Overview2006Tensão NDE

8/9/2019 Overview2006Tenso NDE

1/16

OPPORTUNITIES AND CHALLENGES FOR NONDESTRUCTIVERESIDUAL STRESS ASSESSMENT

P. B. Nagy

Department of Aerospace Engineering and Engineering MechanicsUniversity of CincinnatiCincinnati, Ohio 45221-0070, U.S.A.

ABSTRACT. For a long time, nondestructive residual stress assessment has been one of the greatestopportunities as well as one of the greatest challenges for the NDE community, and probably it willremain so in the foreseeable future. The most critical issue associated with nondestructive residual stressassessment seems to be that of selectivity. Numerous NDE methods have been found to be sufficientlysensitive to the presence of residual stress, but unfortunately also rather sensitive to other spuriousvariations that usually accompany residual stresses, such as anisotropic texture, microstructuralinhomogeneity, plastic deformation, etc., which could interfere with, or even overshadow, the elasticstrain caused by the sought residual stress. The only sufficiently selective NDE method that is more or less immune from these spurious effects is X-ray diffraction measurement, which however does not havethe required penetration depth in most applications unless high-energy neutron radiation is used. It istimely for the community to sit back and ask where we are in this important area. This paper presents anoverview of the various indirect techniques that have been used to measure residual stress in the past. Itis shown that traditional techniques have a number of limitations, which have spurred several recentresearch programs. Some of the new techniques that are presently being examined in the NDEcommunity are reviewed and the current status of these research efforts is assessed.

Keywords: residual stress, surface waves, eddy current, thermoelectricityPACS: 81.40.Rs, 81.65.Ps, 81.70.Ex

INTRODUCTION

Nondestructive residual stress assessment in fracture-critical components is one of themost promising opportunities as well as one of the most difficult challenges we face in the NDE community today. Residual stress assessment is important because there is mountingevidence that it is not possible to reliably and accurately predict the remaining service lifeof such components without properly accounting for the presence of residual stresses. Themain reason for this is that the so-called S-N curve of most materials becomes rather flat athigh number of cycles, therefore modest levels of residual stresses on the order of 10-20%

of the yield strength can significantly increase or decrease the remaining service lifedepending on whether they are subtracted from or added to the primary applied stresses.Unfortunately, residual stresses are usually very uncertain. This is partly due to the fact

22

Downloaded 07 Feb 2007 to 143.107.135.27. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp

CP820, Review of Quantitative Nondestructive Evaluation Vol. 25, ed. by D. O. Thompson and D. E. Chimenti 2006 American Institute of Physics 0-7354-0312-0/06/$23.00


2/16

that they have numerous origins which are highly variable and also because they tend toundergo thermo-mechanical relaxation at operational temperatures, which makes themeven more uncertain. Therefore, really the only way to establish their actual level andspatial distribution is by measuring them. Unfortunately, the only currently available NDEmethod for residual stress assessment is based on X-ray diffraction (XRD) measurementthat is limited to an extremely thin, less than 20 m deep, surface layer [1].

One good example of how the issue of residual stress measurement fits into the bigger picture of life management is the Engine Rotor Life Extension (ERLE) program of the National Turbine Durability Initiative [2,3]. This initiative has a number of very ambitiousgoals, for example, it calls for a two-thirds reduction of maintenance costs, which can beachieved in part by doubling component lives, which in turn can be facilitated by, amongother measures, taking advantage of residual stresses. Presently, we are using differentsurface enhancement technologies to reduce protective compressive residual stresses at and below the surface of critical components, but in fact we do not take credit for this when weestimate the fatigue life of these components simply because, without actually measuringthem, we can not be sure that they will be there when we need them. Presently available NDE methods are not suitable for residual stress assessment in surface-enhanced enginecomponents, therefore there is an urgent and pressing need for the development of new NDE techniques, which is the main topic of this paper.

Stresses are defined at any point in the component by sectioning the component anddetermining the tractions acting on the resulting surface. Because of this conceptual needfor sectioning it is almost impossible to measure stresses directly. Most often we measurestrain instead. In that case the actually measured quantityM , e.g., the angle of maximumX-ray diffraction, is

0 (1 )M M + , (1)

where M 0 is the measured quantity in the reference state of the material and is somemeasure of the elastic strain relative to this reference state. Of course strain has a veryintimate relationship to stress through the stiffness of the material, therefore methods basedon strain measurement can be considered to be semi-direct stress assessment. However,very often we cannot measure strain and we have to rely on even more indirectrelationships, such as measuring magnetic permeability or electrical conductivity, whichhave less direct dependence on the deformation of the material

0(1 ... )M M + + . (2)

In these cases the relative sensitivity of the technique, which is measured by the gaugefactor , could be still relatively high, typically on the order of unity. The real problemwith indirect measurements is the lack of selectivity rather than that of sensitivity. Thismeans that these techniques are sufficiently sensitive to elastic deformation for measurement purposes, but they are also sensitive to other intrinsic variations in thematerial, which is conceptually indicated by the series of unspecified variables in the aboveequation.

Figure 1 shows a list of potential NDE methods available for residual stress

assessment. In some very special cases we can conduct essentially direct stressmeasurements. A good example of this is the assessment of the average tensile stresses inrailway rails. Tensile bias is needed to prevent buckling of the rail on a hot summer day.The rails can be separated from the rest of the structure, which includes the supportingground. By measuring the restoring force as a function of deflection one can directly

23



3/16

assess the tensile force in the rail. At sufficiently low frequencies, the flexural stiffness of the rail is perceivably increased by the so-called string stiffness, which is equal to theaverage tensile stress in the rail, therefore precise sound velocity measurements can beused to assess the prevailing tensile residual stress [4].

In most cases, we have to rely on strain-based semi-direct measurements. This class

of methods can be further separated into two main categories which can be referred to asrelaxation type and absolute measurements. In a relaxation type experiment one measuresthe surface deformation caused by sectioning the specimen using electrical dischargemachining, drilling a hole in the surface, or some other means and then calculates theelastic strain that prevailed in the structure before sectioning using either analytical or finite element techniques. Needless to say that these techniques are inherently destructiveand they are relevant for nondestructive evaluation purposes only in the sense that they can be used to verify NDE results. Then, there are absolute strain measurement methods, suchas X-ray and neutron diffraction techniques, that actually measure the inter-atomicdistances in crystalline grains, which then can be related to the absolute elastic strain in thematerial. In theory, these techniques are nondestructive. However, in practiceconventional XRD has such a small penetration depth that one has to remove layer by layer to get some information below the surface, therefore for all practical purposes it is also adestructive technique. Because of this, we often have to rely on indirect techniques and acouple of the best known methods are listed in Figure 1. The most mature and evencommercially available technique is magnetic inspection, but of course it is limited toferromagnetic materials. This paper is aimed primarily at aerospace applications, thereforein the following we will focus on ultrasonic, eddy current, and thermoelectric techniques.

The major sources of residual stresses can be categorized as (i) material-related, (ii) process-related, and (iii) service-related causes. A good example for material-relatedresidual stresses is the tensile hoop stress building up in metal-matrix composites duringmanufacturing due to the thermal expansion difference between the fiber and the matrix.Other examples include materials with multi-phase microstructure or inclusions. Process-related residual stresses include those formed during welding, casting, quenching, cold

direct semi-direct indirect

stiffness hole drilling magneticvibration analysis contour mapping ultrasonic

etc. X-ray or neutron diffraction eddy currentetc. thermoelectric

etc.

FIGURE 1. Potential NDE methods for residual stress assessment.

strain rosettestress relief holeT T

Time

A m p l

i t u d e

H

B noise

24



4/16

working, cutting, case hardening, etc. Most process-related stresses are detrimental for theservice life of the component, but some are beneficial. For example, different surfaceenhancement techniques are used to produce compressive residual stresses below thesurface, which significantly increases the fatigue resistance of the component. In thefollowing, we will focus on such beneficial residual stresses. Finally, there are service-related stresses, such as those produced by plastic deformation at rolling surfaces. Surface

conditions also play another role in residual stress assessment. They are not only one of the possible sources of residual stress, but they are also the primary cause of acceleratedthermo-mechanical relaxation during service.

Another way to categorize residual stresses is through their spatial distribution. It iscustomary to talk about type 1, 2 and 3 residual stresses. Type 1 residual stresses aremacroscopic average stresses over a length scale which is bigger than the characteristicmicrostructural dimension, e.g., the grain size in a polycrystalline material. Type 2residual stresses are microscopic stresses that vary on the scale of individual grains. Thesestresses are due to the anisotropy of the grains or the appearance of different phases.Finally, type 3 microscopic stresses occur on a much smaller scale below the scale of individual grains and are associated with crystallographic imperfections such asdislocations. In the following, the term residual stress will be used exclusively to refer totype 1 macroscopic or average stresses.

Macroscopic stresses can be further be divided into bulk and surface residual stresses.Of course residual stresses always average out to zero over a large area comparable to theoverall dimension of the component. Surface treatments produce strong compressivestresses within a few hundred microns of the surface and these relatively strongcompressive stresses are balanced by very weak tensile stresses throughout the interior of the specimen. Therefore, in the near-surface region there is a non-zero average residualstress which can be measured by the nondestructive techniques to be presented in this paper later. As an example, Figure 2 shows the residual stress profiles as measured byXRD in cold rolled (a) and shot-peened (b) Waspaloy.

By far the most common way to produce such protective surface layers of compressive residual stress is shot penning, though it is probably also the worse techniquefrom the point of view of damaging cold work. Shot peening (SP) uses a stream of particles called shots aimed at the surface. Upon impact, the shots produce plasticdeformation that leads to compressive residual stress below the surface. Other moresophisticated techniques available today are laser shock peening (LSP) and low-plasticity burnishing (LPB). All surface treatment techniques can produce peak residual stresses

close to the level of the yield strength of the material. Although LSP and LPB producemuch deeper compressive residual stress than SP, their main advantage over SP is that they

a) b)

-1000

-500

0

500

1000

0 1 2 3 4 5 6Depth [mm]

R e s i d u a

l S t r e s s

[ M P a ] .

-2000

-1500

-1000

-500

0

500

0 0.1 0.2 0.3 0.4 0.5Depth [mm]

R e s i d u a l S t r e s s [ M P a ]

Almen 4AAlmen 8AAlmen 12A

Almen 16A

FIGURE 2. R esidual stress profile as measured by XRD in cold rolled (a) and shot-peened (b) Waspaloy.

25



5/16

produce much less cold work, which is usually measured in terms of the equivalent plasticstrain. Cold work is very important because it reduces the relaxation temperature andaccelerates the relaxation speed in the material, therefore the lower the cold work induced by surface treatment, the higher the thermo-mechanical stability of the residual stress is.

The easiest way to measure near surface residual stresses is to use X-ray diffraction.The peak diffraction direction is determined by the absolute elastic strain in the material.

At the same time, as a byproduct of this measurement, we also get some information on the plastic deformation in the material because the widening of the diffraction peak is due tothe lack of periodicity in the lattice, which is related to dislocation density and other latticeimperfections. There is only one big problem with this well-established technique, namelythat its penetration depth is only about 10 microns, which is far less than what is neededeven for surface residual stress assessment, let alone the measurement of bulk residualstresses. Although the accuracy of XRD measurements is quite sufficient for life prediction purposes, the necessity of surface layer removal for subsurface measurementsessentially excludes the use of this method as a nondestructive characterization tool.

There are really only two ways to avoid this limitation of XRD, namely either byincreasing the incident beam intensity or by reducing the wave length, which then reducesthe X-ray absorption coefficient of the material so that one gets better penetration. Today,this latter can be achieved only by using either synchrotron radiation or neutron diffraction,which could increase the penetration depth to a few centimeters. That would be, of course,more than sufficient for surface-treated materials and high enough even for many bulk residual stress assessment applications. On the negative side, the spatial resolution of thesetechniques is not too good. The reason for this is that one has to maintain a minimumdiffraction volume to get sufficient sensitivity and that translates into a depth resolution onthe order of 100 microns. That is still enough, although barely, for surface-treatedcomponents, even for shot-peened ones which exhibit rather shallow compressive residualstress layers. Of course, it is a major disadvantage of these techniques that they requireaccess to a synchrotron accelerator or a nuclear reactor. Because of this limitation the NDE community needs to develop additional indirect techniques to assess residual stress profiles in surface-treated engine components. In the following, we will review threedifferent techniques available for non-magnetic materials, namely the ultrasonic, eddycurrent, and thermoelectric techniques.

ULTRASONIC RESIDUAL STRESS ASSESSMENT

Ultrasonic residual stress assessment is based on the so-called acoustoelastic effect,i.e., the strain-dependence of the acoustic velocity in a nonlinear material. The easiest wayto establish the absolute sensitivity of this method is to do velocity measurements in thematerial under uniaxial tension or compression. Figure 3a illustrates the five differentcombinations of wave velocity and polarization that can be considered in an otherwiseisotropic medium in a Cartesian coordinate system aligned with the principal straindirections. The corresponding velocities can be determined from

2 2 (2 ) ( ) (4 4 10 )ii i j k iv m = + + + + + + + + l , (3a)

2 1( ) ( ) 4 22ij i j k i j k

v m n = + + + + + + , (3b)

26



6/16

where denotes the density, and are the two Lam constants and l , m, and n arethe so-called Murnaghan constants [5]. Under the influence of elastic deformation theotherwise isotropic material exhibits slightly anisotropic behavior. The phenomenon thatoffers the easiest separation between the linear effect of changing dimensions and thenonlinear effect of changing velocity is birefringence, i.e., the polarization dependence of the shear wave velocity in a given propagation direction

4( )

4ij ik

j k ij

v v nv

+

. (4)

As an example, Figure 4 shows the measured acoustic birefringence in Al 2024 T351aluminum under uniaxial tension and compression. The normalized sensitivity of theacoustic velocity to elastic strain, i.e., the above mentioned gauge factor , is close tounity. In other words, one percent elastic strain produces roughly one percent change invelocity. It is a small effect, but certainly measurable. The real problem is not that of sensitivity but selectivity. In addition to stress-induced changes in velocity, there may also be crystallographic and morphological texture-induced changes of similar, or even larger,magnitude. In theory, these effects can be separated. As it can be seen from Equation 3b,

2 2( ) 2 ( )ij ji i jv v = , (5)

i.e., the acoustoelastic relationship is not symmetric as i and j are not interchangeable (vij

v ji). Here, i is the direction of the wave propagation direction and j is the polarizationdirection. The strain-induced anisotropy in the material is a nonlinear effect, therefore itdoes not comply with the reciprocity requirement so that the propagation and polarizationdirections are not interchangeable. In contrast, texture-induced anisotropy is a linear effectin which case, due to reciprocity, the propagation and polarization directions areinterchangeable (vij = v ji). In theory, this fundamental difference between the linear effectof plastic strain and the nonlinear effect of elastic strain could be exploited to separate thetwo effects. Unfortunately, such separation requires measurements in orthogonaldirections, which is not an option in surface-treated materials.

The inherently indirect nature of acoustoelastic measurements causes two main

problems as illustrated by the following equation0 [1 ( ) ...] p p pv v + + + . (6)

a) b)

11v

13v

1 x

2 x3

12v

31v

33v 32v

13v12v

3

FIGURE 3. Five different combinations of wave velocity and polarization that can be considered in anotherwise isotropic medium (a) and the special case of birefringence (b).

27



7/16

First, the gauge factor of the elastic strain itself becomes sensitive to the plastic strain.

This is because the acoustoelastic coefficient itself is a measure of material nonlinearity,which is known to increase with plastic deformation and is often exploited for characterization of fatigue damage and plasticity by nonlinear methods. Second, plasticdeformation causes texture that leads to strong additional anisotropy that usually dominatesthe velocity measurement.

The best way to conduct acoustoelastic measurements on surface-treated componentsis by measuring the resulting surface wave dispersion where the penetration depth can beeasily controlled by the inspection frequency. One particular feature of typical surfaceenhancement technologies, especially shot peening, is that they produce a more or lessisotropic plane state of stress, i.e., the resulting stress is essentially the same in every

direction on the surface. This causes an additional problem because it means that theeffective acoustoelastic coefficient will be the sum of the parallel and normal coefficients.Unfortunately, for most materials these coefficients are similarly in magnitude andopposite in sign, which more or less cancels the small stress-induced effect [6].

Under these conditions, even if the peak compressive residual stress reaches the yieldstrengths of the material, the surface wave velocity is expected to increase by a mere half percent. Still, most evidence in the open literature indicates that the surface wave velocitydecreases at increasing frequencies in surface-treated materials [7]. This apparentlycontroversial trend is also illustrated by the examples shown in Figure 5 [8]. Figure 5ashows dispersion curves obtained from laser shock peened (LSP) and low-plasticity

burnished (LPB) IN100 nickel-base superalloy specimens. Because of the rather deepsurface treatment produced by these technologies, the dispersion is strong below 5 MHz,above which the velocity becomes more or less constant. In comparison, similar curvesobtained from shot-peened IN100 specimens show strong dispersion even above 5 MHz,which is mainly due to the excessive cold work that occurs close to the surface. Thismethod is capable of monitoring gradual changes that occur during thermal relaxation.Figures 5c and 5d show how the dispersion decreases after relaxation for one hour atdifferent temperatures.

These results illustrate that the dispersion phenomenon is dominated bycrystallographic and morphological texture since the velocity always decreases rather thanincreases. Still, ultrasonic surface wave velocity measurements are very useful for characterizing both the degree and spatial profile of the surface treatment as well asmonitoring thermal relaxation. Although these dispersion curves cannot be inverted toresidual stress profiles using the acoustoelastic coefficients of the material, they can be

-0.3

-0.2-0.1

0

0.1

0.2

0.3

-0.2 -0.1 0 0.1 0.2Uniaxial Strain [%]

R e l a t

i v e V e

l o c i

t y C h a n g e

[ % ]

normal polarizationparallel polarization

FIGURE 4. Acoustic birefringence in AL 2024 T351 under uniaxial tension and compression.

28



8/16

exploited to assess the stability of residual stresses, which crucially depends on the amountof cold work present in the material.

In order to eliminate, or at least reduce, the dominance of crystallographic texture inthese measurements, one has to use a different type of inspection which is less sensitive tocrystallographic anisotropy. Ultrasonic measurements are sensitive to the elastic stiffnessof the material, which is a fourth-order tensor, therefore it is highly anisotropic even incubic materials. In contrast, thermal and electrical conductivity and thermoelectric power are all second-order tensors which are fully isotropic in cubic materials, therefore they donot exhibit crystallographic texture at all. It should be mentioned that, with the notableexception of titanium alloys, essentially all structural metals are cubic materials.Therefore, the easiest way to eliminate the adverse effects of crystallographic texture is toswitch to another type of inspection, for instance, to eddy current or thermoelectricinspection, that is immune to this effect.

EDDY CURRENT RESIDUAL STRESS ASSESSMENT

Eddy current inspection is very well established and relatively easy to conduct evenunder field conditions, therefore it has been suggested as a leading candidate for near-surface residual assessment [9,10]. It is a highly accurate and reproducible technique andone can easily change the penetration depths pretty much the same way as in ultrasonicsurface wave dispersion measurements, except for the fact that the frequency has to bechanged over a wider range because the eddy current penetration depths is inversely proportional to the square root of frequency rather than to the frequency itself. Eddy

a) b)

-1.6

-1.4

-1.2-1

-0.8

-0.6

-0.4

-0.2

0

0 5 10 15 20Frequency [MHz]

R e l a t

i v e

V e l o c i

t y C h a n g e

[ % ]

LSP

LPB low

LPB high

IN 100

-1.6

-1.4

-1.2-1

-0.8

-0.6

-0.4

-0.2

0

-1.6

-1.4

-1.2-1

-0.8

-0.6

-0.4

-0.2

0

0 5 10 15 200 5 10 15 20Frequency [MHz]

R e l a t

i v e

V e l o c i

t y C h a n g e

[ % ]

LSP

LPB low

LPB high

LSP

LPB low

LPB high

IN 100

-1.6

-1.4

-1.2-1

-0.8

-0.6

-0.4

-0.2

0


R e l a t

i v e

V e l o c i

t y C h a n g e

[ % ]

SP 6A

SP 8A

IN 100

-1.6

-1.4

-1.2-1

-0.8

-0.6

-0.4

-0.2

0

-1.6

-1.4

-1.2-1

-0.8

-0.6

-0.4

-0.2

0

0 5 10 15 200 5 10 15 20Frequency [MHz]

R e l a t

i v e

V e l o c i

t y C h a n g e

[ % ]

SP 6A

SP 8A

SP 6ASP 6A

SP 8A

IN 100

c) d)

R e l a t

i v e

V e l o c

i t y C h a n g e

[ % ]

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0


225 C200 C150 Cintact

Al 2024 Almen 6A

R e l a t

i v e

V e l o c

i t y C h a n g e

[ % ]

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0



Al 2024 Almen 6A

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0 5 10 15 200 5 10 15 20Frequency [MHz]


Al 2024 Almen 6A

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0


R e l a t i v e V e l o c i t y C h a n g e [ % ] a

300 C250 C

225 C

200 C

150 C

intact

Al 2024 Almen 8A

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0 5 10 15 200 5 10 15 20Frequency [MHz]

R e l a t i v e V e l o c i t y C h a n g e [ % ] a

300 C250 C

225 C

200 C

150 C

intact

Al 2024 Almen 8A

FIGURE 5. Dispersion curves obtained from laser shock peened (LSP), low-plasticity burnished (LPB), andshot-peened (SP) IN100 nickel-base superalloy and AL 2024 specimens.

29



9/16

current residual stress assessment is based on the piezoresistivity of conducting materials,which is basically the strain-dependence of the electrical conductivity. Figure 6 showsexamples of piezoresistivity measurements in Waspaloy, IN 718, and Ti-6Al-4V enginematerials using a directional eddy current probe parallel and normal to the applied uniaxialload. Here, the normalized change in conductivity /0 is plotted against the uniaxialstrain ua. The slope of these curves is referred to as the electroelastic coefficient of the

material. There are only two independent electroelastic coefficients, namely the paralleland normal coefficients. In many materials these coefficients are more or less equal inmagnitude and opposite in sign, which renders eddy current conductivity measurementsuseless for residual stress assessment in the case of isotropic plane stress on surface-treatedcomponents. However, there is a very important group of materials, notably nickel-basesuper alloys, where the two coefficients have similar magnitudes and equal signs, thereforethe parallel and normal effects reinforce each other to produce a rather significant stressdependence [11,12].

Figure 7 shows examples of the measured apparent eddy current conductivity (AECC)spectra in shot-peened nickel-base superalloy specimens. The excess AECC increases withfrequency as the eddy current distribution is concentrated more and more into a shallowsubsurface layer and it also increases with penning intensity. Of course, the intrinsicelectrical conductivity of the material is not frequency dependent. We call the measured parameter apparent eddy current conductivity because of the frequency-dependence of thisquantity, which is due to the depth-dependence of the electrical conductivity and thefrequency-dependence of the eddy current penetration depth. One of the most attractivefeatures of the eddy current method is that it can be used to monitor thermal relaxation. Itwas found that the observed excess AECC completely vanishes in nickel-base superalloyswhen the residual stress vanishes even if some of the cold work lingers [9]. This behavior indicates that this technique is not only sensitive to residual stresses, but it is also rather selective to them.

Now, the main question is whether these frequency-dependent AECC spectra can beinverted for residual stress profiles. Recently, simple approximations were suggested thatallow us to invert the measured AECC spectra first into depth-dependent electricalconductivity profiles and then, using the independently measured electroelasticcoefficients, into depth-dependent residual stress profiles [13]. Unfortunately, these effortshave encountered some difficulties caused by the uncorrected cold work effects. The shape

Uniaxial Strain [0.1%/div]

N o r m a l

i z e d

C o n

d u c t

i v i t y

[ 0 . 1

% / d i v ]

.

parallel

normal

a) Waspaloy


N o r m a l

i z e d

C o n

d u c t

i v i t y

[ 0 . 1

% / d i v ]

.

parallel

normal

b) IN718


N o r m a l

i z e d

C o n

d u c t

i v i t y

[ 0 . 1

% / d i v ] . parallel

normal

c) Ti-6Al-4V

FIGURE 6. Electroelastic measurements in (a) Waspaloy, (b) IN 718 and (c) Ti-6Al-4V using a directionaleddy current probe parallel and normal to the applied uniaxial load.

30



10/16

of the residual stress profile is usually fairly well reproduced, however, the magnitude isgenerally overestimated. The overestimation in nickel-base superalloys is on the order of 50 60%, which is obviously too much of an error for reliable life prediction. Therefore,additional efforts are underway to establish the actual physical source of this discrepancyand to develop an empirical calibration method that goes beyond the measurement of the

electroelastic coefficients in the intact material.Because of the indirect nature of eddy current residual stress assessment, the

measured AECC is affected by not only the elastic strain, but also the plastic strain, whichcould exert its influence in at least in three different ways. It could possibly affect thegauge factor, that is the sensitivity to elastic strain, which we called the electroelasticcoefficient. It might also affect the permeability of the material, and finally it could affectdirectly the electric conductivity. As it turns out, the effects of cold work on theelectroelastic coefficient and magnetic permeability are both negligible. The latter is rather surprising and represents a marked difference from what happens in austenitic stainlesssteels, which are similarly paramagnetic in their annealed state, but become stronglyferromagnetic if they undergo plastic deformation. Interestingly, what really changes innickel-base superalloys with cold work is the electrical conductivity. It was found that theelectric conductivity significantly increases with plastic deformation in these materialsabove 10 % plastic strain. This effect is clearly not due to increasing dislocation density.

a) IN718 b) IN100

-0.5

0.0

0.5

1.0

1.5

2.0

0.1 1 10Frequency [MHz]

A E C C D i f f e r e n c e

[ % ]

Almen 16AAlmen 12AAlmen 8AAlmen 4A

-0.5

0.0

0.5

1.0

1.5

2.0



[ % ]


c) annealed (homogeneous) Waspaloy d) as-received (inhomogeneous) Waspaloy

-0.5

0.0

0.5

1.0

1.5

2.0



[ % ]


-0.5

0.0

0.5

1.0

1.5

2.0



[ % ]


FIGURE 7. Apparent eddy current conductivity spectra in shot-peened nickel-base superalloy specimens.

31



11/16

First of all, increasing dislocation density plays a minor role in the conductivity of metalsat room temperature, and even if it exerted a stronger effect it would reduce rather thanincrease the conductivity. The problem of increasing conductivity is not fully understoodat this point, but it seems to be due to subtle microstructural effects such as changing longrange and short range ordering and changing number density and size of precipitations.

Although there is need for further research in this area, it is clear that the

overestimation of the eddy current method due to cold work should be lower in LSP andLPB specimens, which have much lower plastic deformation than shot-peened ones.Figure 8 shows an example of comparison between eddy current and XRD results in low- plasticity burnished Waspaloy of approximately 15% maximum plastic strain. Figures 8a, b, and c show the measured apparent eddy current conductivity spectrum, the XRD coldwork profile, and a comparison between the XRD and eddy current residual stress profiles,respectively. There is a fairly good agreement between the nondestructively (eddy current)and destructively (XRD) measured residual stress profiles. However, the agreement inmagnitude is somewhat artificial because we had to use an empirical correction factor of 0.65 to eliminate the otherwise significant overestimation by the eddy current method dueto uncorrected cold work effects. In the future, we have to better understand the physicalreasons for this discrepancy so that we can rely on empirical corrections. It is expectedthat such empirical corrections will depend on material properties as well as on the type of surface treatment because different treatments produce different levels of cold work.

THERMOELECTRIC RESIDUAL STRESS ASSESSMENT

In both cases considered above the gauge factor is pretty close to unity, which meansthat one percent elastic strain causes approximately 1% change in the measured quantity.Therefore, for stress assessment with 10% accuracy, measurements should be made with avery demanding, but not impossible accuracy of approximately 0.1%. Ultrasonic velocityand eddy current conductivity measurements are usually doable with such relative accuracyon large, flat specimens, but are much more difficult in the vicinity of corners, edges,fastener holes, etc., where these techniques quickly break down. For such applications weneed techniques that are immune to edge effects. The best candidate for such applicationsis thermoelectric inspection. Essentially all nondestructive thermoelectric inspections are based on the so-called Seebeck effect, which is the underlying physical principle behindthe operation of ordinary thermocouples. Recently, Hinken and Tavrin [14] suggested to

a) b) c)

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.01 0.1 1 10Frequency [MHz]

A E C C C h a n g e [ % ]

eddy current

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.01 0.1 1 100.01 0.1 1 10Frequency [MHz]

A E C C C h a n g e [ % ]

eddy currenteddy current

0.0 0.5 1.0 1.5Depth [mm]

C o l d W o r k [ % ] .

0

5

10

15

20XRD

0.0 0.5 1.0 1.50.0 0.5 1.0 1.5Depth [mm]

C o l d W o r k [ % ] .

0

5

10

15

20

0

5

10

15

20XRDXRD

.

-1400

-1200

-1000

-800

-600

-400

-200

0

200

0.0 0.5 1.0 1.5Depth [mm]


eddy currentXRD

.

-1400

-1200

-1000

-800

-600

-400

-200

0

200

-1400

-1200

-1000

-800

-600

-400

-200

0

200

0.0 0.5 1.0 1.50.0 0.5 1.0 1.5Depth [mm]


eddy currentXRD

FIGURE 8. Inversion of the measured AECC in low-plasticity burnished Waspaloy.

32



12/16

use this method in NDE the same noncontacting way Thomas Johann Seebeck discoveredit in 1821 by detecting the magnetic field produced by the thermoelectric current in metals.Let us assume that we have an inclusion in an otherwise homogeneous material and atemperature gradient is established throughout the specimen as it is shown in Figure 9a.Because of this temperature gradient, different points at the boundary between theinclusion and the host will be at different temperatures, therefore at different thermoelectric

potentials. These minor thermoelectric potential differences will drive local thermoelectriccurrents around the inclusion, which can then be detected in a noncontacting way by asensitive magnetometer.

This technique was originally suggested for the detection of inclusions in the material, but was subsequently shown to be sensitive enough to detect subtle changes in thethermoelectric power of metals due to plastic deformation and the presence of residualstresses. Figure 10 shows an illustration of how this technique could detect weak materialvariations independently of gross geometrical irregularities. First, we prepared a 9.5-mm-diameter semi-spherical cavity in a C11000 copper bar by low-stress milling and found noevidence of any perceivable thermoelectric signature when the specimen was exposed to0.6 C/cm temperature gradient. In comparison, when we produced essentially the samegeometry by pressing a stainless steel ball into the surface, we picked up a very strongmagnetic signature of 18 nT peak magnetic flux density. Even more importantly, when weannealed these specimens, the magnetic signature completely disappeared, which indicatesnot only that the technique is sensitive to material property variations caused by the plasticdeformation, but also that it is completely insensitive to edge effects and other geometricalartifacts, which is a unique, very important feature of thermoelectric inspection.

One big indentation can be considered as a model for many small indentations produced by shot penning. First, we tested this technique on shot-peened copper specimens of high electric and thermal conductivity [15]. Clearly, copper is not a veryimportant structural metal, but it is routinely used as a reference material in electrical andthermal measurements. We shot-peened a series of specimens and then used partial stressrelaxation at 350 C upon which the cold work essentially disappears but there is somesignificant residual stress left in the material, as it is illustrated in Figure 11. We measuredthe magnetic signature of these shot-peened specimens in the horizontal polarizationconfiguration shown in Figure 9b and found that the magnetic signature was essentiallylinearly proportional to the peening intensity. Figure 13 shows the results of noncontactingthermoelectric monitoring of thermal relaxation in shot-peened C11000 copper. It isimportant to notice that after relaxation at 350 C, when essentially all the cold work has

already vanished, a significant fraction of the magnetic signature lingered on and themagnetic signature can be fully eliminated only at around 600 C when both the cold work and the residual stress are completely removed from the specimen due to recrystallization.

a) b)

specimenheat

thermoelectric current

magnetometer

specimenheat


magnetometer

heat


fluxgate gradiometer

heat




FIGURE 9. Noncontacting thermoelectric inspection with vertical (a) and horizontal (b) polarization.

33



13/16

Although these results look quite promising for the feasibility of thermoelectricassessment of residual stresses in surface-treated metals, the crucial question is whether asimilar approach can be adopted to low-conductivity engine materials. Experimentsindicated that such measurements are a bit more difficult to conduct in engine materials because, due to the lower conductivity of such materials, much higher temperaturegradients are needed to produce a given magnetic signature. On the other hand, higher temperature gradients are relatively easy to maintain in these materials using only air heating and cooling exactly because of the low thermal conductivity. Figure 13 shows

before annealing

after annealing

milled pressed plastic zonemilled pressed plastic zone

FIGURE 10. Illustration of the non-conducting thermoelectric technique for detecting weak materialvariations independently of gross geometrical irregularities.

0

1

2

0 0.2 0.4 0.6Depth [mm]

P e a k W i d t h [ d e g ]

-250

-200

-150

-100

-50

0

0 0.2 0.4 0.6Depth [mm]


Almen 4A

0

1

2

0 0.2 0.4 0.6Depth [mm]


-250

-200

-150

-100

-50

0

0 0.2 0.4 0.6Depth [mm]


Almen 8A

0

1

2

0 0.2 0.4 0.6Depth [mm]


-250

-200

-150

-100

-50

0

0 0.2 0.4 0.6Depth [mm]


Almen 12A

0

1

2

0 0.2 0.4 0.6Depth [mm]


-250

-200

-150

-100

-50

0

0 0.2 0.4 0.6Depth [mm]


Almen 16A

0

1

2

0 0.2 0.4 0.6Depth [mm]


-250

-200

-150

-100

-50

0

0 0.2 0.4 0.6Depth [mm]


Almen 4A

0

1

2

0 0.2 0.4 0.6Depth [mm]


0

1

2

0 0.2 0.4 0.60 0.2 0.4 0.6Depth [mm]


-250

-200

-150

-100

-50

0

0 0.2 0.4 0.6Depth [mm]


Almen 4A

-250

-200

-150

-100

-50

0

0 0.2 0.4 0.60 0.2 0.4 0.6Depth [mm]


Almen 4A

0

1

2

0 0.2 0.4 0.6Depth [mm]


-250

-200

-150

-100

-50

0

0 0.2 0.4 0.6Depth [mm]


Almen 8A

0

1

2

0 0.2 0.4 0.6Depth [mm]


0

1

2

0 0.2 0.4 0.60 0.2 0.4 0.6Depth [mm]


-250

-200

-150

-100

-50

0

0 0.2 0.4 0.6Depth [mm]


Almen 8A

-250

-200

-150

-100

-50

0

0 0.2 0.4 0.60 0.2 0.4 0.6Depth [mm]


Almen 8A

0

1

2

0 0.2 0.4 0.6Depth [mm]


-250

-200

-150

-100

-50

0

0 0.2 0.4 0.6Depth [mm]


Almen 12A

0

1

2

0 0.2 0.4 0.6Depth [mm]


0

1

2

0 0.2 0.4 0.60 0.2 0.4 0.6Depth [mm]


-250

-200

-150

-100

-50

0

0 0.2 0.4 0.6Depth [mm]


Almen 12A

-250

-200

-150

-100

-50

0

0 0.2 0.4 0.60 0.2 0.4 0.6Depth [mm]


Almen 12A

0

1

2

0 0.2 0.4 0.6Depth [mm]


-250

-200

-150

-100

-50

0

0 0.2 0.4 0.6Depth [mm]


Almen 16A

0

1

2

0 0.2 0.4 0.6Depth [mm]


0

1

2

0 0.2 0.4 0.60 0.2 0.4 0.6Depth [mm]


-250

-200

-150

-100

-50

0

0 0.2 0.4 0.6Depth [mm]


Almen 16A

-250

-200

-150

-100

-50

0

0 0.2 0.4 0.60 0.2 0.4 0.6Depth [mm]


Almen 16A

FIGURE 11. Residual stress and cold work distributions in shot-peened C11000 copper before (solid circles)and after (empty circles) thermal relaxation or 30 minutes at 315 C.

34



14/16

examples of noncontacting thermoelectric monitoring of thermal relaxation in low- plasticity burnished and shot-peened IN100 nickel-base superalloy specimens. Theseresults indicate that the magnetic signature correlates well with the intensity of surfacetreatment as well as with the depth of the affected region. The thermoelectric techniquecan also be used for monitoring thermal relaxation, which is one of the main applicationsfor nondestructive residual stress assessment. However, it should be pointed out that thistechnique is more limited than ultrasonic and eddy current inspection in the sense that itcannot assess the depth profile of the residual stress. Thermoelectric inspection providesonly one number, which is a near-surface weighted average of the effect. However, thisdrawback is compensated by the fact that it is completely insensitive to geometrical edgeeffects, which is an absolute necessity in many applications where cold working andresidual stress must be assessed in the vicinity of sharp stress concentrators.

before relaxationrelaxation at 235 Crelaxation at 275 Crelaxation at 315 C2nd relaxation at 315 C3rd relaxation at 460 Crecrystallization at 600 C

0

5

10

15

20

25

0 2 4 6 8 10 12 14 16Almen Intensity (A)

M a g n e

t i c S i g n a

t u r e

[ n T ]




0

5

10

15

20

25

0

5

10

15

20

25

0 2 4 6 8 10 12 14 160 2 4 6 8 10 12 14 16Almen Intensity (A)

M a g n e

t i c S i g n a

t u r e

[ n T ]

FIGURE 12. Noncontacting thermoelectric monitoring of thermal relaxation in shot-peened C11000 copper.

0 4 8 12 16Almen Intensity (A)

0

1

2

3

4

5

6

7

8

M a g n e

t i c S i g n a t u r e

[ n T ]

series 1 (intact)

series 2 (intact)

series 1 (565 C)

series 2 (675 C)

0

1

2

3

4

5

6

7

8

M a g n e t i c S i g n a t u r e [ n T ]

Type of Surface Treatment

Almen 6A

Almen 8A

high LPB

low LPB


0

1

2

3

4

5

6

7

8

M a g n e


[ n T ]

series 1 (intact)

series 2 (intact)

series 1 (565 C)

series 2 (675 C)


0

1

2

3

4

5

6

7

8

0

1

2

3

4

5

6

7

8

M a g n e


[ n T ]

series 1 (intact)

series 2 (intact)

series 1 (565 C)

series 2 (675 C)

0

1

2

3

4

5

6

7

8



Almen 6A

Almen 8A

high LPB

low LPB

0

1

2

3

4

5

6

7

8

0

1

2

3

4

5

6

7

8



Almen 6A

Almen 8A

high LPB

low LPB

FIGURE 13. Noncontacting thermoelectric monitoring of thermal relaxation in low-plasticity burnished andshot-peened IN100 nickel-base superalloy (25 C/cm thermal gradient).

35



15/16

CONCLUSIONS

The main advantages and disadvantages of different NDE methods for residual stressassessment have been reviewed. X-ray diffraction is by far the most mature techniquetoday. It is highly selective since plastic and elastic strains exert very different and easilyseparable effects on the diffracted beam pattern. Unfortunately, the penetration depth of

XRD leaves much to be desired and badly limits the feasibility of this method innondestructive materials characterization. This limitation of conventional XRD can beovercome by either increasing the intensity or reducing the wavelength using synchrotronor neutron radiation, but then the availability of the source becomes a major problem.Magnetic techniques offer probably the best indirect approach for residual stressassessment in ferromagnetic materials. They could be extremely sensitive with gaugefactors as high as 10 or 20, i.e., one percent strain in the material can produce 10 or 20 percent change in permeability and other magnetic properties. Unfortunately, this highsensitivity is very much dependent on material variations.

As for nonmagnetic materials, ultrasonic, eddy current and thermoelectric methods

were considered. Ultrasonic techniques are very sensitive to crystallographic texture,therefore generally cannot be recommended for near-surface residual stress assessment insurface-treated materials. However, ultrasonic techniques can be very useful in assessingthe degree of cold work and that by itself is very valuable information for assessing thethermo-mechanical stability of residual stresses as well as for developing corrections for cold work effects for other techniques, such as eddy current conductivity measurements,which are more selective to residual stresses, but still are affected to some degree by plasticdeformation. Eddy current inspection does not work in some materials, such as Ti-6Al-4V,for residual stress assessment because of the unfortunate cancellation between the paralleland normal electroelastic coefficients, but there is an excellent window of opportunity for nickel-base superalloys, which is a very important category of materials for engine lifeextension. Eddy current conductivity spectroscopy is a relatively simple measurement thatcan be conducted with sufficient accuracy and reproducibility even in the field. Finally,thermoelectric techniques are in really their infancy, but show unique advantages over more conventional ultrasonic and eddy current methods. They are completely immune togeometrical artifacts, therefore they offer the best opportunity to study material variations,including those produced by cold work and residual stress, in the vicinity of a fastener holes and other stress concentrators that would exclude the application of other indirectmethods.

Nondestructive residual stress assessment offers great challenges as well as greatopportunities for future research and development. The most important task for the NDEcommunity is to study the selectivity of different NDE methods to better understand theunderlying physics that controls these measurements. In addition, more sensitive and moreaccurate inspection techniques need to be developed. The ultimate goal is to integrate thethree basic building blocks of a good life prediction process. We need (i) better models for material microstructures and damage evolution during service, (ii) better nondestructiveresidual stress measurement techniques, and finally we have to integrate these results with(iii) more accurate and more reliable statistical life prediction models.

REFERENCES

1. V. Hauk, Structural and Residual Stress Analysis by Nondestructive Methods, Elsevier,Amsterdam, 1997.

36



16/16

2. R. John, J. M. Larsen, D. J. Buchanan, and N. E. Ashbaugh, Incorporating residualstresses in life prediction of turbine engine disks, Proceedings from NATO RTO(AVT) Symposium On Monitoring and Management of Gas Turbine Fleets for Extended Life and Reduced Costs (Manchester, UK, 8-11 Oct., 2001).

3. J. M. Larsen, B. Rasmussen, S. M. Russ, B. Sanbongi, J. Morgan, D. Shaw, J. Jira, D.Johnson, S. LeClaire, M. Blodgett, T. Moran, W. Stange, M. Meininger, and T. Fecke,

The engine rotor life extension (ERLE) initiative and its opportunities to increase lifeand reduce maintenance costs, AeroMat Conference (Long Beach, CA, June 12,2001).

4. V. Damljanovic and R. L. Weaver, J. Sound Vibr. 282, 341 (2005).5. T. D. Murnaghan, Finite Deformation of an Elastic Solid , John Viley and Sons, New

York, 1951.6. A. I. Lavrentyev, P. A. Stucky, and W. A. Veronesi, Feasibility of ultrasonic and eddy

current methods for measurement of residual stress in shot peened metals, in Review of Progress in QNDE 19B, edited by D. O. Thompson and D. E. Chimenti, AIPConference Proceedings vol. 509, American Institute of Physics, Melville, NY, 2000, pp. 1621-1628.

7. C. Glorieux and W. Gao, J. Appl. Phys.88, 4394 (2000).8. A. Ruiz and P. B. Nagy, Instr. Meas. Metrol. 3, 59 (2003).9. F. C. Schoenig, Jr., J. A. Soules, H. Chang, and J. J. DiCillo,Mat. Eval. 53, 22 (1995).10. H. Chang, F. C. Schoenig, Jr., and J. A. Soules,Mat. Eval. 57, 1257 (1999).11. M. P. Blodgett and P. B. Nagy, J. Nondestr. Eval. 23, 107 (2004).12. F. Yu and P. B. Nagy, J. Nondestr. Eval. 24, 143 (2005).13. F. Yu and P. B. Nagy, J. Appl. Phys.96, 1257 (2004).14. J. H. Hinken and Y. Tavrin, Thermoelectric SQUID method for the detection of

segregations, in Review of Progress in QNDE 19B, edited by D. O. Thompson and D.E. Chimenti, AIP Conference Proceedings vol. 509, American Institute of Physics,Melville, NY, 2000, pp. 2085-2092.

15. H. Carreon, P. B. Nagy, and M. P. Blodgett, Res. Nondestr. Eval. 14, 59 (2002).

Documents

Overview2006Tensão NDE