acceptedpractices1 -modified layer removal method for evaluating residual stresses in thermal spray...

8/12/2019 AcceptedPractices1 -Modified Layer Removal Method for Evaluating Residual Stresses in Thermal Spray Coatings

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This is a working document under consideration by an ASM International Thermal Spray Society

Committee. It is available solely to obtain comments from interested parties, and may not be relied upon orutilized for any other purpose. Working documents may change substantially in future versions.

1

ASM International TSS

Accepted Practice for Mechanical Properties #1 Published in 2002

AP MP001-02

Modified Layer Removal Method for

Evaluating Residual Stresses in Thermal Spray Coatings

January 1, 2002

ASM International, Thermal Spray Society

9639 Kinsman Road

Metals Park, OH 44073-0002


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Keywords- ASM International TSS Accepted Practice,

Residual Stress Evaluation Procedure,

Thermal Spray Coatings,Residual Stresses, Layer Removal Method

Prepared and Approved by

ASM International Thermal Spray Society

Accepted Practice Committee on

Evaluation of Mechanical Properties of Thermal Spray Coatings

Date: __ July 31, 2001 _

Approved byASM International Thermal Spray Society Board of Directors

Date: __December 31, 2001__

Abstract This Accepted Practice contains the procedure for evaluating residual stresses in

thermal spray coatings using a modified layer removal method. It cites the dimensions ofthe test specimen, the equipment needed, the procedure for applying gages, the procedure

for removing layers, and the method for interpreting the data to evaluate residual stresses.

ASM International World Headquarters


9639 Kinsman Road

Metals Park, OH 44073-0002 USA


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Statement of the Use of

ASM International Thermal Spray Society Accepted Practices

ASM International is not a standards writing organization. Yet, the increased use of

thermal spray coatings and the need for documentation on methods to evaluate mechanicalproperties have generated a need for Accepted Practices. In response to this need, the ASM

International Thermal Spray Society formed three Committees on Accepted Practices. This

document is prepared by the Committee on Evaluating Mechanical Properties of Thermal Spray

Coatings. The purpose of this document is to present a written description of a method for

evaluating residual stresses in thermal spray coatings. The method is the Modified Layer

Removal Method (MLRM). The MLRM is an extension of the well-known Layer Removal

Method to include the Young's modulus and Poisson's ratio properties of the thermal spray

coating material and the substrate.

DISCLAIMER: This document is a collective effort involving a number of volunteer

specialists. Great care is taken in the compilation and production of this document, but it should

be made clear that NO WARRANTIES, EXPRESS OR IMPLIED, INCLUDING, WITHOUT

LIMITATION, WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A

PARTICULAR PURPOSE, ARE GIVEN IN CONNECTION WITH THIS DOCUMENT.

Although this information is believed to be accurate by ASM, ASM cannot guarantee that

favorable results will be obtained from the use of this document alone. This document is intended

for use by persons having technical skill, at their sole discretion and risk. It is suggested that you

consult your own network or professionals. Since the conditions of product or material use are

outside of ASM's control, ASM assumes no liability or obligation in connection with any use ofthis information. No claim of any kind, whether as to products or information in this document,

and whether or not based on negligence, shall be greater in amount than the purchase price of thisproduct or document in respect of which damages are claimed. THE REMEDY HEREBY

PROVIDED SHALL BE THE EXCLUSIVE AND SOLE REMEDY OF BUYER, AND IN NO

EVENT SHALL EITHER PARTY BE LIABLE FOR SPECIAL, INDIRECT OR

CONSEQUENTIAL DAMAGES WHETHER OR NOT CAUSED BY OR RESULTING FROM

THE NEGLIGENCE OF SUCH PARTY. As with any material, evaluation of the material under

end use conditions prior to specification is essential. Therefore, specific testing under actual

conditions is recommended.

Nothing contained in this document shall be construed as a grant of any right of manufacture,

sale, use, or reproduction, in connection with any method, process, apparatus, product,

composition, or system, whether or not covered by letters patent, copyright, or trademark, andnothing contained in this document shall be construed as a defense against any alleged

infringement of letters patent, copyright, or trademark, or as a defense against liability for such

infringement.


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Comments, criticisms, and suggestions are invited, and should be forwarded to ASM

International.

Attn: Committee on Evaluating Mechanical Properties of Thermal Spray Coatings

ASM International World HeadquartersASM International Thermal Spray Society

9639 Kinsman Road

Metals Park, OH 44073-0002 USA

E-mail: Bob Uhl

Phone: 440 338-5151

Fax: 440 338-4634

Photocopy Rights

No part of this document may be reproduced, stored in a retrieval system, or transmitted, in any

form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the

written permission of the copyright owner.


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Personnel


Accepted Practice Committee onEvaluation of Mechanical Properties of Thermal Spray Coatings

Edmund F. Rybicki, Chair University of Tulsa

Oludele Popoola Consultant

Christopher Berndt SUNY at Stony Brook

Joseph DeFalco Sulzer Metco

Humin Gassot Institut de Physique Nucleaire de ORSAY

Warren D. Grossklaus, Jr. GE Aircraft Engines

Ian D. Harris Edison Welding Institute

Robert Hilgenberg PRAXAIR Surface Technologies

Xin-qing Ma Aoyama Gakuin University

Roy T. R. McGrann SUNY at Binghamton

Manuel Maligas FMC Corporation

Sanjay Sampath SUNY at Stony Brook

Elliot Sampson PRAXAIR TAFA

John Sauer Belcan EngineeringAnnie Savarimuthu Hanover Corporation

Mark F. Smith Sandia National Laboratories

David A. Somerville Engelhard

Robert A. Sulit Sulit Engineering

Jan Wigren Volvo Aero Corporation


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Table of Contents

Page No.

Key Words 2

Abstract .. 2Statement of the Use of ASM International Thermal Spray

Society Accepted Practices . 3

Personnel .. 5

Table of Contents .. 6

List of Figures .. 6List of Tables . 6

Scope .. 7

Referenced Documents 7

Terminology 7

I. Introduction and Background 8

II. Equipment and Supplies 10

III. Description of the Residual Stress Specimen 12IV. Procedure for Applying Strain Gages 13

V. Initial Strain Gage and Dimensional Data 13

VI. Layer Removal Procedure .. 14

VII. Data Analysis . 17VIII. References . 21Appendix A. Analysis for the Modified Layer Removal Method ... 22Appendix B. Dimensioned Drawing of the Residual Stress Specimen Fixture .. 25

Appendix C. Acknowledgements 25

List of Figures

Figure 1. Fixture for Holding Four Residual Stress Specimens . 11

Figure 2. Dimensions of the Thermal Spray Coating Residual

Stress Specimen with Strain Gages . 12

Figure 3. Half Bridge Configuration with Temperature

Compensating Strain Gage . 14

Figure 4. Modified Layer Removal Method Program Input .. 18

Figure 5. Longitudinal Residual Stress Distribution .. 19Figure 6. Transverse Residual Stress Distribution 20

Figure A-1. Free-Body Diagram for Modified Layer Removal Method

Applied to a Thermal Spray Coated Specimen 22

Figure B-1. Dimensions of the Residual Stress Specimen Fixture . 25

List of Tables

Table 1. Data Sheet for Strain Gage Readings and Thickness Measurements 16


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Residual stresses: Stresses that exist in a solid material without any external mechanical loadings

applied to the solid. Residual stresses can be generated at room temperature or at higher or lower

temperatures.

Strain gage: A device used to monitor the change in strain of a specimen subjected to changes in

stresses.

Strain rosette: A combination of strain gages used to measure the strains in more than one

direction .e.g. rectangular rosette, delta rosette.

Substrate: A material, which serves as a foundation for a thermal spray coating.

Thermal spray coating: A coating of material sprayed on another material called a substrate.

Thermal spray coatings are used to enhance resistance to wear or corrosion and for dimensional

restoration.

I. Introduction and Background

Thermal spray coatings are used extensively by many industries in applications including

wear and corrosion protection, dimensional restorations, and thermal insulation. The materialsused in thermal spray coatings include metals, carbides, and ceramics. There are several different

commercially available processes to apply thermal spray coatings and newer processes are being

developed. The following four questions and answers were selected to provide an introduction to

residual stresses in thermal spray coatings.

What Causes Residual Stresses in Thermal Spray Coatings?

In most combinations of coating material and application process, there is a temperature

difference between the coating and the substrate caused by heating the coating and having the

substrate at a different temperature than the applied coating. As the two materials cool to room

temperature, they shrink by different amounts causing residual stresses in the coating and the

substrate. Other factors, such as particle impact velocity and splat cooling rate, influence coating

residual stresses. The result is that there are residual stresses in the coating and substrate,and

these stresses can be tensile or compressive. [1]

Why are the Residual Stresses in Thermal Spray Coatings of Interest?

Residual stresses have been shown to affect the performance of tungsten carbide (WC)

thermal spray coated components in gas turbine engines [2] and the fatigue life of HVOF sprayed

WC on aluminum [3] and steel [4]. The bond strength of coatings has also been shown to be

affected by residual stress [5].


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How can Residual Stresses be Evaluated in TSC's using Laboratory Procedures?

There are several methods used to determine the residual stresses in thermal spray

coatings. The X-ray method, the hole drilling method, bending deflection method, the neutrondiffraction method, the Modified Layer Removal Method (MLRM) and other methods have been

used.

What is the difference Between the Layer Removal Method and the Modified Layer Removal

Method?

The Modified Layer Removal Method is an extension of the Layer Removal Method.

The Layer Removal Method was developed in 1945 By Rosenthal and Norton [6]. In 1965, it

was written as a Society of Automotive Engineers Information Report [7]. The Layer Removal

Method was developed for a single uncoated material, such as steel, aluminum, or titanium. The

Layer Removal Method does not recognize the Young's modulus and Poisson's ratio associated

with thermal spray coated materials. Thus, there is a need for a layer removal procedure toevaluate residual stresses in coated materials. The Modified Layer Removal Method was

developed specifically to meet this need for evaluating residual stresses in thermal spray coatings

[1] [8].

The Modified Layer Removal Method has been used to determine the residual stress

distributions through the thickness of the coating and the substrate for a variety of coatings

including tungsten carbide, aluminum, thermal barrier yttria stabilized zirconia, copper, and steel.

[3], [4], [5], [8] [9], [10], [11]

Both the Layer Removal Method and the Modified Layer Removal Method were

developed for isotropic material behavior.

The following sections describe equipment and the laboratory procedure for the MLRM .


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II. Equipment and Supplies

The following is a list of equipment and materials needed for the Modified Layer Removal

Method.

.

ITEM DESCRIPTION

Surface cleaning supplies Silicon carbide paper 220-00 grit, gauze sponges,

degreasers, water-based acidic surface cleaners, and

neutralizers as recommended by strain gage supplier.

Strain gages 90biaxial (2 gages) rosette. One rosette per specimen.

Gage adhesive Follow recommendation of strain gage supplier

Strain gage wires 2-wire, twisted, multi-strand, copper.

Soldering iron Small tip iron

Protective coating Non-conductive, waterproof, mechanical protection,following recommendation of strain gage supplier.

Strain Indicator At least 0 to 2999 microstrain range with 1 microstrainresolution.

Metallographic polishing

machine

Vertical shaft for horizontal wheel, fitted with polishing

head to hold the sample to the polishing wheel for semi-

automatic grinding /polishing.

Polishing wheel 8-inch diameter diamond polishing disc with 125 micron

diamond chips.

Fixture to hold specimens Four-specimen holder to match to polishing head (see

Figure 1).Micrometer 0.0001 inch (0.0025 mm) resolution.

Computer to run MLRM

program

Windows 95 or NT or later to run Excel 97 or later.

Modified Layer Removal

Excel Program based on

References [1], [8] on

MLRM

Contact

Bob Uhl

at ASM, International to download program

.

It should be noted that the fixture shown in Figure 1 is provided as an example of a

fixture that works with polishing equipment of one type. Since there are several types ofpolishing equipment, details of the fixture may have to be different to interface with

different polishing equipment.


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Figure 1a. Specimen fixture showing coating side of four specimens installed in fixture.

Figure 1b. Specimen fixture showing gage wires tucked into cavities above specimens.

Fixture

SpecimensSet screws to

secure specimens

Strain gage wires

Roll pin to match with

polishing head


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III. Description of the Residual Stress Specimen

The residual stress specimen is shown in Figure 2. The specimen is 25.4 mm by 25.4 mm

by 6.35 mm. The coating is sprayed on one of the 25.4 mm by 25.4 surfaces as shown in the

Figure 2. The gages are applied after the coating is applied. Based on experience, the coating

thickness is recommended to be between 0.13 mm and 1.5 mm (5 mils to 60 mils).

a. Isometric view

b. Bottom view

Figure 2. Dimensions of the Thermal Spray Coating Residual Stress Specimen with Strain Gages

Coating

Substrate

25.4mm

25.4mm

6.35mm

h

900Biaxial

Strain Rosette

25.4mm

25.4mm

900Biaxial

Strain Rosette

Substrate


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IV. Procedure for Applying Gages

Procedures for applying or installing bonded resistance strain gages are available from

strain gage suppliers. Also ASTM E 1237-93 (Reapproved 1998) "Standard Guide for Installing

Bonded Resistance Strain Gages" provides good guidelines. Some aspects of applying strain

gages specific to the MLRM are reviewed below.Mount the strain rosette at the center of the substrate 25.4 x 25.4 mm surface. Align the

sensitive axes of the 90rosette parallel to the 25.4 mm edges of the specimen as shown in Figure2. Solder one lead to each solder tab of the two gages on the rosette. Make all leads from all

gages the same length and wire diameter. Follow ASTM E 1237 to check the gage installation.

Label each gage with specimen and gage identification using a masking tape label on the gage

wires or other method. Designate one of the two gages on each specimen as Longitudinal and

the other as Transverse. Apply a non-conductive protective coating to the gage that will keep

the gage, wires, and solder joints absolutely dry and reduce the chance of mechanical damage to

the gage or wires during handling and grinding. Apply a rosette to an uncoated specimen that is

made of the same material as the substrate material and approximately the same size as the coated

specimens to use as a Reference gage and for Temperature Compensation. Wire the

Reference/Compensating gages the same way as specimen gages: make all lead wire the samelength as specimen gage lead wires. Label one of the gages as Reference and the other as

Compensating, and apply protective coatings.

V. Initial Strain and Dimensional Data

Two types of measurements are taken: 1) strain; and 2) specimen thickness. Initial

measurements of these quantities are required before any polishing is done on the specimens.

It is important that strain measurements are made before thickness measurements becausehandling the specimen to make thickness measurements might increase the temperature of the

specimen relative to the temperature of the Compensating gage. Allow time for the substratespecimen on which the Reference gage and Temperature Compensating gage are mounted to

reach room temperature at the location where all strain measurements will be made. Connect the

Reference and Compensating gages to the strain indicator in a half-bridge as shown in Figure 3

and balance the bridge (set output indication to zero). Solid lines in Figure 3 indicate Reference

gage or Specimen gage, Compensating gage, and lead wires. (Dashed lines in this figure indicate

internal components of the strain bridge and are provided here to show the entire bridge

configuration.) Disconnect the Reference gage lead wires from the strain indicator,but leave the

Compensating gage attached. Connect in turn each of the two gages on a coated specimen to the

strain indicator as shown in Figure 3 and record the indicated strains in the first line of the sample

data sheet shown in Table 1. Repeat for each coated specimen. To minimize measurement error

when balancing the strain bridge and when making specimen strain measurements, always insert

the gage wires into the strain indicator terminal post to the same point on the wire.Use the micrometer to measure the thickness of each coated specimen at the four corners

of the specimen outside the area covered by protective coating and enter the data on the first lineof the data sheet in Table 1.


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Figure 3. Half Bridge configuration with temperature compensating gage. (Dashed lines

represent internal strain bridge elements.) Leads, labeled L, should be the same length.

VI. Layer Removal Procedure

After taking initial strain and specimen thickness measurements, insert the specimens into

the four-specimen fixture (Figure 1) and mount the fixture to the polishing head. The specimens

should be secured in the fixture with the coating side down and flat on the polishing wheel of the

metallographic polishing machine referred to in the Table in Section II. The strain gage wires

should be tucked into the specimen holder cavities above the specimens, and the cavities then

covered with duct tape to reduce exposure of the gages to the water used in the polishing process.

Begin the polishing process by pressurizing the polishing head and turning on the

polishing wheel and water. A wheel rotation speed of about 120 rpm works well for most

coatings. Make a brief initial run with a polishing head pressure of zero to provide an idea of the

rate of material removal. Increase the pressure if necessary to increase the material removal rate.

Thickness of layers removed should be between about 0.7 thousandths of an inch (0.017 mm) and

2 or 3 thousandths (0.05 0.08 mm). If the coating layer removed is too thin, the strain change

may be below the noise floor of the measuring system. If this happens, simply polish more

material for that layer before recording the new strain readings. The time required to remove a

layer of sufficient thickness is not fixed, but rather depends on the type of coating and the

condition of the polishing wheel. For a polishing wheel that is in good condition, the time

Temperaturecompensating

R1

R2

L

L

L

L

Strainreadout

Powersupply

Reference orSpecimen


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required can vary from a few minutes (for thermal barrier coatings, to an hour or more for

tungsten carbide wear resistance coatings).

After a layer of sufficient thickness has been removed by polishing, the specimens are

removed from the fixture, dried, and allowed to reach room temperature. No strain measurements

should be made within one half-hour after removing the specimens from the fixture in order to

allow time for the specimen to reach thermal equilibrium with the substrate specimen on whichthe temperature-compensating gage is mounted. Also for this reason, it is important to measure

the strain before handling the specimen to measure thickness. Before measuring the specimen

strains, connect the Reference gage and the Temperature Compensating gage to the strain

indicator in the half-bridge configuration as shown in Figure 3 and balance the bridge (set output

to read zero). To make strain measurements for the specimens, the reference gage is replaced by

specimen strain gages connected in turn into the half-bridge configuration with the Temperature

Compensating gage as shown in Figure 3. To minimize measurement error when balancing the

strain bridge and when making specimen strain measurements, always insert the gage wires into

the strain indicator terminal post to the same point on the wire.

Thickness measurements are made by micrometer at the four corners of the specimen.

Strain and thickness data can be recorded in the data sheet shown in Table 1,starting with the row

immediately below the initial thickness and strain data. The average thickness is calculated incolumn six. The last column in the table is for comments. Usually, the point in the layer removal

process at which the last coating layer is removed from the specimen is recorded in this column.

Continue the layer removal process until the coating has been completely removed from

the specimen. Additional layers can be removed if information on the residual stress levels in the

substrate near the coating/substrate interface is desired. As successive layers of the substrate are

removed, usually the strain measurements stop changing. This indicates that the residual stresses

in the remaining substrate are negligible, and the process can be terminated.


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Table 1. Data Sheet for Strain Gage Readings and Thickness Measurements

Material ThicknessTo CoatSubstrate

* Thickness (in.) * Strain (Date/Initials1 2 3 4 Average Long. Trans.

Thickness of

layer removed(in.)

Comments

* Measure the strain first and then the thickness.

SpecimenNumber

Gage Resistance Gage Factor


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VII. Data Analysis

An Excel spreadsheet program with macros for the analysis shown in Appendix A has

been written. After the data are recorded on the Data Form values for the average thickness

column on the data sheet can be calculated and written on the data sheet. The data are then

entered into the spreadsheet program.

An example of the spreadsheet is shown in Figure 4. The modulus of elasticity and the

Poisson's ratio for the substrate and the coating are inputs. One method for evaluating the

modulus of elasticity and Poisson's ratio is described in Reference [12] The thickness of the

coating is an input. The other inputs are the thickness of the specimen (the average of 4 thickness

measurements) and the strain gage readings. The program will calculate the through-thickness

residual stress distribution for the depth that layers were removed.

- The Program, "MLRM for Residual Stresses" can be downloaded from the ASM,International website: www.ASM-international.org/

- An Example Case is shown in Figures 4, 5 and 6.

A few notes about the program are helpful. The purpose for the program is to make the

computational part of the Modified Layer Removal Method easier for the user. Thus, while the

graphics are perhaps neat looking for the transverse and longitudinal stress distributions for the

example case, any new case may require some changes in the fonts, axis scales, units, ranges,

location of the inset, or other aspects of the graphs. This will have to be done by the user. The

inset is left ungrouped, so the user can modify it. The user can remove the line connecting the

data points. The procedures of Excel apply. The text boxes are not grouped so the user can add

the specific type of coating or substrate. The dashed vertical line denoting the interface between

the coating and the substrate is moveable for different coating thicknesses.

The program calculates a table of through-thickness residual stress values, in the

longitudinal and transverse directions, in the main page, Figure 4. The values are plotted inFigures 5 and 6. Notice the thermal spray coating is plasma sprayed Ni-5Al. The substrate is

steel. Some of the characteristics of the stress distributions in Figures 5 and 6 are mentioned to

help the user. The vertical axis of Figure 3 shows the residual stress in psi. Positive numbers are

tensile residual stresses and negative numbers are compressive residual stresses. The horizontal

axis of Figure 4 shows the distance from the coating surface. There is tension in the coating.

Notice there are some peaks in the tensile residual stress distribution. This could be because

measurements are not exact or because the residual stresses are changing in that region. Recallthat 4 specimens are recommended to examine reproducibility of the residual stresses. There can

be some specimen-to-specimen variation in the residual stress distributions. When this happens,

data from the 4 residual specimens can indicate a range of reproducibility on the results. The user

can select statistics parameters to examine for example confidence intervals of the data.

The compression residual stress in the substrate, near the interface, is due to "gritblasting" the substrate before the coating was sprayed. The Modified Layer Removal Method

analysis assumes that the last remaining piece is stress free and calculates residual stresses up to

and on the gaged surface.


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Insert data in the Blue Bold cells only Substrate Modulus,E 29000000

Thick ELong ETran Substrate Poisson's Ratio,V 0.3

0.2115 364 735 Coating Modulus,Ecoating 12000000

0.2085 355 725 Coating Poisson's Ratio,Vcoating 0.25

0.2059 342 714 Press Long Coating Thickness,Tcoating 0.023

0.2033 331 705 Button for

0.1973 301 673 graph Z LONG TRAN0.1902 270 645 1.5000E-03 1.05E+04 1.12E+04

0.1861 264 638 4.3000E-03 1.65E+04 1.50E+04

0.1834 270 646 6.9000E-03 1.35E+04 1.21E+04

0.1778 264 636 Do Not insert Rows 1.1200E-02 1.69E+04 1.76E+04

0.1705 242 617 or Columns 1.7750E-02 1.37E+04 1.29E+04

anywhere. 2.3350E-02 -1.39E+03 -7.32E+02

2.6750E-02 -1.63E+04 -1.76E+04

3.0900E-02 -2.24E+03 -6.88E+02

3.7350E-02 2.93E+03 2.21E+031.2625E-01 -1.61E+03 -1.58E+03

2.1150E-01 5.02E+03 4.93E+03

If more rows are needed for data points, the References can be moved to make

room for the data points.

REFERENCES

Greving, D. J., Rybicki, E. F., and Shadley, J. R., "Through-Thickness Residual Stress

Evaluations for Several Industrial Thermal Spray Coatings Using a Modified Layer Removal

Method", The Journal of Thermal Spray Technology, Vol. 3, No. 4, December 1994, pp. 379-388.

McGrann, R.T.R., Rybicki, E.F., and Shadley, J.R., Applications and Theory of the

Modified Layer Removal Method for the Evaluation of Through-Thickness Residual Stresses

in Thermal Spray Coated Materials, Proceedings of the Fifth International Conference on

Residual Stresses, held in Linkping, Sweden, June 16-18, 1997.

The Macros for the Excel Program were written by Mr. Sulaiman Al-Musallami

Figure 4. Input and Output Screen for MLRM Program

RunRun


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Figure 5. Through-Thickness Longitudinal Residual Stress Distribution

-20,000

-15,000

-10,000

-5,000

0

5,000

10,000

15,000

20,000

0.00 0.05 0.10 0.15 0.20 0.25

Distance From Coating Surface, Z (inch)

ResidualStress

(psi)

Coating

Substrate

Z

Coating Substrate


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Figure 6. Through Thickness Transverse Residual Stress Distribution

-20,000

-15,000

-10,000

-5,000

0

5,000

10,000

15,000

20,000

0.00 0.05 0.10 0.15 0.20 0.25

Distance From Coating Surface, Z (inch)

ResidualStre

ss(psi)

Coating

Substrate

Coating Substrate

Z


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VIII. References

[1]. Greving, D. J., Rybicki, E. F., and Shadley, J. R., "Through-Thickness Residual Stress

Evaluations for Several Industrial Thermal Spray Coatings Using a Modified Layer RemovalMethod", The Journal of Thermal Spray Technology, Vol. 3, No. 4, December 1994, pp. 379-388.

[2]. Pejryd, L., Wigren, J., Greving, D.J., Shadley, J.R., and Rybicki, E.F., Residual Stresses as aFactor in the Selection of Tungsten Carbide Coatings for a Jet Engine Application, The Journal of

Thermal Spray Technology, Vol. 4, No. 3, September, 1995, pp. 268-274.

[3]. McGrann, R.T.R., Greving, D.J., Shadley, J.R., Rybicki, E.F., and Bodger, B.E., The Effect

of Residual Stress in HVOF Tungsten Carbide Coatings on the Fatigue Life in Bending of

Thermal Spray Coated Aluminum,Jl. Thermal Spray Technology, Vol. 7, No. 4, Dec. 1998, pp.546-552.

[4]. McGrann, R.T.R., Shadley, J.R., Rybicki, E.F., Kruecke, T.L., and Bodger, B.E. The Effect

of Coating Residual Stress on the Fatigue Life of Thermal Spray Coated Steel and Aluminum,

The Journal of Surface & Coatings Technology, Volumes 108-109 (1998), pp. 59-64.

[5]. Greving, D. J., Shadley, J. R., and Rybicki, E. F., "Effects of Coating Thickness and Residual

Stresses on the Bond Strength of ASTM C633-79 Thermal Spray Coating Test Specimens," The

Journal of Thermal Spray Technology, Vol. 3, No. 4, December 1994, pp. 371-378.

[6]. Rosenthal, D. and Norton, J. T., "A Method of Measuring Triaxial Residual Stress in Plates,"

Welding Journal, Vol. 24 Research Supplement, PP. 295S- 307S , May 1945.

[7]. SAE Information Report, Methods of Residual Stress Measurement, SAE J936, (Dec 1965).

[8]. McGrann, R.T.R., Rybicki, E.F., and Shadley, J.R., Applications and Theory of the

Modified Layer Removal Method for the Evaluation of Through-Thickness Residual Stresses in

Thermal Spray Coated Materials, Proceedings of the Fifth International Conference on Residual

Stresses, held in Linkping, Sweden, June 16-18, 1997.

[9]. McGrann, R.T.R., E.F. Rybicki, J.R. Shadley, and W.J. Brindley, Factors Influencing

Residual Stresses in Yttria Stabilized Zirconia Thermal Barrier Coatings, Proceedings of the

NASA Materials and Structures Base R&T Conference, held at NASA Lewis Research Center,

Cleveland, Ohio, May 1, 1997.

[10]. R.T.R. McGrann, E.F. Rybicki, J.R. Shadley, D.J. Greving, J. Wigren, L. Pejryd, and W.J.

Brindley, Residual Stress Development in Thermal Barrier Coatings, poster presentation at the

Thermal Barrier Coating Workshop, sponsored by the TBC Interagency Coordination Committee,

Cincinnati, Ohio, May 19-21, 1997.

[11]. McCune, R.C., Donlon, W.R., Cartwright, E.L., Papyrin, A.N., Rybicki, E.F., and Shadley,

J.R., Characterization of Copper and Steel Coatings Made by the Cold Gas-Dynamic Spray

Method, Thermal Spray: Practical Solutions for Engineering Problems, Proceedings of the 1996

National Thermal Spray Conference, Cincinnati, Ohio, 7-11 October 1996, Ed. C.C. Berndt

(Metals Park, Ohio: ASM International, 1996), pp. 397-404.

[12]. Rybicki, E.F., Shadley, J.R., Xiong, Y., and Greving, D.J., A Cantilever BeamMethod for Evaluating Youngs Modulus and Poissons Ration of Thermal Spray

Coatings, The Journal of Thermal Spray Technology, Vol. 4, No. 4, December, 1995,pp. 377-383.

[13]. Jones, R.M., Mechanics of Composite Materials (NY: Hemisphere Publishing Corp.,

1975), pp. 170-1.


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22

Appendix A. Description of the MLRM Analysis for one Layer Removed.

The layer removal method [6] [7] which was developed for a single material, has been

modified to handle thermal spray coated materials. The MLRM uses the mechanics of compositematerials analysis for a two-material non-symmetric layered plate [13].

The analysis is based on the free-body diagram shown in Figure A-1. The force acting on

the layer removed in the x-direction is denoted by Fx. The force and moment acting on the

remaining piece are related to Fxby the force and moment equilibrium equations. Figure A-1

shows a layer of coating with thickness h removed. The remaining coating thickness is h. The

thickness of the substrate is H. A biaxial strain gage is attached to the bottom of the substrate as

shown. Note that this figure shows a quarter section of the specimen with the z-axis through the

center of the entire specimen. The dimensions bx and by are half of the actual specimen

length and width. The z = 0 plane is located at the center of the remaining piece after layer

removal, that is, at (H+h)/2.

Figure A-1. Free-Body Diagram for Modified Layer Removal Method

Applied to a Thermal Spray Coated Specimen

The changes in strain in the remaining piece due to replacing a layer (this is the negative

of the change in strain measured at the strain gage) are given by:

x x0 x z y y0 y z (1)

Mx

z

x

Fx

Fx

Mx

Strain

Gage

bx

by

h

Hz = 0

y

hFx

Fx

Layer Removed

Mx


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23

where:xand

yare the curvatures.

For plane stress, the stress-strain equations are:

x

y

x

y

E

1

1 (2)

where: E is the following function of the material properties:

E

Eb

b

b1 -

for the substrate, and

EE

c

c

c1-

for the coating.

The resultant forces and resultant moments, defined as the force per unit length, Fx

and Fy

, and

the moment per unit length, Mx

and My

, are related to the stresses by:

F

F z

x

y

x

yH + h

H + h

2

2

d and

M

Mz z

x

y

x

yH + h

H + h

2

2

d (3)

Substituting Eqn. (1) and Eqn. (2) into Eqn. (3) gives:

(4)

where: A11 = A22= Eb H + Ec h ; A12= b bE H + c cE h ;

B11= B22= E Ec b Hh

2; B12=

c c b b Hh2

E E;

D11 = D22=

E E

b 2 c 2H

12H h

h

12h H3 3

2 2

D12=

b b 2 c c 2H

12 H h

h

12 h H

E E

3 32 2

Using Eqn. (1) and the strain readings at the strain gage to derive an equation for x0

and y0

gives: x0 xG xH + h

2

F

F

M

M

A A B B

A A B B

B B D D

B B D D

x

y

x

y

11 12 11 12

12 22 12 22

11 12 11 12

12 22 12 22

x0

y0

x

y


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Appendix B. Dimensioned Drawing of the Residual Stress Specimen Fixture.

Using the Modified Layer Removal Method requires a fixture to hold the 4 specimens during

polishing to remove each layer. Below is a drawing of the fixture with dimensions.

Figure I Dimensions of the Residual Stress Specimen Fixture in Inches.

Appendix C. Acknowledgements .

Acknowledgements for Contributions to the MLRM Program.

The MLRM program is based on the analyses described in References [1] and [8]. The

main computational program was written by Dr. Daniel Greving, currently at Honeywell. The

macros for the Excel program were written by Mr. Sulaiman Al-Musallami. The Graphics were

done by Ms. Annie Savarimuthu, currently at Hanover Corp. All contributors named above were

Mechanical Engineering graduate students at The University of Tulsa.