xray residual stress

7/27/2019 Xray Residual Stress

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Beth Matlock, Senior Materials Engineer

Materials Testing Division

Technology for Energy Corporation (TEC)


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History

1974 Pre-Med Student Working at JC Penney Converted a scissor sale into a career


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Dreams

New Corvette My own horse - On my own farm Be self-sufficient


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Background

NCSU XRD for element and compoundanalysis, developed Co-Ga-B ternary diagram

B&W XRD pole figures on Zr-4, failureanalysis, B4C research

TEC XRD residual stress and retainedaustenite measurements, diffractometer design

and development


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What are Residual Stresses?

Residual stresses are stresses that remain in the materialafter all external loads are removed. They result fromdifferential cooling rates (welding, heat treating, etc.) andplastic flow (shot peening, cold working, etc.)


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How Do They Differ from Loading

Stresses?

Loading stress results from applied load: weight, torsion,

bending, pressure, pushing, pulling, etc.

You can measure total stress (residual stress and loading

stress) using x-ray diffraction


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Whats so Great about X-ray Diffraction

Residual Stress Measurements?Nondestructive* surface measurement technique

Measures strain, stresses are calculated

Gives absolute stress value

Measures residual strain if sample is unloaded

If component is loaded, resultant stress will be the residual plus applied stress

Measurements can be made on most metals and ceramics; samples must be polycrystalline with

relatively randomly oriented medium to fine grains

Measurements can be made in a few minutes to an hour

*Depth profiling can be destructive Large samples need to be sectioned if using conventionalequipment


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History of Residual Stress by X-ray

Diffraction Techniques


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Why Are Residual Stresses Important?

Residual stresses may be harmful or beneficial Tensile residual stresses at the surface are

normally harmful sometimes leading to brittlefracture in fatigue

Compressive residual stresses at the surfacenormally increase fatigue strength

Residual stresses can be controlled in themanufacturing process


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How Does It Work?

Strain:

L/L0 =d/d0

Stress/Strain Relationships:

ii = 1/E(ii)

JJ = -/E(ii)

ii = 1/E(ii) - /E(JJ + kk )

Where L = length

d = d-spacing = strain

= stress

= Poissons ratio

E = Youngs modulus


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Stress-Strain Equation for X-Ray

Diffraction

(33) = (d d0) / d0

(33) = a3k a3lkl

coscos sincos -sin

aik = -sin cos 0

cossin sinsin cos


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Relative Axis


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Diffraction

(33) = (d d0) / d0 =

11cos2sin2 + 12sin2sin

2 +

22sin2sin2 + 33cos2 +13cossin2 + 23sinsin2 =

1/2S2(11cos2 + 12sin2 + 22sin

2)sin2 +

1/2S233cos2 + S1(11 + 22 + 33) +

1/2S2(13cos + 23sin)sin2


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Diffraction

Assuming 33 = 13 = 23 =0

(d d0) / d0 = 1/2S2(11cos2 + 12sin2 +

22sin2)sin2 + S1(11 + 22)

(d,=0 d0) / d0 = S1(11 + 22)

(d d0) / d0 - (d,=0 d0) / d0 = (d d=0) / d0= 1/2S2sin

2

1/2S2 = (1+ )/E S1 = -/E


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Measurement Direction

Stress is a tensor

and has magnitude

and direction


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Braggs Law Used To Measure

Crystal Lattice Parameter, d


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Effect of Tensile Stress on a

Crystal Lattice Structure


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Diffraction Pattern


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Benefits of X-Ray Diffraction Residual

Stress Analysis

Can be nondestructive

Gives a direct measure of strain and provides anquantitative value for stress

Fast

Accurate stress versus depth with electropolishing Repeatable


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Benefits of X-Ray Diffraction Residual

Stress Analysis

Understand and quantify grinding burn to preventcomponent failure

Analyze residual stress versus fatigue to aid incomponent life estimation

Find cause of warping in machined components

Analyze heat affected zones of weldments todetermine need for stress relief


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References

B. D. Cullity, Elements of X-ray Diffraction, Addison-Wesley,Reading, MA, 2nd Edition, 1977, pp. 447-477.

Residual Stress Measurement by X-ray Diffraction SAE HS784,SAE, Warrendale, PA, 2003.

Noyan and Cohen, Residual Stress Measurement by Diffractionand Interpretation, Springer-Verlag, New York, 1987.

Metals Handbook, ASM, Metals Park, Ohio, 9th Ed. Vol. 10, 1986,pp. 380-392.

SEM Residual Stress Seminar Notes, SEM, Bethel, CT. Northwestern University Residual Stress Course Notes, NU,

Evanston, IL

TEC Operation and Maintenance Manual, X-ray DiffractionSystems, Knoxville, TN.


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Parting Thoughts

Develop your talents Encourage, support and mentor your

sisters Be active and contribute to your

technical societies

Get involved with VolSTEM or similarorganization


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Parting Thoughts

Know what you want dream big Believe in yourself Work (and play) hard Continue to learn and grow Dont let your education hold you back


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Thank you!

Questions?

[email protected]

865-218-5848


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Comparison to Other Techniques

Sectioning, Material Removal

- gross residual stresses over large distance, depths- restricted to simple geometries

Hole Drilling- uniform stress fields

- complex geometries can be restrictive

Barkhausen- limited to ferromagnetic materials

- must have calibration standard

Ultrasonics- requires calibration standard

- sensitive to preferred orientation, grain size, second phases

X-Ray Diffraction- more expensive than above techniques

- surface technique

- sensitive to large grain size and preferred orientation


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Retained Austenite Analysis

Nondestructive phase analysis of steels Can be performed in minutes Large, complex-shaped parts can be analyzed The only method for accurately measuring

retained austenite in the 0% to 10% range


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Major phases in steel:

-Austenite (FCC)-Ferrite (BCC) / Martensite (BCT)

-Carbides


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By measuring and comparing intagrated intensities ofeach phase the amount of austenite can be calculated

% + %M + %C = 100%

Assuming no carbides gives:

% = (IRM) / (IM R)


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Areas under diffraction peaks areproportional to amount of each phase


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8 < IM211 : IM200 < 11 (A)

1.1 < I220 : I200 < 1.8 (B)

1. If intensity ratios are outside (A) and (B) thenlarge grain size and/or preferred orientation are

probable.2. If the intensity ratio is within (A) but higher than

(B) then expect interference with 220. (If lower

than (B) then expect interference with 200.)

3.

If the intensity ratio is within (B) but higher than(A) then expect interference with M211. (If lower

than (A) then expect interference with M200.)


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Applications:Bearings - Dimensional stability

- Preventing premature

failures

Large structures Dimensional stability


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Retained Austenite Correction Flowchart


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Production of X-rays

Electronic transitions in an atom (schematic).

Emission processes indicated by arrows.


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Characteristic X-rays

Spectrum of Mo at

35kV. Line widthsare not to scale.

Resolved Kdoublet is shown

on an expanded

wavelength scale

at right.


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X-ray Spectrum

X-ray spectrum of molybdenum as a function of applied

voltage. Line widths not to scale.


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a0

dl

Crystallographic Planes

h+k+l

ad =

0

(for cubic only)


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Comparison of the

spectra of copperradiation before and

after passage througha nickel filter. The

dashed line is the

mass absorptioncoefficient of nickel.

Filters


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Production/Properties of X-rays

For x-ray diffraction, x-rays are produced inevacuated tubes by bombarding metallic targetswith electrons

Two types of radiation- Bremstrahlung (slowing down of electrons)

- Characteristic (electrons ejected from innershells of target material)

Comparison with radiography

X-ray absorption due to electron ejection from K,L, M shells

I=I0e-t


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Crystal Planes


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X-Ray Diffraction Basics

Braggs Law - N = 2dsin

Penetration - t (infinite thickness) =3.45 sin

(/)

Fraction of scattered intensity from surface layer x: - Gx = [1-e-2x/sin]

Back reflection region

d = 0 =(-2dcos) + 2dsin or = (d/d) tan

for a given strain (d/d), peak shift () is greater when 2 is larger

Correction Factors: Lorentz PolarizationAbsorption

BackgroundEtc.


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Cold Work (Microstrain)


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Measurement Pitfalls and Interpretation

Theory predicts (assuming biaxial stresses) linear d-spacing

versus sin plots

Cause Diagnoses Solutions

1. Large grain size 1. Move sample lateral

and look for diffraction

peak intensity changes

1a. Use harder (more

penetrating) radiaition

1b. Use oscillation

techniques

1c. Use larger beam size

1d. Use divergent beam

2. Preferred orientation

(texture)

2. Intensity changes

with -angle but

not sample displacement

2a. Use less sensitive

reflections, i.e.,

{h,0,0}, {h,h,h}

2b. With PSPC systems,

Oscillation techniques

usually help

2c. Analytical Methods

A. Nonlinear d-spacing versus sin plots


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Measurement Pitfalls and Interpretation (Cont.)


3. Surface Distortion 3. Broad diffraction peaks 3a. Use harder radiation

3b. Electropolish

B. Curved d-spacing versus sin plots


1. Stress Gradients 1. Curvature near=0 1a. Use harder radiation

1b. Use Triaxial analysis

techniques


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Measurement Pitfalls and Interpretation(Cont.)

C. Sin splits


1. Sample geometry 1. Split changes according to

beam size

1a. Use smaller beam

1b. Mask sample

2. misset 2. Rotate sample 180 to see

if curve inverts

2. Check sample

diffractometer alignment

3. Focusing circle effects 3. Rotate sample 180 to see

if curve inverts

3a. Use higher back

reflection angle3b. Use only positive

angles

4. Sheer Stresses,

Triaxial Stresses

4. Rotate sample 180 to see

if curve inverts

4. Use analytical techniques

to separate components of

stress tensor

xray residual stress

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