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Esco Optics is a leading manufacturer of custom and catalog optics, precision optical components, and ITAR optics in the United States.
Posted by Steve Rowe on
Custom pressure windows
The design of pressure windows is a multi-faceted task. When planning a pressure window for a vacuum chamber, detection equipment, underwater viewport, or other application your material selection should be your starting point. Optical properties are always a consideration, but when dealing with pressure forces, the focus shifts to the mechanical properties of glass. The mechanical properties of glass determine the amount of stress the glass can withstand. Stress is defined as the perpendicular force per unit area applied to an object, in a way that compresses (compressive stress) or stretches (tensile stress) the object.
All glass has defined mechanical properties which are measured in a variety of ways:
So where do you start? First, you'll need to know if your application will have the window clamped or unclamped. Unclamped (figure 1) is the window resting on a shelf similar to a manhole cover or will it be secured on both sides in a clamped fashion (figure 2).
The next step is to gather information for our equation. Circular and rectangular windows have slightly different parameters. With any pressure application, it is important to understand potential failures. Glass just like many other materials can experience material fatigue, scratches, sub-surface damage which can lead to failure. It is critical to understand your application's risk to determine a “Safety factor” value. The safety factor scale is a guide from 1 to 7 with 4 as a modest value sufficient for many applications and 7 as the highest-rated for safety.
Below are the mechanical properties of the most common materials for pressure windows.
Sapphire | Mechanical Properties |
---|---|
Modulus of Rupture | 65,000-100,000 psi ~ 448-689 MPa |
Knoop Hardness | 2000 kg/mm^{2} (9 Mohs std) |
Young’s Modulus | 400 GPa @ 20°C (63 x 10^{6} psi) |
Compressive Strength | 2.0 GPa Creep @ 100 MPa |
Fracture Toughness | 2.0 MPa (m½) |
Flexural Strength | 900 MPa |
Bulk Modulus | 240 GPa |
Shear Modulus | 175 GPa (26 x 10^{6} psi) (Rigidity Modulus) |
Tensile Strength | 300 to 400 MPa |
Poisson's Ratio | 0.29 |
Density | 3.97 gm/cm^{3} |
UV Fused Silica | Mechanical Properties |
---|---|
Modulus of Rupture | 7,600 psi ~ 52 MPa |
Knoop Hardness | (100g load) 522 kg/mm |
Young’s Modulus | 73 GPa |
Compressive Strength | 1.14 GPa |
Bulk Modulus | 35.9 GPa |
Shear Modulus | 31 GPa |
Tensile Strength | 54 MPa |
Poisson's Ratio | 0.16 |
Density | 2.20 gm/cm^{3} |
Quartz | Mechanical Properties |
---|---|
Modulus of Rupture | 7,000 psi ~ 48 MPa |
Knoop Hardness | HK 0.1/20 600 kg/mm |
Young’s Modulus | 7.2 x 1010 Pa (10.5 x 10^{6} psi) |
Compressive Strength | 160,000 psi |
Bulk Modulus | 3.7 x 10^{10} Pa (5.3 x 10^{6} psi) |
Rigidity Modulus | 3.1 x 10^{10} Pa (4.5 x 10^{6} psi) |
Tensile Strength | 4.8 x 10^{7} Pa (N/m^{2}) (7,000 psi) |
Poisson's Ratio | 0.17 |
Density | 2.2 x 10^{3} kg/m^{3} |
N-BK-7 | Mechanical Properties |
---|---|
Modulus of Rupture | 2,400 psi ~ 16.5 MPa |
Knoop Hardness | 610 kg/mm |
Young’s Modulus | 82 GPa |
Compressive Strength | 1103 MPa |
Bulk Modulus | 34 GPa |
Tensile Strength | 48 GPa |
Poisson's Ratio | 0.206 |
Density | 2.51 g/cm^{3} |
Summation
In many cases, we've had customers specify sapphire as their default material due to its hardness and strength. While sapphire can be a good choice for some applications there are alternative materials that will perform just as well for a fraction of the cost. Pressure window applications can be both large and small and many involve gaskets to properly seal between housings. We are here to help alleviate the high-pressure and stress of your custom machined window.
Contact our sales team today to get started.
Esco Optics is a leading manufacturer of custom and catalog optics, precision optical components, and ITAR optics in the United States.
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