Concentric Axial Loading

OPTI 521LKal Kadlec

Rachel Ulanch

Overview

• Background– Reactions of Mounting an optical element

• Setup– Experimental Setup for testing stress of an optic– FEA

• Results– Strain vs. Applied Load– Stress Birefringence

Background

• Lens Housings, Retaining Rings and Spacers– Axial Loading on Lens– Force Couple– Radial Four Point Bending

[1]

Mechanical Axis

Background

• Four Point Bend Test– Minimal Shear Stress– Zero Shear Stress in

Center– High Constant Bend

Stress in Center• Great for quantifying

strength of glass over three point bend test due to stress concentrations in glass

[2]Shear-Moment Diagram

Background• Stress Birefringence– Changing of path length for two incident

waves (n|| or n⊥) recombine to create

interference like pattern

[4]

[3]

[5]

Background

• Strain Gages– Measures Voltage– Calculate change in

resistance– Calculate strain

[6]

Strain Gage

Gage Factor Equation

Background/Setup

• Wheatstone Bridge– Quarter Bridge

• Power Supply

Wheatstone Bridge Equation

Wheatstone Bridge

[6]

Initial Setup

• Measure Lens Deflection– 4 inch Meniscus Lens– Compare Concave and

Convex• Glass is extremely stiff!!– No Strain Gauge Reading– SW Confirmation

Setup

Setup Strain Gage

Light Box with Polarizer

Outside Diameter

Inside DiameterMultimeter

Power Supply

Polycarbonate Sheet

Wheatstone Bridge (Quarter Bridge)

Setup

• Polycarbonate Sheet– Thickness = .0585”

• Total Weight = 29.5 Lbs.• Initial Voltage = 5.090 V

D=3.24 in

D=4 inD=2.5 in

2.5 in Diameter 1st Principal Stress

2.5 in Diameter Strain

Results

Results (Removed possible error)

Stress Birefringence

Stress Concentration (Burr from machined part)

Stress gradient

*No stress gradient could be seen inside tube due to weights being on top and constant strain over surface

Conclusion

• In general when designing a lens mount, avoid force couples. – Plastics are prone to adverse effects

• Glass is stiffer and can undergo stress with minimal reactions. – Precision optics can be affected by these minimal reactions

• FEA might not be accounting for possible inelastic set of polycarbonate sheet or adhesive

• FEA mesh not allowing accurate strain value• Actual loading is not a line contact

References1. http://lovedigitalphotography.com/wp-content/uploads/2013/07/

lens_elements_cross_section.jpg2. http://www.testresources.net/application/composites-3-and-4-point-flexural-bend-test-

equipment3. http://www.olympusmicro.com/primer/lightandcolor/birefringence.html4. http://teamuv.org/tag/stress/5. http://fp.optics.arizona.edu/optomech/Fall14/lab/files/Class%20notes/Lab%206/tie-

27_stress_in_optical_glass_us.pdf6. http://www.ni.com/white-paper/3642/en/