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PS-100-MW-FOPOLARIMETER SYSTEM

Instructions for Use

STRAINOPTICS, INC.108 W. MONTGOMERY AVE.

NORTH WALES, PA 19454USA

TEL: 215 661 0100FAX: 215 699 7028

www.strainoptics.comstress@strainoptics.com

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I. INTRODUCTION

Congratulations on your purchase of the PS-100-MW-FO polarimeter system. With onlya little practice, you will be able to use this precision instrument to obtain accurate,repeatable residual stress measurements on a wide range of transparent or translucentmaterials.

The Strainoptics PS-100-MW-FO polarimeter enables visualization of residual stress orstrains through the use of polarized light and a proven and widely accepted measuringtechnique known as photoelastic stress measurement. For technical information onphotoelasticity, please email our sales department at stress@strainoptics.com.

The instrument features a 75mm diameter, polarized fiber-optic illuminator/sample stagewith variable 150W power supply, and a column-mounted MWA-100 wedgecompensator with polarizing microscope and adjustable rack-and-pinion focusing mount.

The illuminator in this PS-100 system provides Plane (Linearly) Polarized Light.

II. SETUP

Eyepiece

MWA-100Measuring wedgeassembly

Microscope tube

Objectivelens

Lightsource

Figure 1. PS-100-MW-FOSystem Components

Position samplewith direction ofstress parallel tolong axis ofcompensator

Glass

FocusingMount

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The MWA-100 Measuring Wedge Assembly is a precision Babinet compensator,designed for measuring retardation in stressed transparent materials. It is aa single-wedge assembly with a built-in eyepiece that fits a standard microscopeeyepiece tube. The MWA-100 can be used to measure retardation and stress in avariety of applications requiring a microscope to observe an enlarged image andaccurately measure the stress. Measuring of surface and mid-layer stresses on across-section can be accurately performed using the MWA-100. Similarly, glass-to-metal stress due to the mismatch of expansion coefficients, and annealed products canbe inspected to verify the residual stress level. This manual is generally specific tomeasuring stress in float glass slices; however, the instrument is well suited to otherapplications. Consult Strainoptics for further assistance.

III. ASSEMBLING THE INSTRUMENT

1. Slide the MWA-100 assembly into the microscope tube, perpendicular to thereference axis of the polarimeter as shown in Figure 2.

2. Insert the microscope into the focusing mount.

3. Slide the focusing mount/microscope assembly onto the illuminator column.

4. Turn on the fiberoptic light source and adjust light intensity to maximum.

5. The instrument is now ready for operation.

IV. TAKING A MEASUREMENT

1. Refer to Figure 1 for the correct position of MWA-100 and sample. Long axis ofglass slice MUST be parallel to long axis of compensator as shown.

Figure 2. Typical configuration for measuring stress distribution infloat glass slices. The sample should be cut to approximately 1 x 6 in(25 x 150 mm) strips in cross section (or according to companyprocedures). Thin samples must be supported or fixtured to ensure anabsolute vertical light path through the thickness of the strip. Ideally, thetop and bottom surfaces of the strip should be ground smooth andpolished. Small cracks and imperfections will make measurementsdifficult. Use of an immersion cell fixture or placing a drop of indexmatching fluid (RI = 1.52) is recommended. These items may bepurchased from Strainoptics.

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1.5 Figure 2 shows the approximate distance between the sample and the objectivelens of the microscope that is necessary to obtain a clear image. (Area of sampleto be measured will dictate the correct distance.) Use the rack-and-pinionfocusing adjustment knobs to move the assembly up or down in order to obtain aclear image, focused on the TOP EDGE of the glass.

(NOTE: When evaluating a large glass cross-section, both surfaces and mid-layer region may not beable to be viewed at the same time. Also, it may be necessary to move the sample under theobjective lens to enable the operator to evaluate different sample regions.)

2. Look through the eyepiece and turn focusing ring, if required, to bring the crosshairinto sharp focus. A black fringe should be observed when the counter indicatesapproximately 000 (Figure 3).

Figure 3. Typical Image for measurement of mid-layer tensionseen through eyepiece when counter = 000

3. To measure maximum mid-layer tension in a glass slice cross-section, rotate the dialknob “counter-clockwise” and note that the reading increases from 000, 001, 002,003, 004, etc. Look for the “peak” of the parabola formed by the curved black fringe(Figure 4). Keep turning the dial knob counter-clockwise until the center of the peakof the curved black fringe is positioned at the intersection of the crosshair reticles.This is the point of measurement.

“Black” fringe

Verticalcrosshair

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Figure 4. Typical image seen for reading mid-layer tension

4. To measure surface compression at the edge, position the point of measurement atthe intersection of the crosshair reticles. Rotate the dial knob “clockwise” and notethat the reading decreases from 000, 999, 998, 997, 996, etc. Note: 8 7 0 is 870and not 8.70. Keep turning the dial knob clockwise until the center of the black fringereaches the edge of the glass at the intersection of the crosshair reticles.

5. Take a Reading.

Mid-Layer Tension. If you turned the dialCOUNTER-CLOCKWISE, the dial reading, r,changed from zero and “increased”, that is,moved in the direction: 000, 001, 002.By convention, this counter-clockwise,increasing dial reading indicates TENSIONand the numerical sign of stress is consideredto be positive (+).

Surface Compression. If you turned thedial CLOCKWISE, the dial reading, r,changed from zero and “decreased”, thatis, moved in the direction: 999, 998, 997.By convention, this clockwise, decreasingdial reading indicates COMPRESSIONand the numerical sign of stress isconsidered to be negative (-).

Note the counter reading, (r).

For COUNTER-CLOCKWISE readings, multiply the counter reading by thecompensator constant (nm/count) provided with your MWA-100 to arrive at theretardation in nanometers. Retardation, R = r x b

For CLOCKWISE readings, subtract the counter reading from 1000 and multiply theresult by the compensator constant (nm/count) provided to arrive at theretardation in nanometers. Retardation, R = (r-1000) x b

From this retardation, you can calculate the stress, which is proportional to retardation.

Move black fringe in thisdirection for mid-layer tension.

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V. CALCULATING STRESS

Remember: If the dial reads a number decreasing from 000, e.g., 870, the correctcount to use is r = (870 – 1000) = -130 counts). This is a negative numberand properly indicates compression. But if the reading is 130, the correctcount to use is +130, properly indicating tension.

After obtaining the retardation value (R), use the following equation to arrive atstress units (MPa or psi):

Where:S = Stress (MPa)r = Dial countb = Compensator constant, nm/countR = Retardation (nm)T = Thickness of material at measuring point, in mmCB = Material constant, Brewsters (2.6 for soda-lime glass)

Note: Multiply MPa by 145 to arrive at psi.

Some common stress-optic material constants:

Glass Stress-Optic Constant, CB

_ (Brewsters ) ___Soda lime 2.6Zerodur 3.0Borosilicate 3.8Aluminosilicate 2.660% Pb 0 2.0

Calculation examples:

a. Find maximum mid-plane tensile stress in psi for 6 mm soda lime glass.The compensator reads counter-clockwise to 130 (r). Compensator constant,b = 3.0 nm/count. Thickness of slice (light path) is 20 mm

Use: S = r x b = 130 x 3 = 390 = 7.5 MPa (tension)T (mm) x CB 20 x 2.6 52

Multiply by 145 to arrive at stress psi units (7.5 MPa x 145 = 1088 psi)

T * CB

r * bSMPa = = R

T *CB

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b. Find surface stress in psi for 6 mm soda lime glass, 20 mm thickness (light path).The compensator reads clockwise to 870 (r = 870 - 1000). Compensatorconstant, b = 3.0 nm/count.

Use: S = r x b = -130 x 3 = -390 = -7.5 MPa (compression)t (mm) x CB 20 x 2.6 52

Multiply by 145 to arrive at stress psi units (-7.5 MPa x 145 = -1088 psi)

VI. USING THE OPTIONAL IMMERSION CELL

Figure 5.ImmersionCells

To assist in directing the light through the sample, we recommend using an immersioncell which requires that the glass slice be submerged in an refractive index matchingfluid. This fluid (RI: 1.52) will optically “smooth” out rough edges.

To use the immersion cell, position the first sample to be measured in the cell andadd enough index matching fluid to completely cover the top surface of the sample.For subsequent samples, add more fluid if required. (Avoid skin contact and clean anyspillage promptly with alcohol). The index matching fluid may be stored and re-used.

Strainoptics offers custom immersion fixtures (Figure 5, at right), that provide adjustablesupport and leveling of glass strips to ensure a vertical light path through the thicknessof the sample.

When evaluating stress distribution in glass cross-sections (slices), using the PS-100-MW polarimeter, three factors are extremely important:

a. The sample must be prepared properly.b. The glass slice must be placed strictly perpendicular to the light path

with its long axis perpendicular to the reference axis of the polariscope(parallel to the MWA-100 compensator.)

c. The sample height should be uniform

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For best results, the sample should be cleanly cut into 25 x 150 mm sections, with novisible chips. The edges may be polished after cutting to minimize chipping.Replacement bottles of index matching fluid may be ordered from Strainoptics, part no.MIF-1.52.

VII. USING THE OPTIONAL CAL-GAGE

The optional Cal-Gage is a uniform-field retarder with a knownand certified retardation value indicated on its housing. Alsoindicated is the direction of tensile stress in the retarder. TheCal-Gage is used to verify instrument measuring accuracy, andfor operator training purposes.

Place the Cal-Gage on the sample stage of the PS-100-MW sothat “tension” is perpendicular to the reference axis of the

polarimeter (parallel to the MWA-100 compensator). With the MWA-100 counter at 000,turn the dial counter knob (increasing from zero) until the center of the black fringe isaligned with the intersection of the vertical crosshair and the horizontal number scale.Take a reading just as you would any sample and calculate the retardation. The resultshould match the stated value of the retarder within +/- 5 nm. To order the optional Cal-Gage, specify Strainoptics part no. Cal-Gage-C.

VIII. MAINTENANCE

The optical surfaces of the analyzer, polarizer/screen, compensator, and microscopeobjective lens should be cleaned occasionally, using a soft tissue and glass cleaningfluid (Windex or equivalent). Periodically, a drop of light machine oil should be used tolubricate the bearing surfaces.

DO NOT use bleach, acetone, or other solvents to avoid damaging the polarizers.

Please contact Strainoptics for spare parts and calibration services as needed to ensurethe best performance of your instrument.

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IX. LIMITED WARRANTY

Strainoptics, Inc. warrants all instruments it manufactures to be free from defect inmaterials and workmanship, and agrees to repair or replace any instrument that fails toperform as specified within one year after date of shipment.

This warranty shall not apply to any instrument that has been:

i. repaired or altered by persons unauthorized by Strainopticsin such a manner as to injure, in our sole judgment, theperformance, stability, or reliability of the instrument;

ii. subjected to misuse, negligence, or accident;iii. connected, installed, adjusted, or used in a manner other

than in accordance with the instructions furnished by us.

At no charge, we will repair, at our plant or at an authorized repair station – or, atour option, replace – any of our products found to be defective under this warranty.

This warranty is in lieu of any other warranty, expressed or implied. Strainoptics, Inc.sole liabilities, and the buyer’s sole remedies, under this agreement shall be limited tothe purchase price, or at our sole discretion, to the repair and replacement of anyinstrument that proves to be defective, when returned to our factory, transportationprepaid by the buyer, within one year from the date of original shipment.

Return transportation charges of repaired or replacement instruments under warrantywill be prepaid by Strainoptics, Inc.

This warranty constitutes the full understanding between the manufacturer and buyer,and no terms, conditions, understanding, or agreement purporting to modify or vary theterms hereof shall be binding unless hereafter made in writing and signed by anauthorized official of Strainoptics, Inc.

Strainoptics periodically offers courses on the subject of the measurement of strain intransparent products. In-house programs are also available. For more information,please contact us at:

Strainoptics, Inc.108 West Montgomery Ave.

North Wales, PA 19454 USA215/661-0100 Fax 215/699-7028

stress@strainoptics.comwww.strainoptics.com