PHOTOELASTIC COATINGS

Photoelastic Coatings• PhotoStress coatings can be applied to the surface of

virtually any test part regardless of its shape, size, or material composition.

• For coating complex shapes, liquid plastic is cast on a flat-plate mold and allowed to partially polymerize.

• While still in a pliable state, the sheet is removed from the mold and formed by hand to the contours of the test part (as shown below).

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Photo stress coating being contoured to the surface of a vehicle water pump

casting

Photoelastic Coatings

• When fully cured, the plastic coating is bonded in place with special reflective cement, and the part is then ready for testing.

• For plane surfaces, pre manufactured flat sheets are cut to size and bonded directly to the test part.

Optical Arrangement

Optical Arrangement for commercial Reflection Polariscope

Commercial Reflection Polariscope

• Portable and commercially available• The angle of oblique is usually of the order 4

degrees.• Engineering approximation starts right from

the data collection stage.

Commercial Reflection Polariscope

Photo Stress pattern revealed on a mechanical controlled linkage system in a passenger Air craft

Typical sample that have been coated for conducting test in lab at universities

PhotoStress contoured shells ready for bonding to engine mount bracket casting.

Some Examples

• A plastic beverage bottle coated for comparison of PhotoStress and finite element analysis.

• Chair showing coating applied to areas of seat which was designed for greater flexibility.

• A complex-shaped automotive frame support member with PhotoStress coating applied.

Photo stress coating applied to a complex shaped augmentor fuel casting from a jet

engine

Some Examples

• A large pressure container with PhotoStress coating applied to a “ribbed” reinforced area.

• PhotoStress coating applied over the entire surface of a fan which was dynamically tested.

• Coating ready for bonding to geared ring.

Biomechanics Application

• PhotoStress coating applied to human skull and jaw. (a) Subjected to shock loads representing blows from sharp and blunt objects.(b) Compressive forces were applied to simulate biting action.

A section of steam turbine blades coated for P.S. analysis under dynamic test conditions

Industrial Case HistoryApplications

• Airplane Window Frame. Several tests have been performed on the window frame of a jet passenger transport.

• Next Slide shows the color fringe pattern in the window frame when it was subjected to 98 percent of the maximum load.

• Note the stress concentration around the holes.

The color fringe pattern in the window frame

Note the stress concentration around the holes

P.S.pattern on a section of wing under 90000 lb load.

Landing Gears

• The landing gears for nearly all modern aircraft have been stress analyzed by covering the entire gear surface with PhotoStress coating.

• Landing gears are fabricated from forged and machined high-strength steel.

• The gear is a complex assembly of parts subjected to various static and shock loadings.

• Occasionally, certain parts are exposed to as many as six different loading conditions.

Landing Gears

• Because the landing gear is used only twice during a flight and represents dead weight the remainder of the time, any weight reduction is of great benefit.

• At the same time, safety is obviously of paramount importance; large safety factors must be employed unless the stress distribution is accurately known for all significant modes of loading.

Landing Gears

• Next slide shows PhotoStress testing in progress on the main landing gear of the Airbus A330/A340 passenger aircraft.

• In this case, the landing gear itself is a scale model made of an epoxy resin material for early design testing.

Landing Gears

• After a thorough survey and analysis of the surface strain distribution on all structural components is completed, suggested changes are incorporated into the initial metal prototype.

• Additional Photostress analysis is then performed to help establish final design criteria prior to manufacture and acceptance testing of the actual landing gear.

P. S. Testing under progress on a main landing gear of Airbus A 330/A340 passenger air craft

Photo stress fringe pattern at a specific area of an Airbus gear during a static test sequence

Final prototyping test on a landing gear from a military fighter jet aircraft and P. S. fringe pattern at several sections of the Landing gear

Photo Stress fringe pattern on a partially coated prototype of Boeing 767 main landing gear

Coated area on Jet engine FramesStrain pattern at a specific location of fuel pads and struts

Coated Jet engine augmentor control casting & surface stress pattern during the pressurization sequence

Aluminum welded joints on space shuttle and fringe pattern at 38000 psi

A typical pure bending specimen with its fringe pattern at 17 000 in-lbs (1,920 N·m)moment. Welds in these specimens were 1.40-in (35.6-mm) thick (made in nine passes).

Assembly Stresses in a Traffic light post

Diesel Engine Flywheel

Redesign of the flywheel (where it mated to the shaft of the diesel engine) significantly reduced the initial assembly stresses

Effect of fiber reinforcement on strain distribution1. fringe patterns appeared as smooth

unbroken lines for the homogeneous material (aluminum)

2. while for the heterogeneous material (fiberglass), they were discontinuous, with a more-or-less scotch plaid appearance and

3. strain pattern on simply loaded honey comb beam

Coating Materials

• An ideal photo elastic coating material should have :

High strain coefficient K ( small coating is sufficient to give enough optical information)

Low Young’s modulus (Even thicker coating does not reinforce the specimen)

Linear stress strain & strain fringe relationEasy bond ability to various specimen material

Coating Materials

possession of good machinabiltySufficient pliability to permit use on curved

surfaces of intricate components• Ideally coating thickness should be as small as

possible so that the interpretation of the coating stresses to specimen stresses is simple and direct (mathematical simplicity)

Coating Material Thickness

• However, the chosen coating thickness should be sufficient to produce a meaningful number of fringes for easy measurement.

• In principle, one can increase the optical response by increasing the applied load.

• However, for elastic stress analysis, the loading on the specimen an not be increased indefinitely.

Coatings Suitable to High Modulus Specimen Material

CoatingMaterial

Young’sModulusGpa

Poisson’sRatio

K Strain Limit%

Max.UseTemp

Suitability

Polycarbonate 2.21 0.28 0.16 --- -- Flat

PS-1 2.50 0.38 0.15 10 150 Flat

PS-2 3.10 0.36 0.13 3 260 Flat

PS-8 3.10 0.36 0.09 3 to 5 200 Flat

PL-1 liquid 2.90 0.36 0.1 3 to 5 230 contourable

PL-8 liquid 2.90 0.36 0.08 3 to 5 200 Same

polyester 3.86 --- 0.04 1.5 -- Flat

Epoxy with anhydrate

3.28 ---- 0.12 2.0 -- Flat/contourable

Coating Materials

CoatingMaterial

Young’sModulusGpa

Poisson’sRatio

K Strain Limit%

Max.UseTemp

Suitability

Coatings Suitable to Medium Modulus Specimen Material

PS- 3 0.21 0.42 0.02 30 200 Flat

PL-2 Liquid

0.21 0.42 0.02 50 200 Contourable

Coatings Suitable to Low Modulus Specimen Material

polyurethane

0.004 ----- 0.008 15 ----- Flat

PS-4 0.004 0.50 0.009 > 50 175 Flat

PL-3 Liquid

0.014 0.42 0.006 > 50 150 Contourable

Maximum Fringe order obtainabe

• If the specimen principal stresses are of opposite sign and if it follows Tresca yield criteria, then for a yield strength Sy of the material, the maximum value of Principal stress difference in specimen is

Nmax = Sy

Maximum Fringe order obtainable

• Maximum fringe order obtainable is linearly related to the thickness of the coating, strain coefficient of the coating material and the yield strength of the specimen material.

• For high strength alloys(Aircraft materials), any N can be obtainable, but for low strength aloys n obtainable is given by above equation.

Maximum Fringe order obtainable

• The range N max for various Specimen materials with h = 1mm, K=0.15, λ = 577 nm are given in the next slide.

• N max represents when load applied to the yield point

• Thumb rule is if you see rich colors, it indicates a very high stress. Fringes observable in P. E. coatings is always less and use white light illumination for better observation.

Material Sy , Gpa Es, Gpa Poisson’s Ratio

N max obtainable

Steel MaterialsHR1020 240 207 0.292 0.78

CD1020 310 207 0.292 1.0

HT 1040 550 207 0.292 1.78

HT 4140 900 207 0.292 2.92

Maraging 1720 207 0.292 5.58

Aluminium1100 H16 140 71 0.334 1.37

3004 H34 200 71 0.334 1.95

2024 T3 345 71 0.334 3.37

7075 T6 500 71 0.334 4.88

Glass 21 46.2 0.245 0.29

Prospher bronze

515 111 0.349 3.25

Calibration of The Coating Material

• Evaluation of strain coefficient K for a photoelastic coating material is known as Calibration of the coating material.

• Cantilever beam under bending is the preferred calibrating specimen.

• The experiment can be either load controlled or displacement controlled.


• The calibration apparatus is compact for the displacement controlled arrangement.

• The precious coating material can be saved by applying only where sufficient level of stress occurs.

• To obtain K accurately, usually the fringe order N is measured for various loads/ displacements as the case may be.


• A best fit straight line is constructed through the data points.

• The slope of the graph gives the ratio of N/P or N/Y0 which is to be used in the above mentioned equations (previous slide) for determining K.


• Coating materials meant for low or medium modulus materials, it is generally recommended to perform the calibration test directly on the material itself.

• “prepare a tension specimen out of coating material and stretch it and then find material stress fringe value. Use relation between Fσ and Fε

• Then find K from Fε = λ/K.

Methodology of Brittle coating

• In this technique, a suitable material which is brittle in nature is sprayed on the specimen under test to form a thin coating.

• The coating is allowed to dry.• An estimate of the maximum load to be

applied is made and load increments to reach this load decided upon.

• The specimen is loaded to the level of the first interval , inspected for cracks and then unloaded.


• The specimen is then unloaded for about five minutes before loading to a load increased by the desired increment.

• As the crack pattern progresses with increasing loads, the locus of points of crack tips is marked with a grease pencil. These are isoentatics where the value of principal stress value is approximately the same.

• Few slides are given to understand the crack pattern with increment in load.

Cantilever Beam under load on its free end

• Bending stress is in longitudinal direction hence cracks are formed parallel to width of the beam.

• Free end is not loaded under applied load, no cracks are seen in the free end.

• What is important in this problem or demonstration?


• Knowledge of isoentatics help in determining the principal stress magnitudes.

• First on set of cracks for crack zone• The crack patterns themselves are important

record of information in a brittle coating test as they represent the stress trajectories / isostatics.

• The most critical cracks in this experiment are the initial cracks that form at the reduced section /stress concentration regions etc.


• Considerable care should be exercised in obtaining the load at which the initial cracks initiate as they represent zones of high stressed regions.

• “Coating is designed to fail at a particular strain. If the crack formed at a given load what it indicates is the strain level is atleast this and it could be more than this.”


• From the inspection of crack patterns formed, one can comment whether the stress field is uniaxial, biaxial or isotropic as the case may be and therefore the number of strain gauges can be reduced.

• For large industrial applications, use of brittle coating combined with strain gauge technique provides excellent results I a short time.

Crack patterns by direct loading• These cracks indicate

the principal stress direction of σ2 .

• By looking at cracks formed, it indicates the uniaxial state or combination of uniaxial state

• Cracks are perpendicular to σ1 direction

Both positive stresses

• Pressure vessel is an example

• Longitudinal stress is much lower than hoop stress

• Second set of cracks form at an appropriate pressure and a bit latter than first set

Isostatics and isoentatics for a pressure vessel

Isotropic stress field

• An example for isotropic field

• Every direction is a principal direction

• Random crack pattern in white wash of building

• Also called as craze patterns

Steps In Brittle Coating Test

1. coating selection2. Surface Preparation3. Under coating4. Application of the coating5. drying• Let us get the details of each step in the

following slides

Step 1: Coating Selection

• Brittle coatings are designed to fracture at a specified strain, of the order of 500 to 1000 micro strain.

• Temperature and relative humidity of the testing room influence the coating fracture strain.

• Coating manufacturers produce coatings for different applications and use at a variety of temperature and humidities.

• Coating choosen should be appropriate for the test conditions

Step 2: Surface Preparation

• Surfaces of the test part and calibration specimens must be clean of all dirt, grease, loose scale and any point that is softened by the coating thinner.

• Plastic surface that are softened by the thinner can be protected by the brittle coating under coat.

• Previously used B.C. can be removed by scrapping, wire brushing or sand blasting followed by using an appropriate cleaner.

Step 3: Under coating

• The under coating is used to provide an easy to see surface under the brittle coating and to eliminate directional reflectance characteristics of the test surface.

• Under coat is composed of aluminum particles in a carrier.

• Apply several thin coats of under coat.• The individual thin undercoats dry in 3 to 5

minutes.

Step 3: Under coating

• Allow at least 30 minutes drying time for the entire undercoat before applying the brittle coating.

• At the same time the model is sprayed and a number of calibration specimens are also sprayed and all are allowed to dry in the same environment.

Step 4: Application of the coating

• The brittle coating must be build up slowly by applying several light coats on the test part and the calibration specimens simultaneously.

Each coat should be applied in one spray pass. Spray passes should be quick and steady from a distance of

about 15 cm. A minimum of one minute drying time should be used between

spraying passes to allow for solvent evaporation (for proper sticking).

If the coating is applied slightly below the design temperature or above the specified humidity, more solvent release time must be used.


• Coats should neither be applied so wet/thick that they run nor so dry that they appear dusty.

• Excessive coating thickness causes sagging, running and trapping of large air bubbles.

• The first coat may not cover the surface evenly but subsequent coats should even out the coating.

• The final coating thickness should be 0.06 to 0.11 mm.


• Uniform color is a good guide to coating thickness. A good coating while it is still wet will appear glassy pale yellow.

• Coating thickness can be measured by measuring the calibration specimen thickness before and after the coating is applied.

• Light dust is acceptable and heavy dust can be dissolved by rapid spraying of a 50/50 mixture of coating thinner.

• Best practice is to apply the coating at 3 to 5 degree C above rated design temperature.

Step 5: Drying

• Brittle coating should dry for at least 24 hrs.• Best practice is to hold the coating at the

elevated application temperature for drying and then to slowly cool it to the test temperature.

• At the same time the model is sprayed a number of calibration specimens are also sprayed and all are allowed to dry in the test environment.

Principal stress direction in Low Stress region by Refrigeration Technique

• In low stress regions cracks do not form. The only information available is that stresses are below fracture stress.

• It may be required to determine the stresses precisely in these regions using strain gauge technique.

• The no. of channels could be reduced if the principal stress directions are known. This can be easily achieved by brittle coating.

• A very significant temperature change of the coating can be achieved by passing a stream of compressed air through a box of dry ice before it is directed on to the surface of the coating object

Principal stress direction in Low Stress region by Refrigeration Technique

• Due to rapid cooling, thermal stresses are introduced which have no preferential direction and are isotropic in nature.

• The combined load and thermal stresses are sufficient to produce coating failure and the cracks are coincident with one of the principal stresses of the original loading.

• Since the direction of principal stresses are known, the no of strain gauges are considerably reduced.

Crack Patterns produced by Relaxation• Situations are encountered where either one or

possibly both of the principal stresses are compressive.• A load is applied to the coating specimen before it has

had the opportunity to dry.• The loading is maintained until drying is over.• If a portion of the load is relieved, the specimen will

stretch, since it was previously compressed, and tensile stresses will develop in the coating which are sufficiently high to indicate regions of compressive specimen stress.

Stress Coat• Stress coat is the most widely used brittle

coating.• Stress coat consists of 150 to 300 parts by

weight of carbon disulfide, 100 parts by weight of zinc resinate base and 0.5 to 30 parts by weight of dibutyl phthalate as plasticizer.

• Shellac and alcohol is the first coating used.• The plasticizer is added to control strain

resistivity of the coating.

Stress coat• Unplasticized coating will tend to craze but, on the

other hand, excessive plasticizer is likely to lower the residual tensile stress to the extent that cracks once formed will close upon the release of the strain which caused the crack formation.

• Strain sensitivities of stress coat; 300 to 3000 micro strain.

• Strain sensitivity is a function of environmental temperature and humidity at the time of the test.

• Plasticizer should be appropriately controlled.

Analysis of Isoentatic data• Isoentatics represents lines of constant stress and are

analogous to contour lines on a topographic map.• For each increment of applied load, the respective

isoentatic lines are carefully drawn.• Let the reference load be Ls and the stresses for an

isoentatic line corresponding to load Li needs to be evaluated.

• If the stresses are linear w.r.to the load, then

• σ1s = (Li / Ls ) Es εd

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PHOTOELASTIC COATINGS