LOAD ANALYSIS BY PHOTOSTRESS STRESS EXPERIMENTAL ANALYIS TEAM MEMBERS

LOAD ANALYSIS

BY PHOTOSTRESS STRESS EXPERIMENTAL ANALYIS

TEAM MEMBERS

Tavera Guerrero Carlos Alberto

Gonzales Gil Arturo

Pérez Galicia Paola Adriana

López Cárdenas Ricardo

PhotoStress

Introduction

The PhotoStress method is a popular and widely used technique for measuring surface strains to determine the stresses on a part or structure during static or dynamic testing. With the PhotoStress method, a special strain-sensitive plastic coating is first bonded to the test part. Then, as test loads are applied, the coating is illuminated with polarized light from a reflection polariscope. When viewed through the polariscope, the coating displays the surface strains on the part as a colorful informative pattern which immediately reveals the full-field strain distribution over the entire coated area. Using an optical transducer attached to the polariscope in combination with computer software, quantitative stress analysis is quickly and easily performed at any point on the test part. Also, with the use of digital video-imaging technology, the PhotoStress patterns revealed during a test, along with the calculated results obtained, can be viewed simultaneously by others by transmission over computer networks.

PhotoStress testing provides an accurate and economical means for stress analysis of any part or structure, regardless of the part’s complexity or material composition.

With PhotoStress we can:

Instantly identify critical areas, highlighting overstressed and understressed regions.

Measure principal stress directions and principal stress magnitudes.

Accurately measure peak stresses and determine stress concentrations around holes, notches, fillets, and other potential failure sites.

Optimize the stress distribution for minimum weight and maximum reliability.

Test repeatedly under varying load conditions, without recoating the part.

Make stress measurements in the laboratory or in the field unaffected by humidity or time.

Detect yielding, and measure assembly and residual stresses.

Figure 1 Photostrees analysis in a can

The applcation of photoelastic coatings to irregular surfaces is a common requeriment. Coatings may be applied by brushing, dipping, or spraying; however, with these methods it is generally impractical to accurately determine the nominal coating

thickness. Further, the coating thickness varies markedly with position on the coated part. Coatings in small fillets might be significantly thicker than in areas removed from Sharp geometric irregularities. It is also an impractical to archive coating thicknesses sufficient for elastic analysis of many materials. i

Photoelastic sheet procedure

Introduction

Instructions for Casting and contouring phtooelastic sheets

Contouring is accomplished best in a reasonably clean area at ambient temperature between 65°F (18°C) and 85°F (30°). Precautions should be taken to avoid or minimize the following factors:

Direct sunlight or direct radiant heat

Extreme drafts of hot or cold air

Dust or particle contamination

Moisture

Contaminant in general

The following procedures presents an organized approach that will lead to a successful contouring operation. Approaches that are less than thorough can sometimes yield satisfactory results; however, for consistent success, the instructions given here should be followed.

The contouring procedure may be divided into seven principal steps:

1) Preparing the casting plate

2) Preparing the plastic (resin and hardener)

3) Pouring the plastic

4) Polymerization cycle

5) Removing the semi-polymerized sheet from the casting plate

6) Contouring the sheet to the test-part surface

7) Removing the cured sheet from the test part.

We are going to treat individually each step and in chronological sequence, the materials that will be used are the following:

1) One Teflon-coated heated casting plate model 012-H temperature-controlled.

2) One Balance.

3) One Snap-Together Frame.

4) Hardener PLH-1

5) Resin PL-1

6) Mixing cup

7) Stem thermometer

Preparing the casting plate

1) Work table. - First the Model 012-H temperature –controlled Teflon-coated heated casting plate has to be placed on a rigid table.

2) Cleaning and Applying Releasing Agent. – Clean the Teflon surface with a gauze sponge wetted with isopropyl alcohol or acetone and wipe dry with a clean, dry gauze sponge. Apply a film of releasing agent to the Teflon surface using a clean gauze sponge.

3) Leveling the Casting plate. – Carefully level the casting plate in two perpendicular directions with a machinist’s level, using the three leveling screws, NOTE: do not move the casting plate from its leveled position while the cast sheet is curing.

4) Preparing and assembling the snap-together frame. - The silicone rubber snap-together frame should be cleaned, and a thin film of releasing agent applied to the narrow surfaces which will contact the liquid plastic, also apply releasing agent to the bottom surfaces, and then assemble the frame on the Teflon surface to the size of the desired sheet which is 5’’x4’’.

5) Covering. – Cover the plate with the Plexiglas cover, this protects the prepared surface from dust and other contaminants.

The temperature of the casting plate is a critical factor in the production of high-quality contourable plastic sheets during the casting process; the casting plate should be heated to within a prescribed temperature range prior to pouring resin for optimum results. Preheating to the specified temperature aids the flow-out of the resin on the casting plate, reduces humidity effects during curing and significantly improves the surface quality of the cast sheet for contouring. These preheating temperatures are shown in the next table:

Photoelastic coating materials for contouring application

Resin type PL-1 PL-2 PL-3 PL-6 PL-8

Hardener PLH-1 PLH-2 PLH-3 PLH-6 PLH-8

Handling data

Amount of Hardener

(pph) 18-20 100 150 70 14

Casting plate temperature

(°C) 32-43 46-52 52-57 52-57 32-43

Mixing temperature

32-43 46-52 52-57 52-57 32-43

Pouring temperature

52-55 52-57 57-63 50 52-55

Approximate time on

casting plate to contourable

stage

1.5 hr 2-3 hr 2-3 hr 2.5 hr 2 hr

Time to complete

polymerization 18-24 hr 18-24 hr 24 hr 12-18 hr 18-24 hr

Finished material data, typical

“k” Factor 0.10 0.02 0.006 0.001 0.08

Modulus of elasticity

[GPa] 2.5 0.21 0.014 0.0007 2.9

Maximum elongation

3-5% 50% Over 50% >100% 3-5%

Poisson’s ratio

0.36 0.42 0.50 0.36

Table 1

Last table of the document

The casting plate model 012-1H, has an integral surface heater and thermostatic control which provides a uniform and closely regulated temperature over the entire plate’s area.

Figure 2 Temperature controller

The procedure to warm the casting plate is the following:

1) Set voltage selector on the controller to the appropriate power supply (115 or 230V, 50/60 Hz)

2) Set the controller power switch to OFF

3) Plug temperature sensor from the casting plate into the controller

4) Plug heated casting plate power cord into controller.

5) Connect controller input power cord to proper voltage source.

6) Set the temperature control switch on the controller to the desire casting plate temperature. According to the table shown above (Table 1)

7) Set the controller power switch to ON

8) As the temperature of the casting plate begins to rise, the heater indicator light will be illuminated until the set temperature has been reaches. The heater light will then blink on and off as the pre-set casting plate temperature is being maintained.

Preparing the plastic

If the resin and/or hardener were kept refrigerated, bring them to room temperature prior to opening the container. If the hardener was not refrigerated and/or has been stored for extended periods, it is important to check its appearance. If the hardener is cloudy or contains foreign particles, discard in favor of a new bottle free of such impurities, is necessary to know that in our practice the resin and the hardener were not in operational conditions due to both have been stored for twenty years, therefore beyond this point we have to consider that the procedure might not be as it is shown.

The first step that should be made is determining the amount of resin and hardener, this will be done with the aid of the following equations:

( )

(

)

( )

According to our requirements of area and thickness we can calculate the total weight as follows:

The amount of hardener is indicated in “parts per hundred” or “pph”, e.g. if 20 pph of hardener means 20 gm of hardener for 100 gm of resin, if we have a total amount of hardener and resin of 23.125 gm, the resin and hardener calculations are mad as follows:

The second step is weighing the resin and hardener, by taking and adequate size mixing cup in order to weigh out the needed amount of resin on a balance scale. Avoid using uncoated or wax-coated paper cups and shallow, large diameter containers.

The third step is warming the resin and hardener, prior to adding the hardener to the resin, warm the resin and hardener to the temperature indicated in the Table1. Intermittent, gentle stirring with a steam thermometer will help to maintain uniform resin temperature during warming, by warming the resin lowers it viscosity and facilitates a more uniform mix with the hardener. Warming can be accomplished by placing the resin-hardener containers on the surface of the heated casting plate, or with a heat lamp.

The fourth step is adding the hardener to the resin; pour the hardener into the resin, being careful to avoid introduction of air bubbles during pouring, add all of the hardener to the resin container. If excess hardener is accidently added, it can be conveniently withdrawn with a medicine dropper, or the excess can be absorbed using a paper towel or gauze sponge.

The fifth step is mixing the hardener and resin, this step is very important in obtaining a quality contoured sheet. It is essential to stir thoroughly but slowly and do not use a whipping action which will introduce air bubbles. Stirring to produce a clear, nonstreaking mixture is best accomplished using the technique illustrated below. The stem thermometer should be brought into line contact with the sides of the container several times during mixing. Line contact should be maintained while making several passes around the inside of the container. The temperature increase, produced by the exothermic chemical reaction during mixing, is easily observed on the stem thermometer, continuing stirring until the pouring temperature is obtained, now the mixed plastic is now ready to be poured onto the

previously prepared casting plate. NOTE: immediately prior to pouring the mixed plastic, set the temperature control switch to the OFF position.

Pouring the plastic

The first step is pouring, the cover from the casting plate has to be removed and pour the plastic. When pouring, hold the mixing cup close to the casting plate surface and pour gently. This procedure will minimize bubble formation, whereas pouring, it is advisable to move the cup to form and “X” or “S” pattern which will improve flow to fill the mold, this is particularly advantageous when pouring thinner sheets, from 1.5mm or less. The final portion of the plastic should be poured along the outside boundary close to the frame. The stem thermometer or a wooden tongue depressor may be used to help spread the plastic over the entire plate surface. Place the cover over the poured sheet to protect it from dust and other foreign mater during polymerization.

The second step is the bubble removal, in order to accomplish it, wait several minutes for air bubbles, when present, to rise to the surface and burst. Surface bubbles which remain can be burst using a pointed scribe or dental probe. A medicine dropper can be used to vacuum air bubbles which might remain at the bottom of the sheet and the cover must be replaced.

POLYMERIZATION CYCLE

The liquid plastic will pass through several steps before arriving at the desired semi-polymerized condition for contouring. At the contouring stage it is semi-stable, but also highly flexible and formable. It has no geometric or photoelastic memory and can be readily contoured to conform to both simple and compound curved surfaces. The time span to reach this contourable state is dependent upon the:

Ambient room temperature

Casting plate temperature

Type of plastic

Thickness

Plastic temperature when poured onto the casting plate

These parameters make it impractical to accurately predict the time span between pouring and removal of the sheet from the casting plate, even though in Table 2 is shown a typical information when casting the more widely used Type PL-1 and Pl-8 liquid plastics.

Conditions PL-1 PL-8

Room Temperature 22°C 22°C

Casting plate temperature 38°C 28°C

Plastic temperature when poured

52°C 52°C

Sheet thickness 2.3 mm 2.5 mm

Time until contourable 1.5 hr 2 hr

Table 2

The following sections present a chronological sequence of the different states of polymerization leading to the time when the semi-polymerized sheet is ready for contouring.

First the early stage of polymerization, the plastic is in a viscous liquid state, when probed, the plastic still behaves as a liquid. It adheres to the probe and a viscous string of plastic can be pulled up with the probe.

The second stage of Polymerization, the plastic is no longer liquid but adheres to the probe. The surface is easily deformed with little or no pressure on the probe and feels very sticky to the touch.

The third stage is almost approaching the contourable stage, the plastic can be depressed with light-to-moderate pressure on the probe. A corner of the snap-together frame can be removed with some difficulty but the edges of the sheet will remain square with no tendency to flow onto the casing plate. The corner can be raised using a quick snap-like finger action. The underside will feel slightly sticky to the touch.

The fourth stage of polymerization, the plastic can be depressed with moderate pressure on the probe. A corner of the sheet can be picked up easily from the mold, with little or no stretching, the plastic underside feels dry to the touch, and the sheet can also be cut easily with scissors, without sticking to the cutting edge. The plastic at this time is flexible, mechanically stable, and in an ideal condition for contouring.

Removing the semi-polymerized sheet from the casting plate

The surface of the test part should be prepared well in advance of sheet removal for contouring. All foreign matter such as paint, scale, rust, oxides, weld splatter, etc. must be removed. Surface preparation prior to contouring consists of the first degreasing and cleaning the surface with acceptable solvents; and second, applying mineral oil to the cleaned surface.

First remove the silicone rubber from around the plastic. If the frame does not release easily, the plastic is not yet ready for contouring.

Second lubricate both hand, scissor blades, and the surface of the part with mineral oil. Mineral oil should be applied to the top surface of the sheet as well. Do not press on or deform the sheet when spreading the mineral oil.

Third, lift one corner of the sheet using a flicking action with a finger. Lift and area large enough to allow grasping between the thumb and fingers. Remove the entire sheet using a continuous, quick, smooth lifting motion. Do not use a slow steady pull. This will cause

stretching and produce unnecessary changes in sheet thickness. Inver the sheet and place it top side down on the casting plate. Now apply mineral oil to this side of the sheet.

Finally use a sharp, lubricated scissors to cut off a 6mm border around the entire edge of the sheet. The thicker edge meniscus is contained along the border and should normally be discarded.

Contouring

It will be necessary to cut the sheet into smaller pieces when separate areas of the part are to be coated with plastic taken from a single sheet. Smaller pieces may also be needed if the part is very complex and irregularly shaped. Place the sheet on the part with the original surface in contact with the test part, the sheet’s top, this is important due to the bottom surface was in contact with the releasing agent on the casting plate, and it is desirable that the opposite surface be in contact with the test par for eventual bonding.

In case that the part to be coated is not very complex, it is convenient to bring one edge on the sheet in contact with the part, and then gently and progressively contour the sheet to the remaining surface. If air pockets form under the plastic while contouring, lift the plastic in that area, add a little more mineral oil, if needed, and progressively contour a second time. The plastic, where contact has no yet been made on the test part, should be hand supported in a manner to negate sagging under its own weight. DO NOT press, push or stretch the plastic into place so as not to cause undesired changes in thickness.

When contouring has been accomplished, the unused portion of plastic can be trimmed, to roughly match the boundary of the part.

After the plastic has been contoured to the shape of the test part, it must be allowed to continue its polymerization cycle to full cure for an additional 18 hours or longer before removing it for trimming, cleaning and eventual bonding. At the end of this 18-hour period, or anytime thereafter, the plastic will be hard, and of the same size and shape as the surface of the test part. ii

Calibration procedure

Introduction

In order to translate measured fringe orders in a photo elastic coating into strains or stresses in the coated test object, it is necessary to introduce the strain-optic sensitivity of the coating. In reflection photoelasticity, the basic relationship between strain and fringe order is:

Where:

= principal strains

N= fringe order

Λ=wavelength of tint of passage in white light

ț= coating thickness

K= strain-optical coefficient of photo elastic plastic

Ƒ= fringe value of plastic coating

It is important to know the distinction between the coefficients K and Ƒ. The strain-optical coefficient K defines a fundamental property of the photoelastic material itself, and it is independent of the plastic thickness or the length of the light path. The fringe value Ƒ specifies the strain optic sensitivity of a particular photoelastic coating.

For typical photoelastic plastics used in the stress analysis of structural materials, K varies from 0.08 to 0.15, with larger coefficients corresponding to the more optically sensitive materials. The fringe Ƒ can be adjusted to suit the stress analysis problem but for more practical cases will fall in the range from 500 to 3000 μ in/in per fringe, with the low fringe values representing the more sensitive coatings.

The simple way to calibrate a photo elastic coating is to use a photo elastic division’s model 010-B cantilever calibration fixture, it consists basically of a rigid cast frame for mounting and deflecting cantilever beam. The beam is loaded at its free end by a precision micrometer, permitting accurate measurement of the deflection. When a beam is mounted in the calibrator

and then deflected by a predetermined amount, a known state of strain is imposed upon the coating. Measurement of the resultant birefringence in the coating provides the

Figure 3 Calibrator

necessary information for relating fringe order to principal strain difference. Following are the step-by-step procedures for performing the calibration and reducing the data.

Installation of calibration specimen on beam

1. The standard beam for use with the calibrator is a 2024-T4 or 7075-T6 (or equivalent) aluminum-alloy bar with the following dimensions: 0.250 +/- 0.001 x 1.0 x 12.5 in (6.35 +/-0.0250 x 25 x 305 mm) and we need to verify that the thickness of the bar is within the specified

tolerance.

2. Cut a 25 x 76 mm calibration specimen from the sheet of photoelastic coating, the calibration specimen is the placed back on the casting plate and allowed to polymerize completely in the form of a flat strip. Before bonding the specimen to the calibration beam, measure and record the coating thickness, then clean and degrease the specimen thoroughly.

3. Then we Mix a small batch of adhesive, and apply a thin layer of the adhesive to the calibration beam where the coating is to be bonded.

4. Apply the calibration strip to the beam Surface by first placing one end in contact with the adhesive and then pressing down progressively along the length of the strip, squeezing out the excess adhesive in the process.

5. Finish the installation by building a fillet of adhesive at each end of the strip. Allow the adhesive to cure at room temperature for the time specified in the instructions for that adhesive.

Figure 4

Calibration measurements

1. Back the calibrator micrometer out sufficiently to clear the calibration beam when it is mounted in the fixture.

2. Insert the beam all of the way into the mounting clamp, center the free end of the beam between the siderails of the calibrator and clamp firmly in place.

3. Using a fine-pointed grease pencil, pen or scriber, mark the coating with a small

cross + on the centerline of the beam and directly in line with the index line son the siderails of the calibrator.

4. Set up the calibrator and 030 series polariscope for normal incidence measurements. On serve the coating at the calibration point while slowly rotating the micrometer head. When the spindle of the micrometer contacts the beam, slight birefringence will start to appear in the coating. Continue rotating until the micrometer Reading reaches a convenient round number (0.025 or 0.050)

5. Accurately measure the fringe order at the premarked calibration point for the initial micrometer setting, and record the result. Rotate the micrometer head four full turns (0.100-indeflection increment) and make a new fringe-order measurement. Repeat the operation, marking a measurement after every 0.100-in increment deflection and continuing for five increments to obtain a total of six readings including the preload measurement at the initial micrometer setting.

Figure 5

Load Analysis

When the plate had already been prepared according to the procedures mentioned above with the photoelastic sheet, with the aid of a test bench we will apply a tension load to our plate so as to visualize the stress distribution nearby the main bore and also nearby the clamping bore according to the photostress theory mentioned in the introduction. The following pictures shows the chronological order of how the stress distribution appears and how it changes by incrementing the load until the plate fails.

Figure 6 Element without load

Figure 7 Element with load applied

Figure 8 Element with increase load applied

Figure 9 Element with more increase load applied before fracture

Figure 10 Element deformed doe the load before fracture

Figure 11 Element fractured

Strain Distribution

In addition to its capability for obtaining accurate strain measurements at preselected test points, photoStress provides another equally important capability to the stress analyst. This is the facility for immediate recognition of nominal strain (and stress) magnitudes, strain gradients, and overall strain distribution including identification of overstressed and understressed areas. This extremely valuable attribute of photoStress, described as full-field interpretation, is unique to photoelastic methods of stress analysis. Its successful application depends only on the recognition of fringe orders by color, and an understanding of the relationship between fringe order and strain magnitude.

The PhotoStress fringe pattern is rich with information and insights for the design engineer. If, for example, a part is being stress analyzed as a result of field service failures, the overall PhotoStress pattern will usually suggest corrective measures for preventing the failures — often involving material removal and weight savings. Because of the full-field picture of stress distribution generated, it may be noted that the overstressed zone responsible for the failures is surrounded by an area of near-zero stress; and a slight change in shape will redistribute the stresses so as to eliminate the stress concentration, while forcing the understressed material to carry its share of the load.

Figure 12

Fringe Identification

White light, generally used for full-field interpretation of fringe patterns in PhotoStress testing, is composed of all wavelengths in the visual spectrum. Thus, the relative retardation which causes extinction of one wavelength (color) does not generally extinguish others. When, with increasing birefringence, each color in the spectrum is extinguished in turn according to its wavelength (starting with violet, the shortest visible wavelength), the observer sees the complementary color. It is these complementary colors that make up the visible fringe pattern in white light.

The next table shows the complete color sequence.

Table 1 Color sequence.

Because of simultaneous multiple extinction of colors, the higher order fringes become fainter than the first, and falls in the transition area between red and green. Fringe orders above 4 or 5 are not distinguishable by color in white light. Although fringe orders higher than 3 are rarely encountered (or needed) in stress analysis with PhotoStress coatings.

Photoelastic fringes have characteristic behaviors which are very helpful in fringe pattern interpretation. For instance, the fringes are ordinarily continuous bands, forming either closed loops or curved lines. The black zero-order fringes are usually isolated spots, lines, or areas surrounded by or adjacent to higher-order fringes. The fringes never intersect, or otherwise lose their identity, and therefore the fringe order and strain level are uniform at

every point on a fringe. Furthermore, the fringes always exist in a continuous sequence by both number and color.

It turns out that the characteristics of photoelastic fringes are the same as those of constant-level contours on a colored topographic map. The magnitudes of the strain levels, as indicated by the fringe orders, correspond directly to constant-altitude levels on a topographic map. And the fringe pattern depicts peaks and valleys, plains and mesas with “sea level” represented by the zero-order fringes.

If there is a zero-order fringe in the field of view, it will usually be obvious by its black color. Assuming that the coated test part has a free square corner or pointed projection, the stress there will always be zero, and a zero order fringe (spot) will exist in the corner, irrespective of the load magnitude, but shrinking in size slightly as the load increases. When there is no zero-order fringe evident, the first-order fringe can often be recognized because of the bright colors adjacent to the purple tint of passage. As an alternative, when the test object can be loaded incrementally from an initially stress-free state, the starting zero-order fringe which covers the entire coating can usually be followed throughout the loading process as it recedes toward unstressed points, and regions where the difference in principal stresses is zero.

Once one fringe has been identified, orders can be assigned to the other fringes, making certain that the direction of increasing fringe order corresponds to the correct color sequence — i.e., yellow-red-green, etc. By this process the observer can quickly locate the highest fringe orders and, generally, the most highly strained regions.

Areas of closely spaced fine fringes will usually attract the observer’s attention, since regions of steep strain gradient ordinarily signify high strains as well. The stress analyst will also note any large areas where the pattern is almost uniformly black or gray, usually indicating a significantly understressed region.

Measurements of stress at a point

Colors may be identified as stress values; however, to obtain quantitative measurements, a measuring device (compensator) should be used. The measurement is done by turning a dial of the compensator until the colors disappear at the point of measurement.

Figure 1 Compensator.

The number indicated on the compensator is then translated to stresses by a computer.

To measure the directions of principal stresses, another dial is turned until a black line convers the point of measurement. Directions of principal stress are then indicated on the structure by a laser light.iii iv

Examples of measurements in fieldv

Figure 13 Airplane Window Frame

Figure 14PhotoStress pattern on a section of wing under

Figure 15 PhotoStress fringe pattern on a partially

Conclusion

Paola Adriana Pérez Galicia

The PhotoStress analysis method is basically by visualization in colors of the intensity of the forces in a surface, this is done thanks to a special paint in the material that modifies its optic properties reflecting the light in function of the forces applied to it. One of the most common issues whit this process is the difference between the images obtain in different materials, as the colors might not be the same and the color chart that its used would be unuseful for a wide range of color variations.

There are some methods to analyze manually the images but is difficult too to our eyes sometimes to difference between certain colors or set transition areas. Even all, all of the methods analyze the colors from the center to outer rings. The black areas are those one where there is no force, thats because the methods analyze from in to out.

Tavera Guerrero Carlos Alberto

This analysis method is still used even though it has a significant disadvantage against the photoelastic method, the main one, is the different perspective of colors according to each person who is analyzing the tested part, and also that is not possible to analyze a parts of big dimensions; but on the other hand the great advantage is that is an economical process and the results might be close to another method whereas the analyzer collect quite a few years of practice or by a software. During this practice we had several factors that affected our process the first and the most relevant one was that the resin and the hardener were not in the optimal conditions due to the several years that they were stocked, therefore the process could not be followed step-by-step as the references. The results were not analyzed so as the calibration process neither due to a lack of equipment.

Ricardo López Cárdenas

I believe that the experimental validation of expected results on the prototype test which refers, to efforts can be supported reliably in experimental methods like photostress.

In this method one can clearly see the efforts that occur and to apply force as this may cause a malfunction. Although I think this method has limited area for which would have to use another method for larger pieces.

Gabriel Arturo Gonzalez Gil

In this practice we analyze using a photoelastic test a test piece made from a resin and was somewhat experimental and we did not made the actual test itself so we assume many things, because the resin was already expired the pot in which we mix the catalyst with the resin some small bubbles were generated and in the end these would affect to measure forces to which we subjected in our test piece. in this practice we learned how to use the different equipment we have in the laboratory and also learn how to use it in other types of photoelastic tests in the future.

i Science and education publishing Searched on February 26, 2014 from:

http://pubs.sciepub.com/ajme/1/7/35/

ii Instruction Bulletin IB-221-C, Measurements Group, Vishay; USA, April 2001.

iii Tech Note TN-702-2, Precision Group, Vishay, Jun. 2011

iv http://www-sipl.technion.ac.il/new/Archive/Annual_Proj_Pres/sipl2012/Posters/1-

2011/poster12a.pdf

v PhotoStress Micro-Measurements, Precision Group, Vishay.