Should Extended Range Viewports Have Protective Covers?

The table below is based on various flexural strength test results* for the expected mean strength over an effective area corresponding to ~ 2” diameter.

Material                                         MPa   kpsi

Fused SiO2                                    105    15.2 

Calcium Fluoride (CaF2)                32       4.6

Zinc Selenide (ZnSe)                       51       7.4

Zinc Sulfide Multispectral (ZnS)   65       9.4

Zinc Sulfide (ZnS)                            70    10.2

Magnesium Fluoride (MgF2)         92    13.3

Sapphire c-plane w/Grafoil         1415    205

*Naval Air Warfare Center Weapons Division – NAWCWD TP 8806

“Weibull Analysis and Area Scaling for Infrared Window Materials”

Go to https://apps.dtic.mil/sti/tr/pdf/AD1018793.pdf

 Compare these viewport material properties to polycarbonate plastic

 Polycarbonate has an ultimate tensile strength of 70 MPa (10 kpsi) and exhibits a percent elongation at break of 100%. As a result, PC has an outstanding impact resistance – estimated to be 250 times greater than that of glass.

The combination of high ultimate tensile strength and high elongation leads to materials of high toughness.

UTS = 70 MPa (10 kpsi)

Elongation = 100%

Impact Resistance = 250x glass