Photoelasticity Photo-elasticity is a visual technique for measuring stresses. When a photo elastic material is strained and viewed under polarized light, beautifully colored pattern can be observed. This colored pattern provides information on stress-state of the strained material. Photoelasticity

Plane Polariscope It consists of two linear polarizer (which transmit light only along their axis of polarization) and a light source. The linear polarizer nearest the light source is called the polarizer, while the second linear polarizer is known as the analyzer.

Photoelasticity Circular Polariscope This polariscope employs circularly polarized light. The photo elastic apparatus contains four optical elements and a light source.

Photoelasticity Visualization of stresses in a straight beam subjected to four point bending.

Photoelasticity · Light source emits light waves vibrating in infinite number of planes. · Polarization Filter (Polarizer): Polarizer restricts the vibration of light waves to a single plane. Click START. Experimental setup

Photoelasticity Analyzer · Analyzer: Second polarizer used to analyze polarized light passed through material in testing. Doubly refracting lights which passed through the birefringent are resolved by analyzer. Speed difference in the refracting lights causes that a certain color from the light disappears (wave shift). Experimental setup

Photoelasticity Solution When stressed, some transparent materials behave as bifringent material. The speeds of the refracted lights are directly proportional to the principal stresses σ1 and σ 2, major and minor principal stresses, respectively. How to determine stress?

Photoelasticity Four point Bending The straight beam is subjected to tranverse loading.The beam is supported at two ends and downward load is applied at the two points shown in figure. Test Specimen Support Sample specimen Dimensions

Photoelasticity Start The Forces acting various sections are indicated as shown in the figure. Click to start animation.

Photoelasticity Due to application of load the beam is subjected to bending in the transverse plane. The red lines indicate the new position of the beam. Interpretation of 2-D photoelastic fringe patterns is based on the stress-optic law: σ1- σ2 = (N *F σ )/b Where b is thickness of photo elastic model.

Photoelasticity Calculation of material fringe order. σ1- σ2 = (N *Fσ )/b Hence fs can be determined by plotting the quantity Z versus the fringe order, N

Photoelasticity 2D Photo Elastic fringes under exposure of monochromatic light.

Photoelasticity Photo elasticity is the phenomenon of Elastic behaviour of material in lighting conditions. inducing birefringence in a substance through the application of a stress system. Bending stresses and shear stress determination using light Normal stresses

Photoelasticity What does Δn represent in the equation for the Stress-Optic Law? a Thickness of material b Stress-Optical coeffcient c Birefringence d Stress

Photoelasticity The color pattern indicate the of the stressed material? o Stress o bifrigence o polarization of light o strain

Photoelasticity What are the quantities on the axes of a Michel-Levy chart? a Retardation against thickness of specimen b Birefringence against thickness of specimen c Birefringence against time d Retardation against time

Photoelasticity What is the difference between isochromatic and isoclinic fringes? a Isochromatic fringes are obtained using monochromatic light, whereas isoclinic fringes are obtained using white light. b Isoclinic fringes are obtained when the principal stress direction coincides with the polarisation of the polariser; isochromatic fringes are lines of constant stress difference. c Isochromatic and isoclinic fringes occur in different types of plastic. d Isoclinic fringes are lines of constant stress difference; isochromatic fringes are obtained when the principal stress direction coincides with the polarisation of the polariser.

Photoelasticity Resources Books: Experimental Stress Analysis. Second Edition Dally, J W | Riley, W F McGraw-Hill Book Co., 1978, pp. 571, 1978 Reference Links: www3.ntu.edu.sg/.../photoelasticity/index.html