Educational / Student Teams

Stress/Strain Design Apparatus

ME 328 · Cal Poly · Team of Three · December 2025

Worked on a three-person team to design and prototype a photoelastic stress/strain apparatus capable of demonstrating axial, bending, and torsional loading for future mechanics of materials laboratories.

Prototype stress and strain apparatus in the four-point bending configuration — Full-scale prototype demonstrating the four-point bending configuration.

Overview

This project focused on designing an educational apparatus capable of visualizing three fundamental loading conditions through photoelasticity. The system was designed to produce a distinct purple stress fringe when viewed through a polariscope, whilst remaining simple to manufacture, assemble, and use in future laboratory courses.

My Role

Designed the four-point bending subsystem, completed the analytical calculations, developed an Excel design tool to iterate through geometric configurations, and validated the design using finite element analysis.

Concept & Design

The apparatus was developed as a teaching tool for future students studying mechanics of materials. Each team member designed one loading configuration, whilst my contribution focused on a four-point bending fixture capable of producing a pure bending moment within the photoelastic specimen.

CAD model of the complete stress and strain apparatus — Overall CAD model integrating the axial, bending, and torsional loading configurations.

Exploded assembly drawing of the stress and strain apparatus — Exploded view illustrating the apparatus components and assembly arrangement.

Analytical Design

Hand calculations were used to determine the geometry and loading required to generate the target purple photoelastic fringe. Because the required stress was relatively low, the design achieved large safety factors whilst producing the desired visual effect.

Pure bending moment

15.6 in-lbf

Maximum analytical bending stress

444.0 psi

First page of hand calculations for the four-point bending apparatus — Initial loading and mechanism calculations.

Second page of hand calculations for the four-point bending apparatus — Specimen stress and bending calculations.

Third page of hand calculations for the four-point bending apparatus — Component sizing and strength calculations.

Fourth page of hand calculations for the four-point bending apparatus — Final design and safety-factor calculations.

Excel Design Tool

To accelerate the design process, I created an Excel tool that evaluated different geometric configurations automatically. This allowed the mechanism to be refined efficiently before completing the detailed finite element analysis.

Finite Element Validation

A mesh-convergence study and finite element analysis were completed to validate the analytical calculations. The simulated bending stress closely matched the theoretical prediction, providing confidence in the final design.

Analytical bending stress

444.0 psi

Simulated bending stress

421.4 psi

Finite element stress result for the four-point bending apparatus — Finite element results used to validate the analytical bending-stress prediction.

Engineering Drawings & Prototype

A full-scale prototype was 3D printed and assembled to demonstrate the three loading configurations during the final class presentation.

Engineering drawing for the stress and strain apparatus — Engineering drawing produced to document the apparatus components and dimensions.

Second engineering drawing for the stress and strain apparatus — Additional component and assembly documentation for the final design.

Design Results

The completed apparatus achieved the target photoelastic stress level whilst maintaining high structural safety factors. The close agreement between the analytical and simulated stresses supported the validity of the design approach.

Target bending moment

15.6 in-lbf

Lead-screw torque

0.86 in-lbf

ABS safety factor

11.3

6061 aluminum safety factor

Reflection

This project strengthened both my structural design skills and my ability to collaborate within a multidisciplinary team. Coordinating three independently designed loading mechanisms required frequent communication to ensure the final apparatus functioned as a cohesive system. Developing the Excel design tool also reinforced the value of automating repetitive engineering calculations to accelerate design iterations.