Photo Elasticity

PRESENTATION MEMBERS ARE;1)- Syed Anis Badshah

UW-09-ME-BE--012)- Hafiz Abul Rafay

UW-09-ME-BE--113)- Muhammad Ashiq

UW-09-ME-BE--374)- Muhammad Hashim

UW-09-ME-BE--465)- Hammad Habib

UW-09-ME-BE--43

OutlineDefinition and methodology of

photoelasticity?Theory of PhotoelasticityThe application of photoelasticityPhotoelastic materialsProducts GFP2000 for real-time

photoelasticity PhotoStress Measurement Limitations Conclusions

PHOTOELASTICITY:It is an indirect method of stress analysis, in that a

study is made of the stress conditions in a scale model made from a transparent plastic material, and from this the stresses in a metal component can be deduced, both quantitatively and qualitative results being obtain..

METHODOLOGY: Polarized light is passed through the model in

stress condition and changes the passage of light are thus effected , which are observed on the image of the model at the end of the optical system.

The appearance of an optical stress pattern is illustrated in figure for a pair of mating gear teeth.

These optical results are Related to 1.Stress distribution2.Magnitude and3.Direction in the modelThis provides stresses in the Actual metal component. Model is made of plasticAnd component of metal isOf no significance in Determination of stress in the elastic rangebecause widely different elastic constants would only affect the

determination of deformations and not stresses.

Theory of PhotoelasticityPhotoelasticity applying a given stress state to model utilising the induced birefringence of the transparent material by polarized light examining the stress distribution Natural and polarized light

Light Vector (Amplitude, Direction, Phase Angle), mixed of rays of different frequency

Birefringence(Double Refraction) A transparent material like mica, calcite etc is

birefringent if a ray of light passing through it experiences two refractive indices, usually two rays are “ordinary and extra ordinary rays” The effect of this is to change the polarization state of the transmitted light to form interference fringes

o

e Transparent material

Different refractive indices

Double Refraction

According to snell law

i

t

i

t

t

i

ttii

n

n

nn

sin

sin

sinsineo nn

Optic Mechanics :” the refractive indices are depended on the existing stress state in transparent material”

nlV 2

Phase Angle is

Isochromatic fringes are lines of constant principal stress difference, (P – Q).

Isoclinic fringes happens when principal stress direction coincides with the axis of polarization of the polarizer. The front figure shows appearance of isoclines in a beam during bendingAt 60°, 45°,30°.ORIENTATION Of the polarizer And analyzer withRespect to aVertical datum.

“Photoelasticity can provide both magnitude and direction of principal stress and strain”

Stress trajectories:Isoclines in themselves do not provide a clear

picture of direction of stresses in the model. However from them it is possible to construct a set of curves known as stress trajectories.

Basic Instrumentation for polarized light employed:

Basically three types that isa) Plane polarized, b) circular polarized and c) elliptical polarized

Simplest , but no light pass throughThe analyzer.

Circular Polariscope for Stress Opticon

ColorsLight source Polarize

r

Circular Polarizer

Circular Polarizer

Specimen induced stressed

Analyzer

Fringes in circular polariscope:

Figures are the fringe pattern for a beam built in each end and carrying a uniformly distributed load.

Fig(A) represents whole order fringes and Fig(B)

represents half order fringes

The application of photoelasticity Non-contact optical method for determining stresses and load paths in components or structures

coating 2D investigation 3D investigation

Impact Testing Assembly Stress Analysis Glass Inspection Flaw Detection Residual Stress Analysis FEA Model Verification

Residual Stress in Glass

Photoelastic Materials:Most widely used photoelastic material is an

“epoxy-based resin known as ARALDITE, which is also termed a thermosetting resin”.

In U.S.A several well tried and successful materials for both two and three dimensional work namely “ BAKELITE, FOSTERITE AND KRISTON..” however these materials are rather expensive and show little advantage over ARALDITE.

Materials such as celluloid and Perspex , while exhibiting birefringence , are optically too insensitive for normal photoelastic use, although they are sometimes convenient when determining isoclines.

Requirements for good photoelastic materials

High optical sensitivity.Tensile strength and modulus.Absence of optical and mechanical creepTime edge stressAbility of casting and machining.

Although photoelastic properties tend to vary somewhat from one batch of material to enother , some typical values for various substances are given in the table below.

substances Tensile strength.(lbf/in2)

Modulus of elasticity(lbf/in2)

Poisson’s ratio Material fringe value, lbf/in2. per fringe for 1 in. λ = 5461 A

ARALDITE(U.K) 20 ºC 135 ºC

12000300

0.45 * 10^60.0019 * 10^6

0.3 -

521.4

BAKELITE(U.S.A) 20ºC 110ºC

16000400

0.62 * 10^60.0011 * 10^6

0.360.5

853.3

FOSTERITE(U.S.A) 87ºC

520 0.024 * 10^6 0.48 4

KRISTON(U.S.A) 20ºC 135ºC

8200680

0.54 * 10^60.0138 * 10^6

--------

806.25

CELLULOSE NITRATE(U.K)

6000 TO 8000

0.25 * 10 ^6 TO 0.32 * 10^6

0.34 220

GELATINE(14% GLYCERINE)

---- 14 0.5 0.17

Products GFP2000 for real-time photoelasticityGFP2000 for real-time photoelasticity

•Ability to work with various types of coating materials

•Dynamic Testing •Possible use in dynamic rigs •Real-time images of stress patterns •Compact size camera head and flexible light source and projection head •Use in production line environment, e.g. monitoring the quality of glass or plastic

Key features System Performance

Typical Strain Resolution

~ 20 microstrain (1/100 fringe order)

Spatial Resolution ~ 320x240

Frame Rate

~ Real-Time Calculation: 10frame/s

Capture Video/Post Process: full

frame rate

Image Acquisition

PCI Frame Grabber Card

ProductsGFP2000 for real-time photoelasticity

Tension

Compression Compact Camera Head

Remote Light Source and Projection Head

GFP2000 Test SetupGFP2000 Test Setup

•Orientation of ellipse measures direction of 1

• Ellipticity measures magnitude

ProductsPhotoStress Measurement: PhotoStress Measurement: “ “ full field solution for stress-analysis based on full field solution for stress-analysis based on photoelasticity.photoelasticity.

Verification of FEA model by photoelasticity

Principal stress direction and magnitude measurement

Sets of Photoelastic Instruments))

LF/Z reflection polariscope system

Coating kit

Hand-held portable operationTripod mounted

LimitationsData Acquisition for principal stress

magnitude and directionCoating material and process for damp

concrete-problem from bond failure and unknown shrinkage stress effects in concrete

The difference between thermal coefficient in coating material and concrete leads to errors to field investigation

Too sensitive to the Young Modulus and poisson’s ratio ( local effect from coating process)

Conclusion Photoelasticity theory and its application are given.

The examples of products from venders are described. The most practical use of photoelasticity is to qualitatively investigate the stress-strain pattern in complex specimen. The limitations of this technology is to measure the direction and magnitude of principal stress of a number of interesting locations is impractical due to the data acquisition process. The unreliability in applying the coating material in the field investigation of reinforced concrete structures due to shrinkage, thermal effect, bond, and local effect of material properties.