Experimental stress analysis module 4 Part-B (ME832) notes by Dr. Mohammed Imran

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GHOUSIA COLLEGE OF ENGINEERING
RAMANAGARAM-562159
EXPERIMENTAL STRESS ANALYSIS
[15ME832]
Dr. MOHAMMED IMRAN
ASST PROFESSOR
DEPARTMENT OF MECHANICAL ENGINEERING

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Module-4 Part-B
Photo-elastic (Bire-fringent) Coatings
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.
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.
o Bire-fringent coating that produces a photo-elastic fringe pattern
related to the coating stresses.
o Brittle coating that fails by cracking when the coating stresses exceed
some threshold value.
Birefringent Coatings (Reflection polariscopes)
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-

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strain differences, is determined by employing a reflected-light polariscope to record
the fringe orders, as illustrated in Fig.
Reflection polariscopes commonly used in photoelastic-coating measurements: P-
polarizer; A-analyzer; λ/4, quarter-wave plate.
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 non-destructive,
and since the coatings can be applied directly to the prototype, the need for models is

4. eliminated. Through proper selection of coating materials, the method can be made
applicable over a very wide range of strain.
Properties which an ideal coating should exhibit
 A high optical strain coefficient
 A low modulus of elast
 A high resistance to both optical and mechanical stress relaxation to ensure
stability of the measurement with time
 A linear strain-optical response to minimize data
 A good adhesive bond to ens
and specimen
 A high proportional limit to increase the range of strain over which the coating
can be utilized
 Sufficient malleability to permit use on curved surfaces of three
components
Effects of Coating Thickness
When a photoelastic coating is bonded to a specimen, only in a few instance
strains transmitted to the coating without some modification or distortion. More
realistically, the coating is considered as a three
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Through proper selection of coating materials, the method can be made
applicable over a very wide range of strain.
Properties which an ideal coating should exhibit
A high optical strain coefficient K to maximize coating response
A low modulus of elasticity Ec to minimize reinforcing effects
A high resistance to both optical and mechanical stress relaxation to ensure
stability of the measurement with time
optical response to minimize data-reduction problems
A good adhesive bond to ensure perfect strain transmission between coating
A high proportional limit to increase the range of strain over which the coating
Sufficient malleability to permit use on curved surfaces of three
f Coating Thickness
When a photoelastic coating is bonded to a specimen, only in a few instance
transmitted to the coating without some modification or distortion. More
realistically, the coating is considered as a three-dimensional extension of the
Through proper selection of coating materials, the method can be made
K to maximize coating response
A high resistance to both optical and mechanical stress relaxation to ensure
reduction problems
ure perfect strain transmission between coating
A high proportional limit to increase the range of strain over which the coating
Sufficient malleability to permit use on curved surfaces of three-dimensional
When a photoelastic coating is bonded to a specimen, only in a few instances are the
transmitted to the coating without some modification or distortion. More
extension of the

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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