EXPERIMENT 10: PHOTOELASTICITY TENSION
INTRODUCTION
Photoelasticity is a valuable stress analysis technique that allows for nondestructive examination
of stress distribution within materials. It relies on the optical property called birefringence, which
is exhibited by certain transparent polymers. The key instrument used for this purpose is the
Plane Polariscope. It comprises a light source, a polarizer, the specimen, and an analyzer that is
consistently oriented perpendicular to the polarizer. Additionally, there is a circular polariscope
variant that includes a light source, polarizer, 45º quarter-wave plate, specimen, a second
quarter-wave plate, and an analyzer. The two quarter-wave plates are aligned in a crossed
configuration to minimize inaccuracies stemming from defective plates. The analyzer can be
oriented either parallel or crossed with the polarizer.
OBJECTIVES
The primary aim of this experiment is to visualize the emergence of stress within a material by
observing the fringe patterns that form within it. This technique is particularly applicable to
materials known as photoelastic materials and provides insights into deformations. When
polarized light is passed through a typical specimen, it generates fringe patterns, which are
commonly used to analyze two-dimensional planar stress scenarios.
METHODOLOGY
1. Material Selection: Many polymers exhibit photoelastic behavior due to their
birefringence. However, polymers such as PMMA and polycarbonate may be too brittle
or sensitive to stress. Homalite-100, an optically high-quality material available in thick
sheets, or the newer material PSM-1, which offers good machining properties and fringe
sensitivity, are suitable choices.
2. Template Creation: Before producing multiple identical parts, it is advisable to create a
metal template. A template can be used to manufacture several photoelastic specimens
with the same shape, ensuring consistency.
3. Sample Machining: When creating a specimen from scratch, take care to avoid
overheating the edges. Use a sharp milling cutter and make light cuts. To reduce heat
generation, you can employ coolants such as ethyl alcohol, kerosene, or water.
4. Viewing Loaded Specimen: After cleaning and removing the specimen from the
template, it is ready for loading. A polariscope is essential to observe stress-induced
fringes. Ensure that the components of the polariscope are properly aligned to allow light
to reach the specimen. Insert a loading frame between the first and last components of the
polariscope to apply the desired load. While monochromatic light is preferable for
obtaining clear fringes, the light source need not be coherent, and the specimen may or
may not be cooled.
5. Fringe Pattern Capture: The patterns of fringes can be recorded using either a still
camera or a video camera.
6. Material Testing: The sensitivity of a photoelastic material is determined by its fringe
constant, denoted as fσ. This constant establishes the relationship between the fringe
value (N) and the specimen thickness (h) in the direction of light propagation. It allows us
to relate primary strains in the plane normal to light propagation to Nfh. To determine the
fringe constant, perform an experiment using a basic geometry model under known loads.
OBSERVATIONS AND CALCULATIONS
S.No
Load (P) (in kg)
Fringe Order (N)
1
22.5
1
2
45.3
2
3
63.6
3
Using points 1 and 3 we get the slope of the P-N graph as :
t=9.47mm
w=15.85mm
Slope = (y2 -y1)/ (x2 -x1) = (63.5-22.5)/(3-1)=20.55kg/fringe=P/N
Fσ (material fringe value) = P/N*w;
Fσ= 20.55/ 1.585
Fσ=12.965 kg/ fringe-cm;
SUMMARY
The theoretical values are in accordance with the experimental values and verify the nature of
our calibration experiment.