lab report 3: photoelasticity

Introduction

Reflection Photoelasticity is used to investigate stress patterns on complex parts. Using a plastic coating named PS-1B, a special strain-sensitive plastic coating is first bonded to the test part. Then, as test or service loads

are applied to the part, the strains on the surface of the part are transmitted

to the coating which assumes the same strain condition as the part. Photostress analysis can be useful on almost anything engineered such as automotive control arms, frame supports, and even turbine blades (Ref:Vishay Precision Group).

Photoelasticity is used for observation of stress concentrations within a part. The light is projected onto the part and the reflection is viewed through a polariscope to view the fringe count on the surface. Under a constant tensile load, we neglect stress concentrations at the pinned ends and use a plastic plate of sufficient length to distribute the tensile load evenly before approaching the region that we were interested in finding the stress concentrations.

Methods and Procedures

To conduct the reflection polariscope analysis, one must record dimensions of the model and prototype. The specimen with the photoelastic plastic coating is set up in the reflection polariscope and the light is adjusted for optimum viewing. Blocks of weight were added to load the model to 74.7 lbf. Then the compensator device was clamped into place of the reflection polariscope. Then the fringe compensator’s longitudinal axis is aligned with the most tensile principle directions of the four points being analyzed. The fringe dial compensator is turned until the zero order fringe compensator passes over the point. Stress concentration factors are used around the filleted radii by using a graph (see Figure 2). Each group member participated in reading the fringe order from the different points of interest.

References

http://www.efunda.com/designstandards/plastic_design/radius.cfm

-used for Figure 2

Photostress Micro-measurements PDF

-used in introduction paragraph

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