ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation

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• The area in which stresses are to be measured must be accessible to a rather bulky drilling or coring

alignment device.

• Preparation of the surface for strain gage adherence may induce residual stresses that introduce

substantial error to the subsequent measurement (Ref 71).

In conclusion, the drilling and ring coring methods are nearly nondestructive variations of the destructive

mechanical stress relief techniques and require only rather simple equipment and instrumentation. The state-of-

the-art is relatively well developed compared to many nondestructive methods, some of which require

considerable research and development work before they will ever be suitable to general application in terms of

alloys and stress field conditions. Technological advancements in hole drilling and ring coring have largely

been due to advancements in the more general areas of mechanical stress relief methods and research in new

metal removal techniques for metal fabrication.

Indentation Methods. For more than six decades, engineers and scientists have proposed the use of indentors,

such as those used to perform hardness measurements, as a means to measure or detect surface residual stresses.

Kokubo in 1932 reported that stresses applied under bending load changed the apparent Vickers hardness

values in carbon steel rolled sheets, both as rolled and annealed. He showed that tensile stresses tended to

decrease the apparent hardness, and compressive stresses tended to increase the hardness. The stresses applied

in tension and compression were sufficient to cause 0.3% strain.

Two decades later Sines and Carlson (Ref 72) proposed a method that required various amounts of external

loads to be applied to the component in which residual stresses were to be measured while hardness

measurements were made. The loads were made to cause both tensile and compressive applied stresses. The

quality—that is, whether the residual stress was compressive or tensile—was then revealed by comparing the

effect of the applied stress and whether the applied stress was tensile or compressive on the hardness

measurement. At about the same time, Pomey et al. (Ref 73) proposed that residual stresses could be measured

by pressing a ball-shaped penetrator into the component in which residual stresses were to be measured and

establishing the relationship between the pressing load while it was progressively increased and the electrical

resistance at the interface between the penetration and the component. He maintained that a smaller decrease in

electrical resistance indicated that portions of material under the ball were plastically yielding and that the

corresponding load on the ball could be related to the existing residual stress.

Later, Chiang et al. (Ref 74) provided a critique of several existing indentation analyses and proposed an

interpretation of indentations exhibiting hemispherical plasticity. Nevertheless, the applications illustrated in

this article were focused on brittle materials and not metals.

There have been numerous papers published proposing various approaches to interpreting the indentation loads

and shapes so as to estimate the residual stress field on the surface and near-surface regions of materials.

However, indentation methods have not earned the degree of confidence of XRD or hole drilling methods for

general applications and, thus, are rarely applied.

Spot Annealing. Another semidestructive method that has been proposed to measure residual stresses in metal

surfaces is to reduce the residual stresses in a small volume by annealing the metal in the volume. It has been

proposed that this annealing be performed by intense laser light (Ref 52). This technique was envisioned to be

similar to relief of residual stresses by removal of the material as accomplished in the hole drilling techniques.

However, as Cullity discussed (Ref 53), such localized heating would induce high surface residual tensile

stresses in the heat-affected region, and this would be detrimental to the component being tested.

References cited in this section

9. A.J. Bush and F.J. Kromer, “Residual Stresses in a Shaft after Weld Repair and Subsequent Stress

Relief,” Paper No. A-16 presented at Society for Experimental Stress Analysis (SESA) Spring Meeting,

1979 (Westport, CT), 1979

52. C.S. Vikram, M.J. Pedensky, C. Feng, and D. Englehaupt, Residual Stress Analysis by Local Laser

Heating and Speckle-Correlation Interferometry, Exp. Tech., Nov/Dec 1996, p 27–30

53. B.D. Cullity, Elements of X-Ray Diffraction, 2nd ed., Addison-Wesley Publishing Co., Inc., 1978, p

469–472

59. J. Mathar, Determination of Initial Stresses by Measuring Deformation Around Drilled Holes, Arch.

Eisenhuttenwes., Vol 6, p 277–281 and Trans. ASME, Vol 56 (No. 4), 1934, p 249–254

60. “Determining Residual Stresses by the Hole Drilling Strain-Gage Method,” E 837, ASTM, 1983

61. H. Wolf and D.C. Sauer, “New Experimental Technique to Determine Residual Stresses in Large

Turbine-Generator Components,” presented at the American Power Conf., (Chicago, IL), 1 May 1974

62. H. Wolf and W. Bohn, Origin, Measurement and Assessment of Residual Stresses in Large Forging for

Turbines and Generators, Arch. Eisenhüttenwes., Vol 42 (No. 7) 1971, p 509–511 (in German)

63. H. Wolf, E. Stucker, and H. Nowack, “Investigations of Residual Stresses in the Turbine and Generator

Industry,” Arch. Eisenhuttenwes., Vol 48 (No. 3), March 1977, p 173–178

64. J. Lu and J.F. Flavenot, “Applications of the Incremental Hole-Drilling Method for Measurement of

Residual Stress Distributions—Experimental Techniques, paper presented at the Society for

Experimental Mechanics (SEM) Spring Conference,” 5–10 June 1988 (Portland, OR), SEM

65. M.T. Flaman and B.H. Manning, Determination of Residual Stress Variation with Depth by the Hole-

Drilling Method, Exp. Mech., Vol 25, 1985, p 205–207

66. R.A. Kelsey, Measuring Nonuniform Residual Stresses by the Hole Drilling Method, Proc. Society for

Experimental Stress Analysis, Vol 14 (No. 1), 1956, p 181–184

67. A.J. Bush and F.J. Kromer, Simplification of the Hole-Drilling Method of Residual Stress

Measurement, ISA Transactions, Vol 12 (No. 3), 1973, p 249–259

68. N.J. Rendler and I. Vigness, Hole-Drilling Strain-Gage Method of Measuring Residual Stress, Exp.

Mech., Vol 6 (No. 12), Dec 1966, p 577–586

69. J.W. Dini, G.A. Beneditti, and H.R. Johnson, Residual Stresses in Thick Electro-deposits of a Nickel-

Cobalt Alloy, Exp.l Mech., February 1976, p 56–60

70. F. Witt, F. Lee, and W. Rider, A Comparison of Residual Stress Measurements Using Blind-Hole,

Abrasive-Jet and Treppaning Methods, Exp. Tech., Vol 7, Feb 1983, p 41–45

71. P.S. Prevey, “Residual Stress Distribution Produced by Strain Gage Surface Preparations,” 1986 SEM

Conference on Experimental Mechanics, 1986

72. G. Sines and R. Carlson, “Hardness Measurements for the Determination of Residual Stresses,” ASTM

Bulletin 180, Feb 1952, p 35–37

73. J. Pomey, F. Goratel, and L. Abel, “Determination des Cartraintes Residuelles dans les Pieces

Einentees,” Publication Scientifiques et Techniques du Ministere de l'air, 1950, p 263

74. S.S. Chiang, D.B. Marshall, and A.G. Evans, The Response of Solids to Elastic/Plastic Indentations I:

Stresses and Residual Stresses, J. Appl. Phys., Vol 53 (No. 1), 1982, p 298–311

Residual Stress Measurements

Clayton O. Ruud, The Pennsylvania State University

Nondestructive Procedures

The methods for strain measurement described previously all measure the change in some dimension (strain) of

the component produced by the removal of a finite volume of stressed metal from that component. Thus, these

methods measure the strain induced by removing material so as to perturb the residual stress field. On the other

hand, the nondestructive procedures measure a dimension in the crystal lattice of the metal or some physical

parameter affected by the crystal lattice dimension. Whenever a mechanical force resulting in stress that is less

than the yield strength is placed on a solid metal component, that component distorts (strains) elastically. That

elastic strain results in a change in the atomic lattice dimension, and this dimension, or change, is measured by

a nondestructive stress measurement procedure. For example, the diffraction methods, x-ray and neutron,

measure an actual crystal dimension, and this dimension can be related to the magnitude and direction of the

stress that the metal is subject to, whether that stress is residual or applied. Subsequently in this section, the

following methods of nondestructive stress measurement are described: XRD, neutron diffraction, ultrasonic

velocity, and magnetic Barkhausen noise.

X-ray diffraction techniques exploit the fact that when a metal is under stress (applied or residual), the resulting

elastic strains cause the atomic planes in the metallic crystal structure to change their spacings. X-ray

diffraction can directly measure this interplanar atomic spacing; from this quantity, the total stress on the metal

can then be obtained.

Because metals are composed of atoms arranged in a regular three-dimensional array to form a crystal, most

metal components of practical concern consist of many tiny crystallites (grains), randomly oriented with respect

to their crystalline arrangement and fused together to make a bulk solid. When such a polycrystalline metal is

placed under stress, elastic strains are produced in the crystal lattice of the individual crystallites. In other

words, an externally applied stress or one residual within the material, when below the yield strength of the

material, is taken up by interatomic strain. X-ray diffraction techniques can actually measure the interatomic

spacings, which are indicative of the elastic strain in the specimen. Stress values are obtained from these elastic

strains in the crystals by knowing the elastic constants of the material and assuming that stress is proportional to

strain, a reasonable assumption for most metals and alloys of practical concern. An article published in Journal

of Metals describes the XRD method and instrumentation in some detail. References 8 and 75 are excellent

sources of practical, more detailed information on XRD stress measurement.

There are three basic techniques for measuring stresses, based on the XRD method. They are the double

exposure (or two-angle) technique (DET), the single exposure (or one-angle) technique (SET), and the sin-

square-psi (or multiangle) technique. The angle of exposure referred to is that between the incident x-ray beam

and the specimen surface normal. It should be noted that in any XRD stress measurement technique, x-ray

peaks in the far back-reflection range, that is, peaks with Bragg (θ) angles of near 90°, are much preferred

because they show the greatest effect with a given amount of applied or residual stress. This is illustrated in the

following equation:

(Eq 34)

In Eq 34, θ

is the Bragg angle of the planes diffracting at ψ

; θ

is the Bragg angle of the planes diffracting at

. In Fig. 13, it can be seen that as θ

increases, its cotangent decreases; therefore, a larger difference (2θ

2θ

) would result from a given σ

to maintain an equality.

Fig. 13 One-angle arrangement or the single exposure technique (SET). N

is the specimen normal, and β

is the angle that the incident beam makes with N

. N

and N

are the normals to the diffracting planes 1

and 2 respectively, and ψ

and ψ

are the angles between N

, N

, and N

respectively. η is the angle

between the incident beam and the diffracting plane normals. R

is the camera radius, and O is the point

of incidence of the x-ray beam of the specimen. 1 and 2 represent the diffracting planes at various

attitudes to the specimen surface. S

and S

are the measured parameters representing the distance from

a reference point of known distance from the incident beam and the diffracted x-ray beam position. S

and S

are directly related to the Bragg angles, θ

and θ

. The stress being measured is parallel to the

specimen surface and in the plane containing the x-ray source vector and the specimen surface normal,

For a residual stress measurement, the diffracting angle, θ, of interatomic planes of at least two different psi (ψ)

angles with respect to the surface normal must be measured (Fig. 13). These planes are crystallographically

equivalent (same Miller indices, hkl) and in the unstressed state of the metal would have the same interatomic,

d, spacing for the planes labeled 1 and 2 in Fig. 13 (Ref 53, 76, 77). In a stressed material, however, the two or

more orientations of diffracting planes are selected so that they are at different angles to the surface; thus their

normals are at different (ψ) angles to the surface normal. Then, depending on the angle of these planes to the

stress vector, their interplanar atomic spacing is increased or decreased by varying amounts.

The most common sources of errors and misapplications in stress measurements by x-rays are related to stress

constant selection, focusing geometry, diffracted peak location, and cold-working, crystallographic texture,

grain size, microstructure, and surface condition. The source, significance, and correction techniques for these

errors are not elaborated on here; details may be found in an article by Ruud and Farmer (Ref 78) and (Ref 8).

A point of interest in the error sources listed above concerns cold working and microstresses. Microstresses are

usually considered to be those manifested by strain variation across single metallic grain. This strain variation is

detected in the XRD method as broadening of the x-ray peak—a distinctly different phenomenon from the peak

shift caused by residual stresses. However, microstrain variation can be measured simultaneously with stress.

This microstrain phenomenon has been proposed as a means of judging cold work, dislocation density, and

fatigue damage (Ref 79).

Despite the facts that x-rays provide stress readings only to a depth of less than 0.025 mm. (0.001 in.) and that

the error sources listed above must be considered, the noncontact XRD method is presently the only time-

proven, generally applicable, truly nondestructive method for measuring residual stresses. Its reliability has

been proved and documented by thousands of engineers and scientists over the past four decades beginning

with the classic work of Bolstad et al. at Boeing using x-ray film cameras (Ref 80). This documentation

includes measurement of stresses in the Brooklyn Bridge (Ref 20) and tempering evaluation of carburized

steels. The Society of Automotive Engineers (SAE) considers the method of sufficient practical importance to

have printed three handbook supplements on the subject (Ref 8), and another supplement is under revision.

Even so, this nondestructive technology has been largely restricted to the laboratory because of the general lack

of knowledge regarding the state-of-the-art instruments and the limitations of the more widely known and

available conventional scanning XRD equipment. Instrumentation for bringing this technology into the field

and manufacturing area has advanced rapidly in the last two decades, especially toward increased portability,

compactness, and speed of operation.

As shown in Fig. 14, instrumentation has been developed and is commercially available for stress measurement

in situ on the inside diameter of 10 mm (4 in.) diameter pipe (Ref 39). Position sensitive x-ray detectors have

been largely responsible for these improvements to both laboratory-based and field deployable residual stress

measuring instruments (Ref 10, 21, 22, 39, 81, 82, and 83). Also, with the speed of data collection being less

than 0.1 s with conventional x-ray tube sources in some applications, XRD stress measurement can be

performed on moving components (Ref 12). Nevertheless, many engineers have been frustrated in applying

XRD to residual stress measurement. This has been largely due to crystallographers inexperienced in residual

stress measurement, attempting to apply conventional scanning x-ray diffractometers and techniques to residual

stress measurement. For example, in conventional XRD analysis and crystallography, sharp resolution of the

diffracted spectra is very beneficial. However, in XRD stress measurement, the need to measure (ψ) angles that

are not zero defocuses the beam, and attempts to refocus lead to significant error in the stresses read (Ref 84).

In XRD stress measurement, what is more important than sharp resolution is the repeatable ability to measure

the position of a defocused diffracted x-ray peak (Ref 85). Thus, it is recommended in most cases that XRD

residual stress measurement be performed by trained technologists using x-ray instrumentation specifically

designed and built for stress measurement, not conventional scanning diffractometers. Software packages

specifically for residual stress measurement used with conventional scanning diffractometers do not in most

cases eliminate the mechanical and focusing problems of applying these instruments to residual stress

measurement. It is necessary to mount the component (or specimen) in which stresses are to be measured on the

conventional scanning diffractometer, which usually requires sectioning of the component and which

complicates and adds error to the measurement procedure.

Fig. 14 Photograph of a miniature x-ray diffractometer for the one angle technique arrangement of XRD

stress measurement. This device incorporates a Ruud-Barrett position sensitive scintillation detector and

is capable of being inserted in a 101.60 mm (4 in.) inside diameter for measuring residual stress (Ref 39).

Neutron diffraction (ND) allows measuring the elastic strains induced by residual stresses throughout the

volume of relatively thick steel components with a spatial resolution as small as 1 mm

. Such capability

provides for the measurement of residual stress inside of components without the necessity of sectioning or

layer removal. Principal ND methods, like the XRD methods, measure the spacing between crystallographic

planes in a component, and this spacing is affected by residual and applied stress. The spacing between a

selected set of crystallographic planes (φ) is related to the angle of incidence and diffraction of the neutron

radiation, θ, which are equal, and the wavelength of the monochromatic radiation (λ) by Bragg's law:

λ = 2 d sin θ

(Eq 35)

The elastic strain, ε, induced by the residual stress perpendicular to the diffracting crystallographic plane then is

related to d by:

(Eq 36)

where d

is the distance between the unstressed crystallographic planes. If the orientation of principal stresses is

known in the component, the stress in any principal direction may be calculated by:

(Eq 37)

where σ

, σ

, and σ

are the principal stresses and ε

, ε

, and ε

are the strains measured in the corresponding

principal stress directions. E and ν are the Young's modulus and Poisson's ratio, respectively. If the principal

stress directions are not known, strains in at least six directions must be measured to determine the residual

stresses acting on the volume of material in which strains are being measured.

For residual stress measurement in most alloys, the unstressed spacing (d

) between crystallographic planes at

the exact point of strain measurement is not known and not easily measured. This means that d

or θ

in Eq 36

cannot be precisely established, and this leads to various degrees of error in the accuracy and precision of ND

residual stress measurements. This condition is aggravated by the fact that the elemental composition, and thus

, vary considerably within a component and markedly within the phase (e.g., martensite, austenite, and ferrite

for steel) of the alloy at various locations. Additional limitations are that the component must be brought to a

nuclear reactor, each strain measurement requires over an hour, a single stress determination in one small

volume of the component requires at least three strain measurements, and the measurements are very costly.

Nevertheless, the ND methods have been applied to residual stress measurements in weldments (Ref 86),

cylindrical forgings (Ref 87), plastically deformed plate (Ref 88), rocket case forgings (Ref 86), and many other

types of components.

Ultrasonic Velocity. The principle underlying the measurement of stress and thus elastic strain by ultrasonic

(acoustic) techniques is the phenomenon of an approximately linear change in ultrasound velocity with applied

stress. It has been shown that under certain restricted conditions, residual stress can be measured by exploiting

this phenomenon. Stress is measured by inducing a sound wave in the frequency of several megahertz in the

metal specimen and measuring the time of flight or some other velocity-related parameter. Because many other

characteristics of metals besides stress-induced elastic strain affect velocity, their effect must be sorted out, but

neither the technology nor the fundamental knowledge for such sorting is usually available. The great interest in

ultrasonic techniques for residual stress measurement stems from their promise for three-dimensional

nondestructive measurements within the material.

Principle. A number of velocity-related phenomena have been used in various methods to measure stress effects

by ultrasound. All utilize the deviation of the reaction of the metal from the linearity of Hooke's law of

elasticity, σ = Mε, where σ = stress, ε = strain, and M = elastic modulus. This has been referred to as the

anharmonic property of the solid and may be represented by a power series σ = Mε + Cε

+ Dε

+ …, where C

= third order anharmonic constant, D = fourth, and so on. Most research done for stress measurement has used

expressions in which terms past the third order constant, C, are dropped. Of the several anharmonic property

effects that may be used to measure stress, the following are probably the most exploited: velocity dependence

on the elastic modulus; dispersion of frequency amplitudes in surface waves; birefringence of orthogonally

polarized shear waves; and harmonic generation in surface waves.

A very simplified form of the anharmonic stress strain law has been written as σ = Mε + Cε

and rewritten as σ

= ε(M + Cε). The term in parentheses is approximately related to the velocity of sound as ρV

M + Cε, where

ρ is the density of the medium and V is the velocity of sound. This may be approximately rewritten in terms of

velocity dependence on strain as:

(Eq 38)

Then, to solve for strain, ε = 2(V ρ − 2M)/C (Ref 89).

A simple view of the dependency of ultrasonic velocity on the elastic modulus and density may be shown by

rewriting the equation πV

= M + Cε in terms of V, differentiating and dividing by V to yield an expression for

ΔV/V. The result will readily show that a fractional change in elastic modulus or density would affect the

velocity. The density of metal, for which the Poisson ratio is near 0.3, obviously changes as a compressive or

tensile stress is placed on the specimen, and it is reasonable that the speed of sound would then change.

Limitations and Applications. Ultrasonic technology offers a number of types of wave modes in which to probe

metals; these include bulk waves, such as longitudinal and shear, and surface waves, usually confined to

Rayleigh type. Each mode offers many unique parameters for extracting information. As has been discussed,

the primary effect of stress—induced strain on ultrasonic propagation in metals—is on velocity. This may be

detected in a number of ways, including measurements of wave velocity, shear wave birefringence, and

dispersion. However, there are other characteristics of metals that affect the ultrasonic velocity to the same

degree as stress. These include crystallographic texture, microstresses, multiple phases, coherent precipitates,

composition gradients, and dislocation density and distribution.

Crecraft (Ref 90) discussed velocity effects, manifested as texture, induced birefringence, and the marked

change seen with ultrasonic frequency. He also reported birefringence due to cold work in nickel-steel

specimens but did not attempt to separate the cold-work effects in terms of texture, dislocation density, and so

on. In the early 1950s, Bradfield and Pursey (Ref 91) and Pursey and Cox (Ref 92) reported showing the

influence of small degrees of texture on ultrasonically measured elasticity in polycrystalline bars. They showed

how the true isotropic elastic constants can be determined by using measurements of both longitudinal and

shear wave speeds along several directions. They presented stereographic charts that illustrated the relationship

between elastic behavior of cubic crystals and results of x-ray texture determinations.

McGonagle and Yun (Ref 93) noted the cold-work effects in an article comparing x-ray diffraction results with

Rayleigh wave velocity measurements. Boland et al. (Ref 94) also recognized that other material properties can

affect ultrasonic velocity and recommended that methods be developed to distinguish stress-induced velocity

changes from those from other sources.

James and Buck (Ref 95) pointed out that since the third order elastic constants for most structural materials are

not readily available from the literature, ultrasonic stress measurement must be calibrated relative to the

particular material being investigated. In the same paper they discounted the possible effect of mobile

dislocations on the sound velocity in structural engineering metals with high yield strengths due to the short

dislocation loop lengths prevalent. However, they did mention that crystallographic preferred orientation

(texture) during deformation or fatigue is capable of severely modifying the elastic constants on which the

sound velocity depends.

Papadakis (Ref 96) noted marked velocity changes for ultrasonic waves in various steel microstructures, and

Moro et al. (Ref 97) measured the effect of microstructural changes caused by tempering on the ultrasonic

velocity in low-alloy steel.

Tittman and Thompson (Ref 98) evaluated the near-surface hardness of case-hardened steel with Rayleigh

waves; because hardness in this case is a combination of composition, microstress, and macrostress, the

velocity change was due to a combined effect.

The temperature sensitivity of ultrasonic stress measurements has also been cited as an important source of

error. Salma et al. (Ref 99, 100) proposed that this dependence be used as a means to measure stress but also

noted the marked effect of dislocations and did not address a methodology of separating the stress from the

dislocation effect.

Much of the work cited above is concerned with attempts to measure the effects of a variety of material

properties on the changes in ultrasonic velocity. However, there apparently is no comprehensive study that

demonstrates the capability of quantitatively separating stress effects on ultrasonic propagation from other

variables found in structural metals, such as dislocation density or crystallographic texture. Furthermore, most

of the studies cited observed velocity changes in bulk waves. Velocity measurements on these waves must be

measured through the thickness of a component and, as most metallurgists recognize, obtaining uniform

properties through thicknesses greater than a few millimeters, especially in steels, is difficult. The subtle

property variations to which ultrasound velocity is sensitive, the inherent lack of homogeneity in engineering

metals, and the high residual stress gradients often found in manufactured components present additional

serious problems for through-thickness stress measurements.

In spite of the microstructural variations in manufactured steel products, success in the application of ultrasonic

methods to residual stress measurement has been achieved in specific cases. One is in the measurement of hoop

stresses in railroad wheels (Ref 101). Here changes or variations of the residual stress in the hoop direction is of

concern, while that in the radial or axial direction can often be assumed to be constant or negligible. Some

techniques, then, for the measurement of the residual hoop stresses have relied on normalizing the hoop

velocity against the axial velocity (Ref 102). Also, European railroads have monitored ultrasonic velocity along

the wheel rims during use and attributed changes to residual stress changes (Ref 103). Schramm in his article

mentioned a number of approaches for the application of ultrasound to the measurement of residual stresses in

railroad wheels, and these examples may find application in the measurement of residual stress in other axially

symmetric shapes (Ref 101). Ultrasonic residual stress measurements have also been applied to rails as reported

by Egle and Bray (Ref 104) and Bray and Leon-Salamanca (Ref 105).

Magnetic Barkhausen Noise. The Barkhausen noise analysis technique (BNA) is concerned with measuring the

number and magnitude of abrupt magnetic reorientations made by expansion and contraction of the magnetic

domains in a ferromagnetic metal. These reorientations are observed as pulses somewhat random in amplitude,

duration, and temporal separation, and therefore roughly described as noise.

Applications. A few applications of BNA to ferromagnetic metallic components have been made. Gardner (Ref

106) mentions a number of applications, which include helicopter rotor blade spans, autofrettaged gun tubes,

gas turbine engine components, and rolling element antifriction bearing components. In these examples the

change in residual stresses caused by known service histories was measured.

Chait (Ref 107) qualitatively measured the residual stress condition of a high-hardness laminar composite steel

weldment and compared some of the BNA data with XRD stress readings. Sundstrom and Torronen (Ref 108)

applied their BNA method to a number of microstructural measurements, including evaluation of grain size

measurement for low-carbon ferritic and ferritic-pearlitic steels, evaluation of anisotropy in deep drawing and

textured steels for electrical applications, measurement of the degree of aging in rimmed carbon steels, and

pearlite morphology in steel wires. These researchers have also measured iron loss in magnetic material used

for transformers and have proposed using BNA for residual stress measurements, pointing out that quantitative

results can be obtained if the material and its fabrication history are known and calibration is possible.

Most studies and applications of BNA to stress measurement have focused on the uniaxial stress state.

However, Sundstrom and Torronen (Ref 108) implied that the instrumentation they used could simultaneously

measure stress in two directions to give biaxial stress conditions for magnetic inspection of roller bearing

components, including BNA for monitoring residual stress change.

The BNA method certainly has been demonstrated to be sensitive to the stress condition in ferromagnetic

materials (Ref 109). Nevertheless, its possibilities for application are limited by the condition that the material

must be ferromagnetic, the narrow total range of stress sensitivity (i.e., ±40 ksi, or 275 MPa), and the shallow

depth of measurement. The latter condition might be relieved by using magnetomechanical-acoustic emission

(MAE) (Ref 110), an ultrasound analog to BNA. However, the sensitivity of either of these techniques to other

characteristics of metallic components and the consequent need for calibration with a nearly identical specimen

severely restricts the general applicability of BNA and MAE. Many misapplications have been made that have

severely damaged the reputation of the BNA methods (Ref 107, 111). Such restrictions can be removed only if

the basic phenomena responsible for the effect of microstructural properties on BNA and MAE are understood

and quantified in terms of the signal.

BNA is not recommended where variations in elemental composition, phase composition, grain size, strain

hardening, crystallographic texture, grain shape, grain orientation, carbide size and distribution, and other

microstructural characteristics accompany variations in residual stress. A recent evaluation of BNA by Allison

and Hendricks (Ref 112) confirms the uncertainty of BNA residual stress measurements.

References cited in this section

8. Residual Stress Measurement by X-Ray Diffraction-SAE J784a, Society of Automotive Engineers

Handbook Supplement, Warrendale, PA, 1971

10. M.E. Brauss and J.A. Pineault, Residual Strain Measurement of Steel Structures, NDE for the Energy

Industry, NDE Vol 13 (Book No. H00930-1995), D.E. Bray, Ed., American Society of Mechanical

Engineers, 1995

12. C.O. Ruud and M.E. Jacobs, Residual Stresses Induced by Slitting Copper Alloy Strip, NDC of

Materials VI, Plenum Press, 1994, p 413–424

20. M. Brauss, J. Pineault, S. Teodoropol, M. Belassel, R. Mayrbaurl, and C. Sheridan, Deadload Stress

Measurement on Brooklyn Bridge Wrought Iron Eye Bars and Truss Sections Using X-ray Diffraction

Techniques, Proc. of 14th Annual International Bridge Conf. and Exhibition, Engineering Society of

Western Pennsylvania, Pittsburgh, ICB-97-51, 1997, p 457–464

21. M.G. Carfaguo, F.S. Noorai, M.E. Brauss, and J.A. Pineault, “X-Ray Diffraction Measurement of

Stresses in Post-Tensioning Tendons,” International Association for Bridge and Structural Engineering

(IABSE) Symposium, 1995 (San Francisco), “Extending the Life Span of Structures,” IABSE, ETH

Honggerberg, Zurich, Switzerland, Vol. 71/1, 1995, p 201–206

22. J.A. Pineault and M.E. Brauss, In Situ Measurement of Residual and Applied Stresses in Pressure

Vessels and Pipeline Using X-ray Diffraction Techniques, Determining Material Characterization:

Residual Stress and Integrity with NDE, PUP-Vol 276, NDE-Vol 12, American Society of Mechanical

Engineers, New York, 1994

39. C.O. Ruud, P.S. DiMascio, and D.J. Snoha, A Miniature Instrument for Residual Stress Measurement,

Adv. X-Ray Anal., Vol 27, Plenum Press, 1984, p 273–283

53. B.D. Cullity, Elements of X-Ray Diffraction, 2nd ed., Addison-Wesley Publishing Co., Inc., 1978, p

469–472

75. C.O. Ruud, X-Ray Analysis and Advances in Portable Field Instrumentation, J. Met., Vol 31 (No. 6),

10–15 June 1979

76. H.P. Klug and L.E. Alexander, X-Ray Diffraction Procedures, 2nd ed., New York, John Wiley & Sons,

1974, p 768–770

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