Concentric Axial Loading OPTI 521L Kal 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

Strain Gage Light Box with Polarizer Outside Diameter Inside Diameter Multimeter Power Supply Polycarbonate Sheet Wheatstone Bridge (Quarter Bridge)

Setup Polycarbonate Sheet – Thickness =.0585” Total Weight = 29.5 Lbs. Initial Voltage = V D=3.24 in D=4 in D=2.5 in

2.5 in Diameter 1 st 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

References 1. content/uploads/2013/07/lens_elements_cross_section.jpg 2. equipment _stress_in_optical_glass_us.pdf 6.