Summary of "ME 124 Lab-1A Photoelasticity"

Summary of ME 124 Lab-1A Photoelasticity Video

This video, led by TA Brandon, introduces the first lab of the ME 124 course focused on qualitative Photoelasticity. The main goal is to observe stress patterns in various specimens using a photoelastic setup.

Main Ideas and Concepts

Photoelasticity Overview Photoelasticity is a technique used to visualize stress distribution in transparent materials by observing fringe patterns formed when polarized light passes through a stressed specimen.
Lab Equipment and Setup
- LED Light Box: Provides backlighting for the specimens.
- Stress Opticon: A device consisting of a mount with five pins and a platform to hold the specimen and two Polarizing Filters.
- Polarizing Filters: Placed around the specimen to reveal stress fringes when backlit.
Specimens
- Eight specimens are available: four standard and four optional for exploring different loading conditions.
- Specimens include beams with various features such as holes and notches, and different support and loading configurations.
Using the Stress Opticon
- Loosen knobs slightly to slide polarizers underneath.
- Tighten knobs just enough to hold polarizers in place (not too tight).
- Place the specimen on the platform, secure it with pins, and apply the second polarizer on top.
- Turn on the light box to observe fringes indicating stress.
Interpreting Fringe Patterns
- Fringes appear as lines or bands that indicate stress concentration and distribution.
- Different loading and support conditions produce distinct fringe patterns.

Methodology / Instructions for the Lab

Setup
- Loosen knobs on the Stress Opticon to insert Polarizing Filters.
- Place the specimen on the platform, secured by pins.
- Apply the second polarizer on top and tighten knobs lightly.
- Turn on the LED Light Box for backlighting.
Applying Loads and Observing Fringes
- Apply pressure/load to the specimen according to the desired loading condition (cantilever, simply supported, Four-Point Bending, opposed concentrated forces).
- Observe the fringe patterns that appear as the specimen is stressed.
- Note changes in fringe patterns with different specimen features (holes, notches).
Analyzing Different Specimens and Loading Conditions
- Simply Supported Beam: Contact at three points; observe fringe patterns as loads are applied.
- Four-Point Bending (Constant Moment Beam): Equal pressure at two points; fringes are parallel in the center.
- Opposed Concentrated Forces: Loads applied at two points not touching the specimen; distinctive “X” shaped fringe patterns.
- Cantilever Beam: Load applied at free end or center; fringes appear along the beam length under bending stress.
- Specimens with Holes/Notches: Larger holes cause more perturbation in fringe patterns; notches introduce pre-existing stresses visible in fringes.
Observations
- Fringe patterns correspond to stress distribution and help identify stress concentrations.
- Changes in specimen geometry (holes, notches) affect fringe patterns and stress distribution.
- Different loading conditions produce characteristic fringe patterns useful for qualitative analysis.