfocal radiograph (redarrows point to the root crack and yellow arrows point to themicrocracks)

Figure 7. (a) Digital radiograph; (b) tomogram of typical cross-section of blade


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It can be concluded that the in-service inspection of blades poses

greater technical challenges and demands complementary methods

for inspection and quantication. The possibilities of applying

advanced techniques and methods have also been explored.

5. Acknowledgements

The authors express their gratitude to the Director, GTRE, for his

continuous support in carrying out this work and giving permission to

publish the results. The authors also acknowledge the Quality Assurance

Group, Turbine Group, Materials Group, Vibration Engineering Groupand Structural Mechanics Group for their extensive technical support.

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