Birfringent Coatings

Stress and Strain Sensitivity Davalsab.M.LR15MDD10

M.Tech (MDD)Daval.techno@gmail.com

Introduction • In the application of coating methods, one applies a thin layer

of a reactive material to the surface of the body that is to be analyzed. • The thin coating is bonded to the surface and displacements at

the coating-specimen interface are transmitted without amplification or attenuation. • These displacements at the interface produce stresses and

strains in the coating and the coating responds. • The analyst observes the coating response and infers the

stresses on the surface of the specimen based on the observed behavior of the coating.

Advantage of Coating method • The capability of applying the coating directly to the prototype: • The "whole"-field response of the coating: Stain gages respond

over small regions of the field and give approximations to strain at a point. Coatings respond over the entire surface of the specimen and give field data rather than point data. • There are two coating methods that are used in stress analysis.

Bire-fringent coating that produces a photo-elastic fringe pattern related to the coating stresses.

Brittle coating that fails by cracking when the coating stresses exceed some threshold value.

Birefringent Coating • The method of birefringent coatings represents an extension of

the procedures of photo elasticity to the determination of surface strains in opaque two- and three-dimensional bodies. • The coating is a thin sheet of birefringent material, usually a

polymer, which is bonded to the surface of the prototype being analyzed. • The coating is mirrored at the interface to provide a reflecting

surface for the light-When the prototype is loaded, the displacements on its surface are transmitted to the mirrored side of the coating to produce a strain field through the thickness of the coating. • The distribution of the strain field over the surface of the

prototype, in terms of principal-strain differences, is determined by employing a reflected-light polariscope to record the fringe orders, as illustrated in

Theory of Photoelasticity• Birefringence A transparent material is birefringent if a ray of light passing

through it experiences two refractive indices. 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

i

t

i

t

t

i

ttii

nn

nn

sinsin

sinsineo nn

nlV 2

Phase Angle is

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

THE SEPARATION OF MEMBRANE AND BENDING SHEARS IN SHELLS WITH TWO BIREFRINGENT COATINGS

Apparatus

Application

coating 2D investigation 3D investigation

Residual Stress in Glass

Non-contact optical method for determining stresses and load paths in components or structures

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