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4/4/2014 Hareesha N G, Asst Prof, DSCE, Blore 1

Birefringent coatings

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This presentation gives the information about: Birefringent coatings, covering syllabus of Unit-6, Sub: Experimental stress analysis for BE course.

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Page 1: Birefringent coatings

4/4/2014 Hareesha N G, Asst Prof, DSCE, Blore 1

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UNIT-6: Photo-elastic (Bire-fringent) Coatings :

• Birefringence coating stresses

• Effects of coating thickness

– Reinforcing effects, Poisson’s ratio mismatch

• Stress separation techniques

– Oblique incidence, Strip coatings

08 Hours

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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.

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ADVANTAGES OF COATING METHODS

• 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.

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BIREFRINGENT COATINGS • The method of birefringent coatings represents an extension

of the procedures of photoelasticity 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 Fig.

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(Contd…..)

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Reflection polari-scopes commonly used in photoelastic-coating measurements: P-polarizer; A-analyzer; λ/4, quarter-wave plate.

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BIREFRINGENT COATINGS (…..Contd)

• The birefringent-coating method has many advantages over other methods of experimental stress analysis.

• It provides full-field data that enable the investigator to visualize the complete distribution of surface strains.

• The method is nondestructive, and since the coatings can be applied directly to the prototype, the need for models is eliminated.

• Through proper selection of coating materials, the method can be made applicable over a very wide range of strain.

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Photo stress coating being contoured to the surface of a vehicle water pump

casting

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Commercial Reflection Polariscope

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Photo Stress pattern revealed on a mechanical controlled linkage system in a passenger Air craft

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P. S. Testing under progress on a main landing gear of

Airbus A 330/A340 passenger air craft

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Photo stress fringe pattern at a specific area of an Airbus gear during a static

test sequence

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Final prototyping test on a landing gear from a

military fighter jet aircraft and P. S. fringe

pattern at several sections of the Landing gear

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Photo Stress fringe pattern on a partially

coated prototype of Boeing 767 main

landing gear

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Coated area on Jet engine Frames

Strain pattern at a specific location of fuel

pads and struts

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Properties which an ideal coating should exhibit 1. A high optical strain coefficient K to maximize coating response

2. A low modulus of elasticity Ec to minimize reinforcing effects

3. A high resistance to both optical and mechanical stress relaxation to ensure stability of the measurement with time

4. A linear strain-optical response to minimize data-reduction problems

5. A good adhesive bond to ensure perfect strain transmission between coating and specimen

6. A high proportional limit to increase the range of strain over which the coating can be utilized

7. Sufficient malleability to permit use on curved surfaces of three-dimensional components

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COATING STRESSES

• Refer class notes

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EFFECTS OF COATING THICKNESS • When a photoelastic coating is bonded to a specimen, only in a few

instances are the strains transmitted to the coating without some modification or distortion.

• More realistically, the coating is considered as a three-dimensional extension of the specimen which is loaded by means of shear and normal tractions at the interface.

• These tractions vary so that the displacements experienced by the coating and the specimen at the interface are identical (as dictated by perfect bonding).

• Thus, in the most general case: – The average strain in the coating does not equal the strain at the

interface. – A strain gradient exists through the thickness of the coating. – The coating serves to reinforce the specimen.

• It is evident that these effects of thickness tend to vanish as the coating thickness approaches zero.

• However, coatings with finite thickness (usually 0.50 to 3.00 mm, or 0.02 to 0.10 in) are required to obtain a high fringe count for accurate fringe-order determinations.

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Traction refers to the maximum frictional force that can be produced between surfaces without slipping