• 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
• 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
– 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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• 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
• 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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4/4/2014 6Hareesha N G, Asst Prof, DSCE, Blore
Reflection polari-scopes commonly used in photoelastic-
λ/4, quarter-wave plate.
BIREFRINGENT COATINGS (…..Contd)
• The birefringent-coating method has many
advantages over other methods of experimental
• It provides full-field data that enable the
investigator to visualize the complete distribution of
• 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
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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
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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
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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
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-
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• 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
• Thus, in the most general case:
– The average strain in the coating does not equal the strain at the
– 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-
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Traction refers to the maximum frictional force that can be
produced between surfaces without slipping