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Background Lens Housings, Retaining Rings and Spacers – Axial Loading on Lens – Force Couple – Radial Four Point Bending [1] Mechanical Axis
Citation preview
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/