photoelasticity report

4RD SEMESTER, MECHANICAL

PRESENTATION: PHOTOELASTICITY.

GROUP MEMBERS: 1. SYED ANIS BADSHAH ME-0012. HAFIZ ABULRAFAY ME-0113. MUHAMMAD ASHIQ ME-0374. HAMMAD HABIB ME-0435. MUHAMMAD HASHIM ME-046

SUBMITTED TO: ENGR; MUAZAMA ARSHAD

OUTLINE: Definition and methodology of photoelasticity? Theory of Photoelasticity

DEPARTMENT; MECHANICAL ENGG;


The application of photoelasticity

Photoelastic materials

Products

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 (A) for a pair of mating gear teeth.

These optical results are related to; 1. Stress distribution2. Magnitude and3. Direction in the model This provides stresses in the Actual metal component. Model is made of plastic And component of metal is

Of 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 PHOTOELASTICITY: Photoelasticity is the application of given stress to the model which

is made of plastic or any other transparent material. After this we pass polarized light through the model which is composed of a transparent material and we utilize an induce birefringence (double refraction) , through this we examine the actual distribution of stresses of the material.



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. Ordinary ray are energetic and extra ordinary ray are more energetic.

Mathematically:R∝t ( f 1−f 2) ¿>R=Ct ( f 1−f 2)…….>(1) ; where R= relative retardation in wavelength

C = optical constant; f 1 and f 2 = principle stressesEQUATION (1) is called "STRESS OPTICAL LAW" and is the basis of photoelasticity.

Isochromatic fringes : These are lines of constant principal stress difference, (δP – δQ). "Photoelasticity can provide both magnitude and direction of principal stress and strain”



Isoclinic fringes .It 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 bending At 60°, 45°, and 30°. ORIENTATION Of the polarizer and analyzer with Respect to a Vertical datum.



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 is



a) Plane polarized, b) circular polarized and c) elliptical polarized

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



APPLICATIONS OF PHOTOELASTICIRTY: Non-contact optical method for determining stresses and load paths

in components or structures. Impact Testing Assembly Stress Analysis Glass Inspection Flaw Detection Residual Stress Analysis FEA Model Verification

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 creep Time edge stress Ability of casting and machining.

Although photoelastic properties tend to vary somewhat from one batch of material to another, 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: o GFP2000 for real-time photoelasticity.

KEY FEATURES SYSTEM PERFORMENCE



LIMITATION: Data Acquisition for principal stress magnitude and direction Coating 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 poison'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.



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photoelasticity report

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